Power active pre-amp through TRS cable?

[Rob22315]

5. You have to modify the bass. It can only be used with your breakout box unless it is restored to the original wiring configuration.

Most bass preamps use only a tiny bit of current, so is this really a problem that needs to be solved?

I too would like to get away from using 9V batts in my onboard preamp but am using a different approach. I'm adding a DC 5.5/2.1mm switched jack to the control cover plate to provide an alternative way to get power into the bass. I use the switch built into the DC jack to switch out the onboard 9V batt when powering externally. This way, I can power the onboard preamp from any external source when I want. The downside is that I need to run an extra power cable to the jack. However, in a quick pinch, I can tape or velcro a 9V batt to the back of the bass and have power restored in a matter of seconds (rather than opening up the back of the bass) or I can run a 9V pedal supply chord twisted around or otherwise attached to the signal cable and power the preamp from an effects board.

I've already put the jack into my frankenbass and it works reasoanbly well except that the cheapo 9V power supply I'm currently using isn't filtered particularly well so it add some noise to the guitar output.

Note, as soon as I unplug from the DC power jack, the bass essentially returns to it's original configuration and is powered off the internal battery if it's still good.

Thoughts?

 

[rumblinbass]

use a TRS.
no extra wires to the bass.

Power input from the preamp to the (+) battery terminal.
(-) battery terminal to the Ring.
Sleeve to preamp GND.
Make a dummy battery that shorts the battery terminals.
Use the Tip for signal, Ring for (+) power and Sleeve for GND.

This way you don't have to modify the bass in any way, unless you have to re-wire the bass. You can still use batteries if needed.

Oh, since the external power supply is quite some distance away from the preamp, make sure you have proper power supply decoupling on the preamp.

 

[Rob22315]

use a TRS.
no extra wires to the bass.

Power input from the preamp to the (+) battery terminal.
(-) battery terminal to the Ring.
Sleeve to preamp GND.
Make a dummy battery that shorts the battery terminals.
Use the Tip for signal, Ring for (+) power and Sleeve for GND.

This way you don't have to modify the bass in any way, unless you have to re-wire the bass. You can still use batteries if needed.

Oh, since the external power supply is quite some distance away from the preamp, make sure you have proper power supply decoupling on the preamp.

That's a good idea too, I think I'll add this to the design of my DIY pedal board (not started yet but soon) so the guitar could get it's power from one of the standard power bricks via a cleverly wired TRS cable. Note my main bass (not yet modded) has an 18V preamp. Thanksfully the standard power bricks usually have 18V available. I'll probably custom build the effects switcher and will look into adding this capability there. Then the TRS cable only has to run from the switcher to the guitar as long as I incorporate the power feed there.

Any issue with transients as the 9/18v ring slides past the sleeve contact?

 

[rumblinbass]

That's a good idea too, I think I'll add this to the design of my DIY pedal board (not started yet but soon) so the guitar could get it's power from one of the standard power bricks via a cleverly wired TRS cable. Note my main bass (not yet modded) has an 18V preamp. Thanksfully the standard power bricks usually have 18V available. I'll probably custom build the effects switcher and will look into adding this capability there. Then the TRS cable only has to run from the switcher to the guitar as long as I incorporate the power feed there.

Any issue with transients as the 9/18v ring slides past the sleeve contact?

Until the cable sleeve makes contact with the sleeve contact your bass is not really grounded. It's floating. Shouldn't be an issue.
But, if you prefer, add a switch on your pedal board...or even on your bass.

 

[Rob22315]

Until the cable sleeve makes contact with the sleeve contact your bass is not really grounded. It's floating. Shouldn't be an issue.
But, if you prefer, add a switch on your pedal board...or even on your bass.

Great ideas, thank. I already have a switch on my frankenbass. I can experiment with that before putting my main bass to the test. Of course, frying the preamp on my main bass would be an excuse to upgrade the preamp along the way.

 

[relwof]

I put a diode in every circuit that I can. It's the hip thing to do these days.

It's actually already beyond hip...

 

[Rob22315]

I put a diode in every circuit that I can. It's the hip thing to do these days.

Cures male pattern baldness too.

 

[Spoiler'82]

"AFAIK the ring connects to the (-) battery terminal. The sleeve goes to the (-) power connection of the preamp. "

So what goes to where? You've got two negatives...a "double negative" so to speak...

In 82 my Alembic, the battery would embarass me at the worst times---during performance. I modified my bass using a 4 pin xlr jack setup similar to what you see on Alembics 15K basses. Using 4 conductor Belden Brilliance mic cable, the two blue conductors carry my Plus and Minus 9v, and the two white conuctors double-up to be my signal, and ground is ground-the shield. I get NO hum, as as the amp end of the cable, I 'surgically' pulled the two blue conductors out of the cable a foot or so from the amp, and soldered in a 9v battery snap and velcro 'pouch' of sorts to hold the now EASILY replaceable 9v batt.
I also have a 4 pin xlr-xlr cable and power supply, should I need it. Use GOOD 9v regulated 3-pin grounded power supplies!, the kind you order from Jameco electronics...and they dont hum. Yeah, they're 21 bucks apiece, but WORTH it. Every 99 cent wall-wort power-supply I've ever gotten from boss or dod, or guitar center had SO MUCH RIPPLE when looked at on an oscilloscope that they're TRASH.
I AM going to check out PMT's TRS powering setup for my Fender P-Bass Special though. For 69 bucks its alot cheaper than going through all the rigamoro I went thru to power my Spoiler.
Having my Spoiler done up this way did also garner a cool wireless setup for me in the end...I modified a Samson built shure sm-58 wireless transmitter, by using the 9v out it had for a condenser mic connection at the mic-head poit, i was able to power my bass, and the wireless transmitter off the same 9v battery. Now THATS where I put a fresh 9v in before every critical performance.
Have MORE that one path,
more than one bass,
more than ONE bass head,
---put that spare in the Van---
Be a good boy scout and Be Prepared!
Because NOTHING KILLS a Gig Like NO BOOM-BOOM BASS!!!

 

[Rob22315]

So I have been battery free for 2+ years without incident or mishap. Danelectro makes a cheap power supply with very little noise. Those of you who like doing all the power drain analyses, you are still missing the point - it's about eliminating all 9v batteries from my signal chain for the environment, not for convenience. I play live once a week in church so it's not a full concert but I am not a bedroom wanker either.