Modifying output level on an acoustic guitar piezo preamp.

[VanGenz]

I have quite a lot of different acoustic instruments (6 & 12 string guitars, octave mandolin, weissenborns, oud etc) that I use for live playing. I use a Boss GT-1000 (for all the instruments) and had a plan for making different patches for the different instruments and setting the gain level for the instrument within the patch. But this became to complicated (to many patches). Now I have a few different patches where I can load different effects.. On my main live acoustic guitar (an old Taylor with a Fishman piezo preamp) the level is set to 12-13 o'clock (around 50-60% of max. On some of the other instrument the output level is so hot so the level is maybe just 10-15% of max and setting the correct level is difficult. I am thinking of putting an attenuator (trim pot?) after the preamp so the normal/right level is around 50%. Maybe mount it on the enpin jack inside the instrument? Anybody have seen a solution like this? What value should a trim pot have?

 

[Tubetec]

You could make up instrument cables with a preset pot adjustment at the amp end ,
Clearly label everything , so each instrument has it own specific cable ,
Anything without a preamp , ie strip only probably wont need attenuation ,

 

[ccaudle]

I am thinking of putting an attenuator (trim pot?) after the preamp so the normal/right level is around 50%.

You could also trace out the circuit and find a point to reduce the gain. With most common circuits you could do that with a resistor change.

What value should a trim pot have?

For an attenuator after the pre-amp circuit it will depend on the input impedance of the Boss unit and what the pre-amp is capable of driving (since the attenuator pot is in parallel with the Boss input).
I just found the specs of the Boss and it is 1M, so should be OK with a normal solid state preamp to use something like 10K or 20K ohm pot for the attenuator.
You could try something like this:
Bourns 1 turn cermet 20K trimmer

I would solder some wires to the contacts, then glue the trimmer down on the edge of the preamp board in the guitar.
If you want easier to access you can find something similar with a thumb wheel.

 

[VanGenz]

You could make up instrument cables with a preset pot adjustment at the amp end ,
Clearly label everything , so each instrument has it own specific cable ,
Anything without a preamp , ie strip only probably wont need attenuation ,

I have been thinking of this solution but its not very "practical" to have this extra cable inserted into the instrument when you quick change instrument between songs, sometimes even when I speak.

 

[VanGenz]

You could also trace out the circuit and find a point to reduce the gain. With most common circuits you could do that with a resistor change.



For an attenuator after the pre-amp circuit it will depend on the input impedance of the Boss unit and what the pre-amp is capable of driving (since the attenuator pot is in parallel with the Boss input).
I just found the specs of the Boss and it is 1M, so should be OK with a normal solid state preamp to use something like 10K or 20K ohm pot for the attenuator.
You could try something like this:
Bourns 1 turn cermet 20K trimmer

I would solder some wires to the contacts, then glue the trimmer down on the edge of the preamp board in the guitar.
If you want easier to access you can find something similar with a thumb wheel.

Thank you for good suggestions.