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Old 25th January 2003, 08:43 AM   #1
Mark25 is offline Mark25  United Kingdom
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Default help required with p3a

Hi All,
Could somebody please help me with a problem I have with my two p3a’s. They have been working fine for 3 months or so. But now, when connected to a cd player or pre amp, something goes drastically wrong! They (sometimes) sound like a tiny transistor radio.

These are some observations I have made during the intermittent fault which seem strange to me:

1. The voltage representing the dc bias current shoots through the roof during the fault. It goes from a value representing 100mA to value representing about 2A, instantaneously.

2. The electronics “sings”, along with the music during the fault. Even with no speakers connected.

3. If I put my dvm on the amp o/p with no music playing, on the frequency range, it goes from 300Hz “normal” to 500KHz or OL during the fault. Max F on the meter is 4MHz.

I guess this last observation MAY be a big clue, but I don’t understand enough to trace/diagnose things further.

Please help, I’m pulling my hair out here, even pointing in the direction of a good book, to aid my understanding, would be appreciated. Maybe getting access to the hidden part of Rod’s website is the answer?

Mark
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Old 25th January 2003, 09:16 AM   #2
halojoy is offline halojoy  Sweden
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Arrow Oscillating Amp

This I think is a typical case of Oscillating amplifier.
It is not the LOAD causing this.
So the Zobel filter at the output can't cure it.

It is the amplifier itself.
You might have to do one or both things of this:

- Lower the freq of the Lowpass Filter at the input.
means incresing the value of the little cap
that goes from input to ground.
This cap/filter prevents too high freq to enter the amp.

- Increase the compensation cap, in the negative feedback.
It is also a little cap - in the pF range.
For stabilisation and to avoid such oscillations, that you describe
This would be the first I should try.

halo - knows a little of amplifiers , at least
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Old 27th January 2003, 02:07 PM   #3
Mark25 is offline Mark25  United Kingdom
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Hi Halo,
Thank-you very much for your reply, it has been most useful to me. I looked at my caps and found the one around the class A driver (C4 ESP p3a) and the one around one of the class B driver transistors (C6) had a problem. The capacitance seems to drop with temperature, which all sounds great, because the fault happened when I disconnected some cooling fans that were not required. So, I replaced the caps with more stable types, but still had the fault. I then increased the value of C4, the one around the class A driver, to 200pF and now the value the bias current jumps up to is 0.5A instead of 2A. So I guess I’m on the right track. I have now increased all three caps by 100pF, but still have the fault, but to a lesser extent. So I have some questions for you;

I was just wondering whether the “compensation cap”, you refer to is the one around the class A driver C4, or the one around the class B driver C6? or both?

Why does only one of the class B drivers have a cap around it?

When should I stop increasing the values of C? when it stops oscillating I suppose! Although it’s working, something “feels”, wrong here.

Thanks again
Mark
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Old 15th November 2006, 12:38 AM   #4
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You could try .1uF (100nf) between Q5 and Q6 bases.
That certainly stabilised my P3A's
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Old 15th November 2006, 10:18 AM   #5
AndrewT is online now AndrewT  Scotland
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Hi,
it operated OK for three months.
That seems to indicate that the circuit is correct.
But, something is now intermittantly shorting or going open circuit.
I think you have a soldering fault buried in there.
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regards Andrew T.
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Old 15th November 2006, 11:41 PM   #6
clem_o is offline clem_o  Philippines
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Quote:
Originally posted by AndrewT
Hi,
it operated OK for three months.
That seems to indicate that the circuit is correct.
But, something is now intermittantly shorting or going open circuit.
I think you have a soldering fault buried in there.

The circuit is apparently pretty "on the edge" and any slight changes in operating condition(s) may cause it to begin oscillation.

See:

P3A oscillation

Cheers
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