Preamp Battery Bypass

[pbasswil]

I recently got a passive bass, after years of only active ones. I recently had a thread where I was looking for a compact outboard buffer/preamp to velcro to its bass strap -- I was looking to keep the bass passive.

But now I find I'm missing the convenience of onboard preamp EQ; and I still need some kind of buffer to drive long-ish cables anyway.

I've decided to install an Audere Classic 4-band preamp. But I want to add a battery bypass switch -- this is so I can leave the bass plugged in around the house without running through 9Vs all the time.

Is it merely a matter of using an extra switch to interupt one of the leads to the battery? Does it matter which lead?

Here's what the Audere guy emailed me; but I didn't really understand it:

>You can select either wire for the AND condition below
Note the preamp is normally powered up when black is shorted to grey/white by the mono jack body.
So if you switch in-line with the black then you have an AND function - jack in AND switch on
If you switch from black to grey/white you have a switch only function
If you switch the black to grey/white in parallel then you could also create an OR function (not what I think you want to do)<

Thanks for any clarifications!

-- Peter

[line6man]

The preamp is normally switched on when you insert a mono guitar cable, because the cable shorts the ring and sleeve terminals together which connects the negative lead from the battery to ground.

What you need to do is connect the negative lead from the battery to one side of the switch, and then connect the common terminal to the ring terminal of the output jack.

[walterw]

yeah, i think that would be the "and" scenario, where you have to have the bass plugged in and the switch thrown the right way for the preamp to power up (and for the bass to work).

i'd rather rig it up so a switch bypasses the preamp and kills the battery power at the same time. i think you'd need a TPDT switch, so you can use 6 contacts for true-bypass switching out of the preamp, and two of the leftover 3 contacts to disconnect the battery at the same time.

you'd still want to run the battery's negative through the ring terminal of the jack, so that it couldn't power up unless plugged in.

the only caveat about all this is that switching the preamp on and off with the amp fired up could make for a popping noise through the rig.

[line6man]

Originally Posted by walterw

i'd rather rig it up so a switch bypasses the preamp and kills the battery power at the same time. i think you'd need a TPDT switch, so you can use 6 contacts for true-bypass switching out of the preamp, and two of the leftover 3 contacts to disconnect the battery at the same time.

Yeah, that seems like a good idea, that way you would operate the bass in passive mode when the battery is off.

This might be problematic if you wanted to switch between active and passive modes in the middle of a gig or something.
You will get a loud pop when switching to active mode.

[pbasswil]

Thanks a lot for the insights, folks.

I don't suppose anyone has the ability to do a diagram of how the TPDT might be wired in? Personally I'm a big zero on wiring; and I'll need to communicate clearly with a guitar tech, who may well turn out to speak more Canadian French than English.

Thanks in advance,

-- Peter

[walterw]

well here's the true bypass part; it's a diagram for pedals, but the scheme is the same, either straight through, or using the "effect" (in this case the preamp):

(use the bottom one)

the leftover contacts would just interrupt the battery negative at the same time the preamp is switched out.

[pbasswil]

Walter,

This is invaluable! Thanks so much for going to the trouble to lay that down. Armed with your diagram I can be confident that there'll be no confusion at the luthier, even if we end up meeting somewhere in the linguistic middle with some Frenglish and some Franglais. ;^>

To minimize ambiguity, I guess I'll just give him the bottom version, as per your recommendation.

Cheers buddy!

-- Peter

[walterw]

i found it rather than drawing it, but i did save it for just such an emergency!

the "input jack" would be coming from the pickups through the passive controls.

the bottom diagram is better because it shorts the input of the active circuit to ground when bypassed, eliminating oscillation and switching noise.