Piezo and magnetic pick up electronics

Hi Jonny,

This topic is pretty complicated and I'm no expert. But I'll give you what I understand to be pretty much the standard circuit. Be aware that builders tend to experiment a lot, and the values of the components they use may vary from the ones I'll give you. Part of the difference depends on the "sound" you are trying to achieve, and part is what components you can easily lay your hands on. So, for the standard circuit, here's what you'll need:

1.) The value of the tone control potentiometer and its capacitor depended on what type of magnetic pickup you are using. If it is a single coil pickup, then use a 250K ohm potentiometer with a linear taper and a 0.20uF capacitor. If it is a double coil ("humbuckler") pickup, then use a 500K ohm potentiometer with a linear taper and a 0.47uF capacitor. You probably should also experiment a bit with the value of the capacitors to see how it affects the quality of the sound on your particular instrument and how well the tone control works.

2.) For the volume control, use a 250K ohm potentiomerter with a logarithmic taper if you are using a single coil magnetic pickup, and use a 500K ohm potentiometer with a logarithmic taper if you are using a double coil magnetic pickup. Sometimes the term "audio taper" is used instead of "logarithmic taper", but it is the same thing and this type of tape is required because of how the human ear works. I don't know the technical details.

3.) For piezo pickups, I've seen a variety of different values ranging from 250K ohms to 1M ohm. However, most people don't bother with an on-guitar volume control for a piezo pickup unless it is part of a built in pre-amp. The reason for this is that there is a big impedance mismatch between a piezo pickup and the guitar amp. There is not much signal strength to overcome this mismatch and much less if you put a volume control on the guitar. My recommendation would be to put in a preamp and use a volume control built in as part of the preamp circuit.

When driving a guitar amp with a piezo pickup without a preamp, you will have to turn the amplifier's volume way up, often to the point where you can hear the hum. A preamp in the circuit will allow you to better match the signal coming from the magnetic pickups and the signal coming from the piezo. Some people also like to use a "blending pot" when mixing piezo and magnetic pickups. There's a good article about blending pots at this URL: http://www.stewmac.com/freeinfo/Electronics/Pots/i-4137.html. The blender pot would be used instead of the 3-way switch in the circuit shown above.

I'd spend a lot of time reading up on these topics. There's a lot more to it than a simple recipe. You can use the search box in the top right corner of each page of Cigar Box Nation to hunt down discussions on these topics that I have just touched on here.

Good luck.

-Rand.

Replies

Jonny Chepe December 16, 2012 at 7:42am

What size is the cap on the tone pot?
- Rand Moore > Jonny Chepe December 16, 2012 at 12:45pm
  
  Hi Jonny,
  
  This topic is pretty complicated and I'm no expert. But I'll give you what I understand to be pretty much the standard circuit. Be aware that builders tend to experiment a lot, and the values of the components they use may vary from the ones I'll give you. Part of the difference depends on the "sound" you are trying to achieve, and part is what components you can easily lay your hands on. So, for the standard circuit, here's what you'll need:
  
  1.) The value of the tone control potentiometer and its capacitor depended on what type of magnetic pickup you are using. If it is a single coil pickup, then use a 250K ohm potentiometer with a linear taper and a 0.20uF capacitor. If it is a double coil ("humbuckler") pickup, then use a 500K ohm potentiometer with a linear taper and a 0.47uF capacitor. You probably should also experiment a bit with the value of the capacitors to see how it affects the quality of the sound on your particular instrument and how well the tone control works.
  
  2.) For the volume control, use a 250K ohm potentiomerter with a logarithmic taper if you are using a single coil magnetic pickup, and use a 500K ohm potentiometer with a logarithmic taper if you are using a double coil magnetic pickup. Sometimes the term "audio taper" is used instead of "logarithmic taper", but it is the same thing and this type of tape is required because of how the human ear works. I don't know the technical details.
  
  3.) For piezo pickups, I've seen a variety of different values ranging from 250K ohms to 1M ohm. However, most people don't bother with an on-guitar volume control for a piezo pickup unless it is part of a built in pre-amp. The reason for this is that there is a big impedance mismatch between a piezo pickup and the guitar amp. There is not much signal strength to overcome this mismatch and much less if you put a volume control on the guitar. My recommendation would be to put in a preamp and use a volume control built in as part of the preamp circuit.
  
  When driving a guitar amp with a piezo pickup without a preamp, you will have to turn the amplifier's volume way up, often to the point where you can hear the hum. A preamp in the circuit will allow you to better match the signal coming from the magnetic pickups and the signal coming from the piezo. Some people also like to use a "blending pot" when mixing piezo and magnetic pickups. There's a good article about blending pots at this URL: http://www.stewmac.com/freeinfo/Electronics/Pots/i-4137.html. The blender pot would be used instead of the 3-way switch in the circuit shown above.
  
  I'd spend a lot of time reading up on these topics. There's a lot more to it than a simple recipe. You can use the search box in the top right corner of each page of Cigar Box Nation to hunt down discussions on these topics that I have just touched on here.
  
  Good luck.
  
  -Rand.
the anonymous pick December 16, 2012 at 3:04am

yup .. do what rand said ...

then rip out the piezo ;-)
Rand Moore December 15, 2012 at 11:27pm

This reply was deleted.