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February 19, 2025, 12:48:05 AM

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Power amp started acting weird with a reduced volume: fixed it!

Started by Kaz Kylheku, January 26, 2025, 01:56:13 PM

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Kaz Kylheku

I've not been playing guitar much due to things going on and new job and whatnot.

Getting back to it, I noticed a substantially reduced volume. Very weird; I'm sure that at a certain master volume knob position it should be a good 3 dB louder, at least (subjectively).

I flip the current feedback switch on the back of the amp, and it goes much louder. Wow, there shouldn't be such a big gain difference between voltage and current+voltage. It's as if there is much more current feedback, due to a higher impedance to ground where the current sensing resistor is.

Another funny thing: I flipped the power switch of an equalizer in the rack on and off. A pop could be heard and the volume improved, temporarily. The equalizer is completely disconnected. It is not in the signal chain and its inputs/outputs are not connected to anything. All it shares with the amp is the power bar.

I took the amp out and opened it. My current feedback jig is on a little circuit board, with some Molex type connectors attaching it to the rest of the amp, such as the speaker return. I took some resistance measurements: current sensing resistor is okay, the coupling 1 k Ω resistor is okay.  I noticed an unusually high resistance between a ground point on the current feedback circuit board and on the amp main board; the meter was reading 1.6 Ω. Damn, it's that two-conductor Molex that takes the speaker return and goes out to ground again!

I unplugged and reconnected it a few times. Took another measurement and it was like 0.2.

Closed up the amp, re-racked it and everything is good. Much more volume in current mode; less of a difference between that and voltage mode.

So yes, there must have been a contact resistance to ground making the current feedback resistor look many times larger than it is, hitting the amp with much more negative feedback.

So the best hypothesis is that the inrush current from turning on the unrelated device (equalizer) caused a ground spike which provided wetting current to momentarily improve the bad contact.

Now when I toggle the EQ's power switch, there is barely any audible pop. So that louder pop must have been from the bad contact fritting in the power amp.
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