Can A/B Boxes Work With TRS Cables?

[Bryan R. Tyler]

Never tried this and I don't have an A/B on hand to do so.

[bongomania]

Most don't. I think Radial makes a balanced A/B, and it seems like it would be not too difficult to build or mod such a thing.

[JimmyM]

huh? what for?

[Bryan R. Tyler]

Originally Posted by JimmyM

huh? what for?

Using an expression pedal to control two different boxes with exp. inputs. The A/B box would need TRS in and outs to work.

I spent an hour after asking this researching it (should have done that first- sorry), and there's little on the market for it. One elegant solution I found was the Little Lehle 2.

[JimmyM]

wow, too complex for me! i keep looking for solutions that make things easier for me, not harder

[Bryan R. Tyler]

Originally Posted by JimmyM

wow, too complex for me! i keep looking for solutions that make things easier for me, not harder

I keep looking for solutions that make things smaller. 3 expression pedals on my board, one of which won't be used much, versus 2 and a small switch. Smaller. Hey- it's easier too! You owe me a cookie

[JimmyM]

choco chip or oatmeal?

[Bryan R. Tyler]

At your age I figured the only thing you had were those butter cookies that come in a circular metal tin that sits on your coffee table

[TheMutt]

This thread gave me a good laugh.

IIRC, when I was looking up the same thing, Wounded Paw can make you one if you needed it.

[JimmyM]

Originally Posted by Bryan R. Tyler

At your age I figured the only thing you had were those butter cookies that come in a circular metal tin that sits on your coffee table

you evil bastard! those cookies are quite tasty no matter how old you are. i like to dip them in my metamucil.

[Silent Fly]

Technically speaking, I don't see any major problem in building one.

I am not 100% sure it would work though. At any given time, there is always a unit that has the exp.pedal jack plugged in but no expression pedal connected. As a result, the unit sees the control voltage that should be supplied by the expression pedal floating.

Some unit may be ok with that but I suspect the majority of them may get "confused" by it.

It should be possible to connect to ground the unit that is without expression pedal but it will be like to close (open?) the pedal completely.

[bongomania]

Interesting point. OK this would add to the internal complexity and cost, but what about a sample-and-hold circuit? Whenever the A/B switch got actuated, the device would sample the reading at the exp. pedal input, and hold it at the "inactive" output.

Alternately, if the goal is to make the "inactive" device think there is no expression pedal connected at all (so its own knobs do all the controlling) then I think this might work: The jack is switched TRS, so when a plug is inserted it breaks a connection. If, at the other end of the cable, the tip and ring get tied together, wouldn't that fool the switch into thinking it was closed (so to speak)? If that worked, then the A/B device would just have to have a circuit that ties the tip and ring of the "inactive" output whenever the switch is actuated.

[Silent Fly]

Originally Posted by bongomania

Interesting point. OK this would add to the internal complexity and cost, but what about a sample-and-hold circuit? Whenever the A/B switch got actuated, the device would sample the reading at the exp. pedal input, and hold it at the "inactive" output. (...)

Doable but, as you said, not exactly a walk in the park. Especially considering that the hold value cannot drift.

Originally Posted by bongomania

(...)
Alternately, if the goal is to make the "inactive" device think there is no expression pedal connected at all (so its own knobs do all the controlling) then I think this might work: The jack is switched TRS, so when a plug is inserted it breaks a connection. If, at the other end of the cable, the tip and ring get tied together, wouldn't that fool the switch into thinking it was closed (so to speak)? If that worked, then the A/B device would just have to have a circuit that ties the tip and ring of the "inactive" output whenever the switch is actuated.

I suspect the unit might use a switched jack. In other words the jack needs to be physically removed from socket to break/make the circuit.

Moreover, it is possible that the unit, in order to save CPU cycles, checks the presence of the jack in the exp. pedal connector only at boot time.

An alternative solution could be adding two pots to the A/B box. Every pot replaces the expression pedal when the expression pedal is not connected. In this way the position of the "ghost" pedal can be set to a fixed value (e.g. half open). It is not ideal but it should work.

[bongomania]

Originally Posted by Silent Fly

I suspect the unit might use a switched jack. In other words the jack needs to be physically removed from socket to break/make the circuit.

Well yes, that's exactly what I meant, and I was trying to think of a way to fool that switch.

Originally Posted by Silent Fly

Moreover, it is possible that the unit, in order to save CPU cycles, checks the presence of the jack in the exp. pedal connector only at boot time.

Hmm.... Possibly, and that is the sort of thing that will be completely different from one manufacturer to the next, so a "blanket solution" like mine won't work in all cases.

Originally Posted by Silent Fly

An alternative solution could be adding two pots to the A/B box. Every pot replaces the expression pedal when the expression pedal is not connected. In this way the position of the "ghost" pedal can be set to a fixed value (e.g. half open). It is not ideal but it should work.

I like it. That way each device receives a control input, just selected between the manual pot and the foot-treadle pot. It would be like a bi-directional A/B.
Switch position 1:
treadle --> output A
pots --> output B
Switch position 2:
treadle --> output B
pots --> output A

[Bryan R. Tyler]

As usual, my idea ended up being far more complex than I had hoped

[JimmyM]

Originally Posted by Bryan R. Tyler

As usual, my idea ended up being far more complex than I had hoped

have a butter cookie.

[Bryan R. Tyler]

Can I have some ribbon candy instead, sir?