active to passive piezo conversion- created 11-27-2009

lee, jimmy - 11/27/2009.17:59:31

I have a Steinberger synapse transcale guitar that originally came with active emg pickups and and active piezo pickups with eq. I have pulled the active emg's and installed passive pickups in it. I'm getting the sound I want so far from the passive pickups. They are hooked up to individual 500K volume controls and a 3 way switch.

Now I'm wondering if there is a way to utilize that piezo that is sitting under the bridge and doing nothing so far. I've already removed all the active electronics that unfortunately could not be salvaged.

I was messing around with the piezo today. I hooked it up to a 500k volume pot just to see how it would sound, and to my surprise not so bad. It however quite BRIGHT.

Should I be matching the impedance somehow by choosing the correct pot value for the piezo?. If that's the case what range works when used in a passive configuration?.

Would I be able to wire in a tone control just like a regular guitar pickup to maybe tame that bright piezo sound a little?. what would be some good values to use for the pots.

I'm not really looking for a convincing acoustic sound from this pickup, just another sound different from the magnetic pickups.

You are right with it being an impedance issue. Piezos have a very high impedance, and need to see a very high one in order to get any kind of frequency response out of it. Lots of times they will work fine going into a tube guitar amp, but transistor ones will be an issue, tone wise.

Kevin O'Connor has an acoustic type of preamp in his book "Tonnes of Tone". I built on into a small box, salvaged a broken quarter inch plug, and made it so that it plugs right into the end pin jack. That way I didn't have to cut holes in my Martin, and I can always try a different circuit whenever I find one.

Here are a few shots of it so that it's a bit clearer.

This first one is of the whole thing, with the pot unscrewed and lifted out of the box so that you can see the board and the battery snap.

This is one of the front, so that you can see how the plug sits in the cover of the box. This plugs into the end pin jack on the guitar

And this final one is so you can see the other side of the plug. I cut off a short section of the barrel of the plug for a nut. It seemed easier than finding one in the junk box.

One thing about this one, though, make sure that you use his choice of transistor (it's an FET, but I don't recall the number, right off hand. NTE replacements are NOT acceptable, and will fry quickly). All in all, it's a really inexpensive circuit, with just a few resistors and caps, a battery clip and some jacks. The box will cost you more than the whole circuit.

Can the output from that circuit be mixed with passive magnetic pickups?. I intend to install another 3 way switch on my guitar to select active, passive or both.

where can I find a layout diagram for this?. I've never built anything electronic before.

Generally, I don't find that it's a good idea to mix passive and active. It's once again a matter of impedance. You can't mix highs and lows, and actives have lows, passives have highs. It's a really complicated topic. A pickup is a coil, and there is more going on in them than you can imagine.

Building stuff isn't hard, but you do have to be careful about polarities and parts. Oh, and picking up the soldering iron from the right end (most people don't make that mistake more than once).

Get a copy of Craig Anderton's book "Electronics Projects for Musicians" and read the first several chapters before you pick up an iron. He explains everything you need to know pretty well.

As to this circuit, it's in Kevin O'Connor's book "Tonnes of Tone", which is a fun book. It's all projects, and he also tells you how to go about soldering and stuff. It's possible that either or both of these books would be in a local library.

Will, I don't know if you are referencing Tom Tillman's ubiquitous little FET "Preamp in a Plug" circuit - that thing turns up everywhere on the web and in circuit notebooks - but here is a link to the circuit itself for Jimmy:

Happy Buildin'.

I have used this one with a blend pot in my bass guitars and it worked for my ears...

Alan: No, it's not the same circuit, but thanks muchly for putting that up. I'm going to have to try that one, too. It looks quite easy and I like putting it right in the cable like that.

Michael: Thanks as well to you for putting up the link. I'll probably try that one, before too long, too. I'm always looking for a better way to do things like this, and that one looks pretty good, too.