Alembic preamp problem

[Bernardduur]

This is a weird one

I have an Alembic rehousing project now on the bench; just a simple Alembic preamp just out of the bass of a friend. Easy job, I thought......

Not!


My main problem is that the unit want to be fed with the signal of both the hot and cold  (so tip and sleeve of a normal cable) BUT the cold (sleeve) can't be connected to the ground.
So now I have two options:
- Use an isolated stereo jack. Connect the middle lug to the cold section, connect the tip to the hot section. Result: I have a signal in "FX on" mode, I have no signal in bypass mode.
- Use a normal jack. Result: I have a signal in the bypass mode but have no signal in the "FX on" mode.

Anybody has an idea? I was thinking about switching the ground also w/ an isolated stereo jack............

[Rob Strand]

> BUT the cold (sleeve) can't be connected to the ground.

Why?

Most preamps you can ground the cold input.   If you tried it and it didn't work I suspect the preamp is DC coupled and grounding the cold is upsetting the DC bias - if that's the case connect the cold input to ground via a cap.   100nF would be a good start but if it's too small you might lose bass, in that case increase the cap to perhaps 10uF; there's a trade off between losing bass and how long it takes for the DC conditions in the unit to stablize.

[Bernardduur]

Thanks man! I will try it soon!

Edit: That did the trick! Now I will experiment with some values!
Thanks again!

[Bernardduur]

OK, some update;

I stuck with a 1 uF, non polarised. When I polarise the cap this does not do the trick.

Now I have another issue; popping...........

It is a really loud popping, much louder then normal popping. I already tried a pull down, but that does not work. Any more ideas??