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Threshold 400A: 60Hz square @45mV pulse train left channel
Threshold 400A: 60Hz square @45mV pulse train left channel
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Old 6th December 2018, 03:39 PM   #11
ampexperts is offline ampexperts  United States
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The output DC offset is under 50mV for both channels.

The amplifier functions normally, has a flat frequency response and seems fine in all other respects, excepting the RF-induced hum. Something common mode is at play here.

I'm quite certain no transistors are failed, but I'm not certain that some haven't degraded or developed some form of leakage current. It's not likely it left the factory like this either. 30mV of square pulses would be quite audible and result in an immediate service call unless the buyer were seriously deaf.

I see there's two 4.7K resistors at the input, in conjunction with a 300pf capacitor, forming a low pass filter. By increasing those resistors, you reduce the bandwidth of the input stage, which may offer more stability.

Mechanically, I'm finding it to be a bear to work on because of inadequate service loop on the wiring. Already had a fuse holder on the right channel break because the wire tugged on it a bit and the metal must have been brittle. Now I need to replace a fuse holder.
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Old 6th December 2018, 05:05 PM   #12
MEPER is offline MEPER  Denmark
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If you ground the input is the problem still there?
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Old 6th December 2018, 05:09 PM   #13
ampexperts is offline ampexperts  United States
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Default Breakthrough Solution--Grounding

I just cut the ground connection from the power supply common to the chassis. All hum, buzz and RF oscillation ceased.

What led me to this path was noticing that when I moved the loosened left heat sink, depending on its connection to the chassis, the pulse train would alternate to the right or left channel. That convinced me it was a ground loop within the amplifier.

I've seen this sort of thing on Hafler 500s and XL600s. It is necessary to change the grounding topology to make them stable under some conditions.

So I have the ground wire disconnected from the chassis ground and the back plate connected to the rest of the chassis and hum and noise dropped well below .5mV in both channels.

I need to replace two fuse holders (the other speaker fuse holder terminal just broke while I was in there unsoldering the ground from the bottom of the chassis and I have had to jumper across both fuse holders temporarily to continue testing), but I think this is a satisfactory solution.
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Old 6th December 2018, 05:51 PM   #14
Nelson Pass is offline Nelson Pass  United States
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Threshold 400A: 60Hz square @45mV pulse train left channel
That is helpful.

You will want some sort of connection from ground to chassis, so I
suggest a power thermistor, something like the Digikey 570-1055-ND.
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Old 6th December 2018, 08:13 PM   #15
ampexperts is offline ampexperts  United States
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Some systems just use a 10 ohm resistor. This amplifier has a 10 ohm resistor from each input ground to chassis. Seems to be doing the job.
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