DIY Foot Drum

[JohnH]

This is an excellent idea for a project. I have a whole box of cheap pickups kicking around in a box somewhere, and now I'm tempted to try the bass pickup version

[It seems my previous post vanished, so if you can all see two posts I apologise for the double post]

[jugglindan]

This is an excellent idea for a project. I have a whole box of cheap pickups kicking around in a box somewhere, and now I'm tempted to try the bass pickup version

[It seems my previous post vanished, so if you can all see two posts I apologise for the double post]

Just one post here. If you do build one I will be very interested in your results. This is pure experimentation for me.

[Simon Barden]

I'm not sure how well standard volume and tone controls would work with a speaker as a transducer. You're looking at 8 to 16 ohm for the speaker impedance, rather than 6+k ohms for a guitar pickup. So a much lower volume pot resistance would be required and you'd have to look at the circuit maths as to whether you could get a standard passive tone control to work at all. I suppose it should do.

[jugglindan]

I'm not sure how well standard volume and tone controls would work with a speaker as a transducer. You're looking at 8 to 16 ohm for the speaker impedance, rather than 6+k ohms for a guitar pickup. So a much lower volume pot resistance would be required and you'd have to look at the circuit maths as to whether you could get a standard passive tone control to work at all. I suppose it should do.

Good point Simon. The output from the speaker will be so much lower than a pickup, so the voltage drop across 250k might be way too much. It is hard to see how it could work well without a preamp. And if I have a preamp I may as well do the tone controls in that (apart from a simple low-pass filter after the speaker), or in the main amp.

My plan has evolved now into stomp box --> piezo preamp (should work well enough for speaker transducer as well I hope) --> cigar-box amp.

If the cigar box amp doesn't have enough thump, I might be able to cook something up from an old PC speaker sub-woofer. But now I can't decide between a Little Gem, a Ruby, or a Noisy Cricket. Will have to breadboard all three and compare.

[Cliff Rogers]

Not entirely correct.
Output to Input impedance matching is a subject not well understood.
Below is a link to a page on microphone impedance that may help you understand if you want to plough through it.

In a nutshell, low impedance output into high input works, high impedance output into low impedance input doesn’t.

It has more to do with the signal level presented to the input than the impedance of the output.

100K, 250K & 500K volume pots are better for matching small signals to a high impedance input than a lower value pot.

Here are some links.
https://mynewmicrophone.com/microphone-impedance/
https://sound-au.com/project01.htm

[jugglindan]

Not entirely correct.
Output to Input impedance matching is a subject not well understood.
Below is a link to a page on microphone impedance that may help you understand if you want to plough through it.

In a nutshell, low impedance output into high input works, high impedance output into low impedance input doesn’t.

It has more to do with the signal level presented to the input than the impedance of the output.

100K, 250K & 500K volume pots are better for matching small signals to a high impedance input than a lower value pot.

Here are some links.
https://mynewmicrophone.com/microphone-impedance/
https://sound-au.com/project01.htm

Thanks Cliff. That's why I love these projects and this forum: they help me learn things

I have used the sound-au pages for lots of other information, so I will start with that one.

DC

[JohnH]

That's great, thanks Cliff. Everything about this is confusing for me, but honestly the learning is most of the fun!

[Simon Barden]

A 500 ohm pot might work for volume. You don't want to add too much extra resistance in to the signal path at part-volume. If you keep a similar resistance ratio as a standard pickup e.g 8k to 500k, then 8 ohms to 500 ohms is about right. Whilst the pot acts as a voltage divider, and in theory the output voltage level should be the same with any pot, the current produced with a 500k pot compared to a 500 ohm pot will just be so much less, making it far more prone to picking up induced noise and making pot operation scratchy. The pot just needs to have a high enough resistance so that it doesn't 'load' the speaker, and a minimum 1:10 ratio is normally stated as being sufficient for that.

The pot resistance will form part of the input impedance seen by the speaker, but with a guitar amp impedance normally being around 1 meg ohm, the speaker will only really see the impedance of the pot.

A modern mic preamp will typically have an input impedance of between 1200 ohms to 3000 ohms. Older mic pre designs (and modern copies) tend to have impedances towards the lower end of the scale It would obviously be just as easy to make the input impedance a lot higher, but they don't.

A typical dynamic microphone will have an output impedance between 150 ohms and 300 ohms. So some dynamic mics may be slightly loaded by the lower input impedance mic pres, and in fact some stand-alone mic pre designs have adjustable input impedances so you can load the mic output for a different frequency response (less highs with lower impedances).

Speaker-based drum mics like the Yamaha Subkick (originally the LF driver from an NS-10M in a round enclosure), will run directly into the mic pre of a mixing desk, so will be running into that circa 3k input impedance. But that's a 7" speaker and the signal level produced by sticking that on a loud kick drum is going to be significantly greater than that made by a much smaller speaker resonating due to being given a small knock by your foot.

Personally I'd think about active controls, and look at getting say a cheap Behringer EQ700 pedal, stripping it down and mounting it in the enclosure. So you'll have level and tone shaping controls available. It won't take you to zero volume, but you won't need that. In fact you may welcome the ability to boost the output level!

[jugglindan]

I found this excellent article by Jack Orman that boils it down to voltage dividers and Ohm's law and finally it makes sense why low output and high input impedance is the way to go. The reverse leads to signal loss proportional to how large the mic impedance is relative to the input impedance.

Jack is talking about pickups and guitar pedals but that detail is irrelevant. It's really about a signal source and a sink. He also simplifies impedance to resistance by assuming it has a fixed value at all frequencies, but it still gets the concept across. And he indicates later how the effect is greater at higher frequencies.

As for EQ, building something is doable pretty much from parts I already have. I am not sure where to put the EQ, or how many to have. I can tack active 3-band EQ to the preamp built into the stomp, and the Noisy Cricket (or maybe a Ruby Tuby if I get extravagant, although reviews are mixed) will also have some EQ in the cigar-box amp.

DC

[jugglindan]

I probably need to start a different build diary for this, but I went shopping at The Green Shed (not the Big Green Shed) but a place in Canberra next to the landfill site that sells reclaimed stuff for next to nothing. I was looking for parts for the cigar-box mini amp that I am building for the foot stomp (or other things).

For the princely sum of $5 I got this old Sanyo speaker:


It's as ugly as anything I can imagine, but paint can fix that, and it's the right size and the 6" 5W speaker is usable. It likely won't give anything like a solid thump, but this mini amp might end up being used for a completely different purpose anyway.

I also picked up the chassis of a Kustom Sienna 30W acoustic amp, again for $5:


It looks like someone let the magic smoke out of a cap in the power supply section:


While I can replace the cap and fuse, I would be worried about why it blew in the first place and whether there are other faults. I don't have a chassis, and if I went to the trouble to build one, I would want to put a homebrew tube amp in, not this. So I might strip it for parts. I can salvage all the pots, the knobs, jacks etc. Most passive components are not worth enough to desolder, but there is a nice JRC13700 opamp sitting on an adapter board that is just ready to use in a pedal. There is also a spring reverb unit that looks OK. Too large for a pedal but I might be able to use it somewhere else, like in the mini amp.