Noises when i hit a stompbox

[wcampagner]

Hello,

I've mada a few stompboxes and i notice something strange.

When i hit the stompbox box with my fingers or any other thing i notice some noise on the amplifier... this is noticeable just when the amp volume is very loud... but i think it should not happen. Also when the bypass switch actuator is pressed i hear this noise... i'm using electronic switching with push-button and switch actuators (Sansamp Style).

Is there anything i could do to avoid it?

Thanks,
Wagner.

[R.G.]

It's called microphonics. Many materials exhibit the piezoelectric effect to some degree or another. If you mechanically deform them, a voltage appears across the material. It's trivial or not detectable for most materials, but a bit thing for a select few. That's why piezo(electric) pickups are made from titanium niobate or something equally obscure.

However, if you have a high gain, high impedance amplifier like that guitar amp you're using following your circuit, then you can pick up the piezoelectric effect in things in your circuit. Ceramic caps are the big offenders, but other parts can do it too. There are some guitar cables that are microphonic all by themselves. I was shocked (emotionally...) the first time I tapped a rubber-cased flexible cable and heard the speaker make noise each time I whacked it.

[wcampagner]

Thanks for the answer R.G.,

But it happens because of the amplifier or because of the stompbox desing?

If it happens because of the stompbox, is there a way to solve it? Maybe change the ceramic capacitors?

Thanks again,
Wagner.

[Mark Hammer]

Well, let's be frank here, and admit that we don't always use the best jacks, pots, switches, IC sockets, or tighten the securing nuts optimally, or necessarily secure the circuit board appropriately.  Indeed, there are a myriad of ways that we create opportunities for electroical continuity noise to happen.

Is THAT what we might be talking about here?

[wcampagner]

Hello Mark,

I don't know if this is the case.

I always try to use what i can find best... but what i can afford also.

I think all the components are very good tight... my stompbox doesn't have any offboard wiring... everything is onboard and the board is very tight inside the box...

Thanks again,
Wagner.

[Mark Hammer]

Well, then maybe it IS the sort of microphonics that RG describes.  I don't wish to depict this as an either/or distinction.  Both of these are challenges to any builder, and achieving a unit that is resistant to impact-induced noise or crackling is a constant challenge.

You have to wonder to what extent some manufacturers use "goop" not to hide their designs, but rather to provide some immunity to such noise.  I'm not saying they have NO concerns about IP, but that noise immunity may well be one additional reason for gooping.....unless it has no bearing on microphonics.

[wcampagner]

Thanks Mark,

I think it is going to be very hard to find the problem... but i'll try... i'll make more tests.

Thanks again for all the help,
Wagner.

[markm]

I've had ceramic caps become microphonic to the extreme. 

[John Lyons]

I had a build recently that sounded like a coin inside a small metal box when I wiggled the bypas switch. I thought it was the switch that was microphonic but it turned out that it was a ceramic cap on the board (which was wedged against the switch...)
To see if you have any microphonic caps (assuming you have any ceramic caps on the board) is to tap around the board with somthing hard. When you find the microphonic part you will know it because it will be a very loud ping or clank sound. You may wany to really turen up the gain and amp to zero in on the part. Ceramic caps are know to do this but it may be another component if you don't have any ceramic parts.

John

[R.G.]

But it happens because of the amplifier or because of the stompbox desing?
If it happens because of the stompbox, is there a way to solve it? Maybe change the ceramic capacitors?

It happens because Mother Nature said that some materials make electricity out of mechanical vibrations. It's not really the stompbox design or the amplifier, excepting that these both make for huge gains which highlight even very tiny signals. But I'm thinking that no one wants to design for lower gain, eh?

The solution is to use non-microphonic materials in everything. That includes not using Z7R ceramic materials or any ceramic at all if you can avoid it, and to not use high dielectric materials from different sections of the triboelectric series so they can't rub and make static sparks, and to prevent any mechanical vibrations with damping material.

Or to make some compromises and live with whatever you can't get rid of.

It's a continuum. Quiet pedals, like life, is a journey, not a destination.

[Paul Perry (Frostwave)]

I agree with everythng above. The thng is, if you want to track it down, you will have to open it up & keep hitting all the components hard & soft until it is obvious.
Same as the dentist
I saw a plan for a car engine "sthethoscope" that uses a ceramic bypass cap as the "microphone".

[wcampagner]

Thanks guys for all the help,

I think the idea of BasicAudio (John) is very good... i'll try to find which cap is producing this effect tapping the board with something hard... and then i'll put the results of this test here. Most of the caps that i'm using are ceramic.

Thanks again,
Wagner.

[wcampagner]

Hello,

I'd like to thank the help of you guys... the problem is solved.

I started to tapping with a screwdriver all the components of the stompbox... i noticed that all ceramic caps in the signal path were producing the microphonics effect.

I replaced all the ceramic with poly film caps... all the caps in the signal path and the problem was solved!!

I'm very happy with the results...

Thanks again for all the help,
Wagner.

[TELEFUNKON]

So: it`s PROVEN that you can hear the SOUND of different capacitors!?!