these are just work-in-progress photos but the concept seems to work and so I wanted to share it before the project is fully done.
the idea is to have a pimeta board be the center of a home preamp. that's not all that unusual but what's a bit different is that I've 'stolen' the 3rd 'ground channel' and used it for a bass channel! or rather a subwoofer amp mono channel.
I was willing to let go of the 'high end' virtual ground buffer channel and use that set of opamps/buffers for driving my subwoofer. all I had to do was rewire the pimeta board a bit and instead of the ground channel taking its input from 'ground', I let the input of that single mono channel come from a summed (left+right) that came from 2 resistors from L and R. since I wanted the bass channel to be based on the main 2 L,R channels, the input to the bass channel comes from the output of the L,R and so when you vary the volume on the L,R pair, the input to the bass channel also goes louder, as you'd want.
I have a volume pot on that point, between the L,R outputs and the bass channel input. that lets me 'trim' the bass amount as you would normally do on a home theater stereo or a preamp with a bass level control.
pics:
first photo shows the area of the pimeta board that needed modifying. in the center where its marked C6G is where a new resistor (1M) goes which is the feedback loop, as the other 2 L,R stages have. there's a 15k R on its side (marked 15) and that's similar to the R4 gain setting resistor in the L,R channels.
a leg on the R3G spot is lifted and similarly one is lifted on R1G. a new R is added on its end in the R4G spot and a new junction is soldered 'on top' in place.
all this is to replicate as closely as possible the same topology of the circuit as the L and R has. the ground channel was always a little different (unity gain, etc) but I wanted to convert it to being the same as L and R so that my bass channel was as closely matched as possible.
the orange wire going into and out of the pot on the far left is the bass wire. you can see the 2 temporarily mounted (!) resistors on the 2 gold rca output jacks that form a summing network that just combines both channels into a mono single channel, ready to send into the bass-volume control pot. the long thick green wire is a ground bus.
