I have a few 3 and 10 watt pcb based boards, the gain is set with a resistor (my luck its SMT), can I use a PWM and transistor to replace that to make a gain control?
With no schematic of your amplifiers, who can tell? Are we supposed to use the crystal ball? Or parhaps tell you yes and then what will you do?
Typically, you'd use a [u]digital pot[/u] as a volume control on the input. (Make sure the digital pot works with AC audio.)
Without knowing anything about the board, no there is no way to help. Could be class D, class B, a DAC, using a preamp, whatever. The precise board needs to be identified and even then it needs to have good enough documentation.
I know, it was a bad question. I dug up the link to the modules I have kicking around.
I plan to drive a paging-style 20W horn with PC audio, AC square wave from ESP, Ard, etc would be input.
I have these exactly. Thing is, the CHIP has a volume control, but it’s not broken out. The datasheet is for the CHIP, not the module it sits in, which is basically a bunch of decoupling caps and a few resistors (the latter being of interest).
Yes, that’s the assessment I made. You’ll need a digipot on each input then.
The chip on that breakout has a fade up/down input suitable for a push-button, which is not brought out. It would not be easy to control that automatically anyway as there is no way to know the current gain setting.