If anyone out there is building your own piezo pickups, feel free to post your ideas and experiences on this thread.
I have always just mic'd my classical guitar when playing live. But I finally got sick of the feedback problems. I found that I usually needed a little more gain than I could get before feedback, and also It became too tedious having to keep the guitar-mic position in the same spot for consistency of sound.
Anyway, I did some research and built this piezo set-up.
It is a unity-gain op-amp running on two 3v batteries encased in a cheap Hammond box. The piezo element is something I scavenged from a alarm notification device and attached to the guitar body with sticky picture tape.
Here it is mounted to the guitar.
The tone of the guitar through the PAS with Preset 00 is not very good when the low, mid, and high setting on the Bose controller are set flat (all of them at 12 o'clock). But, when I turn the highs all the way up, the mids all the way down, and the lows all the way up the tone in my opinion is very good.
There is no eq section in the piezo pre-amp.
I think I incorporated a Sallen-Key type high-pass filter design in the op-amp circuit, but I would have to find my notes to be sure about that.
It was simpler to design the preamp without an eq section since the preamp high/low/mid controls on the PAS remote served to shape the tone. What do you guys think about that? It's a bit strange to be having to max out the high and low controls while turning the mid control all the way off, but that's how is sounds the best. Would it be better to eq the signal prior to the PAS, or is there a preset that might have an eq curve that would work better?
It's true about the PAS making every little element of sound going into it very apparent. There is a small bit of harsh clipping that will occur with this pre-amp when I play very aggressively, it's kind of like the sound of a record-needle hitting little bits of dust; just sort of an annoying tick'ish sound. If I play this preamp through a guitar amp, the sound is not even noticeable. But through the PAS it's as clear as a bell, but much more bothersome...I'll have to fix that.
Due to the overwhelming response to this thread I thought I should give an update on the cause of the annoying distortion artifact.
It seems the two lithium 3v batteries do not offer quite enough headroom for the signal, especially when the batteries are a bit decayed from use.
I have now modified the black box by deleting the 3v lithiums, replacing them with two 9v's attached with the same beautiful painter's tape to the exterior of the case.
This arrangement has eliminated the problematic clipping sound, resulting in an absolutely pristine and angelic guitar sound.
You may also wish to note that N style batteries available from Rat Shack are a nice "small" 9 volt cell. Roughly 1/2 length of a AA and slightly smaller diameter.
I previously used them for mic preamps that I used to build for tight space onboard applications. 9V N battery has pretty decent mAh rating and commonly found in electric lighters.
I'd suggest adding some EQ capability too or simply "tune" the response to the individual instrument so that you don't need to do much external tweaking.
Thanks ASAT. I'll check into those. The big 9v batteries taped to the outside of the case looks like I've got a a bomb stuck on my guitar.
Eventually I might try to power the thing with the phantom power available on the PAS.
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