Wiring a foot switch to two OpAmps?

[syzygy]

Is it possible to get good results by hooking up a foot switch to change between two different 8 pin OpAmps in a distortion circuit (Insanity Box)?  In my situation, I only need to make 5 connections to each OpAmp: Power, Ground, In+, In-, Out.

I was thinking of getting a 3PDT foot switch and always leaving the Power and Ground tied to both chips at the same time.  That leaves the In+, In-, and Out to tie to the foot switch.  Is this a safe bet?  Am I missing any considerations?

Thanks!

[R.G.]

Why?

[syzygy]

Both OpAmps sound distinctively different and equally good in their own right, and having a switch will allow me to choose the sound I want to record with (home recording) depending on the musical situation.  The two OpAmps I chose have different pinouts, so I can't just swap the chips out of their sockets, and anyway I want to make both sounds available permanently.

Have I just asked a question that no one has ever considered before?   :shock:

[R.G.]

Have I just asked a question that no one has ever considered before?

I haven't seen it, but I suppose it must have been asked.

OK, you got some reasonable reasons.

Here's the problem. When you switch from one to the other, in the 20mS interval while the switch is switching, what happens? Does the +, the - or the output come loose and/or make contact first? And what's the originally-loafing, but now working opamp been doing? Is it's output lolling down at ground or up near one of the power supplies? Or oscillating?

The likelihood is that sometimes it'll switch fine, sometimes it'll GRITCH!! loudly.

If I had to do that, I'd want control.  I'd hide both opamps behind something like a CD4053 triple SPDT CMOS switch, and included a separate couple of CMOS switches to make the "off" opamp have an output-to-inverting input short while it's not working, just to keep it from doing funny stuff while it's loafing, and also a "mute" switch to mute the output of both opamps while the switchover is happening. By using !FAST! CMOS switches, you can ensure that the mute comes on first, the opamp is switched, and then the mute lasts long enough for the now-working opamp to have time to settle. The resistance of CMOS switches is inconsequential compared to the opamp input impedance, and feedback around the working opamp makes the CMOS resistance not matter - much, anyways. The mute swallows all the CMOS charge injection except its own, and shunt switching is always quieter than series anyway, so this has a good chance of working OK. 8-)

If you only switch while power's off or while the output is not plugged it, it'll work fine with a 3PDT. If you want on-the-fly switching, a footswitch is kind of a stone axe way. But stone axes **are** what killed that wooly mammoth and started us on this little quest, so it can work.

[Khas Evets]

What about mounting the opamps and feedback loops for each opamp on the board and switching only the input and output of each circuit? You could ground (or Vr) the input not in use. It would require a dual 1M pot though.

[cd]

Have I just asked a question that no one has ever considered before?

I haven't seen it, but I suppose it must have been asked.

And you've answered as well, R.G.  I'll see if I can't find it in the search, but I know you (or Mark Hammer maybe) explained the whole loud gritching thing.

BTW I'd follow Khas's suggestion above.  If the circuit is worthy of a switch, isolate it to two points and do the switching.  You don't even have to switch the input and output, just switching the output will do if you're feeding the input from a low Z source.

[j.frad]

Hey why not built two circuits? You could put them in the same box with a general bypass switch and a insanity1/insanity2 switch.
Might not be easier than 2-3CMOS switches and risking oscillation and pops?

[syzygy]

Thanks for the info guys!  I'm not sure yet which method I'll try.. I would have to do some research on the CMOS switches R.G. is talking about.  I looked CD4053 up on mouser, but I'm not sure the results refer to the same part.

Regards