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Old 14th October 2006, 12:22 AM   #1
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Default P3A oscillation

I wonder if anyone has any suggestions on this problem?
I have built a pair of these on one PCB, with seperate supplies.
Both seem to suffer from oscillation, mainly on -ve half cycle
when presented with a load, loudspeaker or resistor.
Supply leads are 4" or less, I have tried extra decoupling of supplies on the board,
base stopper resistors on the output pair. increased C6, etc etc.
The transistors are MPSA42's, MJE15031 VAS, MJE15034/5
drivers and MJL4281A/4302A outputs.
Increasing the bias current solves the problem for a given drive
level, but increasing drive brings the problem back.
I think what I'm saying is that it's fine in class "A", but not in
class "B", and I don't have a big enough room for an 80 watt
class "A" heatsink!
Help!!
Thankyou,
Clive
(Forgot to say, these amps are built on Rod's excellent PCB.
The heatsinks are earthed and the supply is +- 30v)
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Old 16th October 2006, 09:15 PM   #2
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Default Solved it!

A .1uF fitted in the 2 holes next to Q9 cures the oscillation!
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Old 9th November 2006, 02:24 AM   #3
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I know this is an old thread, but I am so curious to know where is the 0.1uf connected on Q9 (Vbe)? Across the emitter and collector?
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Old 9th November 2006, 02:29 AM   #4
clem_o is offline clem_o  Philippines
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Yes, should be...

Cheers
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Old 9th November 2006, 02:31 AM   #5
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so what causes the oscillation? vbe instability?
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Old 9th November 2006, 02:46 AM   #6
clem_o is offline clem_o  Philippines
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Whoops, I meant it's supposed to be across the Q9's Collector to Emitter.

It effectively ties the driver transistor's bases together at HF. Not sure what the mechanism is that causes the problem, but that's been an effective solution on other amplifier circuits as well...

Cheers!
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Old 9th November 2006, 04:33 AM   #7
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Thanks.

Cheers!
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Old 9th November 2006, 07:53 AM   #8
bawang is offline bawang  Malaysia
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clive299087, but my PCBs from Rod does NOT have those holes next to Q9! What revision is yours?
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Old 9th November 2006, 10:39 PM   #9
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The extra .1uf goes between collector and emitter of Q9.
My board is REV-A.
I have since noticed that when approaching clipping with
square wave drive, (probably irrelevant!) there is some
rubbish appearing on the bottom of the waveform.
I seem to have fixed this by changing the VAS from MJE15031
to BD140.
I just wonder if the OnSemi devices are a bit fast for this amp??
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Old 9th November 2006, 11:24 PM   #10
bawang is offline bawang  Malaysia
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Thanks clive299087. Will double check my PCB. Time to repair my antiquated function generator & drag out the scope.....
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