Thanks for the help with this, there is something abit of dilemma about it, I managed to find my stereo pre emphasis circuit using the op amp, stereo sound is unaltered through it sounds really good, just has pre emphasis on it, I didn’t know what the values were until I found the circuit again at least the diagram of it, the capacitor and resistor combinations are known, it was just done to sound good to my ears.

I’ve found there maybe are some problems with the op amp, rf hum is creeping in slightly, more hiss, also whine from the power supply if I turn it up loud. Some of these solved by shortening the leads at the output, reducing the gain, the whine is still there, it was even there when I connected up another stereo encoder, without the op amp my own stereo encoder is dead quiet.

The circuit that was due to be put in on tuesday, had to be taken out again, because of the problems mentioned above. I got 2 npn transistors instead, I don’t notice the whine there, having to test for different things, I got abit tired of it, I left it there. This might be not needed as when a compressor is used it is likely loud enough with just the passive circuit, most of the time you don’t listen to it next to it passing things through neat. If anyone wants to try and help with a little amplifer circuit, that would be immune from noises from the power supply, hiss, rf pick they are welcome to try.

With the stereo encoder I have, ferrite beads from the left and right audio channels, from the positive rail, (these are on the board so no leads) 470uf cap over the power rails. I had another ferrite bead between the ground audio input, found it was not needed, it made it more quiet connecting it up without, seems to me like most of the hum comes down the positive rail.

For RF pickup ferrite beads on inputs/outputs will help. Another circuit I've used is a pair of 1k or 2k2 resistors in series with the input and 100p cap to ground from the junction of the resistors. You could try both.

For PSU noise this is why I prefer linears to switchers for audio circuits. You might even find a linear reg after the switcher would help. Of course you'll have to use a higher voltage switcher. E.g. 18v switcher followed by 7815 reg for a 15v supply, or 15v switcher and 7812 reg for 12v supply. Put a big electrolytic, say 100uF or 470uf 25v with 100n ceramic in parallel at the reg input. 10uf or 22uf + 100n at reg output.

Make sure the grounds from each board and the PSU join at a single point (star ground) preferably on the chassis ground lug. Don't put ferrite beads in ground wires

I've got an audio processor box with my stereo coder board, rds coder board and two of my limiter boards (for stereo) all in a metal rackmount box with linear psu inside. Its set up in my attic right next to a 1/4 wave antenna running 1w and I only get the slightest hum just noticeable behind quiet audio sections. I think most of the hum comes from the mixer (a cheap behringer that really doesn't like rf) rather than the processor.