Phantom Powered Acoustic

[Dugz Ink]

My Washburn AB-20 acoustic/electric (depicted in my avatar) has an XLR out and a 1/4" out, and it has an on-board pre-amp (Equis II) that runs off of a 9 volt battery. The 1/4" jack is a TRS that has been wired to complete the 9 volt circuit when you put in a plug.

The 1/4" jack has a crack in it, which causes intermittent problems, so I'm replacing it.

But I was thinking: While I have the guitar apart and the soldering iron hot, can I set up the XLR jack to deliver phantom power to the on-board pre-amp?

I have one of those cheap "Tube MP" mic-pres, which isn't a very good mic-pre, but it sounds great as a DI/pre for my acoustic/electric bass... and it can provide 48v phantom power. However, I don't know what would happen if I ran the phantom power to an on-board pre-amp that might be designed to only handle a 9 volt battery.

Any thoughts, ideas, or recommendations?

D~s

[thedoctor]

A TRS plug can't complete the ground because the R to S connection isn't complete. Therefore, I recommend you take a close look at your manual for the preamp cause I think it MAY already take phantom power. That would be way cool!

Please note that I said a TRS PLUG can't complete the ground. I'm sure your jack is a TRS. I just think your preamp may very well be set up to handle phantom power through the XLR connection. Check the manual or go online with the preamp maker.

Edited February 5, 2005 by thedoctor

[Dugz Ink]

Please note that I said a TRS PLUG can't complete the ground.

That's correct, however, a TS plug in the TRS jack does complete the circuit.

I'm not really sure why they put an XLR on it; the "ground" is wired to the 1 and 2 pins.

(Their schematics look like they were drawn by a five year old, so maybe that's "why".)

D~s

[thedoctor]

Oops! Then the output of your preamp is really not set up for phantom power. I think it can be done but to figure it out you would have to tell me what the model of the preamp Washburn put in there is and maybe I could find a schematic. All my Shadows and EVs will take phantom power from the XLR connection and I don't have to take the batteries out or anything to use it so lets look.

[Dugz Ink]

Here are links to the only schematics that Washburn sent to me:

Equis II jack wiring diagram

Equis II pre-amp wiring diagram

Like I said before, they suck... but Washburn tech-support says this is all they have.

By the way... thanks for your time. I really appreciate it.

D~s

[lovekraft]

You can add phantom power to anything, it's just a matter of blocking the DC voltage from the signal portion of the circuits - fortunately, that's exactly what capacitors do. Having said that, powering a 9 volt preamp from a 48 volt phantom supply is going to be far from trivial, and it's debatable that it's worth the trouble. Getting the power to the guitar, and isolating it once it's there is pretty straightforward - simply disconnect the lead from the tip lug on the output jack and wire a large cap between the lug and the wire (that blocks the DC current from your preamp's output). Now you've got 48VDC on that lug, and you can use it to power the preamp, but you'll have to work out all the little problems, like regulating the voltage (you've got a surplus 39 volts to deal with), limiting the current ( flash-frying a preamp isn't good), keeping the noise down, etc. Of course, you could pack all the necessaries into a little box that plugs into your mic preamp, and simply run a cable from it to the guitar - that's the approach Don Tillman used to power his FET Preamp Cable project. Of course, he's using a 9 volt battery, but you could easily build a regulator/corrent limiter to put in its place without too much trouble.

Edited February 5, 2005 by lovekraft