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Re: MoFo

Posted: Tue Dec 11, 2018 5:17 pm
by little eddy
I've been struggling with one channel of this amp, (the other channel works fine).

I have 2 inputs. When using 'In 1' wired directly across the 50k load resistor (taking the bias voltage to the pre transformer output), the audio sounds fine. But when I wire to 'In 2' via the dc decoupling cap, there's noise/cracking that I can't fathom or resolve.

I was thinking it was a poor connection somewhere but have even hardwired most of the 0V line and 'In 2' input circuit but it still persists.

So I am admitting defeat and ordered another set of parts and will build another!

Question for you boffins. The amp/output is silent when there is no input connected. Then when I connect the pre, although there's some hum from the pre that I need to sort out, this is louder when using 'In 1' as opposed to 'In 2'. Any ideas? Might this be because there is additional noise on the bias voltage coming off P1 and therefore might a cap between the bottom of R3 and 0V help? p.s. using a laptop power supply.

Re: MoFo

Posted: Sun Jan 06, 2019 4:19 pm
by little eddy
Might have found the issue - a noisy laptop supply!

Will try and get another to test my theory but if not, although a lot more work have the parts for a CRC supply.

Just one question. I was looking at an F5 power supply schematic and it has a CL-60 thermistor between 0V and the chassis ground. Was going to fit a thermistor to the tx primary but what does this second one do? And if I am going to use one, should I use two to 'separate' the 0V of the 2 channels as I will have a dual mono power supply?

Re: MoFo

Posted: Sun Jan 06, 2019 4:34 pm
by Nick
It works as a highish resistance link between signal and chassis ground (household earth) under normal circumstances, but under fault conditions it will allow current from signal to chassis earth to flow.