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AUDIO
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.
AUDIO
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
AUDIO
38 Elektor Electronics 4/2001
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
AUDIO
394/2001 Elektor Electronics
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-
AUDIO
40 Elektor Electronics 4/2001
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
AUDIO
414/2001 Elektor Electronics
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
AUDIO
42 Elektor Electronics 4/2001
(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.
AUDIO
434/2001 Elektor Electronics
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
44 Elektor Electronics 4/2001
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
454/2001 Elektor Electronics
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