AB Box switch Idea

[manson]

Hi,

I came up with this idea for an AB box. The hot and ground are both switched between 2 outputs. The unused output is grounded to the power ground. The idea was to have this fully isolated, the signal and power grounds are separated. I want to use this to switch between the normal and tremolo channel on my Twin Reverb. It's somewhat based on the Fulltone design.

The question is: will it work (without humming, popping etc).




Please let me know what you think, because I'm relatively new to this and it might be complete rubbish

[Skruffyhound]

Basic wiring works for me.
Don't think it will pop as such but you may get some volume jumps, other things not being equal so to speak. Anyway now you are bumped so now someone clever can come by and tell me if I'm right or not
Check out Beavis, if you haven't already  
http://www.beavisaudio.com/techpages/PedalHacker/OrderSwitcher.htm

Edit: Beavis box is doing something similar to what you want, note the lack of caps and resistors, thats my point, the routing is not relevant for your purpose.

[MikeH]

To switch between channels on the same amp you can use any old a/b design.  Usually you only run into popping issues when you're switching between 2 amps.

[Processaurus]

It's rubbish  .  The drawing doesn't work, but your concept is an interesting idea. An auto ground loop removing A/B switch.  You could do what you want with a 3PDT but no lights (hint, left pole of your drawing is the same, but the input ground goes to opposite lugs on middle and left poles).

4PDT (or use relays) and you get lights.

I can't guess without actually doing it, if it will pop.  Maybe not. There would be a moment as the switch moved between contacts that the channels input and ground would be floating.  Maybe in reality the different poles contacts don't all switch 100% simultaneously.

A ground lift switch on a regular AB switch (with an insulated 1/4" jack for the ground liftable channel) would do the trick as far as being able to cut ground loops.  Then you could ground the unused output to the circuit/chassis ground.  But like Mike H said, there is probably no ground loop problem just switching between channels on a single amp.

[Skruffyhound]

Aw Crap, shouldn't answer posts in the middle of the night, still the bump worked and you got your answer. You didn't have enough poles for an LED and the grounding thing, should have caught that Oh well, nice idea by the way.

[PRR]

> to switch between the normal and tremolo channel on my Twin Reverb.

Then there is no point in switching the grounds; they wind up at the SAME place.

The un-used input should be grounded so that open-cord buzz is not amplified

The case should probably be tied to audio ground. On guitar this is jack sleeve, so you may as well use bare metal jacks.

There is no reason the LED circuit needs ANY ground. It will work a million miles in space. It probably makes no darn difference either way. So why have the extra labor and chance for confusion?



BTW: how do you turn-off your LED and battery at the end of the night?

[Quackzed]

yeah, i'd ground everything... the function is the same in the non-isolated drawing but you dont need to double duty the poles of the switch... but the idea of switching grounds may have some potential advantages...

[R.G.]

Then, let's not forget the big hum issue - the amps probably have "grounds" which are slightly different voltages.

Some pitfalls lie in here.
1. It's going to hum unless you put it into a metal box. Once you do that, unless you buy plastic bushing jacks, the bushing on the jacks will connect the "ground" of each jack together, and you'll get hum on one or both amps for some setups. Which setups depend on the specific amps and how much they leak AC to "ground".
2. However, if you don't attach the "ground" of each amp to the guitar/input ground when it's switched there, the live amp will hum. So you can't leave all the jacks not connected.
3. Probably what you want is to connect the metal box to the input jack ground, and leave the amp output jack "grounds" unconnected to the box. However, what is required then is that the live amp has its ground connected to the input jack ground (and the box) when it's live. When the switch is thrown, you want to connect the input jack ground to the new live amp, and disconnect the muted amp from input jack ground and the box. But at the same time, the muted amp needs its signal and ground connected, and no other connections to the box.
4. The power supply ground for the LEDs is irrelevant, except as it may bring in ripple and hum from its power source if you're not careful.
5. The next thing you're going to do is try to figure out how to introduce a "both" connection. This introduces the necessity of connecting the grounds of both amps together again, and again opens the door for hum.

The approach that works it to ground isolate one of the amp jacks with either an amplifier or a differential amplifier to take the difference of voltages between the "grounds" off the signal that goes to that amp. There are examples of how to do this at GEO.

Multi-guitar/multi-amp switching isn't hard because of the signals - it's hard because of the grounds.

[PRR]

> the amps probably have "grounds"

Manson posted "I want to use this to switch between the normal and tremolo channel on my Twin Reverb."

I read that as _one_ amplifier. With both inputs screwed to the same metal a few inches apart.

I actually think this switching should be done inside the amp, but deleted that section because the solution got messier than Manson wanted.

> Multi-guitar/multi-amp switching isn't hard because of the signals - it's hard because of the grounds.

Yes indeedy; that calls for a much more thoughtful approach. Sometimes you get lucky, but if you need to be lucky every night in every venue even with a back-up amp on deck, it's messy.

> The power supply ground for the LEDs is irrelevant, except as it may bring in ripple and hum from its power source

I was trying to pretend the battery really was a battery. Which can just float. But then I realized the battery would run all the time, so there is still an "issue" to kick at.

[R.G.]

> the amps probably have "grounds"

Manson posted "I want to use this to switch between the normal and tremolo channel on my Twin Reverb."

I read that as _one_ amplifier. With both inputs screwed to the same metal a few inches apart.

So did I. But I took some of the later comments as a generalization of this to any AB switching. Hence my answer.

I actually think this switching should be done inside the amp, but deleted that section because the solution got messier than Manson wanted.

Yep. That's the best way to do this for the case of within one amp.

> The power supply ground for the LEDs is irrelevant, except as it may bring in ripple and hum from its power source
I was trying to pretend the battery really was a battery. Which can just float. But then I realized the battery would run all the time, so there is still an "issue" to kick at.

Yeah. At one time, people really did put batteries in all their pedals. I think that this practice is going extinct.  With good 9V power supplies in the $20-$30 range, you only have to buy ten $2.50 9V batteries to start losing money. So what will happen is that even if the original builder uses a battery, he'll decide to change to a power supply pretty quickly, and then he'll be back here wondering why his system now hums.

It's the people that are the problem... 

[Beros]

I'm bumping this one.

It's rubbish  .  The drawing doesn't work, but your concept is an interesting idea. An auto ground loop removing A/B switch.  You could do what you want with a 3PDT but no lights (hint, left pole of your drawing is the same, but the input ground goes to opposite lugs on middle and left poles).

4PDT (or use relays) and you get lights.

This seems interesting..

Could you please draw a layout for doing this with a 4pdt switch.

Thanks!

[Processaurus]

Feel free to post your own, I can look it over.  You'd just need to figure out with your 4pdt (the green one at small bear looks nice) how to satisfy these rules:
Switch clicked in the A position:

Input + = Output A+
Input - = Output A -
Input + ≠ Output B+
Input - ≠ Output B -
Output B+ = Output B-

In the B position:

Input + = Output B+
Input - = Output B -
Input + ≠ Output A+
Input - ≠ Output A -
Output A+ = Output A-

If I was originally right that can all fit on a 3pdt, leaving a pole open to exclusively switch an LED on or off like a regular stompbox, and I would do like PRR suggested, and not connect the LED power ground to chassis ground.  Let the LED circuit and power be isolated from the rest of the stuff in the box.

I would connect chassis ground to the input jack -.

[Beros]

Ive been scratchin my head for a few weeks now.. Still haven't figured it out. Please help!