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DC offset
DC offset
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Old 17th May 2019, 10:56 PM   #1
alexanderfa is offline alexanderfa  Greece
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Join Date: Jul 2012
Default DC offset

Hi friends,
I recently finished a hybrid circlotron amplifier, schematic you can see on attachment.
Is my first attempt in solid state, I only have experience in tubes, but I liked the idea of this topology.
My first listening test was very impressive unfortunately protection circuit at the output shut down when high level music applied.
I connect my signal generator at the input and a dummy load at the output to take some measurements.
When there is no signal in the input there is no DC offset at the output.
When I start raise the signal in the input, there is DC offset at the output and raise as the input raise until shut down from the protection circuit at 500mV DC. input about 300mV AC 1KHz sine wave.

What may cause this problem?

Some information:
input transformer sowter 3575,
tube ECC82 in PP with LL1660 interstage, at 250V power Supply, cathode bias 1K each triode, 8mA each
Output mosfets Exicon ECX10N20, Matched for Idss, bias 1,5VDC , Power supply 42VDC, run at 1A each one

Last edited by alexanderfa; 17th May 2019 at 11:10 PM.
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Old 17th May 2019, 11:48 PM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
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Join Date: Apr 2008
Location: Carlisle, England
If the applied signal is AC then it can have a dc offset but shouldn't trigger your circuit as it keeps passing through zero volts. If your circuit detects average DC throughout the signal then perhaps the two triodes aren't putting out same size signal and need balancing ? Or maybe one is clipping before the other ? I designed my own DC speaker protect circuit but it only fires if DC above/below +/- 2 volts is there for > 500mS. It doesn't look for average dc level. So if the signal passes through zero then DC level count is zeroed.
PCBCAD51 download or CDROM 2019 version out now http://www.murtonpikesystems.co.uk
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Old 18th May 2019, 12:58 AM   #3
DimitrisMav is offline DimitrisMav  Greece
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Join Date: Jan 2009
Hello !
If you don't mind kindly share this DC protection schematic with us !
P.S. Nice software of yours btw and very economical, congratulations !
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Old 18th May 2019, 02:17 AM   #4
N101N is offline N101N
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Join Date: May 2018
Very nice circuit. I would add source resistors going to output along with the 47K resistors.
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Old 18th May 2019, 02:26 AM   #5
academia50 is offline academia50  Argentina
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Join Date: Jan 2013
How about the rest current of the output transistors?
Do not forget to open the current circuit from + B to collector and intercalate there the milliammeter
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