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Aleph 0s Service Manual Version 1.0 - 1.3 PRODUCT DESCRIPTION The Aleph 0s is a high performance Mosfet single-ended Class A stereo audio power amplifier, intended for maximum performance in reproduction of music. It is a simple design, having only three gain stages: input differential pair, cascoded voltage gain stage, and output followers. All three gain stages are biased by constant current sources from the negative supply. The output stage will operate as a single ended class A system at lower power levels and will operate as a push-pull class A system at levels above the bias point of the constant current source. SIMPLIFIED SCHEMATIC To best understand the operation of the amplifier, refer to the simplified schematic Figure 1. The front end of the amplifier accepts a balanced or unbalanced input signal at two N channel Mosfets operating as a differential pair. They are provided with bias by a current source from the negative rail which operates at a constant 8 milliamps. The output of the differential pair drives a P channel Mosfet which provides voltage and current gain. At the output of this second stage you will see the full voltage swing of the amplifier. This second gain stage is provided with a single-ended Class A current bias from another current source from the negative supply which provides a constant 30 milliamps current. Between the current source and the drain of the P channel device is a constant voltage source which is used to provide voltage bias to the output Mosfet transistors. The amplifier has complementary N and P channel output transistors operated as source followers, so that they provide only current gain. High current single ended Class A bias is provided by yet another constant current source from the negative supply. This current source provides greater than 1 amp of constant current per channel COMPLETE SCHEMATIC For purposes of clarity and simplicity, the complete schematic of the Aleph 0s is broken up into the following sections: Power supply, Front end, and Output Stage. Figure 2 shows the power supply schematic. An IEC standard AC line connector connects to the primary of a toroidal power transformer through an inrush suppression thermistor, fast blow fuse, a power switch, and a thermostat. Fig 2 shows the transformer wired for 120 VAC, and the transformer can be adapted to 240 VAC by connecting the two primary windings in series. 100 volt operation requires a special transformer. The secondary system consists of a bridge rectifier and four 31,000 uF capacitors. The secondary DC voltage is approximately plus and minus 40 volts. The front end circuitry of the amplifier is decoupled from the main supply by RC filters. Figure 3 shows one half of the output stage. Both halves run exactly in parallel.
A
B
C
D
V+
1
1
R205 R207 .33
221
IRF244
2
Z201
9.1V
47.5K
+D
R201
Q201
2
OUT
R208
.33
Q202
G
D
Z202
9.1V
Q203 R203 221 R202 221 IRF9240
IRF244
S
-D
3
3
R204 221 Q204 4.7 C201 R206 MPSA42
Title Size
V-
ALEPH 0S OUTPUT STAGE (1/2)
Number Rev
4
1.3
ISD
4
A
Date Filename Drawn by Sheet
PASS
of
10/10/93 PL10OS10.S01
A
B
C
D
Following are the front end circuits and PC board component placements for Revision numbers 1.0 through 1.3. All are very similar, and while the following description applies specifically to Rev 1.0, the comments apply to all versions. The circuit formed by Q101, Z102, R108 and R107 is a constant current source designed to bias Z101, the voltage reference for the front end constant current sources, and Q7, the voltage gain stage cascode transistor. This current source and reference circuit is common to both channels. Further references are to each channel singly, with both channels having identical circuits and part references. Q3 and Q4 are constant current sources which bias the front end. They are driven by Z101 at 9.1 volts, resulting in approximately 4.5 volts across their source resistors, R3 and R10, giving approximately 8 ma and 30 ma constant current. The input differential transistors Q1 and Q2 are power Mosfet transistors which have been matched to .01 volts threshold voltages at 4 milliamps current. The gates of these devices are connected to differential networks formed by R5, 6, 13-18. These form a true differential amplifier for balanced input and can be operated unbalanced by simply driving the positive input (XLR pin 2) with or without shorting the negative input (XLR pin 3) to ground. Shorting the negative input to ground provides twice the voltage gain over leaving it unterminated, but either method of operation is acceptable. Zener diodes Z1 and 2 protect the input transistors from outside transient voltages. Q1 drives Q6 in common source mode which is in cascode (common gate) connection with Q7. At the same time, Q2 drives the source of Q7 in a folded cascode connection, so that both input transistors drive the secondary gain stage. The DC offset point of this system is set by P1. While the amplifier is primarily biased by the output stage constant current source, the design provides for "pull" operation beyond the constant current bias point with a set of P channel source followers. The bias relation between the P and N channel source follower output devices is set by the constant voltage circuit of Q5 and adjusted by P2. Normally, the P channel output stage will be biased at about 10% of the value of the constant current source, or about 100 ma. C5 provides 10 picofarads of forward compensation in the feedback loop. C6 provides 39 pf of compensation for Q6. Z3 provides protection for the gate of Q7 when Q6 is shut down on a negative waveform clip. Q8 provides current limiting for Q6 during a positive waveform clip. R1 and C7 provide loading at radio frequencies. If R1 is damaged, it is a sure sign of high power at high frequencies, such as full power at 100 KHz or Square waves above 20 KHz. Unless it is on a test bench, the only way the amplifier will experience this will be in system oscillation, where the output of the amplifier is allowed to bleed back to the input. This is generally due to wiring fault in the system.
A
B
C
D
V+
C C W
T1
P1
R109
5K
R11
2.2K
Z102
9.1V
W
4.75K
MPSA92 221 Q8 IRFD9210 R7
THERMISTOR
C W
Q6 MPSA92 Z3 4.75K Q101 R8 221 15K Q7 IRF9510 9.1V 39PF R108 C6
2
R107
4.75
1
50K
R2
1
2
+DRIVE
- INPUT
221
R13
R5 R12 7.5K
4.75K
+ INPUT
P2 IRFD210 5K GND Z2 9.1V 9.1V C5 10PF R18 100K
C W
C8
W
R6
R14
IRFD210
Z1
4.7UF
221
4.75K
+
Q1 IRFD210
Q2
C C W
Q5
OUTPUT
C3
C4
R17
R15
100K
R16
4.75K
4.75K
390PF
3
390PF
.047
C7
3
-DRIVE
C1
390PF
C2
390PF
R9 IRF610 221
R4 221 IRF610 GND
680
R3
R10
4
Z101
9.1V
150
2.7 Q4
Q3
R1
Title
ALEPH 0S FRONT END VSize Number Rev
4
B
Date Filename Drawn by Sheet
PASS
of
12/13/93 PL10FE.S01
A
B
C
D
A
B
C
D
V+
10
R103 R2 10
C106
C C W
T1
R105
P1
220 50V
R109
3.3K 2W
5K
R11
3.3K
Z102
9.1V
W
4.75K
1
MPSA92 221 Q8 IRFD9210 R7
50K
THERMISTOR
1
C W
Q6 MPSA92 Z3 4.75K Q101 R8 221 15K Q7 IRF9510 9.1V 39PF R108 C6
2
R107
2
+DRIVE
- INPUT
221
R13
R5 R12 4.75K
4.75K
+ INPUT
P2 IRFD210 5K GND Z2 9.1V 9.1V C5 10PF R18 100K
C W
C8
W
R6
R14
IRFD210
Z1
4.7UF
221
4.75K
+
Q1 IRFD210
Q2
C C W
Q5
OUTPUT
R17
R15
100K
R16
3
4.75K
4.75K
.047
C7
3
-DRIVE
C1
390PF
C2
390PF
R9 IRF610 221
R4 221 IRF610 GND
680
R3
R104
R10
V-
R102
220 50V
4
3.3K 2W
C105
Z101
9.1V
150
2.7 Q4
OTHER CH
Q3
R1
Title Size
ALEPH 0S FRONT END
Number
4
10
B
Date Filename
FIG 3
Drawn by Sheet
Rev
PASS
of
11/6/93 PL10FE11.S01
A
B
C
D
A
B
C
D
V+
C C W
150K
P1
R109
5K
R11
2.2K
Z102
9.1V
W
4.75K
1
MPSA92 221 Q8 IRFD9210 R7
T1
R2 10
THERMISTOR
1
C W
Q6 MPSA92 Z3 4.75K Q101 R8 221 15K Q7 IRF9510 9.1V 39PF R108 C6
2
R107
2
+DRIVE
- INPUT
221 R?
R13
R5 R12 4.75K 2.2K
C W
4.75K
R?
C3
2.2K
680PF
+ INPUT
IRFD210 5K
W
C8
GND 9.1V Z2 9.1V
C C W
Z1
R?
R17
R15
100K
R16
4.75K
4.75K
3
R18 100K
2.2K
.047
C7
C5 10PF
P2
R6
R14
IRFD210
4.7UF
221
4.75K
+
Q1 IRFD210
Q2
Q5
680PF
C4
OUTPUT
3
-DRIVE
C1
390PF
C2
390PF
R9 IRF610 221
R4 221 IRF610 GND
680
R3
R10
4
Z101
9.1V
150
2.7 Q4
Q3
R1
Title
ALEPH 0S FRONT END VSize Number Rev
4
B
Date Filename Drawn by Sheet
PASS
of
11/6/93 PL10FE.S01
A
B
C
D
A
B
C
D
+V
R103
10
R10 10
R105
220 50V
R9
3.3K 2W
Z102
9.1V
5K
C W
3.3K
W
P1
1
MPSA92
C102
C C W
1
Q8 IRFD9210
Z5 9.1V
Q6
C6
R12 221 Q7
2
R101 15K 2W
39PF
IRF9510
2
+DRIVE
- INPUT
R13
R2
33K 2W
4.75K R14
R8
+ INPUT
P2
C8
5K
R1 C5 10PF
10 Z2 9.1V
C W
220 50V
IRFD210
4.7UF
4.75K
W
C3
Q1 IRFD210 IRFD210
220 50V
100K Q2
100K
C4
C C W
Q5
9.1V
VAL
Z3
R7
OUTPUT
R5
R3
R4
Z4
100K
9.1V
4.75K
4.75K
R19
10
Z1
R20
3
9.1V
R6 100K
.047
C7
3
-DRIVE
C2
390PF
C1
390PF
R15 IRF610 221
R16 221 IRF610
2.7 2W
680
.1 UF
C9
R17
R104
-V
R102
220 50V
4
3.3K 2W
C101
Z101
9.1V
R18
150
R11
Q3
Q4
GND
Title Size
ALEPH 0S FRONT END
Number Rev
4
10
B
Date Filename Drawn by Sheet
PASS
of
4/25/94 PL10FE13.S01
A
B
C
D
Figure 3 shows the output stage schematic. It shows one-half of one channel's output stage, which contains 6 output devices. The top IRF244 is an output follower. The bottom IRF244 Mosfet is a constant current source. The P channel IRF9240 transistor is a follower which contributes beyond the current provided by the constant current source. On each module, R205 supplies current to Q204, which is the driver for the output stage active current source. The gate of the output stage current source Mosfet is driven by the collector of Q204 at about 4.5 volts. This voltage is controlled through current feedback from the source of the Mosfet connected to the base of Q204. The Base-Emitter junction voltage of Q204 is about .7 volt, and the circuit operates to hold the source voltages about .7 volt above the negative rail voltage, which puts .7 volt across the 1.3 ohm source resistor on each current source Mosfet, which controls 580 ma each times 2 modules, or 1.15 amps constant current biasing the output stage. Note that the output devices are matched for Gate to Source voltages at 200 ma on all transistors to within .1 volt. This means that all IRF9240 devices within an amplifier are matched, and that all IRF244's used as output followers are matched, and all IRF244's used as constant current sources are matched. The match voltage of each transistor is written on the case at Pass Labs, and ranges from 3.00 to 4.99. If it is necessary to replace devices in the field, they must be a match. Devices with a particular number may be obtained from Pass Laboratories. Figure 5 shows the PC layout of the front end board. Note that the control and power connections to the output stage are through wires connected by screw-down terminal connectors. The wires coming off the main board are attached to the output stage modules by corresponding connections. ADJUSTMENTS AND SERVICE Initial power up procedures: For an amplifier in unknown adjustment or being powered up for the first time after repair or modification. Essential Equipment: Oscilloscope, Audio signal source, Variable AC power source, AC line current meter, 8 ohm load. A distortion analyzer is very helpful confirming proper operation, but is not essential to adjustment of an otherwise working amplifier. If you do not have an AC line current meter, you may place a .1 ohm 5 watt power resistor in series with the AC line (cold) and measure the voltage across it (1 amp = .1 volt AC), taking care not to electrocute yourself.
Check the AC line fuse Set signal source to .1V at 1 KHz Attach signal source Attach 8 ohm load. Monitor the amplifier output with oscilloscope, Set the AC line source to 0. AC power switch on P1 (offset adjust) should be at mid-position. P2 (bias adjust) should be at mid-position. Slowly turn up the AC line voltage to 1/3 while watching the current draw. Rated power draw: 1.4 A avg. @ 120 VAC, .7 A avg. @ 240 VAC Rated power draw: 1.4 A ~ 120 V ~ 1.4 (power factor) = 240 Watts You will see the amplifier draw current near to rating when the AC line voltage is at 1/3 rating or more. At 1/3 AC line voltage, adjust P2 for minimum current draw (maximum resistance for P2). At this position, the power draw of the amplifier will be that of the output stage constant current source. Different means of measurement of AC current draw will give slightly different results. However the constant current source of the output stage draws most of the power in the amplifier and is quite accurate. On a working amplifier it may be used as the basis or norm for current measurement. If the DC offset at the output is excessive, then you should adjust P1 on the front end board. Initially this will usually be set to middle. P1 will have to be readjusted after a warm-up period of at least an hour. If the power draw is correct and a clean 2 volt signal appears at the output and DC offset is minimized, then the amplifier probably works. You may slowly increase the AC line voltage to full rating while watching the output wave form and the current draw. Then increase the signal level to full output of the amplifier, verifying proper operation up to clipping. With the bias set to minimum, there will be some distortion at higher power levels, which is expected.
After you have verified that the amplifier will drive an 8 ohm load to 40 watts, you can set the bias point. It is quite easy. First, make note of the current draw of the amplifier with both channels at minimum bias and with the amplifier cold. Multiply this figure by 20%. Then without a load or signal, idle the amplifier for an hour or more. Noting the minimum current draw after warm-up, adjust P2 of one channel so that the current draw is one half the difference between the warm value and 1.2 times the cold value. Then adjust P2 of the other channel so that the current draw is not 1.2 times the cold minimum draw. Typical example: The cold AC current draw with the bias at minimum is 1.2 amps. Let the amplifier warm up for at least an hour. Now the bias will be 1.0 amps. Set the bias P2 of one channel to one-half the difference between the 1.0 amps and 1.2 amps plus 20%, or 1.44 amps. One half of the difference in this case is 1.22 amps, so set the first channel for 1.22 amp AC line draw. Now set the second channel so that the AC line draw is now 1.44 amps. At the factory, we set 120 volt units for 1.4 A average AC line draw, with 1.6 A for 100 volt line, and .7 A for 240 volt line. Your voltmeter might be different, which is why we have the above procedure. When you are done, the warmed up amplifier should draw AC line current which is 20% more than the minimum possible line draw with the amplifier cold. Any questions, call the factory. The DC offset must be readjusted after warm-up also, setting it as close to 0 DC as possible. The DC offset will drift for an hour after this, and must be set again. Probably the least precise performance parameter of the Aleph 0s is the DC offset at the output, and this relates to the single-ended nature of the design and the pure DC approach to the circuitry. On a warmed-up amplifier, the offset will generally be 50 mV or so, but it will show cold DC offset of 300 mV or even more. The DC offset must be final adjusted after the amplifier has been allowed to warm up for an hour (heat sink temperatures of 50 degrees C. or so). The top assembly of the amplifier must be closed up during operation. After adjusting, check the DC offset at half hour intervals, readjusting if necessary. At the factory we monitor the offset during a three day burn in, and we expect it to stay within 100 mV with an ambient temperature between 70 and 80 degrees F. At higher ambient temperatures the offset voltage will drift positive, and at lower ambient temperatures it will drift negative. It is permissible and preferred to adjust the offset against a known ambient temperature.
Notes on transistor matching Input Mosfets (IRFD110 or IRFD210) must have their threshold voltages matched to about 10 mV at 4 ma of current. This is accomplished by attaching the gate to drain and passing 4 ma of current through the device, typically with a 13.8 volt source and 220 ohms resistance. The voltage from the gate/drain to the source is measured. The devices to be matched must be under the same conditions and temperature. Output Mosfets (TO-3 packages) must be matched to within 100 mV at 200 mA of current. The same procedure applies, except that 50 ohm and 13.8 volts is used. When requesting single transistors from the factory, note the threshold voltage of the device, which is written on it. No transistors in TO-220 packages must be matched. Pass Laboratories prefers to supply power transistors in matched sets, but if you need to replace a one or more devices, be certain that the numerical code matches. Even if the numerical code matches, at the factory we test the current going through all transistors by measuring the voltage across the source resistances in operation. This absolutely insures the integrity of the match and also catches any other output stage connection or transistor failure.
PERFORMANCE SPECIFICATIONS Gain 26 dB balanced (50 Ohm Source) 26 dB unbalanced (50 Ohm Source) 0 dB @ DC -3 dB at 100 KHz 40 watts @ 8 ohms 20 Hz - 20 KHz 80 watts @ 4 ohms 20 Hz - 20 KHz 160 watts @ 2 ohms 20 Hz - 20 KHz <1 % THD 25 amps ( pulse), 30 volts (peak) .02 ohm @ 1 KHz @ 8V @ 8 ohm 12 Kohm, differential 60 dB @ 1 KHz @ .1V input common ground 10 nanowatt, 20 - 20 KHz, unweighted (-95 dB, ref. 40 watts) 100 mV after warm-up, balanced mode 250 watts at idle 50 degrees C. 1 hour minimum
Freq. Response Power Output
Distortion Maximum Output Output impedance Balanced input Common mode rejection Output Noise
DC offset Power Consumption Operating Temperature Warm up time
1 0/20/95 Aleph 0s Service Manual Update Rev 1.4 Comments: The Aleph 0s revision 1.4 gives the circuit the same topology as the Aleph 1, using all N channel devices in the output stage. The input impedance is increased, and the RCA input connection offers 20 or 26 dB of gain depending on the presence of the jumper between pins 1 and 3 of the XLR input. The published distortion and power specs remain the same, but the circuit actually shows a bit lower distortion. Adjustments are made to four potentiometers through a single hole in the center of the top cover. Offset adjustment occurs by P101 and P201. Bias adjustment occurs by P102 and P202. Bias is adjusted for a total AC line draw of 2 amps RMS (120 volt) or 1 amp RMS (240 volt) or 2.4 amps (100 volt). The bias pots are adjusted so that the bias current is essentially equal between the channels, giving the same distortion curve between the two channels. It is absolutely essential that the AC line current be measured in true RMS for bias adjustment. If a distortion analyzer is not available, start with both bias pots all the way down, and apportion the difference in current between the two channels to give the proper AC line draw current. If the amplifier is cold, set the bias 10% high, to allow a decline during warmup. The amplifier DC offset and bias must be adjusted again after 1 hour warmup. If significant adjustment is necessary after an hour, the adjustment is repeated after another half hour.
A
+ SUPPLY +46V
68.1
B
C
D
5K
R110
Z104 9.1
1
S G
3.32K
R111
W
P101
C C W
1
Q105 R128-9 221 Q110-1
G D
C W
15K IRFD9210
D
R113 R112
G S
IRF244
S
9.1 Q106 221 IRF9620
D
INPUT
100PF
Z105
Z107
9.1
C101
10PF
R114
2
G
XLR3
IRFD210
S W
R102
2.21K
Q107
D
+ OUT
R132-3
1K
.47 3W
RCA
R107
C102
+ DRIVE
2
10K .047 3.3 3W 221 R127 R123 22.1K C103
C W
Q101
D
D
Q102
XLR2
5K 221 R106 100K 9.1 Z106 9.1 R121
C C W
10K
IRFD210
IRFD210
G
G
R101
S
S
10K
10K
221
R103
R105
100K
R104
R108
R109
P102
XLR1
- OUT
Z101
9.1
TH1
C-60
R115
15K
Z102
R120
G
S
R126
3
D
221 Q108 IRF9620
10K
Q112-3
D
D
GNDEARTH
- DRIVE
R116
G
221 R130-1 475 15K
3
G
Q103 R117 221
S
D
IRF244
S
IRF610 221 IRF610
R122
S
Q109 9.1 221 Z103 R118
R125
G
Q104
681
R119
Z108
9.1
MPSA42
- SUPPLY -46V
Title Size
4
R134-5 221
.47 3W R124
CURRENT FEEDBACK
ALEPH 0S AMPLIFIER
Number Rev
4
B
Date Filename Drawn by Sheet
1.4 4/10/95 PL10FE14.S01 PASS 1 of 1
A
B
C
D