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Old 23rd December 2013, 07:04 AM   #1
JMF11 is offline JMF11  France
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Join Date: Dec 2013
Default SOHA II amp tripping

Dear all,

I really need your help to fix a strange behaviour of my SOHA II hybrid headphone amp. The design of the amp was introduced on Head-fi forum some years ago. It is a cavalliaudio design. Al design information are available here: The SOHA II Headphone Amplifier The website provides all information related to the design and schematics.


The amp works well, except that from time to time, especially when the amp is cold, the sound stops. It appears that a DC voltage appears at the output of the amp and triggers the protection board (inspired from the epsilon12 design).


I borrowed a scope for 2 weeks, and I started to investigate. I could catch some events, and I' m really puzzled:
Facts:
- events are between -100mV and + 100mV. They don't always have the same shape/amplitude,
- they are exactly the same on L and R channels, so it should come from a part of the circuit common to the 2 channels,
- I mesure at R8 level, so before e12 => issue should not come from there,
- I measured (with several measures) the voltage at different test points, during the events, and in all cases Voltages from power supllies are steady, when the output of the amp at R8R and R8L derives. I tested: +15 and -15 rails (at J1 and J2), TB+, TR+, TL+, TL-, TH
The events are much more frequent when the amp is cold than when hot.
I would have imagined having one of those voltages moving with the event... but it is not the case.
Below pictures:
- yellow&blue= output at R8L and R8R
- red and green= power supplies (various)
- vertical gid: 100mV, horizontal= 500ms

Would you see other possible common points and tests to perform? I have only basoc knowledge in electronics, and none about tubes... So I'm lost, but I really like this amp.

Regards,
JMF

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Old 28th December 2013, 06:45 AM   #2
JMF11 is offline JMF11  France
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Join Date: Dec 2013
I checked and rechecked. All power supplies lines are clean up to their connection point to the right or left channel. Up to now, I was thinking that my measuring point was maybe not the last one, and that there could be issues between the measuring point and the client.

It is not the case.

Could there be some coupling through the e12 33kR resistors ? It is the last common point that I see in the circuit.

Regards,

JMF
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Old 29th December 2013, 04:37 PM   #3
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Location: Germany
This has got to be something fairly trivial. I'd put my bets on an intermittent ground connection somewhere on its way from power supply to amplifier.
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Old 31st December 2013, 10:10 AM   #4
JMF11 is offline JMF11  France
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Join Date: Dec 2013
I really paid attention to this, and the power supplies really looks good up to th right and left channels...
-15 and +15V (check at each op amp pins ICL/ICR)
TB+ at left and right channels tested at TB+R and TB+L
Heating at each valve...

I really see no event on those power suplly lines when I have events on the amp outputs...

Still very puzzled...

JMF
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Old 1st January 2014, 08:03 AM   #5
JMF11 is offline JMF11  France
diyAudio Member
 
Join Date: Dec 2013
So I consider that I failed. I explored all ideas I had without success.

I disconnected one input of the e12 (common point), and it changed nothing.

I also inspected solders, redone few nice ones. looked for shortcuts between tracks...

All seems OK, but the issue is still here.

So either I will let the amp heat for 30-45 minutes (when hot, much less DC events), or I will retry my DiYparadise Charlize T amp.

Regards,

JM
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Old 22nd January 2014, 03:39 AM   #6
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I have seen similar behavior in my SOHA II but have never been able to track it down. Were you ever able to decipher the behavior?
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