A Guitar Switchbox Circuit

[Paul Marossy]

Not really an effect circuit, but something useful nonetheless. I recently had the need to switch between two different guitars using the same amplifier, so I came up with this simple little circuit:

http://www.diyguitarist.com/PDF_Files/GuitarSwitcher.pdf

Hope it's useful for someone who might need something like this.

[MicFarlow77]

Paul,

I love the Parkers in that PDF!

Speaking of Parkers... Saw Jeff Cook (Formerly of Alabama) playing on TV over the weekend and he had a Brite Neon Green Parker Fly! Looked really sweet!

Thanks,

Mick

[Paul Marossy]

Paul,

I love the Parkers in that PDF!

Speaking of Parkers... Saw Jeff Cook (Formerly of Alabama) playing on TV over the weekend and he had a Brite Neon Green Parker Fly! Looked really sweet!

Thanks,

Mick

Yeah, Parker Flys are so cool. I drew those with AutoCAD using a picture I took of my Mojo Nitefly. I imported that into the program and traced over the image. I've used that method for quite a few things...

[bipedal]

Paul,

Could this also work "backwards", so to speak? -- One guitar, switching between two amps?

Thanks,

- Jay

[Paul Marossy]

Paul,

Could this also work "backwards", so to speak? -- One guitar, switching between two amps?

Thanks,

- Jay

Yes, it could. It grounds out the unused input. So, you could use it "in reverse" with one guitar going to two amps or with two guitars going to one amp.

[Andi]

Erm... am I missing something, or is this the standard AB switching circuit? 

[Transmogrifox]

FYI, I built something identical in function for my car stereo system to select between iPod and CD player.  It automagically detects the active channel and switches accordingly (the circuit uses latching relays).   It works great for car stereo.  Applied to your on-stage environment:  In theory you pick up the guitar you want to play and dig in... no stomping.  It would need a manual override switch to work in reverse.  

However, I think the only advantage of this over the simple circuit Paul has posted is that you don't need a switch or anything on the box -- just a black box with 3 audio jacks (and an LED if you wanted).  This is the kind of thing you could stuff into the recesses of your rack and forget about it.

If anybody is interested in a schematic I will post it.  

PJM -- With all respect, I don't mean to hijack your post.  Just thought it was a fitting thread to mention.

Andi -- Yes, it is just a simple A-B-Y box.  Nothing fancy about it.  Paul's posting this is just a friendly way to make a real common utility easily available for the newbie who may be perusing the forum. 

[MarcoMike]

yes, post it! now I'm curious!!!

Paul, why the need of grounding the unused guitar? do you think the eventual second signal will interfere or bleed into the signal path?

[Paul Marossy]

Erm... am I missing something, or is this the standard AB switching circuit? 

I dunno... I don't recall ever seeing any "standard AB" switching circuits that ground the unused input. I specifically wanted the unused input grounded because I am in playing in a very small area, using heavy distortion tones. I don't want the other guitar sitting there feeding back when it's not being played and potentially squealing like a pig when I switch it on. I don't have time to worry about checking where the volume control is set and all that. If you saw what I had to work with, I think you would understand. 

FYI, I built something identical in function for my car stereo system to select between iPod and CD player.  It automagically detects the active channel and switches accordingly (the circuit uses latching relays).   It works great for car stereo.  Applied to your on-stage environment:  In theory you pick up the guitar you want to play and dig in... no stomping.  It would need a manual override switch to work in reverse.  

However, I think the only advantage of this over the simple circuit Paul has posted is that you don't need a switch or anything on the box -- just a black box with 3 audio jacks (and an LED if you wanted).  This is the kind of thing you could stuff into the recesses of your rack and forget about it.

If anybody is interested in a schematic I will post it.  

PJM -- With all respect, I don't mean to hijack your post.  Just thought it was a fitting thread to mention.

Andi -- Yes, it is just a simple A-B-Y box.  Nothing fancy about it.  Paul's posting this is just a friendly way to make a real common utility easily available for the newbie who may be perusing the forum. 

Huh, your automatic switching idea is cool. However, if it doesn't ground the unused input, what happens with the other guitar? It could be sitting there feeding back and when you switch it on, you might get a big squealing mess... 

[Transmogrifox]

Huh, your automatic switching idea is cool. However, if it doesn't ground the unused input, what happens with the other guitar? It could be sitting there feeding back and when you switch it on, you might get a big squealing mess... 

The open circuit condition of being switched off pretty well prevents it from "feeding back".  There's no feedback until it switches on (same in your case) -- the loop is effectively broken by the open contact.  It doesn't switch on until you play, so if there's a "big squealing mess" as a result of turning it on, you have a problem of a big squealing mess.  I can see grounding the unused jack when using the 1 guitar-->two amps configuration being more useful.  Grounding the unused amp input would keep noise from feeding into the amp.

I do see a potential problem for somebody who has a chain of high gain effects going into the auto-switcher, though.  If you didn't roll the volume off on your unused guitar, then effects of stage vibrations would cause a signal to appear on the sensing input, and possibly override your intended channel (possibly an acoustic guitar). 

That said, I think the auto-switcher would be useful in many situations.

Here's a question for you:
What happens to the effect pedal that doesn't have short circuit protection on its output?  What about a person with an active pickup system in their guitar?  If it's the last pedal in the chain, you burn up its output op amp or transistor -- of course, we all only use DIY FX so of course they all have short circuit protection on the output

For those of you interested, I will post the auto switcher later on when I have had a chance to scan it or otherwise digitally sketch it.

[Paul Marossy]

I'm curious to see the schematic for your auto switcher circuit, sounds cool.

On second thought, I suppose microphonic feedback isn't likely to happen since the unused guitar isn't feeding back into an amplifier, right? Or could it still happen? I still think the possibility exists, so that is enough for me. This is kind of like asking if a tree falls in a forest and no one is around, did it make a noise? 

The bottom line is since a DVD is being made of this musical I'm part of, I don't want to take any chances with unexpected noises. That's the only point I was trying to make. 

[Transmogrifox]

Here's a link to the schematic:
http://www.geocities.com/transmogrifox/DSCN0520.JPG

Let me know if there's a problem viewing it.

I put a star next to some part values (capacitors) in there.  Those relate to the response and release times, which are relatively long.  I also goofed with the 100k resistor.  The 10k and 100k's should be in opposite places. 

Anyway, it probably wouldn't make much difference to the operation, other than a slower attack time.  The attack should be pretty fast, while the release should be pretty slow. 

Notice that there needs to be a moment of silence on both channels before either can switch.  This amount of time is set by the 10k resistor going to the bjt's and the capacitor.  The purpose of that whole setup is to make this thing easy on a battery.  The 10k R slowly charges the capacitor during times of silence.  Then when one channel or the other goes active, it dumps its charge one direction or the other through the relay coil.  Then the transistors saturate and keep the current maxed at about 1mA, normal.  It's nice on the battery charge, but you do have to be aware that you need the one channel to be quiet before switching.

This works out fine for switching guitars because it takes about that much time to take one guitar off and pick up the other.... unless you drop one on the floor in the process...

Anyway, there it is.  The concept is well presented, but the circuit itself may take some tweaking to get the switching delays right.  Think of it as an envelope detector on a compressor or EF.

[theblueark]

Erm... am I missing something, or is this the standard AB switching circuit? 

I dunno... I don't recall ever seeing any "standard AB" switching circuits that ground the unused input. I specifically wanted the unused input grounded because I am in playing in a very small area, using heavy distortion tones. I don't want the other guitar sitting there feeding back when it's not being played and potentially squealing like a pig when I switch it on. I don't have time to worry about checking where the volume control is set and all that. If you saw what I had to work with, I think you would understand. 

I can't see your pdf, but by your description, it seems like the AB box me and many others have been building for ages.

Assuming the switch is:
1 2 3
4 5 6
7 8 9

I connect "input" to 1 and 8. "output 1" to 4. "output 2" to 5. And I ground 2 and 7. That way when input is connected to one output, the other output is grounded.
Using it backwards yields the result where one input will be grounded when the other is selected.

Beavis uses a similar idea in a slightly different wiring method: http://www.beavisaudio.com/projects/ABSwitches/

BTW, your site seems down  I haven't visited it in more than a year, wonder how much has changed 

[David]

Huh, your automatic switching idea is cool. However, if it doesn't ground the unused input, what happens with the other guitar? It could be sitting there feeding back and when you switch it on, you might get a big squealing mess... 

The open circuit condition of being switched off pretty well prevents it from "feeding back".  There's no feedback until it switches on (same in your case) -- the loop is effectively broken by the open contact.  It doesn't switch on until you play, so if there's a "big squealing mess" as a result of turning it on, you have a problem of a big squealing mess.  I can see grounding the unused jack when using the 1 guitar-->two amps configuration being more useful.  Grounding the unused amp input would keep noise from feeding into the amp.

I do see a potential problem for somebody who has a chain of high gain effects going into the auto-switcher, though.  If you didn't roll the volume off on your unused guitar, then effects of stage vibrations would cause a signal to appear on the sensing input, and possibly override your intended channel (possibly an acoustic guitar). 

That said, I think the auto-switcher would be useful in many situations.

Here's a question for you:
What happens to the effect pedal that doesn't have short circuit protection on its output?  What about a person with an active pickup system in their guitar?  If it's the last pedal in the chain, you burn up its output op amp or transistor -- of course, we all only use DIY FX so of course they all have short circuit protection on the output

For those of you interested, I will post the auto switcher later on when I have had a chance to scan it or otherwise digitally sketch it.

Trans:

Yes, please, I'd like to see it.  Oddly enough, I'm doing a community theatre musical under similar conditions to Paul's and I've decided I could simplify my life with some kind of guitar switcher.

Paul:
I like your grounding idea.  I might very well wind up building both of these!

[Paul Marossy]

Huh, your automatic switching idea is cool. However, if it doesn't ground the unused input, what happens with the other guitar? It could be sitting there feeding back and when you switch it on, you might get a big squealing mess... 

The open circuit condition of being switched off pretty well prevents it from "feeding back".  There's no feedback until it switches on (same in your case) -- the loop is effectively broken by the open contact.  It doesn't switch on until you play, so if there's a "big squealing mess" as a result of turning it on, you have a problem of a big squealing mess.  I can see grounding the unused jack when using the 1 guitar-->two amps configuration being more useful.  Grounding the unused amp input would keep noise from feeding into the amp.

I do see a potential problem for somebody who has a chain of high gain effects going into the auto-switcher, though.  If you didn't roll the volume off on your unused guitar, then effects of stage vibrations would cause a signal to appear on the sensing input, and possibly override your intended channel (possibly an acoustic guitar). 

That said, I think the auto-switcher would be useful in many situations.

Here's a question for you:
What happens to the effect pedal that doesn't have short circuit protection on its output?  What about a person with an active pickup system in their guitar?  If it's the last pedal in the chain, you burn up its output op amp or transistor -- of course, we all only use DIY FX so of course they all have short circuit protection on the output

For those of you interested, I will post the auto switcher later on when I have had a chance to scan it or otherwise digitally sketch it.

Trans:

Yes, please, I'd like to see it.  Oddly enough, I'm doing a community theatre musical under similar conditions to Paul's and I've decided I could simplify my life with some kind of guitar switcher.

Paul:
I like your grounding idea.  I might very well wind up building both of these!

David:

I'm in an enclosed space, approx. 8'x8' with an acoustic drum kit behind plexiglass, another electric guitarist and a keyboardist who also sings into a conventional microphone. The bass player is behind a wall on the set that is like an annex to the area where the rest us are. We have in ear monitors, but both guitar amps are miked so we can get feedback at will since it's a requirement for a couple of scenes. The idea is that we are supposed to be like a garage band on one end of the stage. It's very tight for all of us, it's standing room only for the guitarists and bass player.

So, as you can see, there is an opportunity for weird things to happen with all that stuff going on in such a small area. I'm not taking any chances.  So that's why I made this "conventional AB switcher" with grounded inputs.

[David]

Your space and performance requirements are more constrained than mine, thank goodness.  We're basically in a recording studio type of setup.  I stupidly decided I should use multiple guitars, so I've been moving the cord from one axe to the other.  Not easy when you're sitting.

First off, I like the X thing.  Very clever.  I would probably have never come up with that.  OK, does it click, pop, gurgle or make other noises when you switch?  Second,  I assume that I could use two independent LEDs if I wished?

There actually IS a "standard" A-B-Y switcher.  I built one in 2002 that was documented by a guy named John Atchley.  I think it was on www.jt30.com, but that was a long time and many, many "events" ago.  It worked, but IIRC, it thumped.

[David]

Trans, one for you:

What are the triangles in your schematic -- op-amps, comparators, OTAs, spear points?

[Paul Marossy]

Your space and performance requirements are more constrained than mine, thank goodness.  We're basically in a recording studio type of setup.  I stupidly decided I should use multiple guitars, so I've been moving the cord from one axe to the other.  Not easy when you're sitting.

First off, I like the X thing.  Very clever.  I would probably have never come up with that.  OK, does it click, pop, gurgle or make other noises when you switch?  Second,  I assume that I could use two independent LEDs if I wished?

There actually IS a "standard" A-B-Y switcher.  I built one in 2002 that was documented by a guy named John Atchley.  I think it was on www.jt30.com, but that was a long time and many, many "events" ago.  It worked, but IIRC, it thumped.

The X-thing is something I came up with on another circuit that I was goofing around with where I wanted to switch the tip and sleeve on a TRS cable. I adapted it for my purposes here.

It doesn't make any noises at all when switching, for me anyway. I tend to use fairly large value resistors with my LEDs, so I think that helps with avoiding popping noises due to LEDs switching on/off-  I think I used 1.4Ks on mine. You could use two seperate LEDs, too, but I like the bicolor LED because it's simpler as far as installation goes. I don't how well it would work "in reverse", though, haven't tried that. I think you're more likely to have a hum problem in that scenario because you are connecting the grounds of two different amplifiers together.

Here's a couple of pictures of my switch box:

[Andi]

I dunno... I don't recall ever seeing any "standard AB" switching circuits that ground the unused input. I specifically wanted the unused input grounded because I am in playing in a very small area, using heavy distortion tones. I don't want the other guitar sitting there feeding back when it's not being played and potentially squealing like a pig when I switch it on.

Ah - I've always done ABs so that the unused input/output is grounded - I thought that was normal.

I'm not sure how you'd get feedback from the other guitar when its output isn't connected to the amp (whether grounded or not), but I can't see the grounding doing any harm.

[Transmogrifox]

Trans, one for you:

What are the triangles in your schematic -- op-amps, comparators, OTAs, spear points?

Spear heads, definitely.

These represent op amps.  Any generic op amp should work.  LM358 is what I think I used for the car stereo thing.  You'll get the highest input impedance if you use a JFET op amp like the TL072 or similar.