DIY Foot Drum

[jugglindan]

I am hatching a plan for a new pedal build, but something different this time.

I am thinking of picking up a cheap piezo transducer on eBay, and putting it into a wooden foot drum/stomp box sort of thing. I have seen people wire them as a passive circuit with just a volume pot which I will try first, but if I need to massage the output a bit more I will build a piezo preamp and wire it into the wooden box. This circuit might be overkill, but I have all the components and the volume, bass, and treble controls might be handy for shaping the sound.

I just wish my woodworking skills were up to making something fancy. I can probably manage something functional but ugly.


I only have one amp, and don't really want to use a mixer to blend this into my guitar signal so I am hoping that the preamp output will have a suitable level to go into the aux input on my THR. If not I could run a simple headphone amp after the pre-amp. Or I could build a 2 channel mono mixer, but I am worried about adding extra noise into my main signal.

Does anyone have any experience with this sort of thing? Any advice or suggestions?

DC

[Bakersdozen]

Cool project idea! I always wondering how these worked, but never got round to doing any research. I would have thought they would be better going into more of a PA rather than a guitar speaker. Something at least 12 inch, 15 inch being ideal as it is mimicking some of the sub frequencies of a kick drum.

[Simon Barden]

I really don't get the description of the preamp. It says it's designed to work with electret condenser mic systems, but those need DC power supplied to them and a low input impedance. Whereas a piezo hates DC and needs a high input impedance. Ideally 5-10 meg ohms for most piezos for the flattest response. As the input impedance is stated as 2.2 Meg, then it looks like a piezo design, but with some wrong descriptive words.

Some piezos work best with 1-2 meg ohm input impedances, some are better at 10 meg. I have no idea what makes the difference. But too low an input impedance and the bass end will be missing, too high and you get a lot more bass (which is really what you want with a kick pedal).

I'd suggest looking for a preamp with a mid control as well, as that is very important for shaping the sound. For a bass drum sound, you need a fair bit of low end, around the 80-100Hz area, and some high frequency punch around 3kHz, and a bit of a cut in the mids.

The high impedance means that the output lead is very prone to picking up noise, and any capacitance will roll off the top end quite quickly. So any wire should be kept as short as possible.

I'm not sure a guitar amp is the best amp to use for such a pedal, as a lot of bass and a fast attack transient could give a lot more speaker cone travel than it's designed for if used at any volume.

[jugglindan]

Thanks Simon,
I agree the description of that particular preamp is a bit off. I think it was written by marketing not engineering. It's pretty much a piezo preamp with two tone controls as far as I can tell. I might be able to tweak the tone filter values to shift the bass control into the low-mid zone, or I could go with a simpler piezo preamp and run the output of that into a separate 3 knob tone circuit.

Simon and BD, you have me thinking my amp won't cut it. This was meant to be a cheap project, but I might end up needing a bass amp. Better build a bass kit too then! Although my THR has a bass amp channel, so it should be safe but I think it must attenuate the bass frequencies a fair bit.

And yes, I could just buy a Peterman Recession Buster for a bargain price, but where's the fun in that?

[jugglindan]

BD, I forgot to add that there are quite a few designs. Some of the pro models use piezo pickups but I am sure they have some sort of onboard processing, maybe digital in some cases, as well. Because I can't see how stomping on a box would sound like a snare, or hi-hat, or cowbells. The version I am thinking about is sort of like tapping on the body of an acoustic guitar fitted with piezo pickups.

Another interesting design uses bass pickups.

[jugglindan]

Simon, thanks also for the tip about preamp input impedances. I can tune that so will breadboard the circuit first to get the values sorted.

Sent from my moto g(7) using Tapatalk

[Bakersdozen]

Yeah that's the sound I figured you were going for. I did always presume they were made from pickups

[JimC]

You might want to see about including a high pass filter in your circuit so you can filter out subsonics and near subsonics which are major speaker killers.

[Simon Barden]

It doesn't need to be a bass pickup. There's nothing inherently different between a guitar and a bass pickup except the number of coils and the coil spacing. Same magnets, same typical 42AWG wire. So a guitar pickup will pick up just as much bass end. Just use whatever's handy/spare/cheapest. Single coils will probably be better as they have less mid-range emphasis.

No need for a preamp or high impedance input then.

Most piezo-style kick blocks I believe are pretty solid as you really just want the 'thump' of the wood as your foot hits it. But a pickup style unit will need a thinnish top that can resonate - at least a bit. You don't want a long sustain and you don't want it so resonant that you get acoustic feedback when you apply a bit of volume.

[jugglindan]

You might want to see about including a high pass filter in your circuit so you can filter out subsonics and near subsonics which are major speaker killers.

Good idea. I could do that in the tone control stage after amplification. And the circuit I am looking at using (the Barcus Berry circuit has some errors so will use something simpler) has a 0.1uF input filter cap, which combined with the 2.2m half of the voltage divider going to ground should act like a high-pass filter as well. According to Jack Orman's R-C filter calculator, it should attenuate below 0.7 Hz. Which is maybe a bit low. I think it's mainly intended as a DC filter. I could experiment with a lower value cap to raise the corner frequency perhaps. Or just handle it in the tone circuit.