One more for the experts. 
I made a few changes today and plugged the amp in for 2 hours and measured bias. It was fine, e.g. -1.1mV and 0.1mV on the 2 channels.
But I also put meters on the output resistors on one side to make sure the values were near my target (550mV). They were more or less, but the values were fluctuating together, both going up 10mV, sitting for 10 seconds, then going down 10mV, 10 seconds...
What would cause this? I would have thought it would be more or less settled after 2 hours idling, and I'm surprised that it oscillates so much. It's almost like it is breathing!
Is this a pretty common occurrence?
I made a few changes today and plugged the amp in for 2 hours and measured bias. It was fine, e.g. -1.1mV and 0.1mV on the 2 channels.
But I also put meters on the output resistors on one side to make sure the values were near my target (550mV). They were more or less, but the values were fluctuating together, both going up 10mV, sitting for 10 seconds, then going down 10mV, 10 seconds...
What would cause this? I would have thought it would be more or less settled after 2 hours idling, and I'm surprised that it oscillates so much. It's almost like it is breathing!
Is this a pretty common occurrence?
mains fluctuate, temp fluctuate in box when it's open ( JFets are really sensing any breeze)
conclusion - do not breathe close to your amp when top lid is off
It was actually doing it with the lid on and me standing a couple feet away. But now that you mention it, something was heating on the stove, and that thing probably switches off on a similar cycle at medium setting. And when I was biasing it yesterday it wasn't fluctuating noticeably after the lid was back on for a minute or two.
Thanks for the hypothesis!
Quick question about output offset... Several months ago when I finished my F5 I had successfully biased it (0.55V) with zero offset on both channels. I let it warm for several hours before doing the final bias/offset tweaking. Then it sat for a few months unused while I built a Burning Amp 2018 to go with it.
I just powered up the F5 again today and had multimeters on the speaker outputs from the moment of switch-on. Both channels started around -75mV and have slowly climbed to -25mV after 30 minutes or so. I am guessing if I let it warm up for another hour or two it will get closer to 0V at the outputs -- but maybe not all the way since it is a lot cooler indoors now than during the summer (Valencia, Spain).
My question is, is it acceptable for the output offsets to start so far from 0V (e.g. +/-80mV) before reaching equilibrium?
I just powered up the F5 again today and had multimeters on the speaker outputs from the moment of switch-on. Both channels started around -75mV and have slowly climbed to -25mV after 30 minutes or so. I am guessing if I let it warm up for another hour or two it will get closer to 0V at the outputs -- but maybe not all the way since it is a lot cooler indoors now than during the summer (Valencia, Spain).
My question is, is it acceptable for the output offsets to start so far from 0V (e.g. +/-80mV) before reaching equilibrium?
I didn't find any reference to washers on the pages you linked to. Also, to be honest, I didn't understand the previous post, or what split washers have to do with my issue. Or where they would go...
Anyway, I redid the bias yesterday since both channels offsets were only climbing to -20mV at the outputs. I think it was probably because the indoor temperature was 90 degrees when I biased/offset the amp. These days its closer to 60. And that means the initial offset at switch on will be around -60mV which seems reasonable.
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