(Active/Passive) Bypassing an active "board" preamp...

I have the same situation. I am trying to find an answer for this too. Hopefully someone will respond. . . . I will post something if I can come up with it, so far I can picture this as a big switch, but I am still working on it. I'll let you know soon, I hope.

FWIW:

unlike an aftermarket Bart or Aggie, the issue you have there is the PCB runs between the volume/blend, jack, and battery. Never messed with that or looked into it but my guess is you'd have to either tap into the PCB or run multiple switches to basically redirect the flow like the Bart/Aggie setups. Similar threads have been posted in the past and I don't recall a workable response.

I'd contact the manufacturer and see what they have to say, you've got nothing to lose. They may have it as an option or it may not be there cause it's not an option.

I have just put a active/passive switch into a custom shop jazz bass. There area few compromises but it worked quite well and is very simple to do.

You need to follow the signal chain - on the jazz bass is went:

Pups---blend---preamp----jack


On the jazz bass there was a single wire running from the blend pot to the preamp.
So what you need to do is put a SPDT switch in the place of that wire that bypasses the preamp and sends the output straight to the jack socket

Wired like this:


It works but the compromise is that you loose volume and tone controls in passive mode because they have just been bypassed. Its simple to reverse as well because you can just replace the wire between blend pot and pre-amp.

I am not so sure it will work with your pre-amp because you seem to have two wires running from the blend pot to pre-amp - but i think they are just the signal wire (white) and ground wire (black) so the switch would go on the signal wire as described. You can test it by running that white wire straight to the jack socket and taking the batteries out of the bass. If it makes the sound it should you can then add the switch as i described


Hope that helps somebody

Just been in the bath thinking about the switch mod i posted and there is still something bugging me.

In passive mode the pre-amp is bypassed but it is still turned on and still connected to the jack socket - so it may have some influence on ths ound still. There was a big difference in sound between the active and passive modes but i cannot be sure its 100% uninfluenced by the pre-amps presence. The bass worked fine in passive mode both with and without the batteries being in the guitar but i didnt compare to see if that made any difference -which i should have. I will haveto ask the guy whose bass it is.

Anyway . . . if that was a problem there is a really easy solution . . . we just need to turn the battery power off when the guitar is in passive mode so we need a switch between the battery and the pre-amp that will cut the power . . . infact if we use a DPDT switch we can just use one side for bypassing the preamp and one for cutting the power . . . like this:

I am thinking of something like that too. I guess the pre on the main amp can be used for tone adjust instead of the tone on the bass. That is the only thing I can come up with.

Interesting . . . do you know how the controls will function when its in passive mode?

I guess you will have the blend and volume control but still no tone shaping ability

I think what is happening there is very similar to the one i have done on a jazz bass - only different routes to get the volume control functioning properly. Basically they have gone from the blend pot and bypassed the pre-amp (like mine) but on there circuit board the volume must be the last thing before the output jack so they can tap back into it there rather than at the jack like i did.

I take it the original volume would have been 25K rather than the 250K/500k they are suggesting - i wonder how thats going to affect the volume control when in the active mode


They obviously are not worried about the fact the pre-amp is still turned on when in passive mode


...


if changing the pot to 250 or 500k it would be easy to use a push/pull pot for this mod and avoid drilling new holes

hopefully i will have some answers on the pre-amp still being turned on issue soon.

I have emailed the guy with the jazz bass asking him to try out the passive mode both with and without batteries and tell me if there is any difference. . . There is a strong chance i am just finding things to worry about where there isnt a problem, but like i said before- its damn easy to change the switch so it does turn off the pre-amp if there is an issue there

OK . . . Apparently there was no real tonal difference in passive mode with or without the batteries in the guitar so the pre-amp being on isnt colouring the sound in passive mode


. . . but . . .

there was more volume in passive mode without the batteries than with the batteries . . .

So the preamp being present and turned on is having some effect, but maybe not enough to worry about

Aussiephoenix said:

So, what do you do when you have a problem with a specific part?
You email the company about it... this is what they had to say:

[Invalid or Expired Link Removed]

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Thanks for the information. Who did you email? I have a little different problem. I got this pre on ebay, and I do not know the model. My pre has a main volumn, but is missing the balance. I am not sure if that work with the diagram that you got for this thing, because I don't know where the balance input to the board would be. . . . the other information seems to be same as what I have though, so I will take it out of the bass this wend and see if I can make any sense out of it...... thanks a lot for the information.

WezV said:

In passive mode the pre-amp is bypassed but it is still turned on and still connected to the jack socket - so it may have some influence on ths ound still.

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If the input the the active stage is bipolar powering it off could (if NPN) turn the input transistor into a forward bassed diode, and if the signal level exceeds Vf, conduct, clipping the signal if the pickups put out enough level.

Either remove the preamp from circuit totally in passive mode, or leave the pre powered on.

Aussiephoenix said:

So, what do you do when you have a problem with a specific part?
You email the company about it... this is what they had to say:...

Click to expand...

Well there you go. Do it and see what it does. I'm a trial and error guy not techhead and I think A9X is saying it won't work, which may be so. But it's a cheap simple mod and you can verify it's one way or the other by doing it in short order. Looks okay on paper. To my knowledge shouldn't destroy anything at any rate and I'm guessing the maker wouldn't have directed you to do so if so.

In accord with A9X, I'd watch output for clipping though. You always want to start out with minimal output and graduate anyway cause you never know if a mod will make it squeal like a pig.

well that last one from A9X was a bit over my head - are you saying its better to leave the pre-amp turned on in passive mode?

I would advise anyone doing this to check the switch works before they drill any new holes in there guitar for it. SWitches are realtively cheap so if it works its well worth it, if it doesnt you can keep the guitar as stock

I know the one i did works, and it works quite well - i am just curious to know if it could work better!

I just had another switch idea jump into my head, argh!!!

FWIW:

I think what A9X is saying is either decide to run it actively or passively, not both.

At any rate, you don't drill any holes. Just wire the switch up for testing and see what it does. You can leave the control cover off and let the switch stick out the back or tuck it into the control cavity (wrapped in electrical tape/whatever if necessary) to make sure it doesn't short out on anything.

You can drill a hole once you find out if it's of any interest to you. Actually, in this case, no hole necessary regardless, get a p/p volume pot if it's of any use.

I am not trying to run both at the same time - but i am curious to know if the active/passive switch i have already installed, that works pretty well, could work better.

The switch suggested by the pre-amp manufacturers to the original poster of this thread suggested a mod very similar to what i did. SO i am fairly confident that my switch is an appropriate way of making an active/passive switch.

But like said before - - - the pre-amp has an affect on the volume level of the guitar when in passive mode, even though the pre-amp is bypassed.

I would be curious to see if this is normally an issue on basses with pre-amps with an active/passive switch already installed.

Has anyone else seen a volume difference between the passive mode with batteries in guitar (preamp turned on but bypassed), or the passive mode with no batteries on the guitar (preamp-definately turned off and bypassed).


You suggestion to wire up the switch before drilling any holes in exactly what i did. I did not use a push/pull pot because the original pots (for the active circuit)where a much lower value than the push/pull pots i keep in stock (for passive circuits)

WezV said:

I am not trying to run both at the same time - but i am curious to know if the active/passive switch i have already installed, that works pretty well, could work better.

The switch suggested by the pre-amp manufacturers to the original poster of this thread suggested a mod very similar to what i did. SO i am fairly confident that my switch is an appropriate way of making an active/passive switch.

But like said before - - - the pre-amp has an affect on the volume level of the guitar when in passive mode, even though the pre-amp is bypassed.

I would be curious to see if this is normally an issue on basses with pre-amps with an active/passive switch already installed.

Has anyone else seen a volume difference between the passive mode with batteries in guitar (preamp turned on but bypassed), or the passive mode with no batteries on the guitar (preamp-definately turned off and bypassed).


You suggestion to wire up the switch before drilling any holes in exactly what i did. I did not use a push/pull pot because the original pots (for the active circuit)where a much lower value than the push/pull pots i keep in stock (for passive circuits)

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Actually my previous post was in response to your statements but directed to Aussiephoenix - since it's his thread.

I typically don't even have any onboard controls let alone a preamp - just run straight to the jack. But I started out with a wall of active basses. The evolution is pretty much covered in FAQ under Dimento's "onboard" preamp experiments. But it's common for there to be volume variation between preamps in active and passive modes. I was concerned about quality of tone not volume.

In as much, I think what A9X was saying to you was ditch the preamp. I say it's minor surgery, wire it like the maker said and see if you can tell any difference. Won't be a side by side but if you can't tell the difference, it's probably not a major difference in terms of being distingushable - which more often than not means volume to the human ear. Doesn't mean it may not be different in terms of quality of tone. I would also wire the pups straight to the jack for a comparison. That would tell you what the pup's really sound like. You may conclude to in deed ditch the preamp - or not.

There are different kinds of by-passes. If it's not a true by-pass, then removing the battery will break the circuit so that there will be no sound even in passive mode - that's with your typical Bart/Aggie style aftermarket pre. Seems the battery will also continue to drain in passive mode if the bass is not unplugged. A LOT of (perhaps most) factory by-passes behave this way. It may be the preamp controls are by-passed in this type but to me the preamp isn't since you've got to run through it to get to the jack - no PCB/preamp no sound.

yeah i know i hyjacked the thread a little - but only becasue last time i searched for the wiring for one of these switches i got absolutely no where. I thought i would share what i have just done because i am almost happy it works well. And i think it would have worked with the original preamp diagram posted in the first thread.

That why i have posted two switch diagrams to help anyone else who wants to find out how to do one of these switches

Just to clarify though, i fully expected there to be a volume difference between active and passive and there was - no problem there

The volume difference i am trying to investigate is between passive mode without batteries in the guitar and passive mode with batteries in the guitar - i wondered if this was normal but it sounds like from what you are saying most aftermarket pre-amps wont even work in passive mode without batteries because its not a true bypass.

this just got my brain working overtime
, i will go look at that FAQ you suggested and have a read - active electronics are still pretty new to me!!

WezV said:

...
The volume difference i am trying to investigate is between passive mode without batteries in the guitar and passive mode with batteries in the guitar - i wondered if this was normal but it sounds like from what you are saying most aftermarket pre-amps wont even work in passive mode without batteries because its not a true bypass...

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FWIW:


Never compared battery in or out in passive mode with such a preamp cause I've never had the opportunity - never had a passive mode in a pre where you HAD to go through a PCB to get to the jack. With a true bypass it's like the pre doesn't even exist.

Given the assumption the manufacturer knows what they're talking about, I'd wire it like they recommend. Even so I'd still be watching for clipping - especially if wired differently yet still worked cause it sounds like you could run the risk of blowing your speakers.