9 Volt DC Phantom Power Supply for Electro-Acoustic Guitar

[Sam Spoons]

On a typical electro acoustic the output socket is a three conductor TRS (often misnamed as a "stereo") socket and the battery -ve is connected to the ring connector so that when a normal guitar lead's TS plug is inserted it makes the connection between battery -ve and the earth side of the circuit (this is why you battery runs down in a day or two if you leave the guitar plugged in but lasts months of normal playing).

To make this work with a "phantom" power supply that would need to be rewired to make use of the ring to supply the +ve side of the 9VDC supply. Not rocket science if you are reasonably competent. All the info to do the mod is in the thread and I note tha Jay used a TRRS jack socket to get around some unconventional switching in his preamp, not sure if your's will need the same refinement.

A more pragmatic solution might be to get a luthier to replace the internal battery bag with a battery compartment on the rim of the guitar adjacent to the end pin?

[Dheran]

Hey, thank you for your reply!

Yes, I tried the mod easily, because i understand those things you say. But it didn't work... And i guess it's because the element VTC has an aux pickup option which goes to the ring. So, it must have tip for pickup 1, ring for pickup 2, and sleeve to make the short that turns on the preamp when a jack is plugged in. The problem is that it seems that there's no trrs jack on the market (1/4 size). I wonder how the OP managed to use one. So I can't try to insert a TRRS with +V on the second ring.

Maybe the only solution is to take the preamp endpin out of the guitar, replace it with a normal TS socket with the piezo wires soldered in it, and build a box with the preamp and the vol-tone control. It'd be a pity because this is a major mod, instead of the super easy way of just using a TRS cable without touching anything on the guitar.

If I just could find a 1/4 TRRS jack to make the try!!

[Sam Spoons]

I have never seen a ¼" TRRS either, nor an adapter to take a 3.5mm TRRS up to ¼". Jay hasn't visited since May but maybe a PM would get to him to ask exactly how he sorted it.

But my inclination would be to fit a battery compartment that is accessible from the outside. It would look neater and be more convenient than removing the preamp to a remote box.

[zenguitar]

I've just been studying the installation manual and user guide on the L R Baggs site. Not as helpful as it might have been. However, it is clear from the manual that the pre-amp pcb does have a terminal to attach a mic or magnetic pick-up which is connected using the ring terminal of the output jack.

It might be worth an email to L R Baggs support asking them to clarify how the battery is switched on when a jack plug is inserted. Specifically, whether it uses the TRS connections n the socket, or whether it is a socket with mechanical switch operated by inserting the plug. That will give you a definitive answer on whether or not it is possible to provide remote 9V DC through a TRS cable.

The simplest solution to your problem is to get a replacement battery bag. And to prevent it coming unstuck use a stapler to attach it more securely to the heel block. Unless you're leaving it plugged in when not being used, the battery should outlast a couple of sets of strings. So when you are changing strings you have the opportunity to check the battery with a multi-meter to see if it will need replacing soon. If is does, just replace it there and then.

Yes, it would be possible to source and fit a battery box in the side of the guitar. I personally wouldn't recommend it, but that's because I am not a fan of cutting holes in the sides of acoustic guitars in general.

Jay is a very nice and helpful chap. But I imagine he's been very busy at work since covid hit hard. So I'm not surprised that he's not visited for so long.

I've also tried searching for TRRS 1/4" plugs and sockets and can find no references anywhere. I very much doubt that anyone makes them.

Andy

[Dheran]

Thank you both! It´s really great to have this super quick feedback.

When I bought the guitar, I feared all these cons of having a battery bag, so I tried to make the mod from the beginning. Since it didn´t work, I somehow forgot about it and used the guitar. It came with me in long van travels for a year or so, with no issue. Also, this preamp is really low consumption. I suspect that the battery can last years (not kidding). I also use a multimeter every time I change strings and the stock battery was always quite healthy.

Nevertheless, I´d be more confident having the possibility of changing the battery easier, since our soundchecks are looong... so we can keep plugged in for 4 or 5 hours a gig.

So at that time, I e-mailed Baggs guys some time ago. They didn´t tell me anything useful... just told me that they wouldn´t be responsible of any mod and that my idea was interesting so they might just study it. Never heard from them again.

OK, consumption is really low, so this concern passed to a second plane for me... I just can live with it. BUT, suddenly, in this COVID era (no gigs), one day I heared a "thoomp!" sound at home. Battery bag fallen in the inside of the guitar while it was hung and resting. Grrrr...

So for me, now the important thing is too avoid this "little inconveniences".

No, I don´t want to cut anything in the guitar. Nothing. Even a little hole either. I love the guitar and don´t want its wood to be toouched.

I guess that Jay did some solder in the original endpin, or something like that. That could be a solution. Also, It´s not too difficult to take the whole preamp out (as I described) and turn the guitar into a passive one with the preamp in a separate box. For me it´d me much more convenient and the guitar would end up being just wood, a piezo, a TS socket and a few inches of wire. It should go to the luthier, though, since I don´t want to apply my clumsiness in this kind of task.

Again... thank you soooo much for yous help. Didn´t imagine I´d had any reply this quick.

[Sam Spoons]

Is the preamp in the endpin with no controls or is there a control panel? I'd assumed the latter so that taking it out would leave a hole but if it has no controls then removing it will be unnoticeable. If that's the case then I wouldn't put an extra hole in it either, I'd follow Andy's advice and just secure the battery bag by some more secure fixing.

[zenguitar]

The pre-amp is a PCB that is part of the endpin jack. It has a volume and tone control on a fly cable attached to the edge of the sound hole.

Andy

[Dheran]

Yeah, Andy os right.

I've been talking to my luthier. He's already made this thing several times, some of them with the Baggs VTC.

This is what has to be done:

Desolder the wire in the ring of the original endpin.

Solder to that ring a wire, and the other end of that wire to some 9V point of the preamp pcb.

So it's approx what I was guessing (ring is connected to the optional second pickup). And the lead switch is not a short circuit between ring and sleeve, but some kind of mechanical switch in the socket.

This is more invasive that just using a TRS jack, sadly. You have to manage to take the endpin out, solder, desolder, reassemble... So it's not a trivial task for amateur hands.

AND, you're also making more difficult to add a second pickup, should you want to do it in the future.

Based on this, I don't know whether I prefer this option or just the one of having everything out of the guitar (but the piezo, of course). This one would only need to desolder both piezo wires from the pcb and solder them to a TRS endpin... With the disadvantage that there's the possibility of, some day, lose the external preamp box and you'd have a passive guitar. I don't know how this piezo would work with any other preamp, but anyhow, you could always get a Baggs one in such that unlikely case.

Or just leave it as it is... And knowing that it's possible to have the bag jumping inside the guitar once in a while. I don't want to use a stapler or something like that.

[zenguitar]

At least you have a clear idea of the options you have available now. Glad we could offer you a little support, and feel free to come back often

Andy

[Dheran]

Yes Andy. Thank you all!

[Sam Spoons]

Yup, same ear

If you don't want to use staples then better glue for the velcro (or better velcro, the real stuff from 3M is noticeably better than the cheap stuff both WRT the glue and the actual velcro? Otherwise I'd give it to your luthier, remembering that anything he modified he can un-modify back to original spec.

[Dheran]

Just four your (possible) curiosity:

My luthier and I were talking about possibilities. Taking everything out has the disadvantage of letting too much cable length between the piezo and the preamp, so capacitance could generate signal loss, and interferences could cause noise. Some filtering should be needed, so an specific preamp it´s better for these cases. Passive pickups need good pres, in part for these kind of filterings and whatever.

So using the internal Baggs preamp doesn´t seem to be the best with long cable to the pickup, since it´s an internal preamp, not prepared for heavy filterings.

He´s ALMOST convinced that the power switching lead of the endpin works by a mechanical switch (you know, that kind of sockets with additional legs that act as interruptors) instead of shorting ring with sleeve. Actually, in this case, it´d be shorting second ring with sleeve (because first ring is proved to NOT turn on the preamp by getting shorted to ground, since inserting a TRS cable turns on the guitar while insulating ring from sleeve). Not too probable the TRRS thing.

So, if we assume that there´s a switching socket, actually we could implement a solution which would allow me to use external power (just applying +9V to the ring and desoldering-soldering internally just the way I described in a previous post) if battery is not attached in the internal clip... AND, in case of forgotten TRS cable at home, or lost, or whatever... (some emergency or odd situation, I mean) just connect a battery into the internal clip, put a normal instrument cable (TS) and get the job done.

I think I´m going to try this. Will build a box, similar to Jay´s one, and let my luthier mod the preamp-endpin. Let´s see what happens.

[Jay Menon]

I read this thread four years too late - but even though it’s late I thought I would reply. The TRRS and pin jack socket that you need is LR Baggs own strapjack plus model:

https://www.ebay.co.uk/itm/361129745116 ... media=COPY

Cheers
Jay

[Dheran]

Thanks Jay.

I ended up modifying the wiring in the preamp (my luthier just had to desolder one wire from its position and resolder in other position).

That worked, using a TRRS cable and en external little box with a 9V battery compartment and a 9V socket for an optional power supply.

It went perfect. But finally I got fed up of needing the TRRS cable and having care of not connecting the power prior to plugging the cable. I burnt the volume pot because of doing that.

So, for me, the days of active built-in preamps in guitars are over. I've switched to passive pickups (k&k style SBTs). I plug the guitar with an standard TS cable, no batteries, no danger of damaging preamps, into an external pre (which can even be a multieffetcs pedal, because 1MOhm impedance is the only thing you need to make the pickup work fine). It works great, simple, clean and hassle free.

[Wonks]

I found my K&K sounded better into a 10k line input.

[Dheran]

I'm not really surprised... I find it great sounding in 470k inputs... So I believe you!

[Jay Menon]

If I wish to install an ordinary passive undersaddle piezo transducer

and the input impedance of my first unit is 5 MegΩ

how would I make this work please?

https://imgur.com/FVDWxge



https://i.imgur.com/FVDWxge.jpeg

[Wonks]

I'm not really surprised... I find it great sounding in 470k inputs... So I believe you!

My Fishman Loudbox Mini has a 10M input for those piezos that need a really high Z. But with the K&K there was far too
much bass. Running in to a mixer line input and an active PA speaker it sounded far more natural. So I made a pre-amp pedal with a 10k input impedance for use with the Loudbox.

It may be down to the installation on my guitar, and the Loudbox does seem to be quite bassy with a lot of my guitars with preamps. It certainly won’t be ideal for all piezo installations. But if you’ve the chance to try some different input impedance devices with flat outputs, it’s worth a go.

[Wonks]

If I wish to install an ordinary passive undersaddle piezo transducer

and the input impedance of my first unit is 5 MegΩ

how would I make this work please?

https://imgur.com/FVDWxge



https://i.imgur.com/FVDWxge.jpeg

Well, you could simply wire it to an endpin jack on the guitar. Cut the 3.5mm jack off if necessary as it’s just a braided outer sheath with a central core like a lot of vintage pickup wire. That sort of undersaddle piezo normally needs a 5-10Meg input Z to give a flat response.

Or wire to an endpin jack with a built-in preamp.

Or fit a full-blown preamp into the guitar. Several preamps just stick to the under-rim of the soundhole so modifications are minimal.

[Wonks]

If there is no existing undersaddle pickup, then be aware that the pickup element will raise the height of the saddle.

So you’ll either need to lower the bottom of the saddle by the height of the pickup, or, if the saddle slot isn’t very deep and the saddle now angles forwards, you’ll need to get the saddle slot deepened.

Of course you’ll need enough depth in the bridge to be able to do this. Most guitars should be fine but I’ve come across one with a shallow bridge where the bottom of the slot was the actual top of the guitar. So no room for an extra 1-2mm of routing in that instance.

If there is some slight wobble of the saddle in the bridge slot once the pickup element is fitted, then I’d make a new saddle from one that’s thicker than the slot and sand it down to fit. It shouldn’t be too tight and needing to be forced in (or the string vibrations won’t get fully transmitted to the piezo element), but should have just enough grip so it doesn’t fall out easily if you turn the guitar upside down.