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Old 25th October 2013, 10:30 AM   #1
Joku is offline Joku  Finland
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Default Too much DC offset in P3A

Hi,

I built a P3A amp with self-made layout & PCB, it works otherwise fine, but in the other channel there's too much DC offset. It's 16mV in one channel, and 145mV at other. 16mV is fine, but 145mV is a bit too much. The PCBs are identical, so that shouldn't be the issue.. any ideas where to look at?

It's otherwise a normal P3A, but I changed the output stage a bit to use some leftover NPN transistors as output stage.
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Old 25th October 2013, 10:33 AM   #2
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Do we have a circuit to look at ?
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Old 25th October 2013, 10:37 AM   #3
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If you mean Rod Elliots amp then the matching of the input transistors play a big part and so to the current flowing in those transistors (through the current sink).

Its not an amp with any pretensions to DC accuracy. You would need to try a few different transistors and/or tweak the current sink.
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Old 25th October 2013, 10:42 AM   #4
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Quote:
Originally Posted by Mooly View Post
Do we have a circuit to look at ?
Here's the basic P3A from Rod Elliott's page:

http://sound.westhost.com/project3a.htm
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Old 25th October 2013, 10:46 AM   #5
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Quote:
Originally Posted by Mooly View Post
If you mean Rod Elliots amp then the matching of the input transistors play a big part and so to the current flowing in those transistors (through the current sink).

Its not an amp with any pretensions to DC accuracy. You would need to try a few different transistors and/or tweak the current sink.
Oh, ok. I guess I need to do try something like that..


Anyways, I did notice that on one module (the one with higher offset) the LED is brighter, could that have something to do with it?
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Old 25th October 2013, 10:47 AM   #6
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So it was one of Rod Elliots...

OK, so Q1 and Q2 need "matching" and possibly R7 tweaking slightly.

I'm going to replace your picture with a link (specific copyright details on that site)
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Old 25th October 2013, 10:47 AM   #7
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Mooly's right.You could make R7 variable.
Whatever you do, try to match the gain of the input transistors.
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Old 25th October 2013, 10:48 AM   #8
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Quote:
Originally Posted by Joku View Post
Oh, ok. I guess I need to do try something like that..


Anyways, I did notice that on one module (the one with higher offset) the LED is brighter, could that have something to do with it?
Yes... the absolute voltage across the LED determines the current. So if the LED's are different then the current will be different between channels. Measure the volt drop across each to compare.
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Old 25th October 2013, 10:52 AM   #9
Joku is offline Joku  Finland
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Originally Posted by Mooly View Post
Yes... the absolute voltage across the LED determines the current. So if the LED's are different then the current will be different between channels. Measure the volt drop across each to compare.
The voltage drop seems to be quite similar, 1866mV on the brighter one and 1844mV on dimmer one. I guess that would be ok..
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Old 25th October 2013, 10:59 AM   #10
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OK, well the absolute current can be calculated by measuring the volt drop across R7.

I think you are going to have to play around with the input transistors as a first step. They should be closely matched. There is also an imbalance with R2+R3 which should ideally equal R5. That imbalance causes offset due to unequal base currents between the two transistors. I see there is no emitter degeneration on those transistors too. All these things add up...

Also my experience of simple designs like this is that the absolute current (in R7) plays a big part in any offset.

Transistors first though
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