DC on output of Chinese 'current mode' ebay amplifier - diyAudio
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Old 26th May 2010, 08:05 PM   #1
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Default DC on output of Chinese 'current mode' ebay amplifier

I just picked up a couple of cheap assembled amplifier PCB's from a Chinese seller on Ebay.

I've got them all connected up properly, however I have approx 2 volts DC on the output even when the offset potentiometer (W2) is adjusted all the way to the end, and the input is grounded.

I though that maybe some of the resistors values might have been substituted at assembly, but all appears correct when compared to the schematic.

I know that I could probably fiddle it by changing resistors LR8 and LR25 for something bigger, but i'd rather know what's actually the problem.

Can anybody spot any obvious mistakes on the schematic? Any ideas what could be causing this?
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Old 26th May 2010, 08:53 PM   #2
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I think I would rather play with LR3 and LR4 for offset adjustment. Are the two Jfets matched??? This not only impacts offset but the distortion figures too.
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Old 26th May 2010, 09:09 PM   #3
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That might explain a few things... In theory, they should be matched, but I doubt they actually are. Where would I obtain matched 2SK170BL and 2SJ74BL pairs?

If I install a 220 ohm resistor in place of LR8 and do away with LR25 and W2, and install a trimmer in place of LR3 and LR4, should that do the trick?
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Old 26th May 2010, 09:17 PM   #4
AMV8 is online now AMV8  United Kingdom
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Hi
I would try a trimmer in place of LR3 to remove dc from the input and also a trimmer instead of LR8 to remove dc from that stage also. However it should be enough to just put a trimmer in place of LR8. I am assuming that you are finally adjusting when the amplifier is at running temperature.
Don
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Old 26th May 2010, 09:25 PM   #5
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There are some sellers over at swap meet, but even then youd have to trim it a little.

My thoughts exactly.
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Old 26th May 2010, 09:36 PM   #6
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Thanks for both of your advice. I noticed some 'matched' pairs on ebay, but who knows if they are any good or not. I'll keep my eye out on the swap meet. In the mean time, i'll have a go at trimming suggestions.
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Old 26th May 2010, 09:42 PM   #7
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Here, Matched Toshiba FETs For Sale

Not cheap but much more reliable and without risk factor.
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Old 26th May 2010, 09:51 PM   #8
AMV8 is online now AMV8  United Kingdom
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Hi
Have you checked measurements at different points in the circuit to find where the 2 volts originates from?
Don
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Old 27th May 2010, 08:30 AM   #9
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reminds of terrible elektor designs that are simulation products and mostly based on perfect parts that actually dont exist ....

the construction /time/effort /cost/ consumed to pick up the proper parts might be twice as much than the cost of amplifier it shelf .... Titan i thing the miracle was called ...

i will go though with amv8 and see where is this 2 volt originate from ...
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Old 27th May 2010, 09:35 AM   #10
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Hi Ben
i think i can help you because i have made twice the TITAN 2000 and CRESCENDO 2000
and input stage topology nearly matched
YOU MUST MATCH LT1 - LT2 if you don't matched you can't never take 0V Offset
i post my result about TITAN 2000 ON POST 59 ----LEGEND sTAGE MK2 IN MAKING............ SOLID STAGE PAGE 4 NOW
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