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LC audio TEM distorsion?
LC audio TEM distorsion?
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Old 1st November 2020, 11:47 AM   #31
tiefbassuebertr is offline tiefbassuebertr  Germany
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Join Date: Jul 2005
Location: D-55629 Schwarzerden
Originally Posted by A 8 View Post
Hoping for some help, I have a LC Audio The End Millenium that distorts audible with a 1khz test signal on one channel. Oddly the amp sounds fine and will play very loud without one thinking there is anything wrong.

Since the amp plays fine at very high levels and with lower frequencies I am thinking or should I say hoping that its over all function/transistors are still good but maybe there are some other non linearity introduced that should not be there. I have cleaned the board a few times but it did not make a difference.
When I first noticed the distorsion it was fine on low volumes but now it's there at any level.
There is a glass substrate resistor (R4) that according to the documentation lowers 3rd order distorsion compared to a traditional resistor. The distorting channel has a lot of of 3rd order and I cant help thinking maybe something has gone wrong with the glass substrate.
Any thoughts or ideas on how I can figure this out is appreciated.
Unfortunately the schematic isn't completely - one don't have the whole diagram. I miss parts for global NFB, at least the DC rail (I recall this is a version without AC global NFB - input stage differential amp have only a gain factor of 2 through high resistance emitter resistors) so as the parts for DC-servo.
According your explanations I guess, there is a transition resistance anywhere in the plugs/jacks for amp-input, speaker relay contacts, output terminals and input terminals of speakers so as solder joints in the whole signal pad.
To check whether the dc conditions without abnormalities check the DC-voltages on follow resistors without connected speakers and shorted inputs (compare both channels and check for plausibility):
1) R10-11-3-8-18-20-21-22 (quiescent current for input stage)
2) D1-2, R23 (voltage and quiescent current for Zener diodes)
3) R26-R27 (quiescent current for buffer of VAS, T1B and T20
4) R17-19 (quiescent current for voltage amplifier stage T12-10)
5) R228-2-1-RE1-6 (quiescent/idle current of each from four-stage resp. double darlington buffer stage)
Unfortunately I don't understand the theory of operation of DC servo. Please upload the whole schematic include power supply and GND-management-schematic (very important, unfortunately mostly not to find) - then I will give you additional advices.

Last edited by tiefbassuebertr; 1st November 2020 at 12:07 PM.
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Old 1st November 2020, 12:50 PM   #32
A 8 is offline A 8  Sweden
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Join Date: Mar 2003
Location: Gothenburg
I started doing some of what you suggest but did not find anything obvious and could not really let go of the fact that it sounds ok meaning if something was really broken it would be more obvious when listening to it.

As it was affected by the DC offset I focused on anything close by so to speak and eventually I disconnected the power-supply resistors (R42 and R45) to the DC servo AMP.

Did not think it would make a difference as the DC Servo jumper was open but I was wrong, distortion was gone!

I still don't understand why, could it be that the DC servo amp started ringing/oscillated and somehow propagated small amounts of this back on the aux power rails?

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Old 1st November 2020, 01:05 PM   #33
A 8 is offline A 8  Sweden
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Join Date: Mar 2003
Location: Gothenburg
Actually and I hate that it always takes me another beat (or post in this case) before I put stuff together.
This channel was missing C12 which means the servo amp was running without HF feedback so it must have been very noisy so to speak.

Would you agree this explains what was happening?
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Old 1st November 2020, 01:59 PM   #34
MarcelvdG is offline MarcelvdG  Netherlands
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Join Date: Mar 2003
Location: Haarlem, the Netherlands
It certainly explains that you got lots of high harmonic distortion, especially at low signal levels. Assuming that the DC loop was still marginally stable, for very low levels, the DC loop tried to suppress the signal. As it has a limited control range, especially taking into account the low-pass filter after the op-amp output, it failed to do so for anything but very small signals. So basically you got crossover distortion from the faulty DC bias loop.
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