My DIY piezoelectric pickup and preamp sound

[ohnoitsalobo]

Here's a link to sound clips of a few instruments recorded using my piezoelectric contact microphone and preamp that I built. I have a good ear, and I think it sounds good, but I'm curious how it stacks up to more experienced ears.

My biggest validation was playing (viola) in the orchestra for a TEDx event, before the lockdown, where all the violins and violas were amplified using AKG C411 contact mics.
I just used my pickup/preamp, and I think it sounded comparable to the C411, if not better.

Side note: I recently found out that, while using my preamp, I can generate a signal as low as a few Hz by shaking my viola. So that tells me the bass response is pretty good - and absence of bass is the main complaint with piezo mics, lol. This low frequency output could have the potential to clip the signal or damage speaker outputs, luckily it is easily avoided with a high pass filter which is standard on most mixers.

[sounds_good]

Thank you for doing the tests.

In raw mode how is the pickup connected to the recording system?


Thanks,
sounds_good

[mandroid]

You got just the brass disc with the Piezo wafer on it & soldered on the wires?

[ohnoitsalobo]

In raw mode how is the pickup connected to the recording system?

All recorded using a M-Audio Fast Track Ultra. "Raw" audio is the piezo directly connected to the line input, while the "preamp" audio uses the mic connection for phantom power.

You got just the brass disc with the Piezo wafer on it & soldered on the wires?

Pretty much. Cheap chinesium ones, like these. The magic happens in the preamp, the page has links to the circuits that I started from.

[sounds_good]

All recorded using a M-Audio Fast Track Ultra. "Raw" audio is the piezo directly connected to the line input, while the "preamp" audio uses the mic connection for phantom power.



Pretty much. Cheap chinesium ones, like these. The magic happens in the preamp, the page has links to the circuits that I started from.

The input impedance of that M-Audio box is way too low for a piezo transducer.
It loads the piezo so it cannot drive an adequate signal level. I think all DI
boxes designed for piezo use have pre-amps in them. It seems you already
have a pre-amp set up. For those considering piezo pickups (and I would)
who are less interested in soldering I suggest buying a commercial DI box
or pre-amp made specifically for piezo use.

Frankly, while I like the effort, the comparison recording you are doing
with the load mis-match is a waste time. I think it would be interesting
if you experimented with the location of the pick up on the instrument
to see if there are locations that are noticeably better than others.
Maybe you already did that.

[ohnoitsalobo]

I did experiment a little with placement, but most of the instruments aren't mine so time was limited. I went with the simplest guaranteed outcome - on the body close to the bridge. Except for the kalimba, which got way too much finger noise - I moved that much closer to the sound hole.

The M-Audio instrument inputs are 1 MΩ according to the manual, which is similar to a lot of stage and recording DI boxes. That's what I used here, it seemed pretty representative.
That said, I've used my piezo with commercial DI boxes (as well as seen others do the same with clip-on and other commercial pickups), and they all still have the same problem of dropping the bass - they can't handle the piezo even with supposed 1 MΩ impedance, and end up sounding "quacky" and shrill (BSS AR133 is a commonly used one where I am). Most mixer line ins are much less than 1MΩ, so of course the problem only gets worse.
I have a friend who has the Fishman V200 pickup, and it has the same problems despite its price tag (but the fact that it's a bridge pickup is also responsible for the lack of good "body", I feel)

The only inputs I've seen that handle piezos well are rated for 10 MΩ or more, and they're surprisingly rare. The only one I've had the chance to use personally is the Fishman Loudbox and it sounded pretty good.
My preamp also has the added benefit of balanced audio, which doubles the signal amplitude and rejects noise, so that helps plenty. I haven't seen very many commercial piezo preamps that do this.

[pops1]

All my preamps have balance audio to the PA. I prefer 1M input to 10M. For mandolin I don't want a lot of bass below 250 hz. For guitar either, as it muddies up the sound in my opinion.

[ohnoitsalobo]

For mandolin I don't want a lot of bass below 250 hz. For guitar either, as it muddies up the sound in my opinion.

To each their own, of course.
My piezo and preamp is meant sort of as a one-size-fits-all, which means it needs to have full response especially in the lows. After all, if there's too much low frequency you can always cut it. You can't add lows that aren't there though, which is the main problem with most piezos and unmatched preamps.
(I haven't had a chance to test it on a double bass yet, but I'm optimistic about the performance given how well it performed on my modified guitalele in the 'cello range.)

[pops1]

My preamps have plenty of lows, and the low control on most PA's is shelved at 80 hz. which is not the rolloff I want. I would like a control or a pre for mandolin that rolls off the 250 hz and below. Usually it is not a problem as I am using a RedEye pre and it sounds great and I usually roll off the 250 slider on the board as most rooms have excessive response in that frequency anyway.

[ohnoitsalobo]

Yes I agree about yours, and that's good. But your pickup is yours, and good pickups are very expensive in my country so most people use whatever they can afford, which too often are the cheap piezo ones that don't have preamps. Like I said: you can always cut the excess on higher-end mixers or in the computer, but you can't fix what doesn't get recorded, which is the main problem with affordable piezo pickups with no preamps - often all the lows get cut off, sometimes as high as 1kHz or more.

In fairness most mandolins produce relatively little sound below 250Hz, since the low G is 196Hz and the power of the sound is in the harmonics above. You can hear mine in the recordings, unedited; the lows aren't excessive at all. That's usually a room problem, not a pickup/preamp problem.

Mainly, my pickup is meant for more than just mandolin, so I designed it to have the lowest possible bass response. The ideal scenario being that I could use the same pickup design all the way from double bass range up to violin range and get good performance, which I think is possible based on my testing. I even designed it to be safely removable / reusable on different instruments.

[sounds_good]

I do not have an M-Audio, but I was able to find the user manual for
the Fast Track Ultra. It specifies the input impedance as 20 kOhms.
That feels realistic, and I think it is an issue for piezo transducers.
Perhaps your box is different. Since you are electronically adept
you will be able to measure the input impedance for yourself.

Most DI boxes are about 100kOhms I think. I have some Radial
Engineering DIs. The passive boxes are 140 kOhms and the active
boxes are 220kOhms. I have a Radial PZ-Pre with 10 MOhm input
I use for the fiddle players. It is designed to go to the board, and
has balanced outputs.

I think it is right that most pre amps have single-ended outputs.
I think most are designed to be hung on the musician and connected
through a DI box to the board. Naturally they would use a guitar
cord to connect to the DI box.



I hope you will do some more measurements. Maybe we learn
something.

[ohnoitsalobo]

I do not have an M-Audio, but I was able to find the user manual for
the Fast Track Ultra. It specifies the input impedance as 20 kOhms.

A little further down the page it shows the instrument input impedance as 1 MΩ. That's what I used for the raw piezo recording. I don't know about electronically adept - I don't have the tools to measure a lot of stuff. Just a very basic multimeter, which isn't really enough for advanced stuff.



At gigs where I am, people either plug
1) directly into the mixer line in (impedance usually well under 50 kΩ) or
2) into this specific DI, the BSS AR133 which has a input impedance of 1 MΩ.
Invariably, the problem with piezo pickups remains the same - loss of bass and most of the mids.

I like the look of the Radial PZ-Pre, it definitely must sound good - but that's way outside my budget and a lot of my friends lol (exchange rate 75:1 USD, plus import tariffs). Good to know for future reference, though.

[pops1]

All peizo pickups need a preamp to sound good. A preamp is really the wrong word as the pickup itself has plenty of power and doesn't need a boost. What it does need is an impedance match, and that is what the preamp does. Yes ohnoitsalobo is correct that plugging straight into the board will loose lows and sound thin and terrible. A tube amp with 100K input impedance will not sound too bad.

I think the piezo got a bad rap from the old days when we started using them but there were no preamps. They didn't sound that great and I would use them mostly for the monitor so I could only put the mic in the mains and get more volume. I didn't care, well I did, about how bad it sounded in the mains as long as I could hear myself. There are a lot of options these days and better sound with them.

[Monte Barnett]

This might help: http://scotthelmke.com/Mint-box-buffer.html

Mismatched impedance will steal the body & breath of the signal. The Fishman Loudbox Artist specs show a 5M impedance on the 1/4" inputs. I have yet to find a piezo that needs a preamp when plugged into the older Loudbox 100 I have.

[ohnoitsalobo]

I actually sort of love the Mint Box Buffer. I might eventually make some of those, separately. I tried a couple of transistor circuits but I had problems with noise, so I decided I wanted to make a balanced signal and just sidestep the need for shielding.

I made mine with op-amps specifically
1) for the balanced signal, because I get literally zero noise. Balanced signal is a lot easier with op-amps.
2) to run from phantom power because I was tired of batteries, and there are very few performance venues without phantom power available, and if phantom isn't available you can still use it as a plain piezo in a pinch. Also phantom power means I have room for a much stronger signal without distorting, as compared to a battery.

[pops1]

My preamp can use phantom power, which I use most of the time. It also can run on the internal 9v battery inside. Some pre's need 48v phantom, which not all systems have, mine simply needs the 9 or 10v similar to the battery. I like having the battery as I can always use it. I have one that only uses phantom and would have to make sure I had 48v phantom or bring along a power supply to power the pre.

[cartershilts]

Sounds great! If you're looking to do a little experimentation you should try running an impulse response before your preamp. There's some great IR loaders out there like the Nux Solid Studio and Line 6 HX Stomp, and there's a bunch of vintage mic IRs available for free online. I've been running a K&K Twin through a Beyerdynamic M260 IR into a Fishman Platinum Pro and have been really happy with the results. Might be a new avenue to experiment with!

[ohnoitsalobo]

try running an impulse response before your preamp

I'm not sure I understand, can you describe what the signal chain would look like?
Right now my setup for recording is piezo pickup --> preamp --> M-Audio.
Putting the preamp anywhere else defeats the purpose it was built for, so I'm not sure how I'd use the IR before the preamp.

I know what IRs are conceptually, I'm not an audio engineer though so I've never used them. Also, I have kind of a shit computer, if you're talking about experimenting live through the DAW, it wouldn't be possible. :-/