pre-amp for foot stomp box

[alparent]

I think I need a pre-amp for my DIY foot stomp box. (Just a piezo on a wood plank.)
I'de like something that would allow me to also shape the sound. (I basically want to boost the base.)
I want it to sound like a base drum.

Thanks

[GibsonGM]

You could just make an LPB-1, and make the input cap larger to allow more bass through....simple circuit to make with few parts, that are readily available...

[R.G.]

You need HIGH IMPEDANCE for piezos. Generally, you have to use a FET input of some sort, although more complex bootstrapped bipolar circuits work. You need a minimum of 1M input impedance, and that is including the device input impedance and the biasing resistors in parallel. Probably some high cut after that.

[chi_boy]

I have seen it recommended and have used the Tillman preamp with a piezo in a cigar box guitar.  Worked well in that application.  Doesn't have a bass control, but you use it as a fragment for a bigger circuit.

 

[GibsonGM]

Good points, guys (R.G., esp)  yeah, you might want to use something more like an AMZ Mosfet boost (see link at top of page).  As chi says, you can put some tone shaping in there, or even between 2 stages....could do a buffer input, tone shaping, then boost....

[smallbearelec]

Bare-Ass Boost:

https://www.smallbearelec.com/Projects/BareAssBoost/BareAssBoost.htm

Raise the 1 Meg input resistor to 2.2 meg or more.

[Mark Hammer]

The old EHX Mole was supposed to be a bass booster.  Underneath, it was really an LPB-1 with some treble cutting included so that the gain would be applied only to the low end.

Steve's Bare-Ass (apologies to all for that brief mental image!  ) is a better solution for a piezo pickup than the Mole is, but we can steal a few ideas from the Mole.  And one of those is to stick a cap to ground, in parallel with the 1M gate resistor that Steve recommends raising to 2M2.

The cap to ground he shows as 10uf, might be 22uf or even 33uf, because you're attempting to amplify foot-banging rather than guitar strings.  Since that cap between source and ground increases the gain, we'll want to leave it in.  You may want to stick a 1k trimpot in series with it to adjust that gain.  But since the piezo disc will provide a big enough signal, my sense is that you won't need to cascade two gain stages as Steve shows.  You might, I suppose, but part of the reason the Bare-Ass has two gain stages is because the tone control eats up some signal, and because a guitar player would want to overdrive subsequent pedals, or their amp, for tonal purposes.  My guess is that you simply want an audible thump, and nothing else.

The cap from gate to ground will trim off some treble, prior to gain being applied, and a second one just before the volume control will trim off some more.  I leave it up to you to decide what cap values work best for you.  Start with .047uf and go up or down from there.

[Johan]

Google "bo hansen impedance converter" for an excellent easy to build buffer for your piezo