Is it possible to harness amp foot switch power?

[Aentons]

Ok, hear me out.

Some amps have a built-in foot switch that doesn't require a battery or any other external power to light the on-off LEDs. I have a Vox AC30CC and the two switches are for reverb and tremolo. It uses a simple TRS cable. There is DC present on both the tip and the ring. 20V and 13V respectively. It's looks like the hot wires are grounded to turn the effect off.

It's seems like it would be possible use the dc on one of these lines to provide a regulated 9v supply for an auxiliary jack for other effects to use.

Is it possible to split and isolate the DC so that the effect switch still works?

Link to full amp schematic:
https://el34world.com/charts/Schematics/Files/Vox/Vox_ac30cc2_ac30cc2x_2005_sm.pdf

[lars]

Depends on how much current that circuit can provide. You might be able to run a vintage fuzz off it, but I doubt it will be able to handle a Berezny Deluxe Modulay Process Station (with expression jack!)

[mauman]

And it depends on the turn-on threshold of the tremolo transistor (TR-1) and reverb transistor (Q103).  You could test it by putting a  milliamp meter to ground from each source to see what the normal turn-on current is.  Then add progressively larger resistors in series to find the turn-on threshold current (when the effect no longer turns on.)   If it's more than 2-3 mA or so, you could power a fuzz. 

[Aentons]

Ok, The current is only 8 and 10 mA...bummer. I was hoping to be able to squeeze enough out of a DC to DC converter for 2 or 3 pedals but I guess that's not gonna do it

[jwin615]

If this is really an avenue you want to pursue, you could mount a 1 spot in the amp and build a 5 pin cable, din or xlr(or run a separate 2 wire zip tied to your switch cable).
But that all seems overkill...

Also, 50/50 on that DC coming from the amp being very clean.  Possibly just some zener diodes between a DC rail and ground.

[Aentons]

If this is really an avenue you want to pursue, you could mount a 1 spot in the amp and build a 5 pin cable, din or xlr(or run a separate 2 wire zip tied to your switch cable).
But that all seems overkill...

Also, 50/50 on that DC coming from the amp being very clean.  Possibly just some zener diodes between a DC rail and ground.

It's funny you mention using the one spot at the amp because I've already been doing that using a TRS cable. Audio on one line and power on the other. It works great.

My next experiment is going to be running them both on the same line, like true phantom power but, using a combiner   box at the amp and break out box on the floor. It will be unbalanced, but as long as there is some power filtering during the break out, I think it should work.

[Bio77]

Maybe this is dumb, but would there be enough room for a lithium battery in the foot switch?  Could you use the amp's power to charge it?  If so, the battery would make up the difference if you needed more current or just go dead if you over do it.

[Aentons]

Maybe this is dumb, but would there be enough room for a lithium battery in the foot switch?  Could you use the amp's power to charge it?  If so, the battery would make up the difference if you needed more current or just go dead if you over do it.

Hmm, good thinking. I'll definitely look into that.

[jwin615]

If this is really an avenue you want to pursue, you could mount a 1 spot in the amp and build a 5 pin cable, din or xlr(or run a separate 2 wire zip tied to your switch cable).
But that all seems overkill...

Also, 50/50 on that DC coming from the amp being very clean.  Possibly just some zener diodes between a DC rail and ground.

It's funny you mention using the one spot at the amp because I've already been doing that using a TRS cable. Audio on one line and power on the other. It works great.

My next experiment is going to be running them both on the same line, like true phantom power but, using a combiner   box at the amp and break out box on the floor. It will be unbalanced, but as long as there is some power filtering during the break out, I think it should work.

I would advise against that. Issue being that if there is any audio freq noise at all on the power rail, it will end up in your signal path. This is solved in pro audio with the fact that the audio is balanced so any noise is phase canceled.
Also, if your decoupling (DC blocking) ever fails or shorts, you could potentially send DC to your guitar, thus send DC to ground. Also, DC to the input of an amplifier(which *should* have decoupling built in, but still...

I'll defer to Dr. Ian Malcolm:

[Aentons]

If this is really an avenue you want to pursue, you could mount a 1 spot in the amp and build a 5 pin cable, din or xlr(or run a separate 2 wire zip tied to your switch cable).
But that all seems overkill...

Also, 50/50 on that DC coming from the amp being very clean.  Possibly just some zener diodes between a DC rail and ground.

It's funny you mention using the one spot at the amp because I've already been doing that using a TRS cable. Audio on one line and power on the other. It works great.

My next experiment is going to be running them both on the same line, like true phantom power but, using a combiner   box at the amp and break out box on the floor. It will be unbalanced, but as long as there is some power filtering during the break out, I think it should work.

I would advise against that. Issue being that if there is any audio freq noise at all on the power rail, it will end up in your signal path. This is solved in pro audio with the fact that the audio is balanced so any noise is phase canceled.
Also, if your decoupling (DC blocking) ever fails or shorts, you could potentially send DC to your guitar, thus send DC to ground. Also, DC to the input of an amplifier(which *should* have decoupling built in, but still...

I'll defer to Dr. Ian Malcolm:

I hear the warnings and I am hesitant to proceed,  but I think this is one of those things I'm going to have to test for myself.

I think a 15 ft shielded guitar cable with an instrument level signal should have relatively minimal noise compared to the longer mic level cable runs that a balanced signal was designed for.  A high pass and low pass filter on the audio will hopefully take care of any noise.

We do DC blocking caps on the ins and outs of every pedal, so couldn't any pedal potentially fail and send DC to the guitar? Plus, I'm not convinced DC on a passive pickup is an issue. A pickup is basically a 3k-25k resistor. Please correct me if I'm crazy talkin

[jwin615]

More concerned about DC noise that wont be filter out. Don't have the balance to cancel that. Get under the wrong florescent lights at it may be apparent.
Also, big potential for a ground loop as your literally creating a loop.
But go for it by all means.
Also, not sure how charging the signal cable will have an effect on its capacitance?
Not saying it won't work. But there's not a lot of million dollar ideas out there left and pedals have been around for 60 years...
The DC to guitar worry is more concerning active electronics. They should be able to handle a 9v short. But I wouldn't risk it to save running 1 cable.
Not trying to debate or rebuttle. The potential risk/reward just doesn't balance out to me. Not bullet proof enough for my liking.

[Aentons]

Questioning, debate, and rebuttle are keys to knowledge and understanding, so  I'm happy to engage in the back and forth discussion. Thanks

You definitely make some good points that I hadn't thought of. I'm not really looking to build a product to sell. I'm just curious about interesting possibilities. I'll definitely have to do a little more research before attempting anything.

[jwin615]

I feel like it would work, mostly. But then, one day, you'll potentially be next to something emitting the wrong EMI/RFI and your rig will sound like a swarm of bees in a sawmill.
You can't bandpass everything because you'll have no signal so any noise that does find it's way there between 20-20k, it's going to be there.
I've seen interference do weird things in and out of the audio spectrum.
I had an unshielded cat5 hooked to a peripheral monitoring system for an MRI cause major image artifacts a few months ago.  That single weak point leeched noise across the ground plane of the MRI magnet itself and wreaked havoc. Those systems have so many filter and fail-safes, you couldn't count them all and operate in faraday cages. But a new weakness was implimented that the system wasn't designed to handle.
Don't underestimate noise, I guess is what I'm saying.
Use high quality cables, cross AC at 180° and use filters.
This implementation may work fine 99% of the time(as did the 3rd party monitoring system) but when it fails, it won't be pretty.
Or maybe it works fine in the basement but at the one local club, it causes mayhem because there's a transformer or high voltage line under the stage...
It's possible, if done correctly, and a cool bar trick. But it's not anything I'd rely on.
Also, keep in mind, any noise will start at the begining of the chain and pass through every amplification stage in your chain. Even a tiny bit could be a major issue.
And, if the amp doesn't block DC properly due to some failure, 9v will probably punch the cones through the grill cloth, briefly. Until the voice coils melt and dead short. Then the output transformer goes...

Not worth not running an extra cable. Too many potential failure points.
Or, as I said earlier, make a 5 pin cable (shield being 1 pin). Drive it with a good buffer and use quality cable. Or ziptie two cable together. This is all a solution looking for a problem, to be frank.

[Aentons]

Fair enough. It sounds like there are 2 main issues. Unwanted noise in the signal path and potential failure of DC blocking to the amp and guitar.

I have some ideas for potential  workarounds but I'll have to do some more research.