DIY Foot Drum

[Simon Barden]

Technically a low pass filter isn't a standard bass control. It passes all frequencies below the filter cut-off (and actually some above, but it's rolling them off at the filter slope rate and will already be 3dB down at the filter frequency). A normal bass control is typically a shelving boost/cut, which only affects the bass frequencies. But a low pass filter should do what you want in terms of just leaving most off the bass and low mids, especially when paired with that boost to get the level up.

A kick drum normally has a low component and then a high-mid component from the beater that provides some definition to the sound in the 2-4kHz region. I don't know how much of those frequencies your weighted speaker produces, but I wonder if you could somehow pair the low-passed speaker with maybe a high-passed piezo signal to more closely mimic a real kick sound.

Again it's something you could test quickly out in a DAW after recording the two signals to see whether it would work at all.

[jugglindan]

Should I have said "standard tone control"? I was thinking of using a first-order passive RC low-pass filter, which I see in circuits a lot labeled as "tone". I guess "bass control" implies an active control that can boost or cut in a band. I did try a 5 band EQ, but it was easier to get a good result with a one-knob low-pass filter.

To do the piezo mix I would need the booster and low-pass filter on the speaker, and a piezo preamp with high-pass filter, then a 2 channel mixer (either passive or with an opamp) to blend. Sounds like a lot but it could probably all fit on a single stripboard. Maybe.

The DAW test certainly sounds easier than breadboarding all those circuits. Hopefully Audacity can do simple filters, because that's all I have.

But I might be able to get something decent using just the weighted speaker by pairing a Baxandall bass/treble tone control with a clean boost. Time to pull the Rat off the breadboard!

[Simon Barden]

Yes, probably a pedal standard tone pot. Though a standard, that LPF would probably be set a bit higher than what you'd require. You may not need piezo pre-amp if you just work with the sound it produces into a lower impedance input. The piezo preamp provides the high impedance required for a flat response if it's a single source transducer and needs to do full-range flat reproduction. But as a simple sound source, and with you 'throwing away' a lot of the sound it produces, and probably at a fairly low-level in the mix, you shouldn't need a special preamp for it.

Hence the suggestion to play with the two sounds in a DAW where you can adjust the LPF and HPF frequencies and mix levels to see whether you even think it adds anything extra or whether the speaker on its own is fine. As long as you record the two sounds at the same input gain level, you'll also find out how much relative gain one signal would need to the other.

Audacity should have LPF/HPF and shelving EQs available, so you can also test the Baxandall EQ and see what frequency settings/gain levels you'd need without having to even open your resistor box.

[jugglindan]

Still haven't recorded anything, but I built a mosfet-based boost on breadboard tonight. It's sort of a modified SHO (Soulsonic Crackle Not OK) set to max gain (no gain pot). Sounds quite good with guitar, and gets the level of the speaker transducer up high enough without too much noise despite being on a breadboard.

No bassy thump, but I haven't added a tone circuit yet. I think it would make an interesting boost pedal for guitar with the addition of tone and volume controls.