Adding Piezos and A Preamp

My Carvin Bolt on the operating table.

I’ve had my Carvin Bolt guitar for just over 20 years now and ordered it new. The story behind it is I needed a decent guitar for the gigs I was playing in my band back in the day. The only good electric guitar I had at the time was my Carvin doubleneck, so I went to the Carvin factory showroom in San Diego and asked them if they wanted to buy my 1976 doubleneck or give me credit toward a new guitar–they did and I was able to order a custom guitar to my specs! Unfortunately 20 years ago having the option to have your electric guitar sound like an acoustic was not cheap and to looking back didn’t sound all that great.

15 piezo transducers cost $6.49

Fast forward to 2020 and peizo transducers are quite cheap. You can even find some pretty cheap piezo bridges, well under $100, but you will need some kind of preamp for the peizo pickups. Having a straight non-active setup like the factory electronics in my Carvin sounds crappy and just isn’t loud enough.

I have been experimenting with placement of these discs. On my fretless bass, I managed to sandwich one between the neck and the neck pocket of the bass and another one under the bridge. The problem I ran into was if I wired them in series with the pickups that were in the bass, even if they had their own volume, it just wasn’t loud enough to be usable in the real world. I would need two separate outputs so I could run each signal into 2 different amps or different channels in the PA.

After the piezos were in place, one stuck in the bridge pickup cavity and the other under the bridge, I needed a small preamp. One of the things I wanted to not do is cut up my guitar, even adding a battery door on the back or an extra jack to the side of the body. I ended up finding a teeny preamp which was reasonable priced around $30, wired it up using a second output jack and volume control. Check it out in the photo at the top of this page.

The Schatten MicroPre is really teeny!

Following Schatten’s directions for the most part, the preamp worked fine amplifying the peizo signal. But having it wired in series with my stock magnetic pickups was not the solution. I could either have the piezos on or the magnetic pickups on. Schatten also sells a dual input preamp, but I didn’t want my other pickups to be active. What I finally decided on was two outputs. This serves two purposes as an acoustic signal wouldn’t sound very good going through a regular guitar amp with distortion, etc., and the same goes for the magnetic pickups going through an acoustic amp’s settings. I could have both signals going out one output and using a pedalboard to control which I wanted to use, but if I wanted to blend the signals together, it would be tricky without two outputs. How would I separate the signal? Two outputs it is!

I also wanted to use a push/pull pot (potentiometer) because, you know, they’re so cool and it looks impressive when you pull that volume or tone knob out and adjust the signal. My push/pull pot could serve a couple different things. Since I had to lose the coil split switch in order to install a separate jack, I might use a push/pull pot to split the coil. Now that I think about it, I have never really liked or used that feature. Not as a rhythm guitarist anyway. Also on the chopping block was the tone knob. I’ve never been a fan of tone controls on guitars or basses so that left me room for a volume knob in the pickguard.

Exploded view of push pull potentiometer

My solution ended up being using the capacitor from the tone circuit and wiring it to the push/pull pot; similar to the diagram on the right. When the pot is in the normal, down position, the capacitor is in the circuit, rolling off some of the high frequencies. If I want a brighter signal, I can pop the knob (pull) up and now it’s brighter because that side of the switch doesn’t have that capacitor. I can always add a capacitor or fool around with different values of things to get exactly what I want.

One question you may have is why not use a stereo (TRS: tip, ring, sleeve) jack? That way the magnetic pickups could be isolated from the piezos and run into different channels on the PA or different amps. Well, I am using a stereo jack…and a mono jack. Why? The ring switch on the TRS jack turns the preamp circuit on when a cable is plugged in. That way my 9V battery lasts longer and I wouldn’t have to worry about a thump when the preamp turned on. Plus it’s a pain to get this pickguard off and I hate changing strings. That’s one of the reasons I became a bass player. Fun fact, Donald “Duck” Dunn reportedly never changed the strings on his famous P-Bass. But I digress.

I got it all crammed back under the pickguard, trying to be careful when cramming it in there, testing each circut’s signal before each part of assembly. Once I got the strings on, the peizos weren’t working anymore. That’s where this ends for now, until I can get back to it and see what came loose, manage the wires along with the preamp board and 9V battery all in the same cavity as it were.