P3A oscillation

clive299087 · 14th October 2006, 12:22 AM

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)

clive299087 · 16th October 2006, 09:15 PM

A .1uF fitted in the 2 holes next to Q9 cures the oscillation!

JojoD818 · 9th November 2006, 02:24 AM

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?

clem_o · 9th November 2006, 02:29 AM

Yes, should be...

Cheers

JojoD818 · 9th November 2006, 02:31 AM

so what causes the oscillation? vbe instability?

clem_o · 9th November 2006, 02:46 AM

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!

JojoD818 · 9th November 2006, 04:33 AM

Thanks.

Cheers!

bawang · 9th November 2006, 07:53 AM

clive299087, but my PCBs from Rod does NOT have those holes next to Q9! What revision is yours?

clive299087 · 9th November 2006, 10:39 PM

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??

bawang · 9th November 2006, 11:24 PM

Thanks clive299087. Will double check my PCB. Time to repair my antiquated function generator & drag out the scope.....

14th October 2006, 12:22 AM	#1
clive299087 diyAudio Member Join Date: Aug 2004 Location: Gloucestershire	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)

16th October 2006, 09:15 PM	#2
clive299087 diyAudio Member Join Date: Aug 2004 Location: Gloucestershire	Solved it! A .1uF fitted in the 2 holes next to Q9 cures the oscillation!

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9th November 2006, 02:24 AM	#3
JojoD818 diyAudio Moderator Join Date: Mar 2003 Location: searching...	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?

9th November 2006, 02:29 AM	#4
clem_o diyAudio Member Join Date: Jun 2005 Location: Manila	Yes, should be... Cheers

9th November 2006, 02:31 AM	#5
JojoD818 diyAudio Moderator Join Date: Mar 2003 Location: searching...	so what causes the oscillation? vbe instability?

9th November 2006, 02:46 AM	#6
clem_o diyAudio Member Join Date: Jun 2005 Location: Manila	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!

9th November 2006, 04:33 AM	#7
JojoD818 diyAudio Moderator Join Date: Mar 2003 Location: searching...	Thanks. Cheers!

9th November 2006, 07:53 AM	#8
bawang diyAudio Member Join Date: Oct 2001 Location: Malaysia	clive299087, but my PCBs from Rod does NOT have those holes next to Q9! What revision is yours?

9th November 2006, 10:39 PM	#9
clive299087 diyAudio Member Join Date: Aug 2004 Location: Gloucestershire	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??

9th November 2006, 11:24 PM	#10
bawang diyAudio Member Join Date: Oct 2001 Location: Malaysia	Thanks clive299087. Will double check my PCB. Time to repair my antiquated function generator & drag out the scope.....