audio handbook no.3 - the use of audio frequency transformers - norman h. crowhurst (1953) clearscan

8/17/2019 Audio Handbook No.3 - The Use of Audio Frequency Transformers - Norman H. Crowhurst (1953) Clearscan

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HANDBOOK No. 3

To wok ropely an af. tasme, e a valve equis stbect

vaus

Ths oo shos i det how it vaes aetaf tasme pefomaeSipe-tse hat ae povded to d a cit stae anyaf tasrme h the ad of a detahae pea ueA appedx t th paaetes o toc ie b

y

eadg utueput ntetae dve opt ad mtching types

With 44 dagrams drwn by the Author

LONDO: NORM PICE (PUBSHERS) LTD.

I ECK ROOK CO. ITDNew and econdh Rokslr� 5l PULTFNEY STRFE xpnce etADEiID S.


2/66

The Use

of a. ransformers

N H. CROWHURST, A.M.1EE

NORMAN PRICE (Publishers) LTD.283 CTY ROAD, LONDON EC I.


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AUDIO HANDBOOK No. 3: THE SE OF

AF TRANSFMES

irst Publshed 1953

Pined in Great Britain by Thomasos Ld., Ceda ress Hounslow Middx


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CONTENTS

INTRODUCON page vi

I. ASSESSNG THE PERFORMANCE OF AN

AUDIO FREQUENCY TRANSFORMER 9

. CAUSES O DISTORTION 14

3. EQUIVALENT CIRCUS 23

4 MEASURING UP THE EL ECTRICAPROPERTIES OF TRANSFORMERS 30

5. SING THE CHARTS 34

APPENDIX 52

NDEX 61


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ILLUSTRATONS

FIGUE

1 Method of making ratio chk2

Checking the fequency response of an otpu ransrmer3 Checking he equency response of an inpt or inersage ransrmer

4 Osciloscope mehod of checkng r distortion in otpt transrmer

5 Oscilloscope mehod of checking r distortion in an otp orinersage ransrmer

6 Voltage and magnetizing crre waverms as saraion isapproached

·

7 Disribtion of inpt voltage across source impedance and primay8 Presentation of waverm of Fig. 7 on 'scope scrn, sing circui

of Fig 4 or 5

9 Ellipial load lines de to hning c o pimay idtan0 Waverm distorion due to low primary indcanceI I Waverms associaed wih power drive when sorce impedance

is too high12 amped oscillations excited by cessation of grid crren

I3 Typical power limi esponse presenation4 Complee eqivalent circit of tyical adio transfrmer

I 5 ssential eqivalen circi fr deemining insertion loss6 Eqivaent circit r esponse of direc copled ransrmer17 Reone fm o idea low eency ct-of aming ontan

inductance

8 Typical circit fr paralle d iersage transrmerI9 quivaen circit r I.f. respose of paralel d ransomershown in Fig. 18

20 Eqivalent circuits r hf. response of adio transfrme2 Variation of h response with exeal circi resisance vales22 Varation of hf response wih eteal circui resistance values.23 Variation of h.f. response with circi reactance values24 Variaton o h resonse with circit reactance ale25 Measring eecive primary inductance26 Finding leakage inductance and winding capacitance by muta

resonane

27 Finding approximate winding capacitance by resonance wihprma indctance28 Method o nding capacitance ale graphically om reslts

sing the setup of Fig 2729 Use of the detachable scale fom page 46 to see he eec o

aing r30 Use of the deachable scae to s the eec of varying R3 I Use of the deachable scale to see the eec of vaying L32 Use of the deachable scale to see he et of vaying C33 Use of the chart of Fig. 42 o deermine eciency34 Abac r calclating combined resistance of resistances in paralel35 Char r calclating reactance of capacitance or ndctance a

any adio eqency36 Chart r calcuaing imdance transrmation and transrmer

t atio37 Chart elaing load impedance maimm volts and power rating38 Char r identifying esponse shape on Fig 4 and showing

variaion wih circit vale39 Chart r identifying reference freqency r the appropriate

esponse crve of Fig. 4 with h response40 Response crves applicable by means of the hars in Figs. 38

and 39 o all h.f. cto shas

PAGE

10

IIII

2

2

66

17

17 8

19

20

2

23

24

25

25

26

2627

28

29

29

0

31

32

33

33

737

38

38

39

4 2

4 3

4

45

46

47

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4

42

43

4

Chart r idenifying erence equency i .f. caes usig paralelEciency CharThe leered compoet ca b varie in amos imiles com-biaio

qivalent circui fequency respone of circui of Fi 43

9

51

5


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NTRODUCTION

For many years now there has been a general prejudice against

the use of transfrmers in audio circuits except where absolutelynecessary. Quaity enthusiasts have aimed at transfrmerlessamplir, and ircut hav vn bn dvid to liminat ipuand output transrmers.

Much of this preudice is du to lack of understanding of theunctiona· detais of an audio transrmer Plenty of good components are avaiable and properly used they need introduce nomor distortion than the ingenious circuits which have been devisedto elminte them. When propery understood audio transrmers

wl e fund to have a ul pla in audio iruitry Esstialatures in the presentation of this book are the charts fr predictingperrmance in varying circuits and the appendix giving practicaldata of manucturers' products.

The author woud ike to take this opportunity of thankingthe manucturers listed in the appendix fr their co-operaionin making this data availabe When preparation commncedit was hoped that a larger nuber o manucturers could beincluded particulary some of the Amercan companies. To makethis data availabe each manuturer has had to adopt on othre courses: () making the necessary measurements himsefwhich are non-routine om his production viewpoint and hencerequre special cilities not normally catered r; supplyingsamples of each component so that measurement could be made pplying omplt intrnal ata of th ontrution of thtransfrmer om which the necessary data could be derived.The nglish manucturers listed have all co-operated in on ormor of these ways.

The ct that none of the American manufcturers has so rbeen able to cooperate should no be taken as a reection on thembecase of the particular diculties invoved r them in each of thabov ourses. Maing the nssary measurmnts themselvsrequired manhours which were ot availae; supplying samplesr measurement is out of the question due to current trade resrictions while the supplying of intrnal data especialy r relaseoutside their own country is unerstandably against their poicy.he author is condent that this presentation of inrmation aboutaudi transormers will prove its value he fels sure that ourAmeican iends when they have seen the presentation willwant to aval themselves of the opportunity of being include inate ditions Meanwhile American readers can apply the methodspresented in the book to home-bought components by using thesimple measurement procedure outlned in Chapters and 4.

The older reader may have noticed that technica literature hastendd to make circuits go by fshions or instance when the


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ASSESSING THE PERFORMANCE

U IO transrmers have come to be viewed wth a certaiamout of suspicio ad r a log time the polcy hasbeen only to use an audio transrmer where absolutely

necessary. Which usually means at the input and output of anamplier. Probably some of the suspicon s the outcome of someearly eorts at producig audio transrmers, based o the idea

that all an audio trasf9rmer needed was some sort of core wthsome turs wrapped roud it-some even tried makig the corerom attened out cocoa tins! Modern material and desig haveresulted i a rage of audio trasrmers that ca serve mayuseul purposes and, if properly used wil not do ay of the unnyhigs of which they are so often suspected

The rst hig a user wants to do wth an unknow audiotransrmer is to check up its perrmance om the various vewpoits i which it could il to meet hs requiremets.

Ratiohe rst property to check is its ratio The classic methoduses a ratio bridge built round a complicated multiratio transrmerwhich is used as a standard capable of giving steps ratio atextremely smal ntervas By balancig the trasrmer to betested against this stadard in the bridge circuit the ratio can bechecked to such accuracy that one turn too may or too few oneither the primary or secondary widings ca be detected; but theaverage tasrmer user is ot interested n kowig ratio to thsdegree of accuracy Usually, simple measuremets with a voltmeter wil eable he ratio to be calculated with quite enoughaccuracy.

A word or two is ecessary here about the precautions necessaryto prevet arrivig at a seiously wrog aswer he best equecyto use r the test would realy be somewhere near the middle ofthe audio range but it is often simpler to use a voltage obtaiedom the mais d this will give quite good accuracy f precautions are taken to see that the transrmer core is ot being saturated.Frequences above the middle of the audio rage are likely to gve

serously iaccurae results because of the eect produced by thenternal reactances of the transrmer leakage inductace andwndig capacitance.

o esure that he trasrmer is ot saturating the bestplan is to compare the voltage across the primary and secondayat a number of settings of an input potentiomete gure 1 shos

9


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10 TH US OF .F TRNSFORMRS

S

I

Jl

BE

C NPUT

VOT GE

INT MESURE TRANOER SUREPOTENTOMETER IPU UNDER EST OUTU

VOLS TS

FIG. 1 METHOD OF MAKNG RAO CEC

a simple arrangement fr sch a ratio check A reasonably highresitance rectier type voltmeter shold be sed fr making these

checks peably one o the mlti-ange types with at least 1000ohms per voltHaving checked the feqncy ratio the ser now knows

what eect the transrmer will have on impedances conectedto it: they wil be transfrmed in proportion to the sqare of thetrns ratio ; resistances and indctances will be stepped p by astep-p transrmer whilst capcitance will be stepped down bya step- transfrmer and vice ersa.Frequency Response

The next thing to check is the reqency response of therasrmer This is not qite sch a smple matter as chekingratio

Many eqency responses are pblished by manctrersr ther prodcts which are practcally meaningess to the serbecase they do not state the conditions nder which the requencyresponse is taken s will appear rom the later part o this book,it wold be qite possible to adjst circit vales to prodce anextremey nice looking reqency response which might not be

obtained when more practical circit vales are applied Bt thereader shold not inr that manctrers who pblish hesecrves are trying deliberately to mislead. saly the crves havebeen taken in a circit smilar to the one fr which the transrmers itended and which is pobably qite a pactica circt. Evenso the manctrer omits to tell the ser what his circit wasso the esponse crve only indicates that the feqency responsewill be somewhere in that region with the nominal impedancesvales stated in its specication

What seems to be overlooked in specifying the circt impedancesrelaed to transrmer operation is that transfrmer action is atwoway eect Take eample an otpt tansrme notonly does the transrmer mltiply the load applied to its secondaryby the sqare of the trns ratio to prodce a working load r theanode circit of the otpt valves; the ac. resistance of he outptvaves is also transrmed down being divided by the sqare of thetrns ratio to appea as a sorce resistance at the otpt terminals.From the viewpoint of feqency response the important thingo reaise is that the sorce impedance om which the transfrmer


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AEING HE PERFORMANCE 1 1

operates, in th case the ac resiance o the ouput ae a

wel a the oad mpedance into whch t ed both aet tsequency reponeSome epone have been aen by holding he olge he

primary terminals o the transrmer contant a fequeny ivaried and plotting he variation o otage at the econdaryterminals as the equency response. Such a respone ha nopractcal signicance at al l becaue it using zero source mpedancen eect. ll pracca circut wor with ome actual aue oourc mpedance.

To tae a requency response that ha practical ignicancehe input voltage should be applied through a seres reistane othe ame alue as the pracica source impedance to be used gure2 hows he arrangement checking equency response o anoutput tranfrmer and Fgre 3 r checking he equency responeo an input or interstage transfrmer. Input or nterstage ypeare est measured worng ino he grd o the actual alve r whichthey are designed because there will be the ame grd input capaci-tance appied to their econdary termna.

AC R SIST H

LO OFO

VAES S

= r- l

ADJUST VAGE RANSERTO SA REDI UNDER ESAT EACH REQUENCY

MESURE OUTUT

OSFG 2. CECNG THE FREQUENCY RESPONSE OF AN OUUT TRANFORMER

SOURCEINPUT FROM ESISTAE

AUDIO OSCAOR

.•OAD,

·� USED•t'I

RAER NORMA STAGEUNDER. ES OOWING

NSOERWI WIC N ITI , SE NPUT TOVE SAME RENG A EH

FQUEY;

RED PU ICI I O i.

G. 3 CECKING T RUENCY RESONSE OF N INU OR INTRSTAGETRANSFORMER


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12 HE USE OF A.F. TRANSFORMERS

Wavefrm

Having checked the rati f a transformer and its eqencyespnse the net ting t d is t ceck it r distrtin Thispartcuary applies at the lw equency end f the adi specrms these tests shud e cnducted at te lwest equencie thetranrmer is t hane. The same cicit as tat sed reqency respnse measrements shd e sed, ut te wavermat the tput sud e examined n an scilscpe.

The vis methd f cecking waverm is t apply thescilscpe rst t the input terminals whee the input vtmeters cnnected in Figres 2 r 3 and then t transr it t the tputtermnas. If the waverm at the input terminals is a gd sinewave then any distrtin t the sine wave apparent at the tputmst e de t the transrmer; bt if the inpt wavem is nt apure sine wave a change f shape at the utpt des nt necessarilymea te transrme is distrting. The cange may e de t

SOUERESISANCE

INP FROM AIO

OSCLAOR

HIGV UE

POT

L°DRESISANCE

RANORMER OSCLLOSCOEUNER ES

4. OSCILOSCOP MTHOD O CECK O SOO OTU

ASOME.

INP FROM AUDIO OSCLAOR

SORCERESISANC

OSOSOPEF 5. OSCOSCOP MEHO Of CHEK O ISOO N A NT O

NAG RAOM

the eqency respnse f the transrmer causing dierent ampi-catin and phase cange fr the varius cmpnents f the inputwave. Using waveform examinatin as a means f checkig rdistrin the nly satisctry way is t see that the input wave-rm is a pure sine wave sing ltering t get rid f hamnics ifnecessay he classic way measring waverm distrtin


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ASSESSING THE PERFORMNCE 13

is to put n a good sine wave and masu up th components of

vaious quncis in th output by mans of a wav analyseA simple and moe diect method of chckng wavemdstotion is shown at Figus 4 and 5 output and input ointestag typs of tansm espctivly. Using this mthodthe absolute puiy of th input wavefm is not so impotantInstad of using a timbas hoizontal deection of th oscilloscop the inpu wavefm is used and th output wavfm sapplid to th " Y pates in the usual way The advantag ofthis mthod of psentation is that any disoion on the wavem

s caly indcatd by dviation om the saigh lin and theshape of th deviation identis th caus of distotion moe adilythan s possible by the cassical analysis mthod so ths way hasth advantags of both simplicity of mthod and claity of thnfmation convyd.

t Pick-up

Input and intstag tansfmes fr opation at low sgnallevels may be susceptbl to pic-up of stay hum lds o othspuious signals. It is athe dcult to make spcc masuementsof h cacy of dien kind of scening in pvnting thispick-up.

Manucues usualy specfy the amoun of ducion inpickup tha th scning td povids ; this mans that if htansm is mountd in a unim hum ld out in spac thepick-up in th cor is rducd by whatvr numbr of db thspecicaion stats whn th scrn s td ; but pactical humeds a not usually unifm due to the ct that the tansms probably mountd on a chassis and the psence of the chasss

distots the elco-magntic eld causing th touble. So althoughth tansfm gives is statd duction in hum ld unddealisd tst conditions it may not always gv qute such goodesults und pactica conditions This ct should not be makdup as mispesntation on the pa o he tansme manufctue:he cannot know exactly what so of ld is going to b und onthe amplie chassis whee the us is going to x he tansfme

The only test of pactical use is one using th actual ampliechassis whe th tansme is o be mountd. Having mountd

up the ansme and connctd t in cicuit the hum levl shouldbe masued on a snsitiv voltmete and compad with maximumgna output To b saisctoy h hum levl should b notgeate than 3 p cent. of maximum 0tput voltage and pablyit shoud b wl below 1 pe cent

In making a tst of this knd geat car should be takn tose that the hum signal masud s ally due to stay pic-up.The simplst way to make ctain of ths point s to disconnctth pimay of the tansme om its associatd cicuit, twistthe leads togth and teminate thm with a sistance equal to th


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14 TH USE OF AF. TRANSFORMERS

primary source resistance. The transfrmer should be removedfom ts chassis mountng and i i employs a metal case this shouldbe connected to earth by a clp lead. Now the transrmer, whilestll in crcut on its seondary side can be moved around to see theeet o stray pick-up. I moving it around in all directions doesnot at the hum present n the output then this hum s not dueto pickup n the transmer and the cause o the hum should besought elsewhere and remedied bere urther tests are made trnng the transrmer aroud does alter the hum level consderably then t s evident that the transfrmer is pcking up the

hum beng head possble a ouning poiion should be undsuch that the hum is a mnimum and o satisctorly low eve this is not possible a component with better magnetc screeningagainst hum must be sought

2

CAUSES OF DISTORTION

AFTR assessing ts perfrmance, and checing up o the

vaous ponts where distortion o signal coud occur thenet mportant thng is to understand cleary the cause o

these various rms o dstorton

Frequency Respnse

This frm o distortion has someimes been called fequenydistorton. Brtish Standards Institution recommends caling itatteuaton dstortion. Although a poor fequency responseresults in dstortion o the input signa so that the output sgnaldiers rom t the author fels that the designation " requencyResponse is a better heading r onsderation o ths characterstc.

Any audio transrmerin ct any piece o audo equipmentls o peeion n that the epone doe not exend unimyom zero to innty n fequency but possesses both a low equencyand high fequency cut-o. By denng this ilure as a rm odstorton imples that all audio equipment introduces this kind odstoton In point o ct, provded the response is at rombew the lowest requency to above the highest fequency there is

no attenuation distortion to the actua signa handed Theremay be some phase dstortion, at the extreme ends o the rangeparticularly; but this s generally regarded as unimportant anaudio amplier fr its own sake t is true that phase shit can beimportant fom the vewpoint o eedback amplier design (thisis eplaned ully in Audio Handbok No. 2).

The requency response o a audio transrmer n the mddeo its range s almost level so the critical parts o the spectrumom the vewpoint o dstortion are the low requency and high

equency end


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CAUSES OF DSTORTION 15

Factors aectng the low equency response o a transrmerare ts own prmary nductance compared wth the external crcutmpedances and n the case o parallel d nterstage transfrmers,the couplng capactor Factors aectng the hgh equencyresponse o the transrmer are he wndng capactances and theleakage nductance between wndngs

The dea o wndng capactances s not dcult to grasp buteakage nductance s a subject that s somewhat msunderstoodt s due to magnetc ux that gets down between the wndngsnstead o gong rght round both o them through the transrmercore as t shoud. The ux n the transrmer core s governed

by the appled otage nd equency but the leakage u sgoverned by the load current n both wndngs there s nocurrent n the seconday wndng then there wll be no leakageux; as curren n the secondary wndngs s ncreased the leakageux ncreases. Due to ths leakage ux a derence n voltagebetween the wndngs occurs whch s not accounted r by theturns rato o the transrmer. he relaton between the currentn ether wndng and the voltage derence due to leakage uxbecause o that current has the propertes o an nductanc

Transrmer desgners can reduce leakage nductance bysectonalzng the wndng so that t s more dcult r u tocreep between sectons (detals o ths wl be gven n HandbookNo 5 on Audo Transrmer Desgn). The pont to realse heres that the crcut made up by leakage nductance wndng capactances and external crcut values s responsble r determnngte hgh equency esponse.

he tendency o leakage nductance combned wth wndngsel-capactance s to produce a peak at the hgh equency end othe esponse. Ths suggests that the leakage nductance ca beregrded as n paralel wth wndng sel-capactances. Thisvewpont s not qute true The nductance s best vewed asbeng n seres wth wndng capactance the output beng connectedacoss jut the capactance The reader wll remember that analternatng voltage s appled across an nductance and capactancen seres at ther resonant equency a voltage appears across boththe nductance and capactance hgher than that apped acrossthe two together Ths s how the peak appears n the outputoltage whch s eectvely n paralle wth the capactance. hs

wll be better understood ater readng the next chapter onequvalent crcuts

Wavefm Dstortion

Ths s generaly called harmonc dstorton because t s adstorton that ntroduces harmoncs to a pure waverm that werenot there bere A transrmer wll not o tsel produce apprecablewavefrm dstrton n the mddle equences o ts range. the oad that t provdes r the anode ccut o the precedng


17/66

16 THE USE OF A TRANSORMERS

G. 6. VOAGE AND MAGNIZG CRRN WAVFORMS AS SATRAIO SAROACHD

stage is not correct r the ale then istortion may be presentthrughou th whole equency range but hs is not directlydue o the transrmer

The only waerm distortion irectly ue to the transrmerappears at the low equency end of the range an is ue to thenonlinear waverm of he magnetizing crrent. igre 6 showshe corresponding voltage an magnetizing current waveorm ra typical transrmer approaching its saturation poin.

The resul of rawing this peaky current wavefrm rom asource possessing resistance, usually the a.c. resistance of the valve,is to produce a distortion n the oltage wavefrm on both thepimary an the seconary. or this reason it wil be no usecomparng he waverms on pimary and seconary of the transrmer because both will be the sae shape one being largertha the other because of the step-up, or step-own

NPUT VOLTGWAVFRM

VOLTAGE ACROSSPRIMARY

FG 7 DISTRBO O IU VOLAG ACROSS SORCE DACE AND PRARY


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CAUSES OF DSTORTON 17

Fgur 7 shows how a snusodal nput voltag s dstrutdacross th sourc mpdac ad th transfrmr prmary Thdisortd wavrm show as across th prmary wll mry blargd across th scondary

IG 8. PRSNATION O

WAVORM OF IG. 7 ON

'SCOE CREE, USING

CIRCUT O G 4 OR S

ir 8 how how thi warm wl appear o the oclosco crn usng th mthod o conncton shown at Fgur

4 o 5 at ow quncs th wavrm dstorton producd

may ot b drcty du to th transfrmr I th nductacs ot sucnt to mantan th quncy rspons o as lw aquncy as may b dsrd, ths wll ntroduc prmarly attnuato dstorton and phas dstorton Howr th low nductana wl shunt th anod oad nto whch th prcdng valv sworkng and ths may caus th valv to ntroduc dstorto.

Th prsnc o a shunt ductanc causs th load to

bcom liptcal-Fgur 9 shows th ct o addng shutnductanc o progrssvly lowr valu n parall wh th da

:w00�


19/66

18 THE USE O F RNSFORMERS

resstance oad lne, B deal ellipses are drawn wth rerence

to dea valve characterstics, each beng shown dotted where heydepart fom the real valve characterstics. Ths shows where hedtoton comes n. n practce the presence o such a load wlmodiy the operatng point somewhat dependng on the methodo bas used; bu Fgure l() shows the waverm o voltage andcurrent n the anode crcu o the valve. The transrmer wlpass on a waverm smlar to he voltage waverm shown, andthe pattern on the osclloscope creen wll take the rm shownat Fgure ().

IG. 10. WAVEFORM DSOO D O LOW PIMAY INDUCTANC

(a) Voltage and Curt Wavms b Dispy fr thod of Fg. 4 o 5.

At the hgh feuency end o the response the audio transrmercan ever o itsel introduce dstorion, but distortio may be caused

due to the impedance t presents, either to the anode crcuit o thepreceding stage, or to the grid crcuit o e ollowng stage.rom the vewpoint o the precedng stage, t is again the load

impedance provided r the valve that determines when dstortionoccurs I a pea occurs due to resoance between leaagenductance and secondary sel-capactance, the mpedance reectednto the anode circuit wll ll to a value much lower than themd-band value because the resonat circuit, looked at om theprimary, o the series type.

rom the vewpont o the llowg stage, the most lkelyway that a transrmer can ntroduce dstorton is by causngparatc osclato. To explan ths acto the smplest iewpont s to compare it with te tuned-anode/tunedgrd oscllatorsused at rado equencies. These work wthout any delberatecouplng between anode and grd, simply due to the nternacapactance o the vave and what s known as the Mller eecwhich this capacitance produces. At a equency where the anodeload possesses nductve reactance and grid circuit has the dynamcimpedance o a parallel resonan crcut, the valve wll oscllat


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CAUSES OF DSTORTION 1 9

due to edack hough is anode-o-gd capaciance Thsoscilaion s dependen upon: he gain o he vave he vae oinducive eacance n he anode ccui ; and he dynamic mpedancein he gd ccu.

As appled o audio equency ampies he nducve eacancen he anode cicui wil e due o he oupu ansme in cominaion wih is seconday load he dynamc impedance o hegid cicui will e due o he esonance eween seconday selcapacince and leakage nducance o he coupling ansme.

Unde some ccumsances he oupu valve would oscillaehad poaly a a supesonic requency and aemp o pass

audio signa wil esul in a vey oken up epoducion, bausehe oupu vave is compleely ocked y adio fequency oscilaionexcep on he loude passages o audio whch educe he ampliudeo osclaon momenaly sucenly o allow a sho us oaudio o eak hough

When he eec s no so song as hs, he valve may nooscilae seadily u may insead poduce sho uss o osclaiona ceain poin in he audio ycle whee he gain o he opuave s highe han aveage due o he insananeous posion o he

opeating poin on he valve chaaceiscs. Ths eec s ceainlyless noiceale han s moe volen companion u i sil iseviden as a fm o disoion.

A paicula case whee o avoid he om o disoion jusdescied equies vey cael aenion in he design o he inesage ansfme, is ha o oupu sages woked unde powedive condiions i.e. whee he gds o he oupu vaves ae dvenposiive dung pa o he audio cycle The gid cicui mus epovided wih a low impedance soue o peven wavem

FIG. 1 WAVFORM ASOCIAED WH OWER RIV WHN OURC PDANCE

I TOO HI


21/66

20 H US O AF RANSORMRS

distortion at peak amplitde de to grid crrent brsts Scdstortion wld occr wt peak amplitde at any eqency ifte ransrmer ratio and prmary ac resstance were not arrangedto present a scently low sorce impedance to te otpt valvegrds nrtnately te circt ales so prodced almostinevtaby combne so tere is a marked resonance between leakagendctance and secondary capactance at a rado eqency, wellaboe te adio spectrm Even if te otpt valves are qtestable drng te wole cycle of adio waverm te resonance onte econdary of te drie transfrmer wil be scienty prononcedto prodce a damped train of r.f. oscillations eac ime grd crrent

ceases so tat te outpt waem looks ke ge 12. ocre ts: () te drive transrmer mst be desgned especiallyto mnmse te eect of tis resonance; and ( te crcit mst beadjsted to kl any residal resonance at ts eqency wthotintefering wt its ado eqency response

FG 12.

DAMPED OSCLAONS EXIED BYCESSTON OF GRD CUEN

Powr L Response

Some manctrers are now ntodcing nto te lteratreon adio transfrmers, wat as been termed a " power responseTe ator sggests tat te expresson " power limit responses a ittle more eplicit to sow better te distnction om tenormal eqency response.

Te presentaton of a eqency response gnores te qestonof waefrm dstortion, assming tat tere is none Sc afeqency response sold apply r low leel signals at any rateIt may be modied at g leel sgnals de to dstorton settng

n at some eqencies and not at oters bt more mportant tante modcaton to eqency response it cass, is te eect ofdstortion on te reprodcton of tose eqences. It s tondcate ts lmtation tat te so-caled " power response asbeen ntrodced It is a crve tat sows te imit of power wicte transfrmer can ande befre dstorton sets in potted againstfeqeny.

From te ewpoint of te transrmer itself tis limt applesessentialy at te low feqency end of te caracteristic and


22/66

CAUE OF DORON 2 1

withot considering the shnting eect of the indctance on theanode load the limit will be a 6 db pe octave slope epesented by

satation x density in the coe of the transfme. (o mantaina constant x density in a tansme core, as eqency is vaiedvolage must vary poporo to eqency) Sch a pwerlimt response may be strictly tre as regads he tansmer yitself bt it does not tell the whole stoy becase tansrmersare nvariably operated in conjnction with vves, so the eectof the transrmer on the anode load applied to the valve msalso be aken into consideation This means hat the shntingeect of he transme primay ndctance will also conbte

to the power lmt respose at he ow eqencies and that theresponse n the middle of the eqency band wil be level at a limitset by the power otpt of the valve At high eqencies depateof the load vale presented to the valve anodes will agan intodceredction in maximm powe, so that a typical tansfrmer characte-isc showing eqency response and powe lmit " responseto the same scales might appea as at ge 13. As with thepesentation of eqency esponse, it mst be realised that powelm esponse s dependent not only on the tansme tself,bt also on the cicit in which it s opeated, paticlaly on thechaacteristcs of the valve o valves sed

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FG 13. TYPCAL POW LIM RSPOS ESNAIO

Shorted Turns

A tansme n good woing ode will not possess any

shoted tns bt this is one of the lts that can develop n aansfmer de, perhaps to oveheating to opeation in climatccodtos fr which t was not desgned, or to poor workmanshipin he st place In some nstanes shoting tns may be presentwhen the transmer is new, if the manctre's test pocedrehad ot been adeqate fr detecing them Nowadays howeverreptable tansmer manctes gard themselves againstths possbility by careul testing at the vaios stages dingmactre


23/66

22 THE USE OF A TRANSFORMERS

The eect of shortng turns on the perrmance of a transrmerdepends () upon the number of turns shorted and ( their ocaton

n te wndings of the transrmerA small number of shorted turns may not produce noticeabe

eect in the central part of the equency response as the ossesintroduced will be qute small compared to the main transr ofpower om primary to secondary the more noticeabe eec wlbe on equency response due to the way n whch the shortedturns inuence the leakage ux n the wndings. Generally speaking the result wll be evdent in the rm of a gradua slope oneway or the other usually toward the hgh equency end of the

response cuve. The drection nd degree of the slope and theequency at whch t s evdent wll depend on whether the shortedturns are in the primary or scodary and how they are located inrelation to the leakage ux n the man wndings

A greater number of shorted turns wil produce a loadingeect on the transrmer makng the input mpedance ower thant should be Ths wil probably cause dstorton because thewrong load impedance s presented to the valve anode crcuit.A subsidary eect may be to ncrease or decrease the eectve

rato of the transfrmer, accordng to whether the turns shortedare n the primary or secondary wndng Thus, turns shortedin the primary windng of a step-up transormer mght appearon ato test to ncrease ts step-up; but n a practcal crcut thegan n step-up wl varably be lost due to the ncreased odngeect

Change of Frequecy Response Wit Sgnal LeveThs is another frm of dstorton that may be notced when

audo transrmers are used n amplfyng equipment of aboratorystandard For normal ado eproducton the degre of changewt level s small compared wth the wel-known scale distortonproduced by the Fletcher-Munson characterstcs of the humanear. The prncpal eect s at the low equency end due to changeof nducance with sgna leve Generaly speaking nductancerses wth ncreasng ux densty so that the low reuency responses better at hgh levels than at low levels. Ths s frtunate fraudo reproducton because t s at ths end of the spectrum that thescale dstorton o the human ear makes low level low equencysounds dsappear anyway

In theory there can be no change in equency response withevel at the high equency end of the spectrum but n practice somechange occurs. Ths s due not to the transrmer itsef whoseeakage nductance and wndng capactance are independet ofsign evel but to the vave characterstcs again. The eectvea.c resstance of a valve nvarably changes to some extent wthsgnal level and ths change wil produce a varaton in feqencyresponse n just the same way as any other alteraton to externalcrcut values, as explaned n the llowng chapter.


24/66

3EQUIVALEN CRCUTS

TO represent the various aspects o audio transmer

perrmance equivalent circuits are drawn. A completeequivalent circuit can be drawn showing all the poperties

o an audio transrmer and one uch is shown at Figure 14.

C rim

SCapacitae

CSeconday

Self CapacinceC5 ntewndi CapacitanceL5 Pimary Leakage nductaL5 Scoary kg ndctance

Lp Prima Inductance Primar Wndinq Resitancerw2 Secon y Wnding Reistance� Ce Ls de to Eddy CrentRH C L due t Hysterei

G H. COMPLEE QUIVAN CRCUIT OF TYCAL AUDIO RASFORER

When an equivalent circuit is drawn it is assumed that aperct transme with the same ratio as the actul transrmerreplaces it r the purpose o impedance matching or volagestep-up and that all the " imperfctions o the actual transrmerare repesented in the equivalent ircuit. To make quite clearthe idea o the perect tansrmer: it is dened as one with nolosses (.e. the winding resistances are ero and th coe equeno current to magnetize t) and with perct coupling, so that noux can get between the hypohetical windings giving rise toleakage inductance. ts property is to provide unrm transrmation o voltage and curent at all euencies om zero to innity

Having separated the ratio aspect o the transrme in this way

its arious shortcomings ae repesented n the equivalent circuit.It was stated in the rst chapter that the transrmer acts both aystansrming mpedances actually present n the seconday circuitas rerred alues in the prmary and vice versa This action isequally true o the intenal characteristcs o the tansrmeritsel such as winding inductance core loss leakage inductancewnding esistance and capacitance. Because o this to aoid

23


25/66

24 THE USE OF AF TRANSFORMERS

confusion the equivalent circuit must refr all circuit values

internal and external to the transrmer, to the same windng.If te secondary is the chosen rerence wndng, then impedncesactually pesent n the primay ciuit must be refered by the suareof the transrmer ratio, n order to get their eective value in thesecondary crcuitInsertion L

Related to ampler perfrmance insertion loss is usuallygven in db; but fr estimating transfrmer perfrmance t seasier to calculate n terms of eciency and r this reaso thechat provded at Figure 42 gives eciency as a percentage

Figure 1 5 shows the essental atures extracted om Fgure14 r the purpose of estmatng insertion loss in the case of a powertype of audio transrmer Interstage types are not basicallypower transfrmers so the important fature is not the loss of poweras an insertion loss but depreciation of voltage stepup.

MER

LOADREISTANE

FG. 15. ESSNTIAL

EQUIVALNT CIRCIFOR DETERMINIG

NSERTON LOSS

In a power type audio transfrmer the resstance of both wind-ings as well as the core loss contribute to the insertion loss butin a voltage type transrmer only the primary resistance and the

core loss are of importance snce negligible current ows thesecondary, unless a loadng resistor is used fr the purpose ofmantaining a satisctory equency response. Even then theloadng resistor s usually much hgher in value than the nomnalimpedance of the tansme winding to whch t s connected,wth the result that the secondary current is too small f thesecondary winding resistance to contribute appreciably to thetotal losses. The eect of the loading resistance may be consideredas drectly in parallel with the core loss fr all practcal purposes,in an nput or interstage transrmer.

In nput and nterstage transrmers insertion loss, howeverassessed is so small that it is not usually necessary to cosdert t all In power type transrmers used fr output or driveinsertion loss is important because some of the avalable powroutput ils to reach the output circuit on account of it.Lw Fruency Response

Low equency response cannot accurately be representedby a smple equivalent circuit because of the eect pointed out atthe end of the previous chapterective inductance and core


26/66

EQUVALENT CRCUTS 25

losses change with both amplitude and equency Howeverit is usel to have an approxmate reerence so that the generalorder of goodness o the transrmer at the lowequency end can be estimated although itis not possible to gve an accurately denedresonse curve. Figure 6 shows the theoreticalequivalent circuit r an audio transrmerwith its associated external circuit. The resistance r represents the primary source impedance and, with the transrmer direct coupledwill be the a.c. resistance of the valve into

FIG.16 EQUVALNT CR·C FOR F RESPONSEOF DIREC COUD

RANSFORMER

whose anode it is coupled R represents the combined shntresistance o the secondary load and the xed component ofthe core losses (the ltter will usually be so high as to benegligible) represents the average value of primary inductance.otice that R must be reerred to the primary winding the same asall other values.

This theoretical circuit gives a response of the frm shownat Figure 17. The equency at mid-scale is where the reactance

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FG 17. RSOSE FORM OF DEA OW FRQENCY CU-OFF, ASSMG CONSANDCANCE

of is equal to the combined resistance of r and R in parallelHaving worked this out by the use o the resistances in paallelchart and the reactance chart of gures 34 and 3, the responsecurve of Figure 1 7 will give a rough idea of the behaviour o thetransrmer t must be emphasised that this esponse curvecannot be used r detailed plotting because of the variatio ininductance with both equency and level

If the transrmer core is gapped as it normally will be whendesigned fr single anode direct coupling the inductance will be


27/66

26 HE USE OF A TRANSFORMERS

reasonably constant at the current vaue taken by the valve anode

and the response crve given at Fgre 17 will be a reasonablyaccuate presentation o eqency response at the . end. Somepshpul transrmers are also provded with a smal gap r thepurpose o stabilizng inductance so that requency response smore dente at ths end It shold not be thoght howeverthat it wold be better to have a gap under all circmstancesbecause the inductance without a gap when there s no polarzngcurrent is always higher than the ndctance wit a gap; so, althoghthe l response may vary om standard rm when a transmer

is cored up wthout a gap t wil be better at al levels than the sametransrmer would be with a gap.

Parallel Fd

High qality interstage transormers operating om a sngleanode are sally paralle d to revent polarzing current owingthrough the primary wnding o the transrmer Fgre 18shows a typical circit r this purpose and Figure 19 shows theeqialent crcuit r response

In ths crcit there are two re-actacs and the prmary ndctacetends to resonate wth the coplngcapacitor whether this will prodce a peak or not depends pon therelatve vaes o the inductance andcapacitor and the external circuitvaues r and R

The charts ntroduced ater frFIG. 8 TYPIA IRUIT OR

use more particuarly n treating h..PARALE FE NTERSTAGE TRANS-

response can be appied to treat l ORMER

response n parallel d types o transrmer It shoud be notedhowever that the charts are based pon an assmed constant valueo nductance and the above remars abot varation o nductancewll also mean that accurate predicton o response in parale dcases cannot be achieved either.

Cc

r •.R +R ,

lNPU

G 19 EQUIVALENT IRCTFOR RESONSE OFPARALLEL FED TRANSORER

SHOWN FIG 8

High Frquncy Rspons

In practcal transrmers steps areusualy taken to eliminate the eect ointerwinding capacitance by connectg toearth, or the ow signal potential point othe circit the end o one o a par oadjacent windngs so that interwndngcapacitace s eectively fom one side othe other wndng to earth instead o

between the hot sides o both windings his procedre avods

ecessive peculiarity in the response curve


28/66

EQUVALEN CIRCUTS 27

We are now left with two or three reactances that take eect

atthe high equency end Almost nvarably only two of them

ned be seriously considered If the transrmer s workng asa step-up as is usually the case in input o interstage types therelevant quantities are leakage nductance and secondary windingcapacitance If the transrmer s working as a step-down as susually the case fr output types, the relevant quantities are prmarywndng capacitance and leakage nductance. Fgure 20 shows theequivalent circuts r stepup and step-down.

IP LO I T

r

lRSTA

A-UP TYPES (B) STEPDN YPES

FIG 20. QUVALEN CRCUIS FOR H RPON O AUDIO TRANSFORMRS.

An excepion to this rue occurs wth drive transfrmers havinga step-down In this case the pimary wndng capacitance s oflittle importance because the ac resistance in the prmary circuthas to be kept low to avod dstortion which ct ensures thatprmary capacitance has negligible ect within the useul equencyrange On the other hand although the sondary winding hasthe lower number o turns it is connected to the hgher impedancewhc wll usually be almost open circut except when grid currentows So the crcuit still confrms to the nterstage pattern rhigh equency response, although the direction of transrmationrato has been reversed

t s convenient in presenting the perfrmance charts to usethe same symbols both fr input or interstage types and r outputtypes ; bu it will be noticed om Figure 20 that hese symbolshave dierent sgnicance n the two cases In each case r s theseries dampng of the resonant circuit, beng eectively n sereswith the leakage inductance while R is the shunt damping of thecrcut beng eectively in parallel with the capacitance

or input and nterstage transfrmers r s the source resistance,i.e the impedance of the input circuit or ac. resistance of theprecedng stage and R s the load impedance in the grid circuit

Jn output transrmers R is the ac resistance o the outputstage and r is the load resistance.

Eect of Vang Cirit Valun each case certain of the values in the equivalent crcuit are

xed by the audio transfrmer n question, but external crcuitvalues may be adjusted so as to alter the response in some way.The actual rm of varation wll der fr each design of compoent


29/66

28 HE USE O A RANSORMERS

so i is not possible to gve a standard set of curves to show this

variation. As well as varying the shape of equency responsealtering circuit values will aect gan or eciency of power trasfer.The series of hats gven n Chapte 5 have been pepared so theexact eect of crcuit values on ay individual design of transfrmercan be determined quite quickly.

It is realised, however, that most readers will nd it ratherunsatisctory to be given only some technical data and a et ofcharts and be told to work it out fr themselves So to give someidea of variaton in typical cases Fgures 21 to 24 have been prepared,

whih the reader can study r himself. It must be emphaisedhowever that these presentations ae ony typical and cannotbe applied o any particular type In these diagrams the variationof values is taken over a much wder range than would be practicaln any individual transrmer n order to show the completescope of possibility fr the particular examples selected .

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FIG 21. VAIATON OF HF ESPONSE W XTERNAL CCUIT ESSTANCE VALUES L �O

NTIA COMBNAION (EAVY CVE): -=10; -=.13 ; C=5 x 0r CrR


30/66

SP.SE

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EQIVALENT CIRCUITS 29

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FIG 2. VARIATION OF .F RESPONS WI EXRNAL CIRCIT RSISTANCE VLUSR L 10

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31/66

30 HE USE OF A.F TRNSFORMERS

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32/66

MEASURNG UP THE ELECTRCAL PROPERTIES 31

transrmer irons more usually employed fr output transfrmers.For this reason t is wse to alow a sightly larger margin on these

high quaity transrmers although r good reproducton a rmargin shoud be alowed on ordnary transrmer iron becausethe distorton athough increasing more gradually may be sucientto cause ntermodulation at a leve ower than what may be regardedas the saturaton point

The gures given n the appendix as ma:xmum votage freach winding have been derived n a variety of ways Somemanucturers have suppied nrmation about the distortioncharacteristic of the transfrmer; others have supplied samples oftheir products, stil others have allowed the use of ther condentalinternal design inrmation om whch to extract the data requiredn the appendx The ·endeavour has been to present a reasonabyconsistent gure r maximum voltage, based upon knowledge ofthe core materials used in the case where dstortion charactersticshave been given; upon measurements taken in the case wheresamples have been submtted; and upon a suitabe maxmumworkng ux density n the case where nternal data has beensuppied.

n any event maximum voltage gures can be regarded only asgiving a sa lmit, and not as indicating a specic point wheredistortion commences n indivdual samples of the same typethere will be variations in distortion characteristic due to thenconsstency between individual batches of the same core materialSo even had great pans been taken to ensure that the maxmumvoltage gure represented a certain degree of harmonic distortonn all cases, this gure would be subject to certain varaton withindividua sampes

Primary Inductane

Ths property of core mateas s subject to even greatervariation than dstortion characterstic unless the transrmer sgapped For ths reason the gures gven n the prmary nductancecoumn of the appendix can also only be regarded as providing anapproximate indication

To measure the primary nduc-tance of an unknown transrmer,the voltage check aready describedshould be made rst; after whchthe impedance of the primary windingmay be measured by comparison withstock resistors at a low equencyand wth a voltage beow saturationpoint Figure 25 shows the methodof making such a measurementf a calibrated resistance box is

FIG. 25. ASURINGFFCVE PRMARY

INDUCTANC.


33/66

32 THE USE OF F. TRANSFORMERS

avaiable a useu metod is to adjst te esistane until tevoltage aoss te transrme winding and tat aross te resistaneare eqal

From te impedane vale fund and te equeny usedf te test an indtane vale an be alulated om te reataneat It is a good plan to make measurements of tis type atseveal voltages and ten take a average or miminum value.

Le akage Inucance an Winding CapacitanceTe most sessl way o determining leakage indtane

and winding apaitane is by nding te esonant point, and ten

adding extenally, known vales of additional apaitane in orderto nd te atual value of internal apaitane Te diltyere is tat, partilaly fr small outpt transrmers, or igqality designs te natual resonane of te tansfrmer willprobaby be in te radio equeny region, say 10 K/s Largeror less i prodts wil resonate at a lowe feqeny bu maystill be beyond te noma adio range

Figre 26 sows te test iruit best sed r nding te

PUT

�


34/66

MEASURING UP THE ELECTRCA PROPERES 33

NPUT FOMAlO

OILAOSCOSCOP RC

SHNG EC

I 7 INDI AROXI MATE WNDIN CAACIANCE BY RESONACE WTH

RIMAY IA

the known values connected to shift the resonant equency Aypical display indicating resonant conditions is also shown inFigre 27. he dicuty about this method is that the windinginductance tself changes with equency, so the answer cannot beud by a simple rule such as was gven r resonance with leakageinductance The bes method is o add a number of eternalapto whoe vale have been heked on a capatane bdge,and then plot he results in the manner shown at Figure 28 sing

log-og paper wo or three curves are ploted assumng dierentvalues fr winding capacitance and the one representing truewnding capacitance s recognsed as that giving bes approximaonto a straight line or the value i nerpolated beween two plots,eac diverging in opposite directons om straght

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Core Los

Ths can easily be measured sing the same circuit arrangeentand method f balancing out reative components shown i Figure2 To obtain a value of core loss expressed as shunt resistance


35/66

34 THE USE OF A.F. TRANSFORMERS

ethe a esstance box is used the sees esistance, ts valuebeing adusted untl the votage acoss the tansme is equal

to that acoss the esistance o else the equvalent coe loss esistances calculated fom the voltage atio acoss the two pats of thecicuit The advantage of adjusting so that both voltages aeequa is that not only voltmete eos ae elimnated om themeasuement, but also eos due to the shunting eect of thevoltmete impedance in taking the measuement When bothesistance vaues ae equal, connecton of the same voltmeteacoss each will have exactly the same shunting eect and so neednot be taken nto the calculation.

Winding RistaceThe esistance of each winding can eadily be measued by

means of the conventional ohmmete type of instument A moeaccuate esult can be obtaned usng a Wheatstone type bidge(commonly known as a Post Oce Box), but such accuacy is notwaanted this pupose because individual specmens of a gventype of tansme will vary as much as 20 pe cent. Each wndingesistance should be measued and then the esistance of onewndng should be eed by the squae of the tansfme ato

to the othe windng and the two values so obtained, added togetheto give the total windng esstance eed to that widing Thshould be the same windng as used f the measuement of coeloss.

Geneally, moe convenent values ae obtained by usnthe lowe impedance wnding f all these loss eences.

5

USING THE CHARS

THE nmation obtaned om the appendix o from measue-

ments on tansmes, as n the pevous chapte canbe appled by means of the chats hee given ethe to obtain

the best esults om a specic tansme, o to nd out whethea tasme desined f a deent ccut will gve satsctoypefmance f the job n hand, as a makeshft

No attempt should be made to use the chats o the nmatoncontained in them to decde whethe one manuctues tansmes a bette job than anothe's, o even whethe one type s bettethen anothe.

Fom the eale pat of ths book it wll be appaent that eveyapplcation equies individual consdeation; because of thsmany tansme manuctues do not make any stock lines oftansfme at all but pefe to design a component each


36/66

USING THE CHARTS 3

ppliction to customes' speciction. Hndbook No 5 in thissee will give moe detis bout designing tnsme

spic ppliction.Oten it hppens tht the job in hnd is only one-o nd

hence it would not b woth while hving tnsme designedspecily while on othe occsions the ob my be too ugent toow time specil designs to be dwn up nd poduced Thechts nd ppendix e intended to seve in such cses to enblethe use to obtin the best esuts om edymde componento to dpt the neest possible design whee type exctly suitedto his equiements is not vibe

Dieent tues will equie ttention in individu cses,but they e listed below in ode o pnce in the tbutiono the ppendix

·

Impedance Ratio

Nominl impednces e listed in the tble but to aciittedetemintion o the coect tuns tio Figue 36 enbles tunstio to be detemined rom knowledge o the woking impednces,o vice vrsa.

In the cse o output tnsmes the impednces eted bythis tio e the secondy lod impednce nd the optimumod impednce equied by the output vlve(s) on the pimy

n the cse o intestge o input tnsmes the impedncesto be consideed e the souce impednce pesented to the pimryo the tnsme nd the eed vlue o this souce impedncein the gid cicuit connected to the secondy

Use o the bc o Figue 36 enbes st check to be mde tosee whethe the tio o tnsme is ight o within wokbletolence.

Maximum VoltageAl tnsmes will hve limit to sign hndling cpcity;

this is oten listed by mnuctues s wttge pticulyin the cse o output tnsmes but such listing is theunstisctoy expecilly when pplying tnsme to cicuitdieent om tht which it ws designed The citic ctoom the tnsmes viewpoint is the mximum voltag it cnhndle bee distotion commences.

Fo input nd intestge tnsmes genely signl voltget the gid equied mximum sign will be known fom themplie design nd hence the gues given in the mximumvotge tble o the ppendix e diectly ppicbe

Fo output tnsmes voltge swing is equently notegded s the citicl cto but wtts output The cht oFigue 37 is given to id in conveting mximum volts om theppendix coumn with od impednce to wttge ting.

As stted in Chpte 2 mximum votge wil vy withequency the ppendix lists the mximum voltge t 50 cycles


37/66

36 THE USE OF AF TRANSFORMERS

if maximum signal is never required at 50 cycles, an increasegure of maximum voltage can be used based upon the lowes

equency at which maximum signal is required This practiceis not to be recommended becase it makes the transrme theweakest link in the chain unless precautons are taken in circuitdesign to ensure that it is impossible r maximum voltage to eachthe transfrmer at this bottom end of the range On the otherhand if equencies below 50 cycles are required the maximum voltagewill have to be reduced in proportion to the lowest equency gure.

Frequency Chaacterisic

aving ascertained that the ratio is correct or workabl andthat the transrmer will handle the necessary signal level attentioncan be turned to its equency characteristic.

Unless parallel ed is used the important ature r lowequency response is the primary inductance in conjunction wththe parallel combination of the source and load impedance rerredto the primary Figure 34 is a chart that will aid in calculating theequivalent value of resistances in parallel n the case of interstageand output transfrmers this parallel combination consists of the

a.c resistance of the valve and the secondary load impedancerefered to the primary.aving determined the equivalent parallel resstance the

next step is to nd at what fequency the reactance of the primaryis equal to it This is und fom the primary inductance by meansof the reactance chart of Figure 35 Having fund this equencyan approxmate low equency response s gven by gure 1 7 asexplained in Chapter 3

igh equency response is predicted with the aid of the charts

given in Figures 38, 39 and 40 Figure 38 identies which shapeof equency characteristic shown at Figure 40 gives the correctresponse curve Figure 39 xes the relative equency used rFigure 40 indicated on the relative equency scale as l . If desireda sliding equency scale can be constructed to aid in applying theinrmation om Figure 40 to actual equencies r the transrmerin hand.

To use these charts values of source and load impedancererred to one of the windings must be assumed; these assumedvalues are listed in a column of the appendix tabulation and the

R Lcorresponding values of-- nd L r use on he carts

r CrRUse of these three parameters with the charts r determining highequency response will identify an appropriate response curve onFigure 40 and locate it in the equency spectrum If the responseshape does not suit the eect of varying circuit values om thoseused initially can easily be see with the aid of the detacbalescale attached to Figure 38


38/66

USING THE CHAR 37

gures 2 and 30 show how this scale can be used o the

charts of Fgures38

and39

to see the eect of changng values ofr or R on the shape and position of a fequency response Figures31 and 32 show slary how he ohe ed of the detachale scaecan e used o see the eect of aryng L or C

As aectng hgh equency response L is xed, beng heeakage ductance of the transrer and C can only e ncreasedby te connection of eternal capacitors

The charts in Figures 38, 40 and 41 can be used r the pre-dcton of f response n paralel d cases n whch case C can bencreased or decreased a wil, snce it s the coplng capacitorand L wl vay over quite a rage wth sgnal eve he charscan e used in hs case to see how variaion of prary nducanceaects he low equeny respone and o aid in choosg valuesso tat ths variaton s kept to a imu if so required.

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39/66

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Eficiency

The chart gven r ecency is only applcable r udiotransrmers requred to handle power.

Interstage and input transmers intended to provide voltage

transr usualy have an eciency value suciently hgh to beneglected; but i r any reason the precise value is requred wllbe etermned om the prmary restance and core lo o thetransrmer, provided no secodary load is connected. asecondary load s connected then ecency can be caculated omthe ecency chart by rerring the load windng resstance and coreJoss aues a to the same windng.

The method o using the ecency chart of Figure 42 is illus-traed by Figure 33. It will oten be und that load impedances


40/66

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rerred windng resistances or referred shunt loss values do not

come withn the range of the chart ; ths can readily be overomeby multiplying, or dividing, all of these values by a common ctor,some power of ten being usually convenent r ths purposeFor example a transrmer lsted as having a wnding resistanceof .3 ohms and a core loss of 100 ohms rerred to a load impedanceof 3 ohms can use the chart simply by multplyng all these valuesby 0, so that rerred windng resstance s 3 ohms rerred shuntloss I O ohms, and rerred load impedance 30 ohms

Having und the ponts of ntersecton between refrred

winding resistance and rerred shunt loss the eect of dierentload mpedances r the same transrmer can easily be seen byusing derent rerence lines on the rerred load mpedance scale.Maximum eciency will always be acheved when pont B as indicat-ed on Fgure 33, flls on the vertical rerence line idented by thegure I 0 on the REFERRD SHUNT LOSS scale on the chart as thepoit flls to the rght or left o ths vertical line ecenc wllll o as represented by the curve on the chart

Adjustng ResponseThe charts of igures 38 to 41 may be appled r adjustng

the overall equency response by means of external circuit valuesChanging circuit resistances wll eect a correspondng change inr and R accordng to the positon of the resistor n the crcut.The most usual problem wll be that of modifying the response atone end of the spectrum wthot nterfering with the respose athe other end


41/66

40 THE USE OF A.F. TRANSFORMERS

he rst step should be to see that !.f. response s satisctorymprovement of !.f response can be achieved by reducing theassocated circuit resstance values the most eectve vaue tochange being that which aready has the owest rerred vaue.

n the case of a trode output stage ths woud be the acresstance of the output vave. The ony way to reduce this s byapplicaton of negatve feedback rom the anode to some earlierpont in the ampler

n the case of pentode or tetrode output stages the owerresistance vaue wil usualy be the rerred oad resstance whchs naturally xed by matching conditions so the only remaning

option s the reducton of a.c. resstance. Hgh quaty pentodeor tetrode output stages should always have a high degree of negaivefeedback in order to improve loudspeaker dampng Ths willalso mprove !.f response by reducng the shunting eect of thetransfrmer primary inductance

nterstage tansfrmers are not often used with pentode ortetrode preceding stages but where used in drect couped circuitsthe regong remarks r output types wl appy. Where usedwith paralel fed coupling both ends of the response require more

careful consideration and the special methods rerred to nextmay be necessary

o adjust the response at the h.f end of the spectrum thecircuit value whch will have most eect depends upon the postonof the ntal values on the scales of Figure 38. If these result in a

L

value o - greater than unty R s the most eective cicuitCrR

component to adjust. On the other hand if the nital value of

L- s less than unity r is the most eectve circuit value to adjustCrR

Paralel Fed Transfrmers

Fgure 43 shows methods of adjusting the perrmance ofparalel fed couplng, while Figure 4 shows the equivalent circutsrom the viewpoint of f and hf. response The a.c. resistanceof the preceding stage n paralel with the coupling resistor provde

the basic eective vaue of r r both .f and h.f characterstcThe primary shunt resistor Rz contrbutes to the shunting eectof R fr the !f characteristic but assists in reducng the eectvevaue of r r hf. characteristic by being eectvey in paralelwith the two components aready mentioned The secondarshunt resistor RJ occupies the position of R fr both characteristics

The best resistors to modify in any ndividua case dependsupon the postion on the chrt of Fgure 38 r both response


42/66

USNG THE CHARTS 41

characterstics It will be realsed, om a study of the chartsand the illustrations of how to use them that a wide range of possble

adjustment exsts n ths type of crcuit and use of the charts willprove r qucker n nding a satsctory arrangement thanprologed ddlng of iruit values.

Adapting a Trforme

Frequently t is neessary or desirable to use a stock transrmerr an applicaton somewhat dierent om that r which it wasdesgned or speced. The queston then comes as to whch of thetransrmers available will best serve this new purpose, or to what

degree the haraterstics of the transrmer used wll ll short ofideal charactestics th�t might be available if a transrmer couldbe specially wound r the purpose.

The most equent applicaton of this nature is r otputor speaker matchng transrmers.

In ths case the rst requsite s that the transrmer musthave somewhere near the correct ratio.

Next t must have sucen turns to be able to handle themaximum signal This can be checked by means of the maximumvoltage gures associated with the winding

For adaption purposes of this nature the next important fctoris he question of eciency, r whch the eciency chart shouldbe sed

If these three ctors gve satisctory answers the fequencyresponse will in all probability e good enough fr the purposebearing in mnd that the arrangement s an adapton Howeverfequeny response can easly be checked by means of chars 38

to 4 1 .Another application where adapton s necessary i s r nput

or iterstage transrmers. Here, as well as considering maximumsgnal often minimum signal wll prove a more mportant questionas transrmers designed fr use n output circuits usually employa lower grade of core material quite suitable r output circuitsbut not havng sucient permeability r low level input crcuits.For this purpose one of the high grade core materials speciallyproduced to have high ntial permeability should be employed.

Frequency response will be the ext requsite to consider fr thesecases eciency usually being relatively unimportant.


43/66

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THE USE OF .F RANSFORMERS

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44/66

USING THE CHARTS 43

Use of Fig.

The important thing wth ths chart s to remember (a) that

r any one calculaton the gurs on the same side ofthe efeencescales must be used-ethe A or B scales; (b) that the gures mustbe taken to epesent he same ts ah sae. For exampereured the parallel combnaton of l Megohm and 220 K : usngA scales r units of 0 K the result is und to be about 180 KReured the parale combnatn of 5 0 K a nd 33 K : using theB scales r unts of I K, the resut s fund to be 20 K.

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45/66

44 THE USE OF A.F. RANSFORMERS

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A 40/ 1 ransfrmer wil mach 1 5 ohms up to 25,0 ohms,or 3 ohms to approximately 50 ohms

To match 50 ohms o 20, ohms eqires a raio o 20 1


46/66

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5 watts into a load o 30 ohms gives approxmatey 0 volts volt across 15 ohms epesents a powe o about 65 miliwatts.Gven any two o these quanttes, the thd can b fund by

algment with a rule o staight-edge


47/66

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audio handbook no.3 - the use of audio frequency transformers - norman h. crowhurst (1953) clearscan

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