Piezo Pre-amp + volume pot problem

[Jack.Straw]

Hello. I'm building an electric upright bass (pic below) using the circuit pictured below. Without the volume pot it sounds really good. When i introduce the volume pot the output gets reduced quite a bit. Full volume on the pot is less than half the volume of the circuit with no pot at all. Is it because i'm using a linear pot?

The circuit:



The bass:



Thanks for any advice you can offer,
-Jack

[DavidRavenMoon]

That's a fun looking bass! What are the strings made from?

Your output should be from the Drain, not the Source. By putting the pot where you have it, you are reducing the gain of the preamp by putting a resistor in parallel with R2.

So the end of C1 that you have going to the output should go to ground. The side of C2 you have going to ground should be going to the volume pot.

That should fix it. I make similar circuits as this, and R2 is the gain setting resistor.

Here's a circuit I found somewhere that has a similar layout as yours, and you can see where the two wires should swap:

[bongomania]

Very cool-looking! Be sure to post about it over in the EUB forum too, if you haven't already.

[DavidRavenMoon]

I can't take any credit for that circuit... I'm not even sure what web site it came from.

But when I was looking over your circuit I wanted to double check what I thought the problem was, and found that file in with my notes on FET circuits. But it's a fairly common type of circuit.

I use a variation of that type of circuit without the volume control.

[bongomania]

Oh sorry David, I meant the EUB was cool-looking. Of course your circuit is cool too, and as a student EE I can appreciate the simplicity of it.

[DavidRavenMoon]

Oh got cha... yeah the EUB is cool.

His circuit is almost the same as the one I posted... it's a text book FET buffer amp.

[Jack.Straw]

David, thanks!! I'll give that a try tonight. The strings are made from different gauges of weed wacker (trimmer) line