Power through a stereo cable

[Earthscum]

I'm doing a build for a buddy, and he doesn't want to run an ac adapter (sorry if my text is off a bit, I'm mobile). Since this is gonna be his only box, I completely understand not wanting to make a spider out of it.
So, what I was thinking was running a stereo cable from output to his amp. On the amp side, the cable would split to a power box and the input. At the pedal, the output would be normal, but using the ring for dc voltage. This would eliminate the extra string of wire, and wouldn't affect how he would place the pedal with battery supply. Also, the DI circuit won't concern me anymore as far as battery drain.
Given filtering at each end, etc., would there be any problem doing this, besides cable resistance Or is that small enough to become near null, or just assume the resistance as such and use it as part of a pi filter

[amptramp]

There is a fairly long run of cable with power right beside output where it can couple capacitively, so make sure the power supply is regulated to avoid noise on the output.  If the pedal output is low impedance, it should be OK but if it is not, you will run into susceptibility to power supply noise.  Add a buffer stage if there is any doubt.  There should be capacitance at the pedal end even if there is some at the supply end to ensure good filtering and a low impedance on both ends of the power line.

One other thing to check is hot-switching - can you safely insert or remove the plug without blasting the circuit output with the supply voltage?  If not, add some protection (good) or make sure the circuit is never plugged in or unplugged when the amp is on (iffy).  The ring will contact ground when it is plugged in, so the power supply should be short-circuit proof.  The output at the amp end will contact supply voltage, so there should be some series resistance and voltage protection (diodes to the power rails) to avoid destroying the electronics.

Other than that, it is a good idea.  Just make sure he knows to use a stereo cable and not a mono - that would short the power supply output, so make it continuous short-circuit proof.

[Earthscum]

Cool, thanks for the reply. There's a couple things I didn't consider, and am glad I asked. I'm probably going to redo the amp end of the cable to mono (amp input) and 5.1mm standard jack. That way there shouldn't be a mixup and he can still extend cable length with a f/m extension. I'll have to do some thinking on the power supply (decide on wall wart to filtering, or just box a new one, or build protection into the cable and have him pick up a one-spot). Sadly, this actually easier because there would be no logical reason to do anything but block phantom (option he wanted for battery life, but phantom's not always available. Two birds with one conductor).

[PRR]

> extend cable length with a f/m extension.

If that is a "mono" cable, it will short-out the power source.

"Don't work" is one thing, "don't wark and SMOKES" is more exciting. Be prepared for dead-short on power line.

[Earthscum]

Aye, Paul. I was thinking  extend the amp side, but that would entail moving the power supply. Now, if I do dual jacks at BOTH ends, it totally eliminates that, BUT the power supply is gonna have to get extended as well.

I'll just stick him with a 18' cable. If we figure out he needs a couple extra lengths for smaller (10') or larger stages (buffered output to the amp, so no problem with something a little longer, 30' or something), I'll just build him a couple more.

[markeebee]

Just a thought - how about using an XLR cable between the power supply and pedal?  Cables (and sockets) are readily avaiable if your friend needs a replacement in a hurry, fit for use on stage, and only a bit more expensive than jacks. And it'll avoid the short circuit issue. If you were really keen, you could make the power supply box into a DI.

Or, cheaper but less robust, USB or network or phone extension cable.  Or something.

[earthtonesaudio]

XLR cable

Beat me to it!  XLR cable between amp and effect box... if you were extra super really keen you could make the amp-side receptacle one of these and additionally retain the ability to go straight into the amp with a standard 1/4" mono cable.

[amptramp]

I should add to the answer I gave above that if you disconnect the pedal from the amplifier, the input of the amplifier will be driven to the power supply voltage, so some protection may be needed there.  It will definitely give you the grandaddy of all "switch pops".

[Gordo]

XLR is an excellent idea.  I just picked up a Variax guitar on ebay (just too good a price to pass up) and Line6 uses a stereo cord between the guitar and switch pedal.  This gives you phantom power, turns off the guitar's internal batteries, and provides a switchable 1/4" out and XLR out.  In this scenario the XLR goes to either the board or an acoustic amp.  The guitar's acoustic sounds need an acoustic setup.

The downside is that stereo cables tend to be a tad more delicate than standard cables and I'm rough enough (spelled "clumsy") on cables already...

[Earthscum]

I should add to the answer I gave above that if you disconnect the pedal from the amplifier, the input of the amplifier will be driven to the power supply voltage, so some protection may be needed there.  It will definitely give you the grandaddy of all "switch pops".

Quoting to make sure it's well heeded...

I was considering a current regulated "soft-on/off" style circuit. When power is applied, it will have to take about half a second or so to fully power up through the cable. That way if there's no load, also, there will be no current supplied by the power circuit, even if it's disconnected from the pedal side. I have an idea of how to do this, but if someone knows of something tested and proven, that would be awesome.

I've kinda been thinking about XLR, as well. It makes sense, so I may just go that route... also, the box was going to have DI in it, if I make the PSU myself, I could dig throwing it in there. That would reduce most of the current I'd need going to the box. Thanks for the ideas, guys.

This is, so far, a neat circuit. Two preamps into a Vox-modelled tone stack (using Lin pots to give better control of the notch... he likes the notch) into volume. It's going to A/B the pre's so he can have his upright and EB on stage, with their own voicings. Also adding Mute stomp so he can switch out, take a break, whatever. If he only has one bass plugged in, he can A/B it through either preamp, giving him 2 voicings for one instrument. That is just a matter of using switching jacks... neat bonus feature for free, lol.

[PRR]

> current regulated "soft-on/off" style circuit

You think too much.

Use rocks.

How much is this preamp pulling? 10mA at 10V?

What is the smallest power supply worth buying? Maybe 12V 500mA?

Put a rock in it. Or more like a sock in its throat.

24 ohms off a 12V supply, dead-shorted, flows 500mA. The scheme will survive dead-short.

10mA happy-load through 24 ohms drops 0.24V. The 12V arrives at 11.76V.

In fact you can work the other way. With these assumptions you can tolerate 2V drop at 10mA, which is a 200 ohm resistor. Dead-short, 60mA. Even a cheapo 100mA supply won't be stressed. And you can go 1 Watt rating in the resistor (with 12V 24 ohms you need 6 Watt dissipation which is a packaging problem).

Further, 470uFd hanging on that 200 ohms gives 30:1 reduction on 60Hz ripple (60:1 on 120Hz) so even a cheaply filtered supply might be clean-enough at the preamp.

A more complex example. I built a 36V 600mA raw supply. Much of this stays inside the box. An external preamp is tapped. First, 100 ohms to 4,700uFd (big-rock filter). Even if this cap shorts, the 360mA will not burn the power transformer or rectifier. It does give 13 Watts in a 10W resistor: it can be shorted for minutes but not forever. However the main in-box function stays powered while the resistor smokes, and short at this point is unlikely.

Power comes out of the box via a 470 ohm 2W resistor. Worst-case dead-short load is 63mA (actually 100mA for two channels shorted). No problem. The preamp normally draws 36mA. Voltage on the line is 16V. Yes, much less than the raw supply. This leaves much room for self-balance. The preamps (several designs) can pull 30mA, 40mA, or self-voltage to 14V. If they pull more or less, they get less or more. Wild component variations and junkbox parts always tend to land in a usable range.

This is the DC analysis. Conceptually the preamp is a common-emitter amplifier, and the 470 ohm resistor is its load. While the guts of the preamp only have ~~15V to work with, the cable and the signal to the main box can swing nearly 30V peak-to-peak. A 10V signal in 470r termination is so strong that the cable needs no shielding. One design literally is a CE Darlington, another is a 3-transistor compound for a higher gain/distortion spec, and thrifty opamps could be included.

[amptramp]

There is another possible candidate for the preamp and power - an industrial 4 to 20 mA current loop similar to that used in industrial control systems.  It could use a mono cable.  You might have to do some thinking about the design, but it is in use in factories everywhere and it has the benefit of being a bit more immune to radiated noise and inherent immunity to short circuits - when a current line is grounded, the power goes to zero.  The cable will have the outer shield at ground and the inner conductor at a voltage consistent with a power supply into the unit.  The preamp turns voltage at the input into current drain along the power line.