Saddle Piezo Pickups Filtering

[McSeem]

Graphtech saddle pickups produce great, very detailed sound. However, the major disadvantage is a huge low frequency noise, when you just slightly touch strings. With the standard connection and Graphtech preamps it may even overload the input. So, it would be great to filter it out. The problem is that the piezo crystals have very high Z, about 10 mega ohms or so, basically they require FET based input. What I want to do is a high pass filter at the input. I want to cut off everything lower than 40-60 Hz. When I connect about 1 MOhm resistor in parallel, it significantly damps high frequencies, which works as a low-pass filter and spoils the whole idea of saddle pickups, so, it may be tricky to have a high pass filter. What kind of circuity could it be? Of course, I'll try a simple RC, I just want to hear about your opinions.

[psw]

Why do you need to put a filter before buffering the piezos? If you run it through a simple buffer you'd get a low impedance and they you could filter as you pleased.

There are some good DIY piezo circuits around, JohnH form GN2 and here gave a good circuit with the filter graphs and specs for instance...might be of some help.

[McSeem]

The signal level is rather big with piezo, so, I checked, when I heavily tap the strings, the low freq signal can overload the buffer. I want to filter it out before any buffers.

Can you give me some links to JohnH's works?

[Ripthorn]

If you insist on doing it before a buffer, you just need to make it so that you have a really high impedance filter. This means large resistance and/or inductance and small capacitance (I think that's how they are ordered still). My only concern is that going all passive, having a filter that will DRASTICALLY reduce the frequencies less than an octave below what you want to keep is going to require a several-order filter. If you assume a standard butterworth topology, you are looking at 6 dB/octave per order. If we assume you are only getting 2.5 dB of reduction per order at the frequencies you want to get rid of (about half an octave below), you will need probably a 4th order at the minimum, is my guess. That means you will need inductors and capacitors (since you have more than one order) and inductors get really bulky really fast, especially for anything that will give you a cutoff frequency as low as you are talking. But do look into it and let us know how that goes.

[McSeem]

Yeah, you are right, I've just checked a simplest RC filter. A 330pf capacitor and 10m resistor doesn't have any noticeable effect. I even tried 150, 68, and 10pf. In the last case it works somehow, but I also get a huge signal drop. There definitely must be some high order filter, which too much hassle in my case. But after more careful experiments - it's not that bad, and my assumption was wrong. The crystals do not overload the input. My overload was in software mixing, where I get clipping. This is how it sounds:

http://antigrain.com/hex_project/sample_piezo_noise.mp3 - I just damp the open strings with my hand. Don't worry about the white noise, because there're just extreme conditions, with about 45db of extra gain before mixing. So, I'm sure I can filter it out in software, with a steep IIR filter before further processing.

[psw]

Can you give me some links to JohnH's works?

JohnH DIY Piezo circuit and details

This circuit was designed for a specific guitar of course, but sounds fairly decent for the "buzzer piezo" element kind of thing...it's mono of course, but gives you much detail about the filtering and such.

Piezos can have a huge dynamic range, but I was surprised that you'd be trying to alter the piezos before buffering and likely affect the ghost preamp system if trying to mess with that end of things also.

I considered doing a piezo thing in my new guitar, it even has a piezo element built in, but the preliminary sound was not that great and in the end I got a $50 Acoustic simulator pedal that sounds as good as a lot of DIY piezo things I have heard and in many ways more versatile as well...can plug any guitar into it for instance...so abandoned it at least for this guitar.

Along the way, I also use Kahler bridges and trems, so it was in part a project to show how you could piezo up these bridges...there are lots of design features that kind of lend themselves to this kind of thing and even DIY hex systems as well...when, I got word that graphtech are coming out with a midi compatible ghost system for this bridge in the near future. All piezos, and in the bridge even, are going to be subject to handling noise, especially in muting on the bridge and with a trem, possibly springs and mechanical noise showing up...so, for me perhaps not the best way to go and obviously a vast amount of work to DIY and costly with the new systems as well...

[Fresh Fizz]

I don't see a simple solution. Since you are the programmer over here you could write some program that does the following:

1.Send the signal through a low pass filter (24db/octave).

2.Then the signal goes to a comparator. If the signal from the low pass exceeds the reference voltage the comparator's logical level changes ( 0 to 1 or vice versa).

3. When the comparator's logical level is changed a noise gate will switch off drastically reducing the signal.

So what you've done is create a side-chain (1,2). Only the noise gate (3) is in the signal path. Of course you don't need to build this in 6-fold. You mix the signals from the 6 piezos into one signal.

By the way, somewhere else you mentioned some other problem you have with silencing the 'unplayed' strings. If you send some tiny portion of the overall mixed signal to all of the 6 sustainer driver amps it could be just enough to snow under the process of spontaneously developing sustain on 'unplayed' strings.

Hope this helps a bit

FF

[psw]

If you tie a sock around the nut end of the guitar, you get the "moog banjo" effect with a conventional sustainer, as long as you don't play open strings! sorry, couldn't resist, an old recording trick, sustainer or no to keep things quiet...

[Fresh Fizz]

Of course you need 6 noise gates (3), one for each string. No need to build 6 separate side-chains. But on the other hand, why not?

Cheers

FF

[JohnH]

Pete - thanks for pointing to my GN2 thread

I also initially had huge subsonic pulses to deal with, so that if you pressed the bridge the signal would die for a second or more. The thing is, when you press a piezo, it makes a charge, and if you press it alot, its a significant level, larger than the music signal and swamps it. The nature of it is like the piezo is acting as a capacitor, and generating charges across its plates. I think the best way to control these unwanted very low frequency signals is to bleed them off, by lowering the input resistace of the preamp. My piezo (based on a buzzer) has a capacitance of 20nF, so 150k gives a roll off from about 50Hz and below. Other values would apply for different piezos. This fixed the problem in my case and has no effect on the low musical notes. So I think that the idea that very high input impedances are always needed for piezos is not always true. It would only be so if he piezo capacitance is very small.

John