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Piezo Volume, Post Preamp


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I can't seem to figure this out. Let's say I have an endpin preamp for acoustic guitar. Like this one...


And let's say I wanted to add a volume pot. Hypothetically, let's assume this preamp is installed according to its design, so my volume control would be external. What value pot should I use? My gut is telling me 100k, but I really have no idea.


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Do you mean a volume pot between the endpin jack and the mixer/amp it's connected to? Depends on the output impedance of the preamp (details on the GFS website are a bit thin on the ground), but likely something in the vicinity of 10Kohm - 100Kohm will work. Pots are cheap - experiment.

If you mean a volume pot hanging off the pickup itself and before the output jack (ala normal electric pickups), not the best place for it. Piezo pickups work best when Pickup -> buffer/preamp -> volume/tone -> outside world.

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What curtisa said. At their most basic, pre-amps buffer the weaksauce high impedance output of a pickup and offer a meatier lower impedance output. The reason passive magnetic guitar circuits use 250k/500k pots is that these are sufficiently high values to not "load" or expect too much of traditional pickups' limited grunt. Piezos are even weedier than that.

Choosing the value of a pot after a pre-amp is one of the factors in defining the guitar's final output impedance. Lower is generally better (for many reasons) however too low provides no real returns and too high somewhat defeats the point of the pre-amp.

The values curtisa provided are ideal and intelligently chosen. 100k is just lower than the lowest common "passive pot value" and 10k is on the lowest end of the scale seen in active circuits. EMG and Seymour Duncan active circuits use 25k pots for example, and these are hot quiet circuits for driving long cables. These are all after the pre-amp of course which doesn't help with these endpin jobbies....

Graphtech's GHOST system recommends 5MOhm pots for passive (so as not to totally load the piezo) so depending on the input impedance of the pre-amp you should experiment with values in this kind of range between your piezo and the pre-amp. Too low and the pickup's tone will be loaded and sound crap, too high and the pre-amp will become a case of crap-in-crap-out.

Alpha (Taiwanese pot manufacturer) make wheel pots which are those usually used for soundhole-mounted pots. These would work great in an acoustic if you can find a supplier that does them in a sufficiently high value. This is as good as I can find from Mouser (1MOhm):


Personally I would be tempted to see if the endpin pre-amp can be disassembled so that you can splice in a more readily-available low value volume pot. Chance are the PCB in the jack will be solder mounted directly to the connections making a splice near impossible. Time to consider buying a volume pedal? ;-)

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Thanks for the responses. The volume will be after the preamp. I'm aware of the issues you guys mentioned. It's actually all going to be internal. I'm blending a piezo with a mag, and I need a buffer/preamp. I'm build one, but these are so cheap I'd rather save the trouble. I'll try to open it up and see what's going on in there, but it seems to me that if I just go piezo -> preamp -> volume -> switch, and put a switch in the ground line to the battery so it doesn't constantly drain, it ought to work. I'm open to other suggestions if anyone has any.

I guess I'll just start with something between 25k-100k and see how it works.

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David, you've got a PM. Sorry it took me so long to get back to you, haven't been around these parts too much lately. I have done basically what you have done and used a 100k trimmer to balance the magnetic and piezo signals together when blending.

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