Microphonic pedal - strangeness!

Philtre

I have a self-built pedal which uses true bypass. If the output of the pedal is plugged into the amp on TB with nothing in the input socket (and no power connected) and I tap the pedal's case I get a microphonic pinging sound. If I slacken off the nut and washer on the input socket then there is no pinging. Must be some weird grounding issue. I've isolated it to the input and output sockets by bypassing the 3PDT switch and the vero board. Should I put an insulating washer in the input side or do something else?

Philtre

Replaced the input socket and problem alleviated. What could cause that?

I notice that on some of my home made pedals this happens to a degree. Perhaps there's some capacitance formed in the connections of the sockets and the case?

ICBM

Are you using shakeproof washers?

Philtre

ICBM said:

Are you using shakeproof washers?

The sockets are Neutrik - https://cpc.farnell.com/neutrik/nys229/1-4-jack-panel-socket-mono/dp/AV09109

It also happens on my Xotic SP Compressor - pedal off (TB), connect output socket to amp, nothing in the input socket, tap it and it rings. Slacken off the input socket nut to separate it from the case and no problem.

Putting a jack in the input socket (even if not connected to the guitar) and not a problem.

It's as if the case being tapped is transmitting the sound down the input wire to the output connection. Also makes a donking sound if I put a jack in the input socket and quickly pull it out.

Thinking about it - maybe the socket live connector is acting like a spring reverb?

ICBM

Philtre said:

ICBM said:

Are you using shakeproof washers?

The sockets are Neutrik - https://cpc.farnell.com/neutrik/nys229/1-4-jack-panel-socket-mono/dp/AV09109

Yes... but did you use shakeproof washers?

Philtre

ICBM said:

Philtre said:

ICBM said:

Are you using shakeproof washers?

The sockets are Neutrik - https://cpc.farnell.com/neutrik/nys229/1-4-jack-panel-socket-mono/dp/AV09109

Yes... but did you use shakeproof washers?

Only what came with the socket.

Thinking about it - maybe the input socket's live connector lug is acting like a spring reverb?

ICBM

Philtre said:

Only what came with the socket.

Thinking about it - maybe the input socket's live connector lug is acting like a spring reverb?

It's more likely to be the contact between the jack and the casing, which is why you should always use a shakeproof washer... as well as to stop it coming loose.

You're not alone though, a lot of supposedly properly made commercial boutique pedals don't have them either.

Try these - you need 3/8".

https://www.ebay.co.uk/itm/Imperial-A2-Stainless-Steel-Internal-Serrated-Shakeproof-Washers-3-16-to-3-8-/291209692214

Philtre

More testing. It's a physical thing. I dampened the input socket live connector lug with the end of a plastic pen and no pinging sound.

ICBM

Alternatively, you could just stop powering up your amp with nothing connected to the input of the pedal...

Philtre

ICBM said:

Alternatively, you could just stop powering up your amp with nothing connected to the input of the pedal...

True. :-)

I noticed it when disconnecting the pedal from the board. I was just curious as to the cause. After pretty much desoldering my entire pedal and having a straight connection from input to output, checking things with a MM, and wondering if I was going mad (well, OK, yes) I narrowed it down. Wonder if there's some capacitance between ground and signal involved here in the case/socket?

ICBM

Philtre said:

Wonder if there's some capacitance between ground and signal involved here in the case/socket?

Most likely - there's not really any other possible mechanism. It's either between the spring contact and the casing, or between the barrel and the casing if the electrical connection is not 100% tight.

It's actually poor design to leave the input floating with no plug in, but is standard practice. Ideally it should be switched to ground.