crescendo amplifier

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36 Elektor Electronics 4/2001

CrescendoMillennium Editionremake of a famous amplifier

Design by T. Giesberts

In early 1984, Elektormagazine beat the com-petition hands-down by

publishing the design of athen-revolutionary MOS-FET amplifier. Even now,

this amplifier enjoys anenthusiastic following. In

response to manyrequests, we have

brought the design up todate and given some

attention to improvedreliability and operating

safety. The output poweris 90 watts into 8 ohms

or 135 watts into4 ohms, which should

leave little to be desiredfor most users.

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374/2001 Elektor Electronics

Measured results (power supply as shown in Figure 3; quiescent current 200 mA)

– input sensitivity: 1 Vrms– input impedance: 45 kΩ– sine-wave power (0.1 % THD): 90 W/8 Ω, 137 W/4 Ω– power bandwidth (80 W/8 Ω): 1.5 Hz – 300 kHz– slew rate: 60 V/µs (rise time = 1µs)– signal/noise ratio: 104 dB (A–weighted)

(with respect to 1 W/8 Ω) 96 dB (BW = 22 kHz, linear)– harmonic distortion at 8 Ω: at 4 Ω:

(bandwidth 80 kHz): at 1 kHz: 0.002 % (1 W) 0.0026 % (1 W)0.0017 (40 W) 0.004 % (80 W)

at 20 kHz: 0.028 % (40 W) 0.04 % (80 W)– intermodulation distortion: 0.0017 % (1 W) 0.003 % (1 W)

(50 Hz : 7 kHz = 4 : 1) 0.004 % (40 W) 0.007 % (80 W)– dynamic IM distortion: 0.0026 % (1 W) 0.003 % (1 W)

(3.15 kHz square wave with 0.0014 % (40 W) 0.0023 % (80 W)15 kHz sine wave)

– damping factor (at 8 Ω): 460 (1 kHz)330 (20 kHz)

– open–loop parameters:gain: 4,000bandwidth: 25 kHzoutput impedance: 0.5 Ω

protection:DC: + 4.7 V / – 4.3 Voverload (0 V out): + 5.8 A / –5.4 Aswitch-on delay: 8 to 10 sbias compensation: ± 4.5 µA

From the number of zeros after the decimal point, you can see in asingle glance that this is an exemplary set of results. You will notoften come across a better set of figures. The distortion is verylow, the damping factor is very good and the slew rate can even besaid to be remarkably good.As you may expect, we have also measured a number of curvesusing the Audio Precision analyser in order to complement the per-formance figures, which always have a somewhat ‘dry’ taste.Figure A shows the harmonic distortion (THD+N) over the rangeof 20 Hz to 20 kHz with an 8-Ω load, using a measurement band-width of 80 kHz. At 1 W the increase in the distortion level at20 kHz is minimal, but at the 50% power level (40 W is equivalentto 70% of the maximum output amplitude) the effect of the non-linear input capacitance of the MOSFETs can be recognised.Figure B shows the distortion of a 1-kHz signal into an 8-Ω load asa function of the output level in watts, measured with a bandwidthof 22 kHz. The behaviour of the amplifier is more readily visiblewith this narrower measurement bandwidth. Up to 10 W, theTHD+N is predominantly due to supply ripple and noise. A slightincrease in the distortion can be seen above 10 W, but a level of0.1% is reached only at 90 W.Figure C shows the maximum output power into 4-Ω and 8-Ωloads at a distortion level of 0.1% for frequencies between 20 Hzand 20 kHz (80 kHz measurement bandwidth). Both of thesecurves can be said to be practically straight.Finally, Figure D shows the results of a Fourier analysis of a 1-kHzsignal (1 W into 8 Ω) with the fundamental suppressed. At thispower level, the THD is clearly lower than the supply ripple,whose harmonics lie below –100 dB. The 2nd and 3rd harmonics lieat negligibly low levels (–118 dB and –115 dB, respectively).

1

500

2

5

10

20

50

100

200

W

20 20k010001 - C

50 100 200 500 1k 2k 5k 10k

Hz

4Ω

8Ω

0.0006

1

0.001

0.002

0.005

0.01

0.02

0.05

0.1

0.2

0.5

%

1m 1002m 5m 10m 20m 50m 100m 200m 500m 1 2 5 10 20 50010001- BW

0.001

1

0.002

0.005

0.01

0.02

0.05

0.1

0.2

0.5

%

20 20k50 100 200 500 1k 2k 5k 10k010001-AHz

40W

1W

a

b

c

There are surely not very many circuitdesigns that continue to enjoy such a highlevel of interest more than ten years aftertheir original appearance, as does theCrescendo power amplifier from 1984. In part,this is due to its completely symmetricaldesign, which was in fact an unusual featureat that time, but unquestionably it is mainlydue to the use of power MOSFETs in the out-put stage. A lot of people happen to be fer-vent fans of these devices. Even people whoswear by valve amplifiers and are allergic toanything with ‘semiconductor’ in its nameoften have a weakness for MOSFETs, and arethus prepared to make an exception for them.Sadly enough, most of the problems with theamplifier in question had to do with the MOS-FETs. The original types have long since goneobsolete and become unavailable, and suit-able replacements are hard to find. However,there were also other difficulties. The stabil-ity of the amplifier sometimes gave cause forconcern, and users considered the absence of

protective circuitry to be a majorweakness.Consequently, in honour of ouranniversary, we decided to takeanother look at the original design.Our objective was to update thedesign of the amplifier in a way thatwould eliminate the sources of criti-cism without sacrificing the goodcharacteristics of the original design.This objective has been quite suc-cessfully achieved. In addition, wewere able to obtain such a generouslevel of output power using a newpair of MOSFETs that it is not neces-sary to split the new Crescendo into‘light’ and ‘heavy’ versions.

The same concept

Since we have intentionally tried tochange the old amplifier design aslittle as possible, the differences

between the schematic diagrams ofthe old and new versions are mini-mal. The design still consists of aninput stage with dual differentialamplifiers and current sources, acascode driver stage and a MOSFEToutput stage. That may have been arather sophisticated design in 1984,but nowadays it would more likelybe described as a ‘minimal design’.There’s nothing wrong with this, bythe way, since attempting to keepthe signal path as short as possibleis certainly not a mistaken endeav-our in an amplifier design — but wedon’t need to dwell on this point.Since the basic concept of the origi-nal design has been retained, any-one who compares the schematicdiagram of the new version (see Fig-ure 1) with that of the old version(May 1984) will first have to try tofind the differences. Of course, there

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bias

T12

2SK1530

T13

2SJ201

T11

2SK537

R34

0Ω

22

R35

0Ω

22

R1

1M

R2

47

k

R3

470Ω

R5

1M8

R4

1M8

R16

22

k

R19

22

kR17

27

0Ω

R20

27

0Ω

R18

8k

2

R21

8k

2

R8

1k

R9

1k

R13

1k

R14

1k

R6

47Ω

R7

47Ω

R11

47Ω

R12

47Ω

R22

47

0Ω

R10

330Ω

R15

330Ω

R23

12k

R32

220Ω

R33

220Ω

R37

1Ω

R25

33Ω

R24

10

k

R26

10

k

R27

33Ω

R29

12

0Ω

R28

27

0Ω

R30

22

k

R31

22

k1kP1

LSP+

LSP–

tp1

tp2

tp3

C1

2µ2

C4

1n

C5

1n

C3

180n

C2

1n

C10

100n

C14

100n

C12

100n

C11

10n

R36

10Ω

C6

100µ25V

C7

100µ25V

C8

220µ 25V

C9

220µ 25V

C13

1000µ63V

C15

1000µ63V

D1

D2

T5

BC556B

T3

BC556B

T4

T6

BC546B

T1

BC546B

T2

T9

BC550C

T10MJE340

T7

BC560C

T8MJE350

D3

3V90W5

D4

3V90W5

L1

2x

2x

+45 ... 50V

– 45 ... 50V

*

*

*

see text*zie tekst*siehe Text*voir texte*

010001 - 11

CW

2SJ201

G

D

S

2SK1530MJE340

E

C

B

MJE3502SK537

D

G S

Figure 1. In the schematic diagram, the changes from the original version are hardly noticeable at first glance.

too much for the transistor types used for T8and T10 in the old version, they have beenreplaced by the somewhat more robust typesMJE340 and MJE350.Now we come to the output stage. In contrastto the MOSFETs used in the old version, the2SK1530 and 2SJ201 devices used here have apositive temperature coefficient. This meansthat with a constant gate-source voltage, thedrain current increases with increasing tem-perature. This made it necessary to use a dif-ferent design for the quiescent- current cir-cuit. Here the MOSFET T11, which ismounted on the same heat sink as T8/T10and T12/T13, provides the necessary com-pensation.Finally, there are a couple of other significantitems.Insiders will notice that the none-too-attrac-tive bipolar electrolytic capacitor has beeneliminated from the reverse feedback network(R22/R23), which means that DC coupling isused here. To get rid of the resulting outputoffset, we have provided an automatic com-pensation circuit that is located on the pro-tection circuit board. We anyhow intended to

are indeed differences, and it seemslike a good idea to list the mostimportant changes before diving intoa detailed description of theschematic diagram.The most evident change is naturallythe new pair of MOSFETs in the out-put stage. The Toshiba 2SK1530 and2SJ201 are readily available, and fur-thermore they can dissipate so muchmore power than the originaldevices that we were able to boostthe output power of the old ‘Mini-Crescendo’ by a factor of nearly two(90 W into 8 Ω in place of 50 W)using only a single pair of transis-tors.As a consequence of the increasedpower level, the bias currents of thevarious stages must be modified anddifferent transistors must be used inthe cascode stage, as will be seenlater on.The next change is the addition ofthe networks R10/C4, R15/C5 andR30/R31, which represent the resultsof measures that havebeen taken to optimisethe stability of theamplifier. A very impor-tant final item is thatthe amplifier has beenprovided with reliableprotection circuitry andautomatic offset com-pensation, by means ofan extra printed circuitboard.This pretty well coversthe most importantchanges.

SchematicdetailsNow that we’ve seenthe global picture, it’stime to take a moredetailed look at the cir-cuit diagram. Let’s startat the beginning, whichis of course the inputstage.The design of the inputfilter is more or lessstandard. R2 (with R1 inparallel) determines theinput impedance, andin combination with C1it forms a high-pass fil-ter that blocks frequen-cies below around

1.5 Hz. C1 is also needed to isolatethe DC bias of the input stage. Thecombination of R3 and C2 forms alow-pass filter that is dimensionedfor a frequency of more than 300 kHz.This helps prevent TIM (transientintermodulation) distortion and elim-inates possible RF interference.The dual differential amplifier(T1–T4) has been designed to workwith a bias current that is approxi-mately three times a great as that ofthe original design, on account of theincreased output power. The currentsources that regulate this setting, T5and T6, now use LEDs as references(D1 and D2), since this results in lessnoise than using Zener diodes. In theinterest of the thermal stability of theDC setting, D1/T5 and D2/T6 arethermally coupled, as are the tran-sistor pairs T1/T2 and T3/T4.The bias currents of the cascodestages T7/T8 and T9/T10 are alsosignificantly greater than in the orig-inal design. Since this would be a bit

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use the compensation circuit to correct for theoffset caused by the unavoidable asymmetryof the input stage. The necessary compensa-tion circuit consists of nothing more than anopamp wired as an integrator, which mea-sures the output voltage of the amplifier andprovides the proper amount of reverse currentfeedback to the (bias) input. Thanks to thevery high values of R4 and R5 and the decou-pling provided by C3, this correction hasabsolutely no effect on the audio signal.Another essential detail is that the open-loopgain has been made independent of the loadby the addition of R30 and R31. These resis-tors together determine the output imped-ance of the voltage amplifier, and as a resultthe source followers T12 and T13 now oper-ate purely as buffers in the audio range. With-out these resistors, the behaviour of theamplifier is directly dependent on the con-nected load, which is not the way things aresupposed to be.Together with the compensation networks

R10/C4 and R15/C5, the modificationmade using R30/R31 ensures thatthe amplifier is unconditionally sta-ble, so much so that the standardBoucherot network (R36/C11) caneven be omitted.

Protection

The protection circuitry (Figure 2),which is located on a separateprinted circuit board, includes over-load protection, DC protection, aswitch-on delay for the output relayand a voltage detector that directlydisables the output relay when thepower is switched off or any of thetransformer voltages is absent. Theintegrator for the offset compensa-tion is also located on this circuitboard.There are three terminals on thepower amplifier board that provide

information to the protection cir-cuitry: tp1 and tp2 convey the volt-age across the emitter resistors,while tp3 conveys the output volt-age. The actual protection takesplace with the help of two relays(Re1 and Re2), whose switching con-tacts are connected in parallel inorder to keep the insertion resis-tance as low as possible. The relaycontacts are wired in series with theamplifier output via the terminals‘Amp’ and ‘LSP’.The supply voltage for the protectioncircuitry is tapped off from the sup-ply points on the amplifier board.The supply voltage for the integratoris simply derived from the amplifiersupply voltages using a pair of Zenerdiodes (D3 and D4).The overload protection circuit isconstructed in a ‘classic’ mannerusing a voltage divider and a tran-

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T1

MJE340

T5

T6

BC546B

T4

BC546B

T10

BC546B

T2

MJE350

T8

BC556B

T9

BD140

T3

BC556B

T7

BC516

R8

5k

6

R7

1k

R3

18

k

R1

390Ω

R4

390Ω

R2

12

0Ω

R6

18

k

R9

1k

R10

5k

6

R12

1M

R14

15

0k

R5

12

0Ω

R18

12

k

R17

33

0k

R19

33

0Ω

R20

22

0k

R21

22

0k

R22

15

k

R23

68

0Ω

R28

47

0k

R29

3k

3

R11

10k

R13

1M

R1539

k

R16

22

0k

R24

1M

R26

4k7

R27

4k7

R25

10k

R30

10k

R31

10k

C1

150p

C2

150p

C5

47µ63V

C3

22µ 63VC4

22µ 63V

C7

220µ25V

C8

220µ25V

C9

1µ63V

C6

2µ2

D3

20V 0W5

D4

20V 0W5

V+

tp1

tp2

tp3

V– CNY17-3

IC25

4

1

2

6

V+

D8

1N4004

D7

1N4004

Re1

Re2

D1

1N4148

D2

1N4148

Amp.

LSP

biasIC1

OP77

2

3

6

7

4

1

8

V+

D6

BAT85

D5

+20V

V+

V-

–20VV-

+20V

–20V

Re1, Re2 = G2R-1-E

2x

2x

010001 - 12

35V

35V

Figure 2. Schematic diagram of the added protection circuitry and offset compensation circuit.

together form a bipolar electrolytic capacitor.If a sufficiently large positive voltage is pre-sent, T5 is brought into conduction via thevoltage divider R13/R14, and T7 is thenbrought into conduction via R17. With a suf-ficiently large negative voltage, the currentthrough T6 will be large enough to cause T7to conduct. The voltage divider R13/R14, incombination with R15/R17/R18, ensures thatthe positive and negative threshold voltagesare nearly the same. T7 can thus be broughtinto conduction via R12 /R16 and T5/T6.When the supply voltage comes up and nofault is present, the electrolytic capacitor C5will be charged to approximately half of thesupply voltage level via voltage dividerR20/R21. The time delay before the relayengages thus amounts to around 8 to 10 sec-onds. Darlington T8/T10 connects the relaycoils to the supply voltage. If T7 starts to con-duct, C5 is immediately discharged and therelays disengage.An optocoupler is used for the voltage detec-tion circuit in order to prevent ground loopsbetween the transformer ground and the sig-nal ground, as well as other possible types ofinterference. The current for the optocouplerdiode is provided by R29–R31, and the timeconstant determined by C9 has been chosensuch that the transistor in IC2 remains con-tinuously conducting only as long sufficientvoltage is present on both transformer wind-ings. If the voltage drops, T10 starts to con-duct and the relays are disengaged.The offset compensation circuit consists ofonly two resistors, one capacitor, an opamp(IC1) and two diodes, in addition to the supplycomponents. Since the correction current iscoupled into the non-inverting input of thepower amplifier, this integrator must invertthe signal. D5/D6 and R25 provide additionalprotection for the opamp. With an eye on pro-tection we have chosen an OP77 (ultra-lowoffset) opamp, which already has internalinput protection and is short-circuit proof.

A robust power supply

In the description of the original Crescendo,it was already noted that the power supply isone of the most important components of apower amplifier. In fact, the ultimate soundquality depends on the power supply. Thedesign of a good power supply does not haveto be difficult, since the well-known and com-monly used formula of a transformer, bridgerectifier and electrolytic filter capacitors isfully adequate. However, you should not tryto cut corners here, which is why two elec-trolytic capacitors of no less than 22,000 µF(22 mF) are used in the power supply shownin Figure 3. In order to avoid misunderstand-

sistor. T1 measures the voltageacross R34 in the power amplifier cir-cuit via the network R1-R3 and canthus determine whether the 2SK1530has exceeded its safe operating area.The combination of T2 and R4-R6performs the same service for the2SJ201 by measuring the voltageacross R35. The maximum allowablecurrent through the output transistoris linearly dependent on the voltageacross the transistor, up to the pointthat the maximum allowable voltageor current has been reached. Themaximum current limit is set usingthe voltage divider R1/R2 (or R4/R5for the other half), and this limit isdecreased via R3 (or R6) as the volt-age across the output transistor

increases. Since we can assume amusic signal, we have stuck to the100-ms limit, so that the limitingvalue for the load impedance can beset lower without causing problemsfor the output transistors. If T1 (orT2) starts to conduct, the transistorsof the DC protection circuit areutilised via T3 (or T4) to disengagethe relay. C1 and C2 reduce the cir-cuit’s sensitivity to HF interference.R7 and R9 are 5-W types, since theirpower dissipation can be significantin certain fault situations.The DC protection circuitry employsa commonly used principle. Any DCvoltage that is present is receivedvia the low-pass filter R11/C3/C4(roll-off frequency 1.5 Hz). C3 and C4

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2x 35V

Tr

225VA

B

200V / 35A

+49V

–49V

C5

22000µ63V

C

22000µ63V

1A T

F

974078 - 1

010001 - 13

mainspower-on

delay

B1

B250C1500

C2

470µ40V

C3

470µ40V

R3

220Ω

R1

470k

R2

470k

C1

330n250V

Re1

F1

R4

10Ω

5W

R5

10Ω

5W

R6

10Ω

5W

R7

10Ω

5W

K2

K1

*~*

zie tekst*see text*voir texte*siehe Text*

Re1 = V23057-B0006-A201

974078 - 11

Figure 3. The power supplies of mass-produced amplifiers are rather skimpy. Thisone has no such problems.

Figure 4. A mains switch-on delay circuit, such as the one shown here, preventsthe fuse from blowing when the amplifier is switched on.

ings, we hasten to point out that we are talk-ing about a monaural version here, so for astereo amplifier you will have to build two ofthese supplies!The ‘mains switch-on delay’ shown insidethe dotted box in Figure 3 is not mandatory,but it is highly recommended — especially if

a toroidal transformer is used. Thiscircuit does exactly what its namesuggests, and it ensures that exces-sive current surges do not occurwhen the mains voltage is switchedon. Such circuits have frequentlybeen described in Elektor Electron-

ics; the most recent one can be foundin the Summer Circuits issue of 1997,and we have reproduced its

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(C) ELEKTOR010001-1

C1

C1

C2

C2

C3

C3

C4

C4

C5

C5

C6

C6

C7

C7

C8

C8

C9

C9

C10

C11

C12

C13

C14

C15

D1

D1

D2

D2

D3

D3

D4

D4

D5D6

D7

D8

H1 H1H2

H2

H3

H3H4 H4

IC1

IC2L1

P1

PC7

PC8PC9

R1

R1

R2

R2

R3

R3

R4

R4

R5

R5

R6

R6

R7

R7

R8

R8

R9

R9

R10

R10

R11R

11R12

R12

R13

R13

R14

R14

R15

R15

R16

R16

R17

R17

R18

R18

R19

R19

R20

R20

R21

R21R22 R22R23

R23

R24

R24

R25

R25

R26

R26

R27

R27

R28

R28R

29

R29

R30

R30

R31

R31

R32

R33

R34

R35

R36

R37

RE1

RE2

T1

T1

T2

T2

T3

T3

T4

T4

T5

T5

T6

T6

T7

T7

T8

T8

T9

T9

T10

T10

T11

T12

T13

Am

p.

LSP

T

bia

s

~~

0

tp2

tp3

tp1

V+

V-

tp1

tp2

LSP

+

LS

P-

T

0

+-

bia

s

tp3

010001-1

0

(C) ELEKTOR010001-1

Figure 5. The printed circuit boards for the amplifier and the protection circuitry are deliv-ered as a single board and must be sawn apart.

COMPONENTS LISTAmplifier board

Resistors:R1 = 1MΩR2 = 47kΩR3,R22 = 470ΩR4,R5 = 1MΩ8R6,R7,R11,R12 = 47ΩR8,R9,R13,R14 = 1kΩR10,R15 = 330ΩR16,R19,R30,R31 = 22kΩR17,R20,R28 = 270ΩR18,R21 = 8kΩ2R23 = 12kΩR24,R26 = 10kΩR25,R27 = 33ΩR29 = 120ΩR32,R33 = 220ΩR34,R35 = 0Ω22 / 5W low-induc-

tance, e.g., MPC71 seriesR36 = 10Ω / 1W *R37 = 1Ω / 5WP1 = 1kΩ preset H

Capacitors:C1 = 2µF2, MKT (Siemens), lead

pitch 5mm or 7.5mmC2,C4,C5 = 1nFC3 = 180nFC6,C7 = 100µF 25V radialC8,C9 = 220µF 25V radialC10,C12,C14 = 100nFC11 = 10nF *C13,C15 = 1000µF 63V radial

Inductors:L1 = 9 turns 1.5 mm dia. ECW

around R37, inside diameter8 mm

Semiconductors:D1,D2 = rectangular face, redD3,D4 = zener diode 3V9 / 0.5WT1,T2,T6 = BC546BT3,T4,T5 = BC556BT7 = BC560CT8 = MJE350T9 = BC550CT10 = MJE340T11 = 2SK537 (Toshiba)T12 = 2SK1530 (Toshiba)T13 = 2SJ201 (Toshiba)

Miscellaneous:5 off M3 spade terminals, PCB

mount3 off ceramic (or mica) isolating

washer for voor T8/T10/T112 off mica isolating washer for

On the amplifier board, five wire bridgesmust be inserted, and it is a good idea to dothis at the beginning. In addition, there aretwo items on the amplifier board that couldbe considered to be somewhat difficult: thethermal coupling and the output coil L1.For the thermal coupling between the D1/T5and D2/T6 pairs, it is sufficient to mount theLED so that it is in contact with the flat face ofthe transistor. In the case of the T1/T2 andT3/T4 transistor pairs, it is recommended toclamp a small metal ring around each pair.Incidentally, we have discovered that suitablerings can be made by sawing them from apiece of copper water pipe and then bendingthem into a suitable shape.Coil L1 can be easily wound on an 8-mm drillbit. After this you can insert R37 into the coiland then solder both components to the cir-cuit board, after having first removed the lac-quer from the two ends of the coil with theaid of a knife.Transistors T8 and T10–T13 are intentionallyplaced along one edge of the circuit board sothat they can easily be screwed to a singlecommon heat sink. Naturally, the transistorsmust be mounted using insulating washers,

schematic diagram in Figure 4. Itsoperation is simple, and is based onthe fact that the current is initially

limited by R4-R7 immediately afterswitch-on. After the expiry of a timedelay determined by C2 and C3,these resistors are bridged over bythe relay and the full current flowsbetween K1 and K2. The relay usedhere is a type that can switch2000 VA. The supply voltage for therelay is taken directly from the mainscircuit via C1, R3 and B1, so this cir-cuit is dangerous to the touch!

Soldering

The printed circuit board layouts forthe amplifier and protection circuitryare shown in Figure 5. These circuitboards are supplied as a singlepiece, so the must be (carefully)sawn apart. Experienced electronicstypes will not need very muchadvise with regard to the construc-tion of the circuit boards, since thecomponent layout overlay and thecomponents list speak for them-selves. Still, we would like to make afew practical remarks.

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Figure 6. This is how the finished circuit board should appear. Don’t forget the insulatingwashers for transistors T8 and T10–T13!

T12/T13 (e.g., TO-218 sheets size 21x 24 mm)

Heatsink: <0.5°K/W (e.g., Fischer typeSK47/100 mm, Dau Components)

PCB, order code 010001-1Mainsd power-on delay PCB , order

code 974078-1Enclosre, e.g., Monacor (Monarch)

type UC113/SW

*) may be omitted

Protection board

Resistors:R1,R4 = 390ΩR2,R5 = 120ΩR3,R6 = 18kΩR7,R9 = 1kΩ / 5WR8,R10 = 5kΩ6R11,R25,R30,R31 = 10kΩR12,R13,R24 = 1MΩR14 = 150kΩR15 = 39kΩR16,R20,R21 = 220kΩR17 = 330kΩR18 = 12kΩR19 = 330ΩR22 = 15kΩR23 = 680ΩR26,R27 = 4kΩ7R28 = 470kΩR29 = 3kΩ3

Capacitors:C1,C2 = 150pFC3,C4 = 22µF 63V radialC5 = 47µF 63V radialC6 = 2µF2 MKT (Siemens), lead pitch

5mm or 7.5mmC7,C8 = 220µF 25V radialC9 = 1µF 63V radial

Semiconductors:D1,D2 = 1N4148D3,D4 = zender diode 20V / 0.5WD5,D6 = BAT85D7,D8 = 1N4004T1 = MJE340T2 = MJE350T3,T8 = BC556BT4,T5,T6,T10 = BC546BT7 = BC516T9 = BD140IC1 = OP77GP (Analog Devices)IC2 = CNY17-3

Miscellaneous:Re1,Re2 = relay, type G2R-1-E

(Omron), 16A / 24V / 1100 ohm)3 off M3 spade terminal, PCB mount

and as usual it is recommended to smear athin layer of thermal grease on each side ofthe insulator before mounting the transistor.The thermal resistance of the heat sinkshould be less than 0.5 K/W. Figure 6 showsone of the fully assembled prototype ampli-fier circuit boards with attached heat sink.There isn’t much to say about the protectioncircuit board. You should pay attention to thediameter of the electrolytic capacitor C5,which must be no more than 8 mm. If youcannot obtain a suitable type, a 40-V type canalso be used.For the sake of completeness, the printed cir-cuit board layout of the previously mentionedmains switch-on delay circuit is shown inFigure 7. This circuit board was neverincluded in the Readers Services list in thepast, but since this ‘two-stage’ delay can beespecially useful for a variety of applications,we have now added it to the list.

Wiring and set-up

Once you have finished building the amplifierand protection logic boards (or sets of boards)and have carefully checked them against thecomponents list, it is time to start looking for asuitable enclosure. The first decision to bemade is whether you want to build the ampli-fier as a monophonic building block or as astereo version. We chose the latter option forour prototype, which means that what weactually did was to build two mono blocksinto a single enclosure, each with its ownpower supply and mains switch-on delay. Theonly shared item is the mains switch. For theenclosure, we chose a Monacor (in some coun-tries: Monarch) box that provides a generousamount of room for everything, and thenmounted hefty heat sinks (bigger than actu-ally required) on opposite sides of the box.Since there are several circuit boardsinvolved, the wiring of the complete amplifierincludes quite a few interconnections – whichis why we have made a separate wiring dia-gram, as shown in Figure 8. Connect the V+,V–, earth, tp1, tp2, tp3 and bias points on theprotection board to the corresponding pointson the amplifier board using ordinary insu-lated stranded wire. The ‘∼ 35 V’ pointsshould be connected directly to the outerends of the transformer windings, and point‘0’ should be connected to the junction of thefilter capacitors in the power supply.Use lengths of screened audio cable to makethe connections between the input sockets(Cinch sockets) and the input points on theamplifier boards.Flat tab connectors (automotive connectors)are used for the output and supply connec-tions on the circuit boards. The connections

between these points must naturallybe made using heavy-gauge wiring.We used 2.5-mm2 electrical wire forthis purpose. The contacts of relaysRe1 and Re2 on the protection boardare simply connected in series withthe amplifier output by connectingthe output terminal ‘LSP+’ to therelay input terminal ‘Amp’ and the‘LSP’ terminal of the protectionboard to the positive output socket(banana socket). The other (nega-tive) banana socket is connecteddirectly to the ‘LSP–‘ terminal.The necessary connection betweenthe circuit ground of the amplifierand the metallic enclosure can bestbe realised by fitting the Cinch(a.k.a. RCA or ‘line’) input sockets ina ‘normal’ (non-insulated) manner.Take care that there is not any otherunintentional connection betweenthe signal ground and the enclosureground, since this will create anearth loop that can cause stubbornhum problems.It goes without saying that a well-insulated cable, a robust mainsswitch and an equally robust mainsentrance must be used for the con-nection to the 230-V mains circuit.Pay attention to the electrical safetyof the overall assembly, and attachan identification label that lists thespecified values of the supply volt-age (230 V) and fuse to the outsideof the enclosure.

Once you have again thoroughlychecked everything and re-mea-

sured the supply voltages, it’s nearlytime to power up the amplifier.Before doing this, however, you mustturn trimpot P1 fully to the left(counter clockwise). Otherwise yourun the risk that the quiescent cur-rent will immediately rise to a veryhigh level, which is not what wewant.After switching on the unit, firstcheck the amplifier output (test pointtp3) to verify that the voltage is zero.An offset of a few millivolts isacceptable, but if you measure 0.1 Vor more you will have to carefullyreinspect the whole assembly, sincethere is something wrong.Following this, you can set the quies-cent current to the proper value. Theideal value for this amplifier is 200 to250 mA. To adjust the quiescent cur-rent, connect a voltmeter across R34(test points tp1 and tp3) and turn P1slowly until the measured voltage isbetween 0.044 and 0.055 V. Then letthe amplifier warm up for half anhour, and again adjust the current tothe same value using P1.

Listening

Readers who have already taken apeek at the measurement resultsshown in the separate box will havequickly concluded that the Crescendoscores very well as far as the num-bers are concerned. However, weknow from experience that amplifierswith practically identical specifica-tions can sound quite different.

AUDIO


974078-1

B1

C1

C2

C3

F1

H1

H2

H3

H4

K1

K2 O

UT

R1

R2

R3

R4

R5

R6

R7

RE1

97

40

78

-1

~~

~~

Figure 7. The printed circuit board layout for the mains switch-on delay circuitshown in Figure 4.

97

40

78

-1

We thus come to the crucialquestion: how good is thesound of the new amplifier (or,if you will, the ‘refurbished oldamplifier’)?The first thing that struck us inlistening sessions is that theCrescendo can produce anicely spacious and opensound image with all differenttypes of music. Of course, therelative differences betweengood amplifiers are alwaysvery subtle, but the Crescendoclearly revealed itself to be anamplifier with a pleasantlywarm-blooded character.After listening to the amplifierfor a while, we developed acertain understanding of thepreferences of fervent MOS-FET fans, since the sound pro-duced by the amplifier is just abit less reserved and clinicalthan that produced by a typi-cal amplifier with bipolar tran-sistors in the output stage. Anamplifier such as the ‘CompactAF Power Amplifier’, whichwas published in May 1997(and which is one of ourfavourites), offers reproductionthat (according to our convic-tions) can hardly be surpassedin terms of natural fidelity anddetailing, but it still missesthat slight trace of warmththat is so typical of theCrescendo. Can we say thatone of the two is the betteramplifier? No, that would begoing to far. The differencesare too small for such a pro-nouncement, and anyhowsuch a judgement is alwaysvery subjective. ‘Better’ and‘worse’ are qualifications thatdo not have a place here; atmost we can say ‘different’.What well can be consideredto be no less than amazing isthat this Crescendo, in spite of(or thanks to) its simple con-cept and the age of the origi-nal design, can easily hold itsown against many more mod-ern examples of the breed.This amplifier can be highlyrecommended, and not only forMOSFET fans!

(010001-1)

AUDIO


Figure 8. Wiring diagram. Thick wires must be used for the power supply and amplifier output wiring!

B1

C1

C2

C3

F1

H1

H2

H3

H4

K1

K2 O

UT

R1

R2

R3

R4

R5

R6

R7

RE1

974

078-1

~~

~~

C1 C2

C3

C4

C5

C6

C7

C8

C9D

1D

2

D3D4

D5D6

D7

D8

H1H

2

H3 H4

IC1

IC2

PC7

PC8PC9

R1

R2

R3

R4

R5

R6

R7 R

8

R9R10

R11

R12

R13

R14

R15

R16

R17

R18

R19 R20R21R22R23

R24R25

R26

R27

R28

R29

R30

R31

RE1

RE2

T1 T2

T3

T4

T5 T6

T7

T8

T9

T10

Am

p.

LSP

T

bia

s

~~

0

tp2

tp3

tp1

V+

V-

0

C1

C2

C3

C4

C5

C6

C7

C8

C9

C10

C11

C12

C13

C14

C15

D1

D2

D3

D4

H1H2

H3 H4

L1

P1

R1R2

R3

R4

R5

R6R7

R8

R9

R10

R11R12

R13

R14

R15

R16R17

R18

R19R20

R21

R22R23

R24

R25

R26

R27

R28

R29

R30

R31

R32

R33

R34

R35

R36

R37

T1T

2

T3

T4

T5

T6

T7

T8

T9T

10

T11

T12

T13

tp1

tp2

LSP

+

LS

P-

T

0

+-

bia

s

tp3

010001-1

250V

F1 = 1A T

10A

MAINS

010001 - 14

B

C22000µ

63V

C22000µ

63V

B = 200V / 35A

Tr = 2x 35V / 225VA

LSP

crescendo amplifier

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