Lowering output gain from active bass

Why not adjust the trimmer pot on the bass that has one? You're trying to adjust the one that is not adjustable.

You'll need to add a trimmer pot.

 

Ok, that makes sense.

Try a 100k trimmer in series with the signal path, after the preamp. Take the connection that goes to the jack, and wire it to either side of the pot, and then connect the wiper terminal (center) to the jack. Leave the third terminal of the pot open, unless you cannot attenuate enough. If that is the case, ground it.

 

Why not just turn the volume knob down?

 

I'd replace the preamp, but that's just me. Lowering the volume knob is not precise and impossible to set it exactly the same twice, plus it can get moved without you knowing it. It could also affect your tone, as would adding a trimmer pot. All of my basses now have preamps with trimpots.

 

A trimmer pot may alter the tone in a good way,
if it alters it noticeably,
or may need a small value cap in series with it to retain tone.

 

If you can trace the circuit a little, we can try to find the gain set resistor and replace it with a trimpot (and a serial resistor).

So, first, see what kind of opamp is it (probably a dual one). Then you'll try to find out which one of the two stages is the gain stage. Acording to this info we'll try to find feedback (=gain set) resistor.

 

It looks like the preamp is glued into the cavity pretty solidly, so doing a trace could be difficult. I'd worry a little about pulling clad off the board by trying to remove it.

 

You're right. If it's glued solidly, trying to remove it would damge the PCB. It looked like a double sided tape to me.

Anyway, ShowKing asked info on doing it. I tried to guess some info on electronics. He'll let us know about the mechanics

 

I would add a 100k trimpot after the preamp, wired like a volume control. It should work well, with no tonal changes. EQ tweaks like treble bleed circuits are relevant when dealing with high impedance, but the output of the preamp is low impedance, and a simple volume control you can preset will be fine.

Disconnect the output wire from the preamp, either at the jack or the preamp or cut the wire. From the preamp connect to the end terminal of the trimpot. The wire/connection to the jack goes to the middle terminal, and the other outside terminal goes to ground. If the trimpot works backwards, swap the outside connections, but who cares. You will be setting this and forgetting it's in there.

A 50k trimpot would probably be OK too.

And/or, lower the pickup(s) some on the hot bass, and raise the pickup(s) on the not-so-hot-bass.

 

JOOC, what's the bass? What's the pre and is it stock?

Riis

 

The difference is 50k Ohms. Pots come in hundreds of different values, but you need one with a range that is appropriate for the signal impedance. I suggest 100k, because it will allow you to attenuate more, if needed. A 50k pot will not provide the same range of attenuation, but it will spread its range of control throughout the rotation, allowing you to better fine tune the level of attenuation.

The trimmer pot should be of linear taper. Do not use log taper or reverse log taper pots, because they will be very difficult to fine tune, since small rotations cause large jumps in value, at particular points.

Note that I suggested leaving the ground off of the trim pot, unless you cannot attenuate enough with the value you choose. This is because is is preferable to avoid loading the signal with parallel resistance.

 

Since we don't know the output impedance of the pickup, I suggested 100k. Wired as a volume control (voltage divider) it will go from full output down to zero. With a 50k, as long as the impedance is low enough, will act just like the 100k.

You would have a hard time finding trimpots that are anything but linear taper.