Acoustic 370 turn on thump

[Helmholtz]

Note the most recent drawing shows a 10 ohm resistor.

Well, a 10R resistor will have to dissipate 100 times the power of a 1000R resistor (when connected to the same voltage).

[dmeek]

This problem is more complicated than I initially thought. The easiest solution is to live with the 10 second wait time.
A surge thermister would slow the rise of the power supply which should prevent thumping. see msg #17
Years ago I fixed an amp with a surge thermister. I don't remember the brand, not a major brand but not unknown either, anyway the
problem turned out to be the thermister itself burnt out so maybe it would have its own set of problems

[Randall]

I'm perfectly OK with a ten second wait time, after all we'd wait at least that long on a tube amp. That is how I will explain it to him.

[Leo_Gnardo]

This problem is more complicated than I initially thought. The easiest solution is to live with the 10 second wait time.
A surge thermister would slow the rise of the power supply which should prevent thumping. see msg #17
Years ago I fixed an amp with a surge thermister. I don't remember the brand, not a major brand but not unknown either, anyway the
problem turned out to be the thermister itself burnt out so maybe it would have its own set of problems

True that - sometimes I've put a surge thermistor aka inrush limiter in the AC line - note several manufacturers like Peavey and Fender have done the same. Even the high quality hi fi / industrial amp manufacturer McIntosh had these in their tube power amps all the way back in the 1960's. The lower current value thermistors increase the charge up time of the power supply, but they can burn out when asked to pass maximum current. OTOH high current thermistors don't slow the charging of the power supply much but then they don't burn out. At best, they're a partial solution, far from perfect.

[Leo_Gnardo]

Well, a 10R resistor will have to dissipate 100 times the power of a 1000R resistor (when connected to the same voltage).

That's why I think it's not an acceptable engineering solution. But it would certainly shorten the "warm up" time to a fraction of a second. For those who just cannot wait. People are so impatient....

[dmeek]

I was thinking the 10 ohm/5w would absorb the turn-on pulse, but it wouldn't survive audio. If a double pole switch was used the other side could short the input of the power amp when it's on standby.

[Randall]

I converted the ground switch to the standby switch using a 470 ohm 5 watt resistor. I pulled the ground cap and installed the resistor on the terminal strip it occupied. One of the wires was already there, so it was pretty easy. It works like a charm! I will install a new output cap when it arrives, and this customer will be happy.

Thanks as always!