Foot Drum Stomp Box

[antonio.s]

Those two traces you posted look nothing alike.  There's about a 40db difference at 450Hz...

Yes, and do you realize it's in favour of the stompbox? No buffer or preamp is needed here, you might want to use a passive eq, but you don't even need it? (I'll expand this idea later)
After all we're doing a 'stompbox' here, if we wanted a perfect bass drum sound we would have sent a trigger into a drum module.
We have more frequencies than required here, if you heard it short of frequency, chances are you did a biased listening (or... see last paragraph).

The frequency response of the raw piezo element is almost exactly wrong for this application.  The frequency response of the raw mag pickup is almost exactly what we're looking for.

My gosh, did you even check a few numbers before saying that? I'll keep it simple. A good humbucker is almost mute below 400Hz, and a single coil about 600Hz, they both have their resonant peak above 1800 - 2000 Hz; how do you think you're going to use those to produce a sound that sits between 100 and 400 Hz? You might have more luck with bass pickups, but judging from what I hear in the video, where bass pickup has been used, it's a big no to my ears.  If piezos are used to capture the sound of acoustic instruments (including bowed instruments) it is because they capture a wider range of frequencies. And what about good old vinyl records? All the instruments in the world pass through a piezo in a turntable.

How did you generate your "stompbox" graph? Is that the average or maximum frequency response for an actual stomp?

For both the bass drum and the stompbox, it is a peak graph of a single hit.
To make clear the 'impedance' question once for all, my soundcard (which yes, is an external module) has a mic/inst selector for each track to match the impedance of the incoming signal.

It might not even be lack of bass so much as excess treble.  It's all relative, right?

Now Ashcat, finally you said something that really makes sense! However an excess of frequencies doesn't require any active circuitry (if any circuirty at all). We could cut trebles directly on the amp/mixer/PA we're using, or else we could use a simple tone tapering circuit as the one we find on every electric guitar. This would allow adjustments that might be needed depending on the PA, the room's acoustics, etc. Any other tone shaping, in my opinion, could be done with different designs of the hitting surface (that's why I took the solid block of wood you see in the picture, which I didn't use because the box was surprisingly satisfactory.


One more note on the trebles in a bass drum. An untreated, raw bass drum professionally micked in a studio, would also show a spike in the 2000-3000 Hz range if I recall correctly. 
This is not the case of the drumcore, which has already been professionally edited and equalized before serving, but as I said, in the 'raw' condition a bass drum has that sound too, which is usually, very untechnically referred to as 'the clic' or 'the hit' (opposed to 'the thud' or 'the body'). When evaluating the bass drum sound alone, of course the low freqs sound more appealing, but in a complex mix tre treble spike can be useful (and can be restored with active equalizers if your source material is already processed as the drumcore or any other good sample) in order to make the bass drum part more intelligible. The trebles in the bass drum sound can help it cut through the mix, and even if it generally happens when you have a very busy lower range (double kick, bass guitar, strings including contrabasses...), which is unlikely to be our scenario,  I wouldn't discard trebles permanently as in very reverbed rooms or other cases of room acoustics making everything less intelligible, they just might come in handy.

[DrAlx]

I've used those same types of piezo shown in the picture and glued them inside my acoustic guitar under the treble and bass sides of the bridge.  I run them direct into the mic input of a recording box and can get nice sound in stereo (bass strings panned left and treble panned right) with no preamp (well the recording box is the preamp I guess).  Rather long recording here https://soundcloud.com/alex-lawrow/ziur-movements-ii-and-iii

There is a practical problem that I found with these piezos.   The piezoelectric material is sandwiched between a copper disc on the bottom and a very thin layer of aluminium on the top.  The piezos come with wires already soldered to them, but I found that the wire that is fixed to the aluminium breaks off really easily.  Actually its the aluminium that flakes off the piezoelectric material, and I found it impossible to solder on a replacement wire without burning more aluminium off.

So what I do is cover the side of the pickup with the wires in a big layer of epoxy resin so the wires can't rip the aluminium away from the piezo.  Use cable with a grounded screen on the outside.  I know that electronic drum kits use piezos as triggers for a digital sound and the main thing that is getting processed is the amplitude of the strike and the time it occured.  Its not quite the same thing as using one as an acoustic sensor.  Piezos can have very broad frequency response, so theres no issue with missing low end but rather lots of high end.

[jatalahd]

Unfortunately I don't have any detailed instructions to give for building this, but I just want to write a small comment regarding this topic in general:

If I would build this kind of box as a DIY project I would first try to use either the piezo disc as already described, a small electrect mic or a small loudspeaker driver inside a wooden box. Each of these solutions are much more cheaper than the pickup approach, which to me appears as a huge waste of good pickups.

The DrAlx's recording (good playing!) kind of proves that the bass response of a piezo DISC might not become a problem when plugged directly into a "recording box". It is a fact that the capacitance of the piezo driver forms a high pass filter with the input impedance of the following amplifier stage. If the capacitance of the piezo disk is large enough, then the input impedance of a normal recording equipment (but not a PC sound card) might be enough to keep the bass response clean. Bigger piezo disks typically have larger capacitance (because the larger area is directly related to capacitance), so if going towards this path, try to find a "big" piezo disk somewhere.

A small electret mic often have surprisingly steady frequency response even at low frequencies, so that is also worth experimenting. The electret mic requires an external circuit that provides a few volts of operating voltage to the mic. Examples can be found by searching the internet.

To comment on the spectrum analysis comparison between the piezo stompbox and some (emulated) bass drum signal: to me the piezo spectrum seems to contain only a random noise at the low frequencies, which would indicate an error in the frequency analysis. I don't think that a wooden box of the shown size could generate steady amplitude signals at the range 10Hz - 400Hz. Since the spectrum analysis typically requires a periodic signal, the validity of analysing a single hit should be questioned. Additionally the piezo disk senses the vibrations at a fixed location of the box, where signal reflections from edges of the wooden board randomise the signal.

[ashcat_lt]

I find it important to point out that 400Hz is just a little flat from the A on the first string 5th fret.  The fundamental of the fifth string (low A) is 110Hz.  Low A on a bass is 55, and the low B on my big bass is around 30Hz.  I have no trouble hearing (or seeing in analysis) any of those fundamentals coming from any magnetic pickup I've ever tried.  If your humbucker isn't passing your fundamentals, it's wired wrong.

[Ggg]

I'm goofing around with some sensors that came in the mail yesterday, so far I'm getting decent results by sticking one to the underside of a small plank of wood. It definitely does need filtering to get rid of the scrapey high-pitched stuff, but rolling of the frequencies above 70 Hz (or even lower) is giving me a good impression of a bass drum.

PBE6  - I would be very very  interested in anything you wanted tell me about the sensors and other circuits you've had success with!!!

Please post!!
Ggg

[PBE6]

I ended up going with a pretty simple opamp design. Here were the basic functions:

- fixed non-inverting gain stage to boost the incoming signal by about 20dB, with another resistor on a switch bypassing the feedback resistor to create a 10dB input "pad"
- a variable single pole low-pass filter with a range of something like 400 Hz down to 40 Hz
- another fixed non-inverting gain stage to provide makeup gain

All the input, output and gain filters were set to roll off frequencies below 20 Hz and above 100 Hz (I was looking for a dull thud kind of sound). The final version also had both a 1/4" line out and a balanced XLR out, so there was some additional circuitry in there to create the balanced out that I took from Rod Elliott's pages.

The person I gave it to hasn't used it yet but it sounded decent on my tests at home.

The box itself was a little cigar box, probably no more than 6"x4"x2", and I assume it had a piezo pickup inside. I would suggest that a simple piece of soft but heavy wood would make a better instrument if you're using a contact piezo sensor (which I have to admit are surprisingly nice sounding).

[Ggg]

I ended up going with a pretty simple opamp design. Here were the basic functions:

- fixed non-inverting gain stage to boost the incoming signal by about 20dB, with another resistor on a switch bypassing the feedback resistor to create a 10dB input "pad"
- a variable single pole low-pass filter with a range of something like 400 Hz down to 40 Hz
- another fixed non-inverting gain stage to provide makeup gain

All the input, output and gain filters were set to roll off frequencies below 20 Hz and above 100 Hz (I was looking for a dull thud kind of sound). The final version also had both a 1/4" line out and a balanced XLR out, so there was some additional circuitry in there to create the balanced out that I took from Rod Elliott's pages.

The person I gave it to hasn't used it yet but it sounded decent on my tests at home.

The box itself was a little cigar box, probably no more than 6"x4"x2", and I assume it had a piezo pickup inside. I would suggest that a simple piece of soft but heavy wood would make a better instrument if you're using a contact piezo sensor (which I have to admit are surprisingly nice sounding).

Ok! This sounds exactly what I'm looking for! I would like to use a low z mic rather than piezo. One snag...I'm not exactly a whiz with electronics, would you be willing to send me a schematic?

[PBE6]

You might have better luck with a dedicated mic preamp. Here's a link to a schematic for the famous $5 preamp:



http://m.record-producer.com/the-famous-5-preamp-everything-you-need-to-know

If you want to add tone shaping to the end of the preamp, you could try a Sallen-Key low pass filter:

http://en.m.wikipedia.org/wiki/Sallen–Key_topology

For simplicity's sake try keeping R1 = R2 and keep C2 anywhere from 0.5*C1 to 0.2*C1. A good starting point would be setting R1 = R2 = 22k, C1 = 100nF and C2 = 47nF. This will give you a cutoff frequency of about 100 Hz.

[Ggg]

You might have better luck with a dedicated mic preamp. Here's a link to a schematic for the famous $5 preamp:



http://m.record-producer.com/the-famous-5-preamp-everything-you-need-to-know

If you want to add tone shaping to the end of the preamp, you could try a Sallen-Key low pass filter:

http://en.m.wikipedia.org/wiki/Sallen–Key_topology

For simplicity's sake try keeping R1 = R2 and keep C2 anywhere from 0.5*C1 to 0.2*C1. A good starting point would be setting R1 = R2 = 22k, C1 = 100nF and C2 = 47nF. This will give you a cutoff frequency of about 100 Hz.

Okay so I'm going to lay it all on the line here. What would it take for you to build it for me? (everything except the enclosure). i'm in way over my head. Also I would not want any sort of preamp that would require power (battery).