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.
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.
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.
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?
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..
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
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
many of the P3A i make feature no matched parts ... at no time i measured anything more than 45mv given as a fact that Dc supply is between 35-40 volt
Expect something to be produced due to mismatching of LTP transistors but nothing like 145 mv
This actually points to faulty part or some other reason beyond imagination like serious PCB error that produces things from proximity inductance and/or capacitance .
Further more flux left overs on board might also cause some complication . Wash boards properly
Post pictures of your work .
Scope the amp you never know what will come up
Kind regards
Sakis
Expect something to be produced due to mismatching of LTP transistors but nothing like 145 mv
This actually points to faulty part or some other reason beyond imagination like serious PCB error that produces things from proximity inductance and/or capacitance .
Further more flux left overs on board might also cause some complication . Wash boards properly
Post pictures of your work .
Scope the amp you never know what will come up
Kind regards
Sakis
I tested it with scope and signal generator when output was connected to ~7.3 ohm resistive load. Frequency response was quite flat from 10Hz to 20kHz. Square wave at 1kHz was quite sharp, but at 20kHz it was rounded, which can probably be expected. The only thing I noted is that it clips at slightly lower output on one channel, but that's probably because I used different output transistors between channels (I just used some leftovers..). So it would seem to work OK..
I would post pictures of PCB, but looks I have lost the layout files (there was some hiatus between making PCBs and actually building the amp), and I'd prefer to not disassemble it again to take pictures of the amp.
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I didn't really use any high-quality parts in the amp, just some leftovers, so that could a the reason for performance. Here's a pic of the 20kHz square wave:
http://koti.mbnet.fi/~jokunen_/20kHz_square.jpg
P3A
Hi Joku
You can see this if you want.http://www.diyaudio.com/forums/solid-state/164756-p3a-comparsion-table-long-37.html
Best regards.
Thimios.
Hi Joku
You can see this if you want.http://www.diyaudio.com/forums/solid-state/164756-p3a-comparsion-table-long-37.html
Best regards.
Thimios.
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