Active Bass Hiss problem

[pablogilberto]

Hello!



I'd like to ask help on troubleshooting Active Bass with on board Preamp High Frequency Hiss problem.



I hear this high frequency hiss when the amp volume is high.

It gets noisier as I turn the mid high and treble knobs at full position.



I'm sure that the amp and cable is good because the hiss disappears when the bass is disconnected or another instrument is plugged.



The control cavity is shielded.

The metal parts are all connected to the ground.



Any tips on how to find the problem and what is the possible cause?



I will attach an audio file for your reference.



Thank you!

[newey]

Sorry, Pablo, but I had to move your post to here, as it was not germane to JohnH's piezo pickup thread. I don't have any answers to your issues, but someone will be along.

EDIT: Having moved your post, I now see that it duplicates a thread you already started here in Guitar wiring. I have removed the other thread to avoid duplication.

[thetragichero]

is this an off-the-shelf preamp or a home brew preamp or active pickups with preamp embedded in layers of epoxy?

any noise already in the bass will be amplified by a preamp. if it's a boost/cut type circuit then neutral is in the middle of the knobs and of course you'll get hiss if you're amplifying high frequencies by another 15db or so

resistors cause hiss so metal film is a must for this kind of situation. adding to that, fets (and fet-input op amps) tend to be noisier (but with much higher input impedance) than bjts (and bjt-input op amps)

i have used this in two basses to good effect. you can still get hiss with the treble control up but it's also got a decent enough volume level that you can turn everything down:

www.freestompboxes.org/viewtopic.php?t=29425

[pablogilberto]

Thanks!

This is actually a built in on board preamp from a Musicman Bass.

Passive Pickups.

I hear the hiss even if all the knobs are at 12NN

Can you recommend any troubleshooting guide, tutorial or lesson about this?

I have some background in electronics and I'd like to learn more.

Thank you!

[sumgai]

Jun 2, 2020 9:19:49 GMT -5 pablogilberto said:

..... on board Preamp High Frequency Hiss problem.

I hear this high frequency hiss when the amp volume is high.
It gets noisier as I turn the mid high and treble knobs at full position.

You're experiencing either a noisy transistor or op-amp. While Music Man is generally considered to be a top-tier company, there's nothing that says they can't use "less expensive" components in their on-board preamps. (For all I know, they could be simply picking a pre-existing assembly from an off-the-shelf supplier, one which comes at a great price, but has these "little trade-offs", you understand. Keep in mind that I'm just guessing here...)

The only real test of my theory would be to substitute with a known good replacement. Of course, that can get both expensive and messy, workbench-wise. But anything less than that will be either even more trouble, or less conclusive, sorry to say.

If the pickups can keep up with the big boys, then take it out altogether.  If not, then replace it.  Any preamp will do, it doesn't have to be from Music Man.... unless you need to keep the resale value as high as possible.

HTH





sumgai

[thetragichero]

while i love that baja preamp i linked to the next active bass i build will have the original Leo-designed two band preamp using the ridiculously obsolete (but ridiculously low current draw) lm4250:

www.freestompboxes.org/viewtopic.php?f=13&t=431

heck you could even call it an upgrade it you ever go to sell the bass