HELP! Hearing radio interference in simplest unity gain buffers

[antonis]

C4 value (100pF) seems a bit low but if it works , it works..

[Rob Strand]

C4 value (100pF) seems a bit low but if it works , it works..

100pF no problem there.  The Cornish buffer uses 1nF and it's still OK.

Some analysis in this thread but not sure if I posted a frequency response,
https://www.diystompboxes.com/smfforum/index.php?topic=124166.20

[antonis]

100pF no problem there.  The Cornish buffer uses 1nF and it's still OK.

That's exactly what I meant, Rob..

[amptramp]

Is this an AM or FM signal that you are trying to block?  AM is in the range of 530 to 1700 KHz and FM is in the range of 88 to 108 MHz.  You say it is probably AM but the techniques for blocking AM will be different from the techniques for blocking FM.  The solution to the problem is dependent on frequency.

The circuit needs shielding with a cast aluminum box as a minimum and if as you have found, this is insufficient, you need to add ceramic capacitors from the Vb voltage to ground and from the supply voltage to ground.  You have to ground the input and output jacks to the enclosure and use jacks where the the mating surface of the jack is bonded to the mounting point.

[PRR]

Is this an AM or FM signal...

OP says "I do hear (probably AM) radio ..." I agree this is not for-sure.

It is probably worth getting an AM/FM radio, running it with the offending pedal, and scan to find the same program on both. It is usually very clear when two sounds "fall together", even if one is very distorted.

ground the input and output jacks to the enclosure and ...

In general we often need L/R/C networks on the audio jacks also. The most famous(?) is the 34k&100pFd on the input of Fender tube amps. (Two 68k makes 34k; 12AX7 grid has 100-120pFd hidden inside.)

[Rob Strand]

100pF no problem there.  The Cornish buffer uses 1nF and it's still OK.

That's exactly what I meant, Rob..

Only adding to what you said.