descrete op amp papadopou

8/12/2019 Descrete Op Amp Papadopou

1/8

1

The John Hardy Company1728 Brummel St.Evanston, IL 60202

USA

Phone: 847-864-8060Toll Free: 866-379-1450

Fax: 847-864-8076

www.johnhardyco.com

October 1, 2013

The 990 discrete op-amp is the finest op-amp available for audio applications. If you ant superiorsound !uality, the 990 can provide it. The 990 is used in the most critical audio applications. "everalmic preamps and other products usin# the 990 are available from the $ohn %ardy &ompany.

Circuit designof the ori#inal 990 is by 'eane $ensen of $ensen Transformers. 'eane as aarded (.". patent )*,2+,*9 for aspects of thisdesi#n. very aspect of the desi#n and performance of the 990 as optimied throu#h e/tensive computer aided desi#n and analysis. achcomponent of this discrete op-amp as carefully chosen for its specific tas, providin# superior performance compared to monolithic op-amps and other discrete op-amps. or complete desi#n theory, circuit details and specifications, please see the $ensen en#ineerin# report.The & modifications ere developed by "teve %o#an of $ensen Transformers 4no at his on company, The "ound "teard5.

Packaging and production designof the 990 is by $ohn %ardy of the $ohn %ardy &ompany. The desi#n enables this *9-component circuit tobe constructed on a circuit board 1 s!uare, ith final module dimensions after encapsulation of 1.126 s!uare by 0.700 hi#h. Thedimensions and pinouts conform to the 8I 2620 paca#e, alloin# direct replacement in most applications.

990 Discrete Op-Amp

T% $O%: %8;'< &O=8:

1.8 ey component of the 990, the :ational

"emiconductor ?=39* supermatched pair oftransistors, as discontinued in 2010.ortunately, there ere to devices that erevery suitable replacements for the ?=39*,

providin# virtually identical performance> the8nalo# 'evices =8T02 and ""=2210.Ori#inally the =8T02 as a recision=onolithics part, the ""=2210 a "olid "tate=icrotechnolo#y part. =I bou#ht ""= inthe late 19+0s, 8nalo# 'evices bou#ht =I in1990. The same semiconductor chip as usedin the =8T02 and ""=2210, ith =I usin#""= to access broader marets for thesupermatched pair.

2.8nalo# 'evices une/pectedly discontinuedthe =8T02 and ""=2210. This left nosuitable supermatched pairs of transistorsavailable. 8s the story #oes, sales eredeclinin# on these and other old-schoolanalo# parts, so :ational "emiconductorand 8nalo# 'evices decided to discontinuethem and close the outdated fabrication plantshere they ere made.

3.8nalo# 'evices reversed its decision andreintroduced its parts under ne part numbers,

movin# the manufacturin# to a modern fabplant. The =8T02 is no the =8T12, the""=2210 is no the ""=2212.

4.8s ith the ori#inal parts, the reintroducedversions use the same semiconductor chip, theonly differences bein# the paca#in# and the

price. The specifications of the ne parts areidentical.

5. Packaging: The =8T12 uses a throu#h-hole 7-lead TO-+ paca#e, the ""=2212

uses a surface-mount "O-+ paca#e.

6.Price:The ?=39* as alays around @3 atthe 1,000-piece !uantity. The ""=2212 isalso around @3 at the 1 !uantity. The =8T12is around @16 at the 1 !uantity.

"ince the =8T12 is five times the price of the""=2212 4and ?=39*5, yet provides noadvanta#e in performance, the decision asmade to use the loer-cost ""=2212 andconvert the 990 to surface mount assembly.

The conversion to surface-mount assemblyenabled several improvements in components>

1.=ost of the resistors have been up#radedfrom metal-film resistors ith a 1A toleranceand a 60 or 100ppm temperature coefficient tothin-film resistors ith a 0.1A tolerance and a26ppm tempco for improved performance.

2.The three small-value capacitors 4&1, &2and &35 in the si#nal path have been up#radedfrom a 6A tolerance to 1A, still usin# the

superior &OBC:0 ceramic dielectric.

3. The to poer supply bypass capacitors4&* and &65 have been up#raded from theD; ceramic dielectric ith a 10A toleranceto the superior &OBC:0 ceramic dielectricith a 6A tolerance.

4. &7 4in the current-source5 has beenup#raded from a film dielectric ith a 6Atolerance to the &OBC:0 ceramic dielectricith a 6A tolerance.

5.The to 20E% inductors 4?1 and ?25 havebeen up#raded to a ti#hter tolerance in a

smaller surface-mount paca#e, main# ashorter 990 paca#e possible as an option.

6. Transistors F3 and F10 in the currentmirror have been up#raded to a matched-pairfor improved performance.

7. The encapsulant has been chan#ed fromsilicone to an advanced epo/y encapsulantthat has hi#h thermal conductivity and iscompatible ith the special demands osurface-mount paca#in#.

2013: Important Changes and Improvements
http://www.soundsteward.com/http://www.soundsteward.com/


2/8

2

Technical DetailsDiscrete vs. monolithic op-amps.8n op-amp typiGcally consists of doens of diverse components, inGcludin# transistors, diodes, resistors, capacitorsand, occasionally, inductors. The fundamental difGference beteen a discrete op-amp and a monolithGic op-amp is the ay these diverse components are

brou#ht to#ether to mae a orin# op-amp.

A discrete op-amp is made from individual 4disGcrete5 transistors, diodes, resistors, capacitors, and,

occasionally, inductors. These components arebrou#ht to#ether on a circuit board or substrate tocreate the final circuit. ach diverse component isfabricated on a manufacturin# line that is fully opGtimied for that specific part. Therefore, each comG

ponent is the very best it can be. ?o-noise inputtransistors are fully optimied for their uni!ue reG!uirements. %i#h-poer output transistors are fullyoptimied for their uni!ue and very different reG!uirements. recision resistors come from manuGfacturin# lines that are dedicated to main# preciGsion resistors. &apacitors come from optimied caG

pacitor lines. Only after these fully optimied comGponents are fabricated are they brou#ht to#ether ona circuit board or substrate.

A monolithic op-amp starts ith a sin#le chip4monolith5 of silicon that is typically 1C17 s!uare.This chip is the substrate upon hich the doens ofdiverse components are created. :ote that all comG

ponents are created on the same chip, and you simGply cannot have the orldHs best input transistors,and the orldHs best output transistors, and preciGsion resistors and capacitors on the same tiny chip.There are unavoidable compromises due to limitaGtions in the fabrication process. If the process isoptimied for lo-noise input transistors, there illliely be a compromise in the hi#h-poer outputtransistors, etc. ach of the to inductors in the990 4?1, ?2 on the 990 schematic, pa#e 35 is manytimes lar#er than the 1C17 s!uare chip of silicon ofa typical monolithic op-amp.

ven the small sie of the typical silicon chip is alimitin# factor. To fit all of the parts on such asmall chip they must be made much smaller thanmi#ht otherise be desired. The reduced sie causGes a reduced ability to dissipate heat. The closerspacin# of components and circuit traces reducesthe ma/imum volta#e levels that the circuit can tolGerate.

=onolithic op-amps are marvels of technolo#y, buthen performance is critical, they cannot match adiscrete op-amp. 8 discrete op-amp costs more andis lar#er than a monolithic op-amp, but it offers suG

perior performance in many ays>

Lower noise.The 990 is an e/tremely !uiet op-

amp, particularly ith lo source impedances. Thiscan provide as much as +d of improvement in si#Gnal-to-noise ratios in summin# amp applications,compared to the popular 663* monolithic op-amp.

The 990 provides e/tremely lo noise hen usedin mic preamps. The $ohn %ardy &ompany manuGfactures the =-1, =-2, and $ensen Tin "ervoJ

990 =ic reamps, and several mic preamp cardsusin# the 990. The application notes later in this

paca#e include a schematic of the mic preamp cirGcuitry of the =-1 and a discussion of circuit deGtails.

(Trademark, Jensen Transformers).

One of the reasons the 990 is so !uiet is its use of

the 8nalo# 'evices ""=2212 supermatched tranGsistor pair for the input pair of transistors 4F1 andF2 on the 990 schematic5. The silicon chip of the""=2212 is about 1C17 s!uare, the same sie asthe entire chip of a typical monolithic op-ampK Thelar#e sie provides very lo noise. 8nalo# 'evicesused hatever sie chip as re!uired to mae thefinest possible supermatched pair.

The input pair of transistors in an op-amp should

be as closely matched in performance as possible.The ""=2212 is ideal as an input pair because

both transistors of the pair are fabricated on thesame chip of silicon, thus #reatly reducin# perforGmance differences that ould e/ist beteen sepaGrate chips of silicon. This is a uni!ue situationhere the monolithic process is superior to disGcrete, creatin# multiple transistors side-by-side onthe same substrate for optimum matchin#. In fact,there are four transistors on the chip> the upper-leftand loer-ri#ht transistors are connected in parallelto form F1, the remainin# to transistors conGnected in parallel to form F2, further reducin#even the sli#ht variations that mi#ht e/ist across thesame chip.

High output power.The 990 provides much hi#heroutput poer than monolithic op-amps. This is beGcause the =$-1+1 and =$-11 discrete outputtransistors 4F+ and F95 are much lar#er than theones found in monolithic op-amps 4and some otherdiscrete op-amps5, so they can handle much more

poer. They ere desi#ned from the #round up aspoer transistors. They use a silicon chip that is aslar#e as the chip in a typical monolithic op-amp.The chip is attached to a metal bac-plate for imG

proved heat dissipation. ach transistor is about aslar#e as an +-pin 'I op-amp. The 990C+still usesthe throu#h-hole =$-11C1+1 parts.

Then the 990 paca#e comes into play. The metalbac-plates of the =$-1+1 and =$-11 transisGtors are bonded to the aluminum shell of the 990

usin# a hi#h thermal conductivity epo/y. This proGvides e/ceptional heat-sinin# of the transistors.The 990 paca#e has about 1* times the surfacearea of a typical +-pin 'I op-amp, #reatly increasGin# its ability to dissipate heat. It is easy to see hothe 990 can handle much hi#her poer levels thanthe typical monolithic op-amp. In fact, the 990 candrive 6L loads to full output level, hile monoGlithic op-amps are limited to loads of 700L at best,and more typically 2L. "ome discrete op-ampsuse much smaller output transistors than the =$-1+1 and =$-11. The transistors have smallerchips and are lacin# a metal bac plate critical forheat dissipation. They cannot handle as much poGer as the =$-1+1 and =$-11.

The ability to drive loer-impedance loads is imGportant for to reasons. irst, the 990 can easilydrive multiple poer amps, or pots, etc., ith lessconcern for cumulative loadin#. "econd, the resisGtors, capacitors and other parts that are connectedaround the 990 to determine the function of the cirGcuit can be scaled don to much loer impedancesthan those of a monolithic desi#n. This can result inloer noise. "ome monolithic op-amps are theoretGically capable of very lo noise performance, butthey cannot drive lo impedances ithout inGcreased distortion or decreased headroom, comproGmisin# performance.Low noise and high output power.Mhen you have

both lo noise and hi#h output poer in the same

op-amp, you can often eliminate e/tra op-ampsta#es in e!uipment. (sin# the =-1 mic preamp asan e/ample, the 990 provides the e/tremely lonoise that is re!uired in a mic preamp, and the hi#houtput poer that is re!uired in a line driver ormain output sta#e. There is no need to have tosta#es N one for lo noise and one for hi#h output

poer. The si#nal path is shorter, resultin# in lessi#nal de#radation. 'iscrete op-amps cost morethan monolithics, but hen you use feer of them

the hi#her cost is less of a factor.

Higher voltage ratings.The components of the 990discrete op-amp can handle hi#her volta#es thanthose in most monolithic op-amps. This allos the990 to operate ith 2*P poer supplies, hilethe typical monolithic op-amp is limited to 1+Psupplies. It is common for monolithic op-amps to

be operated at 16P, sometimes even 12P. In audio terms, this means that the monolithic op-amphave reduced headroom. The 990 ith 2*P poesupplies is capable of output levels of #reater thanQ2*du, hile most monolithic op-amps clip several d belo that due to the reduced poer supplyvolta#es.

Precision passive parts.The 990 uses 0.1A, 0.6Aand 1A tolerance metal film resistors ith tempcoof 26 or 60ppm, and ultra-stable &OBC:0 ceramic capacitors ith specifications superior to thosetypically found in monolithic op-amps. "ee the special report about &OBC:0 ceramic capacitors on

pa#e +.

It sounds better=ost important of all is the facthat the 990 sounds better than monolithic op-ampsThe 990 does not suffer from the many compromisGes of the monolithic manufacturin# process. "ome

people thin that solid-state e!uipment is cold andharsh soundin#. :ot the 990K

Applications.The 990 offers the finest performancein summin# amps, mic preamps, phono preamps

tape-head preamps, 8C' and 'C8 converterse!ualiers and line drivers. The sensitivity of measurement e!uipment can be increased by the lonoise of the 990. 8pplication notes start on pa#e *.

!volution o" #odels.There are three versions of the990> The ori#inal 990, the 9908 and the 990&. Theori#inal 990 as introduced in 199. The 9908and 990& ere introduced in 19+. The 8 version adds three components to the ori#inal 990 circuit to provide protection in the rare event that the

positive poer supply is lost hile the op-amp idrivin# an e/tremely lo '& impedance such athe primary of an output transformer. (nder thoseconditions, the ori#inal 990 circuit ould consumhi#her than normal current from the ne#ative sup

ply. The 8 modification prevents the e/cess current flo. The 990& is a further development of the8 version, alloin# the op-amp to operate over aide ran#e of poer supply volta#es. Other additional components provide reduced offset volta#e"ee the schematic on pa#e 3 for details. $ote thathe %%&C' is the onl( model in regular production.

#odel ) Description990C+ Standard 0.6 height of potting shell990C+(Short) Shorter 0.4 height of potting shell


3/8

3

%%&C *peci"ications +&d,u &./ 01

(Measurements made with power supply voltages of 24 V!"

#easurement *pec. 2nits

#pen$loop gain% ! to &0' )2* d+,ain error at )00d+ gain 0.4 d+-oise$voltage spetral density% eah transistor 0./ nV1' differential pair ).)& nV1'-oise urrent spetral density ) p1'-oise inde3% )5 soure resistane 0.6 d+7uivalent input noise voltage% 20' 8andwidth% shorted input )60 nV !orresponding voltage level $)&&.9 d+uMa3imum sine wave input voltageat unity gain )&./ Vrms !orresponding voltage level :2* d+u;nput impedane% non$inverting input 1.126 / 1.126 / 0.*00 4?/M/%5 for applicaGtions here space is limited. ins are 0.0*0',#oldCnicel plated.

3eliabilit(.To ensure lon#-term reliability at temGperature e/tremes, most resistors have a 0.1A tolerGance ith a tempco of 26ppm. 8ll capacitors areultra-stable 430ppm5 ceramics ith the &OBC:0dielectric. :OT> lease see the special report onceramic capacitors on pa#e + for important inforG

mation on this superior formulation. &apacitors inthe si#nal path have a tolerance of 1A. 8ll modulesreceive a total of *+ hours of active burn-in at100R& 4212R5.

2pgrades "rom the original 4ensen Design.=any ofthe components listed in the $ensen en#ineerin#report ere up#raded in the 990 made by the $ohn%ardy &ompany to ensure lon#-term reliability attemperature e/tremes> 'eane $ensen specified 6Atolerance carbon-film resistors. These ereup#raded to 1A metal film ith a 60 or 100ppmtempco in 199. In 2013, most resistors ere furG

ther up#raded to 0.1A thin-film ith a tempco of26ppm. &ertain 0.1A resistors are trimmed to ahi#her de#ree of accuracy usin# proprietary trimGmin# procedures.

&1, &2 and &3 are ultra-stable 430ppm5&OBC:0 ceramic capacitors. "ee the report onceramic capacitors on pa#e +. &* and &6, hichare not in the audio si#nal path, ere up#radedfrom the


4/8

*

Application Notes

olloin# are several circuits for use ith the 990 discrete op-amp. Mithproper attention to detail, you should achieve e/cellent results.

6igure 7: 8raditional mic preamp.i#ure 1 shos a traditional transformer-input mic preamp, adSustable from 11.7 to 70d of #ain includin# the inputransformer step-up of 6.7d. The circuit has a bandidth of 160% 4-3d5The $ensen $T-17- 4or $T-17-85 mic-input transformer as desi#nedspecifically for the 990.

;1, ;2 and &1 provide proper termination for the $T-17- input transformer;3, ;* and ;P1 determine the 8& volta#e #ain of the 990.

&3 is used for to reasons. irst, it eeps the input bias current 4thus '&volta#e5 of the invertin# input of the 990 from reachin# the #ain-adSust po4;P15 here it could cause noise durin# adSustment of the pot. 8ll op-ampshave small amounts of bias current floin# at their inputs. "mall '&volta#es develop as these currents flo throu#h hatever '& resistance pathis available 4I/;5. :oise could occur durin# adSustment of the #ain pot ifmore than about 1mP ere to develop.

&3 also eeps the '& #ain of the 990 at unity so that a small differencebeteen the '& volta#es at the invertin# and non-invertin# inputs of the 990ill not be amplified into a lar#e offset volta#e at the output.

8n optional offset compensation circuit is shon. The diode re#ulator andfilter circuit supplies a current to the invertin# input hich compensates forthe une!ual '& resistances seen at the inputs. The offset volta#e at eachinput is found by multiplyin# the input bias current 4typically 2.2U85 by the'& resistance seen at that input. or the non-invertin# input, the '&resistance is the input transformer secondary resistance in parallel ith ;1

47.19L5. or the invertin# input ;3 is the only '& path. "ince the closedloop '& #ain of the amplifier is unity, the '& offset at the output is e!ual tothe difference of the offset volta#es at the to inputs. The compensatin#current re!uired into the invertin# input is the offset volta#e divided by ;3410L5. This compensation ill si#nificantly reduce the '& offset at theoutput for applications ith no output couplin# capacitor.

&2 provides phase-lead compensation ith a hi#h-fre!uency cut-off o16%. &* 8&-couples the output of the 990 to remove any '& offset fromthe output.

The use of capacitors &3 and &* to control various '& problems istraditional. or a superior approach that eliminates these capacitors and thesonic problems they can cause, see the application note for the =-1 mic

preamp on pa#e .

6igure 9: Phono preamp. i#ure 2 shos a phono preamp ith relatedcomponent values and theoretical ;I88 response fi#ures. Bain is *1.d a1%. The circuit provides ;I88 response accuracy of 0.1d. The valuesare taen from a paper by ?ipshit V1W hich covers ;I88 e!ualiationnetors and their proper desi#n.

&olumn 1 shos the e/act calculated resistor and capacitor values. Thenearest 1A resistor values are in column 2. &olumns 3 and * sho the valuesscaled by a factor of 10 to tae advanta#e of the 990Hs loer noise fi#ure aloer source impedances.

&3 8&-couples the 990, causin# '& #ain to be unity. &3 could be eliminatedif offset compensation ere performed. "ee fi#ure 1 for one method. "ee the=-1 mic preamp application note for superior methods. The ferrite beads atthe input are optional to reduce ;I.

REFERENCE: 1. Lipshitz, ., !"n R#$$ E%&a'ization Netorks, Jo&rna', $&dioEn*ineerin* o+iet, -o'. /, 0, 2/3, pp. 4567461.

6igure : 8ape-head preamp.i#ure 3 shos a tape-head preamp. &omponenvalues for 3.6 and .6 ips :8 e!ualiation and a #ain of 60d at 1% arelisted. Other #ains and e!ualiations can be achieved usin# the formulas

provided. Tape head specs and characteristics vary idely, so the valueslisted ill probably re!uire trimmin#. The results should be carefullye/amined.

Tape heads ith e/tremely lo output levels ill re!uire additional #ain. 82nd op-amp should be considered for that purpose. It should have flaresponse. ach op-amp should be set for e!ual #ain at hi#h fre!uencies420%5.

This circuit is similar to the phono preamp, e/cept it is tunable. The ;2-&2netor is at 300% performin# phase-lead compensation rather than ;I88e!ualiation. "ee hono preamp for comments on &3 and ferrite beads.


5/8

6

6igure ;: 8wo-stage mic preamp. i#ure * shos a to-sta#e transformer-coupledmic preamp, recommended for situations here e/tremely hi#h #ain is re!uiredThe first sta#e is the same as the sin#le-sta#e preamp of fi#ure 1 e/cept thema/imum #ain is about *0d. 8 sitchable second sta#e ith 20d of #ain #ivea choice of sin#le-sta#e operation ith up to *0d of #ain 4includin# thetransformer step-up5, or to-sta#e operation ith up to 70d of #ain. The 2ndsta#e could be chan#ed to adSustable #ain. Ideally each sta#e ould have the sameamount of #ain.

Offset volta#e compensation can be performed on the first sta#e as described inthe sin#le-sta#e preamp te/t, or as shon in the =-1 application note. The secondsta#e ill have a lo offset volta#e because the invertin# and non-invertin# inputssee identical '& resistances 410L5. The techni!ues in the =-1 application notecan be applied here too. "ee the data paca#e for the $ensen Tin "ervo J990 =ic

reamp, a superior to-sta#e mic preamp usin# the $T-17- input transformerand 990& op-amp. It eliminates all couplin# capacitors by usin# '& servocircuitry and input bias current compensation circuitry.

6igure /: *ockets.=any types of socets for 0.0*0' pins are available fromseveral manufacturers. The $ohn %ardy &o. uses and stocs the socet shon infi#ure 6, reprinted from the &oncord catalo#. The same part is also available from&ambion, a very similar part 4but ith less retention force5 from =ill-=a/. It canbe soldered in place, or sa#ed 4tool re!uired5. %ere are three sources>

CONCORD ELECTRONICS INC. 212-777-6571 800-847-416230 Great Jones St. Part #09-9035-2-03New York, NY 10012

WEARNES CAMBION, LTD. 011 44 1433 621555United Kingdom 800-947-1256 (USA & Canada)

Part #450-3756-02-03

MILL-MAX 516-922-6000190 Pine Hollow Road Part #0344-2-19-15-34-27-10-0Oyster Bay, NY 11771

6igure


6/8

7


7/8

M-1 Mic Preamp with Input Bias Current Compensation and DC Servo Circuitry

i#ure shos the complete circuit of the=&-1 mic preamp card used in the =-1 and=-2 mic preamps, state-of-the-art micpreamps manufactured by the $ohn %ardy &o.This circuit eliminates all couplin# capacitorstraditionally used in mic preamp circuits, andthe de#radation in si#nal !uality that they cancause. The main difference beteen the =-1and the =-2 is the type of #ain control> a 2-section potentiometer in the =-1, a 17-position rotary sitch in the =-2. "ee the =-1and =-2 data paca#e for further details.

8t first #lance capacitors seem lie idealcomponents to use hen tryin# to eliminatethe '& volta#es that alays mana#e to creepinto audio circuits. &apacitors have essentiallyinfinite impedance at '&, and ero ohmsimpedance throu#hout the audio bandidth ifthe value is lar#e enou#h for the application.

%oever, capacitors also have problems. "eethe special report about ceramic capacitors onpa#e + for a discussion of one problem.8nother problem is dielectric absorption. Thisis a condition here a small portion of the 8&volta#e that passes throu#h the capacitor istemporarily absorbed by the dielectric of thecapacitor, then released a short time later,

causin# a smearin# of the sound. The severityof the problem depends on the type ofdielectric in the capacitor, and otherconstruction details.

The problem tends to be unmeasurable ithnormal test methods, but can be audible. "omefilm dielectrics such as polypropylene,polycarbonate, polystyrene and Teflonminimie the problem. ut hen a circuitre!uires several hundred microfarads, it is outof the !uestion to use them, both from a spaceand cost standpoint. 8 compromise approachhas been to use electrolytic capacitors of there!uired lar#e value, then add a 1.0U or0.1U 4or both5 film capacitor in parallel, thetheory bein# that lo fre!uencies ill be

handled by the lar#e electrolytic capacitor, andhi#h fre!uencies 4here the smearin# ouldbe most audible5 ill be handled by the smallfilm capacitors.

Traditional transformer-input mic preampstypically have to couplin# capacitors in thesi#nal path. ;eferrin# to the traditional micpreamp circuit of fi#ure 1 they are &3 and &*.Their functions are discussed in thatapplication note.

=ic preamps ith transformerless inputs haveto additional couplin# capacitors to eep theQ*+P phantom poer supply volta#e fromreachin# the active circuitry of the preamphere it ould cause dama#e. 8n input

transformer inherently blocs '& volta#es,but does not suffer from the problem ofdielectric absorption that capacitors have.=anufacturers of transformerless mic preampsmi#ht say that these capacitors cause lesssonic dama#e than an input transformer. Thisis true of some input transformers, but notith the $ensen $T-17- input transformerused in the =-1K This is $ensenHs finest inputtransformer, and it is truly superior. "ee the=-1 data paca#e for details.

The =-1 taes a different approach. ;atherthan forcin# the audio si#nal to pass throu#h

various capacitors to bloc the '& volta#es4and, in the process, smear the audio si#nal5,the '& volta#es are nulled usin# specialcircuitry. The couplin# capacitors arecompletely eliminated.

The input bias current compensation circuit4II8"5 on the =&-1 mic preamp cardprovides an adSustable current to each inputof the 990 op-amp. The current is theopposite polarity of the normal input biascurrents of the 990 op-amp. Mhen ;P2 isproperly adSusted, the input bias currents ofthe 990 are nulled so that no '& volta#esare developed at the inputs of the 990.Traditionally a couplin# capacitor 4&3 ini#ure 15 is used in series ith the #aincontrols to eep '& volta#es from reachin#the #ain controls here they could causenoise durin# adSustment of the control. TheII8"circuit eliminates the need for this '&-blocin# capacitor.

"ince all input-related '& volta#es havebeen nulled by the II8" circuit, it is nolon#er necessary to orry about a smalldifference in the volta#es at the inputs bein#amplified into a lar#e '& error or offset atthe output of the 990. Therefore it is not

necessary to limit the '& #ain of the 990 tounity, a function that &3 also traditionallyperforms. 8#ain, &3 can be eliminated byusin# the II8"circuitry.

8 Q16P reference volta#e is applied to thetop of ;P3, a 26-turn trim pot. The trimmedvolta#e is applied to the inputs of the 990 asa current throu#h ;9, ;10, ;11 and ;12.&3 and &* act as noise filters.

The '& servo 4";PO5 circuitcontinuously monitors the output of the 990for the presence of any '& offset, andprovides a correction to the invertin# inputof the 990 throu#h ;16. The final '& offsetof the 990 is determined by the '& offset

characteristics of the servo op-amp 4(25.The 8'06$ as chosen because it hase/ceptional '& characteristics, ith atypical '& offset of 200 microvolts and driftof 2 microvoltsCR&. The '& offsetperformance of the 8'06$ is furtherimproved by an order of ma#nitude throu#hthe use of trim pot ;P3. 4&urrently theO9 is used as the '& servo op-amp.The ?T1012 and ?=11&: op-amps ereused in earlier production5.

The servo circuit itself acts as an ultra-lofre!uency lo-pass filter. The -3dfre!uency is so lo 4ell belo 1%5 thatessentially only '& is passed throu#h thecircuit and applied to the invertin# input of

the 990 as a nullin# si#nal. The to ;C&netors, ;13C&6 and ;1*C&7, alon# ith;16, determine the operatin# fre!uency.The capacitors have no detrimental effect onthe audio si#nal because they only affectfre!uencies in the pass-band of the filter4ell belo 1%5.

Input Bias Current Calibration

Mhenever a 990 op-amp is replaced, itshould be assumed that it has a differentinput bias current than the previous 990.The input bias current adSustment procedure

should be performed as follos>

1. Install the ne 990, turn on the poer andallo the unit to arm-up for at least 16minutes.

2. &onnect a '& voltmeter ith at least 100microvolt sensitivity to the circuit as follosThe ositive lead connects to test point )14T15. This is the output of the 990 op-amp8 lon# #old pin is provided for T1, locatedalon# the left ed#e of the p.c. board near therear. The ne#ative lead connects to #round. 8lon# #old pin is provided for this #roundconnection to the rear of the 990 op-amp.

3. =ove $3 to the 8'$("T 48'$position. $3 is located to the ri#ht of the 990op-amp. This disconnects the '& servo circuiso you can measure the '& offset of the 990op-amp.

*. "et the #ain controls to minimum #ain andmae note of the '& offset measured at T1.

6. "et the #ain controls to ma/imum #ain andadSust ;P2 so that the '& offset readin# isithin 1 millivolt of the readin# taen henthe #ain controls ere at minimum #ain. I

may tae several seconds for this measuremento settle. ;P2 is a 26-turn trim pot labeled II8located to the ri#ht of the 990 op-amp.

7. ;epeat steps * and 6 until the '& offsetmeasurements are ithin 1 millivolt of eachother at minimum and ma/imum #ain. :otethat both readin#s may be several millivolts, oeven tens of millivolts. They mi#ht both bepositive, or ne#ative. The important thin# isthat they are ithin 1 millivolt of each otherand the same polarity.

. =ove $3 to the ;(: position. Thisreconnects the '& servo. This should causethe '& offset volta#e of the 990 op-amp todrop to ell belo 1 millivolt. In fact, the '&

offset should drop to ell belo 100microvolts if the '& servo circuit isfunctionin# properly.

DC Offset Calibration

The final '& offset of the =&-1 mic preampcard is determined by the performance of the'& servo op-amp 4(25. 8n 8'06$ or O9op-amp is used because it has e/cellent '&specifications. It is capable of providin# a '&offset that is typically less than 200 microvoltithout any additional trimmin#. This ie/cellent, but can be improved by an order oma#nitude throu#h the use of trim pot ;P3.

Mhen main# '& measurements belo 100

microvolts you ill need a '& voltmeter itha sensitivity and resolution of at least 1microvolt. =ost meters ill have '& offsetsof their on to deal ith, as ell as drift dueto time andCor temperature. ven the cablesand test probes can introduce errors. ollothe meter instructions very carefully re#ardin#arm-up time and eroin# procedures.

Mhen you have properly armed-up anderoed your meter, adSust ;P3 until the '&offset measurement beteen T1 and #roundis as close to X;O microvolts as possible.


8/8

+

Ceramic Capacitors

&eramic capacitors have a bad reputation in audio circles. It is only partially deserved. =anyen#ineers are unaare that there are several distinctly different #rades of ceramic capacitors, eachhavin# a uni!ue formulation of ceramic dielectric, and a uni!ue set of properties. The three moscommon .I.8. V1W types are>

1. (ltra-stable> &OB dielectric 4also called :O V2W5.2. "table> D; dielectric.3. Beneral purpose> X6( dielectric.

The &OB dielectric is a vastly superior performer. It is also more e/pensive, particularly in valueabove a fe hundred p, and is usually dismissed as cost-prohibitive. 8 common mistae is toshop by price alone and buy the cheaper dielectrics, not realiin# the serious performance

compromises. The en#ineer then condemns all ceramics based on the limited e/perience ith onlythe inferior types. Too adK /amination of the performance #raphs of fi#ure 1 reveals si#nificandifferences beteen the dielectrics. In each case N capacitance vs. temperature, capacitance vstime 4a#in#5, capacitance vs. applied 8& volta#e, capacitance vs. '& stress, and dissipation vstemperature N the D; and X6( dielectrics sho si#nificant compromises hen compared to the&OB formulation.

The D; and X6( formulations trade off electrical performance for increased volumetricefficiency. To achieve this a ferroelectric material is used. erroelectric behavior is comple/. 8ne/cellent te/t by &entre n#ineerin# V3W provides a comprehensive discussion of this and otherceramic properties. ssentially, ferroelectricity causes capacitance to chan#e as the appliedvolta#e to the capacitor is chan#ed. In audio applications the 8& volta#e passin# throu#h aferroelectric dielectric ould modulate the capacitance. In resistorCcapacitor netors ine!ualiers and crossovers this modulation causes distortion hich increases as the si#nafre!uency approaches the cut-off fre!uency of the ;C& netor.

Tests ere conducted ith the &OB, D; and

descrete op amp papadopou

Documents