Multiple amps + single power supply = unstable output

[d0s4gw]

Hi all -
I am testing some amplifiers (very basic INA217 configuration) and I've noticed something very unusual (or unexpected at least).

When I am trying to amplify 1 signal on 1 amplifier, the results are coming back very nearly as expected. When I am trying to amplify 2 different signals on 2 different amplifiers, the results are all over the place; the output is totally unstable.

I am using a PC power supply moded to a bench power supply (http://www.hackaday.com/entry/1234000687042947/). This is for testing purposes only. For the experiment, I am planning on using pairs of 9V batteries for each amp (there is a total of 4 amps).

Now I am also going to be adding some high pass butterworth filters to this, which is also going to require power. Is there something I can do that will allow me to use the same power supply for more than one amplifier or 1 amp and 1 filter?

Thanks,
-- Mike

[GRollins]

You could try using a CRC filter to each channel to isolate them from each other. You may be experiencing motorboating (an old-fashioned tube term) where the current draw from the two channels sets up beat frequencies in the power supply.

Grey

[amplifierguru]

Poor PSRR will do this. If you can't control it in the amplifier design you must isolate or achieve independence of operation in the power supply by aforementioned approaches. Or have seperate PS.
Greg