switching op-amps, as in the Screamer Lab

[ubersam]

Remember the Screamer Lab? I've been contemplating building a similar rig and was wondering what could be the ideal method of selecting which of the op-amps would be "active". In the Screamer Lab the op-amps are wired such that all the same number pins are connected except for the outputs pins 1 & 7. So when the circuit is powered on, all op-amps are on. As an alternative, I thought, why not wire all the same pins together except for the ground pin (pin 4). Then select which op-amp to activate by connecting its pin 4 to ground. This way, only one opamp will be "active" at any given time. What do you guys think about this? Any drawback or things to watch out for?

[aziltz]

i would think this would not work right.  I can give one example.  I don't think you can guarantee that the inputs will act as they should (high impedance) if the opamp is not properly powered.  By switching the output only, you insure the the extra unused opamps don't unnecessarily load down the input to the one in use.  *I Think*

[ubersam]

Good thought. But wouldn't the inputs of the un-powered opamps look like "dead-ends" to the signal? I think that the input of the opamps are high-impedance enough, specially the jfet input opamps, even when the device is off--even more so if the device is un-powered because there is no path to ground.

[aziltz]

that's what I am unsure of, and was trying to suggest.

I *think* I remember being told, that if the device isn't being powered correctly, you can't really invoke the simple "op amp rules" involving high impedance at the inputs, etc...

I could be wrong.  It wouldn't be hard to try it on a breadboard with some jumper and see if notice any different in sound.