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Old 15th December 2008, 08:21 AM   #1
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Default 200W Marshall power amp oscilations

I've build this power amp (Marshall 3520): http://www.drtube.com/schematics/marshall/3520pwr.gif . I replaced output MOSFETs with 2SK1058 and 2SJ062. It works fine but at low input signal levels (~200mV) the amp oscilates on the positive half-cycles. The only component that could influence this problem is C3 capacitor but when I increase it, the problem still remains. What can be the reason for the problem?
Another question is about quiescent current of the amp. It is not specified on the schematic. Currently it is 30mA per transistor. Is it a correct value?

Mark
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Old 15th December 2008, 08:31 AM   #2
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try with a zobel filter across output
this can be needed for some inductive speakers

10 Ohm small resistor + 100nF filmcapacitor, polyester
in series, from output end of the output inductor to GND

if this does not help,
not even when using 47pF, 100pF or even 220pF for C3,
then I do not know
---

Important that R16, 17, 22, 23 resistors (470 ohm) are not too far away from the transistors GATE pins.

30 mA could work
but generally MOSFET likes a bit more .. maybe 100, 150 mA per each MOSFET

How much is your power supply voltage?
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Old 15th December 2008, 08:44 AM   #3
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The supply voltage is +/-52V. I tried Zobel filter but this didn't help. I will change C3 to much higher value and report back. The value was clearly modified on the schematic but I cannot see what was the original value.

Mark
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Old 15th December 2008, 09:07 AM   #4
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you can try to add High Freq lowpass RC filter at input.
1 kohm in series with C1 and one 220pF or 470pF cap across R1 (47kohm)
I do not think this will help, though.

One 47pF across R5 (4k7) fb resistor. One other try.

Is there one resistor in parallel with the output inductor?
If not, try with one 10 Ohm or 4R7
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Old 15th December 2008, 09:19 AM   #5
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Thanks. There is no resistor in parallel to the inductor. I will try all your suggestions in the evening.

Mark
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Old 15th December 2008, 08:36 PM   #6
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I did some tests. Unfortunately I found out that none of the changes helps. But... I found out that the problem does not appear when the amp is cold. It starts after a short warm-up period. Then I checked the transistors used in the amp. And it appears that MPSA92 (used instead of MPSA93) is hot. The simulation in LTSpice shows that the power dissipation of this transistor is 300mW at full power. The datasheet says that maximum power dissipation of this transistor is 625mW. Could it be the reason of the problem? I don't have any PNP transistor to make the test today.

Mark
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Old 15th December 2008, 09:26 PM   #7
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Default it could be

a pcb issue as well .... do you have your own pcb ???? beyond that when amp is getting hot

on the other hand have you performed the circuit as is ???? cause only by looking at it i would say that decoupling and by pass caps are not there ....

are they omited for simplicity ???? or they are omited any way ...

another thing i would do is to for a minute remove the VI limiter to see if make any diference

also simple design like that at this rail voltage might be quiet ready to oscilate with out a reason .....to explain that i mean that if its a marshal design they would have in mind that this amp will work with 50cm speaker cable as used on quitar amplifier ....

longer leads is another issue

there is also a very good posibilty that this oscilation might might be generated from your measuring setup as well
( hapend to me and then realized that my signal generator and scope created a ground loop through my metal made bench )

but then again i was testing a mosfet pcb .....5 years of operation and various tetsings on bipolar boards or comercial amps never seen this ....but with the mosfet it was there
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Old 15th December 2008, 09:54 PM   #8
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2SK1058 and 2SJ162 have different gate-source capacitance (lower for the 2SK). Amps using this output pair usually have some means to equalise this; you may try fitting 220 pF caps between gate and source on the 2SK1058 or increasing their gate resistor value. Hope this helps.
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Old 16th December 2008, 12:00 AM   #9
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Default Re: 200W Marshall power amp oscilations

Quote:
Originally posted by Markus2006
I've build this power amp (Marshall 3520): http://www.drtube.com/schematics/marshall/3520pwr.gif . I replaced output MOSFETs with 2SK1058 and 2SJ062. It works fine but at low input signal levels (~200mV) the amp oscilates on the positive half-cycles. The only component that could influence this problem is C3 capacitor but when I increase it, the problem still remains. What can be the reason for the problem?
Another question is about quiescent current of the amp. It is not specified on the schematic. Currently it is 30mA per transistor. Is it a correct value?

Mark

I usually start with no bias volts and then turn it up until the crossover distortion goes. I test it on a scope with about half working AC on output. This gives sufficient bias to get rid of crossover distortion but not so much you are wasting power into the heatsink.
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Old 16th December 2008, 12:02 AM   #10
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Default Re: 200W Marshall power amp oscilations

Quote:
Originally posted by Markus2006
I've build this power amp (Marshall 3520): http://www.drtube.com/schematics/marshall/3520pwr.gif . I replaced output MOSFETs with 2SK1058 and 2SJ062. It works fine but at low input signal levels (~200mV) the amp oscilates on the positive half-cycles. The only component that could influence this problem is C3 capacitor but when I increase it, the problem still remains. What can be the reason for the problem?
Another question is about quiescent current of the amp. It is not specified on the schematic. Currently it is 30mA per transistor. Is it a correct value?

Mark

I noticed on my own similar design to this that I got quite a bit of hum with that power supply setup. I put a 330R in series with the positive supply to the VAS and LTP stage and a 470uF cap to GND.
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