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Old 20th July 2014, 06:20 PM   #1
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Default DC at output and weird voltages

hi all,
I just finished a P3A and I'm testing it before connecting the speakers.I'm testing with a 12V transformer which gives 20VDC to the amp.

I found two problems:
1) I have 3.5 VDC between output and GRD, which is pretty scary.

2) If I leave the amp plugged in for say 30 seconds, the difference in voltage between + and GRD increases, while the difference in voltage between - and GRD decreases. So far I reached a 15VDC between + and GRD, and a 5VDC betwenn - and GRD.

So far I checked the resitors and they're fine. What do you think the problem could be? E.g. is the different in voltages for +/GRD and -/GRD an indicator of a specific problem?

Thanks
AC
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Old 20th July 2014, 06:27 PM   #2
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Load the output of the amplifier with a resistor. 10 ohms would be a good value.
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Old 20th July 2014, 07:09 PM   #3
Art M is offline Art M  United States
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You need 2 supply voltages +20VDC and -20VDC.
You seem to be trying to run the amplifier with a single supply ?
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Old 20th July 2014, 07:50 PM   #4
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@Art M:
Sorry, let me rephrase.
I have +,GRD,-. With my transformer, I expect a +10,0,-10 VDC configuration on the amp.
And as I switch on the amp, that's what I've got.
But if I wait a few seconds, the voltages begin to shift towards a +15,GRD,-5 configuration.

@Frank Berry:
Isn't it so that you should check the VDC without the load? And that this should be in the order of mV?
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Old 20th July 2014, 09:07 PM   #5
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ok problem solved. transistor mounted backwards
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Old 21st July 2014, 12:59 PM   #6
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will the P3a work properly with such low voltage ? i think not

i am missing something here ?
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Old 21st July 2014, 01:00 PM   #7
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Frank ? 10R load with 3.5 v in the output ? make the math ...Kaboum
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Last edited by east electronics; 21st July 2014 at 01:03 PM.
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Old 21st July 2014, 03:25 PM   #8
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hi,
no it won't work with 12V. But I was performing a smoke test.
I had the driver transistors mounted backwards so..

Btw, once I reversed them I get 150mV on the output. Which is quite a lot in my opinion, so..
I still have to set the bias, would that help lowering the DC at the output? (I used max resistance in the trim, for the smoke test)

Thanks
AC
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Old 21st July 2014, 04:39 PM   #9
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expect the Dc offset to be expected at such a low rail voltage
Increase rails and i expect the offset to drop gradually

No load conditions no signal

Kind regards
Sakis
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Old 21st July 2014, 06:37 PM   #10
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hi,
I connected the planned PSU (24-0-24, bringing 33,0,-33V to the amp).
The DC offset is now 90mV right, 120 mV left.
Still quite high I think.
Would this damage my 8ohm speakers?
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