Help with switching between op amps

[rockhorst]

I'm planning a Screamer Lab kinda thing (sadly, beavisaudio.com is down at the moment?). I'm having a hard time deciding on what to do with the op amps. Dano (from beavisaudio) never got his op amp stack to work (but it may have been a wiring mistake). I've seen other posts contemplating the idea of switching op amps, but no real definite answer. So I thought I'd try one more time .

Let's say I start with a 4558 op amp in the circuit. Also connected is something else, say a TL072. I put pins 3 (+ In 1) and 7 (Out 2) on a DPDT. Would this be sufficient to switch between the opamps, or is more needed? Is it a problem that the feedback loops are still connected before the input? Is there any additional loading of the circuit? Other ways to do it? I'm looking to do this with about 4 opamps on a rotary switch, preferably two pole but four pole is also possible.

After I get this sorted out, I have to decide if I also want to include stacking opamps. So that would mean two rotary switches each going between opamps. Any additional problems with that?

[merlinb]

Are you against each opamp having its own feedback network?

If not, you could let the input signal feed all the opamps at the same time, and the switch just needs to select one of the outputs.

A second pole of the switch could connect the gain control to the relevant opamp.

[rockhorst]

They have to share the same feedback network I'm afraid. There's already 10 switches on there for switching clipping diodes...Imagine having 4 separate feedback loops!

[merlinb]

Yikes! 

Maybe its time to consider CMOS switches...

[rockhorst]

Haven't you seen the Screamer Lab by Dano? Switches galore, uber cool.

Anyways, I mightt have a good workaround: since I'm in favour of just a passive low pass filter as a tone control, a la Zen Drive, I could just switch the halves of the opamps that are in the clipping section, and disable all but one of the other halves and use it as an output buffer. I don't think that'll affect tone too much for comparison purposes. That can be done with a 3 pole rotary.

[amptramp]

As long as you can switch inverting, non-inverting and output connections, it should be OK.  Watch for the buildup of capacitance at the non-inverting input.  Long leads here may cause excess capacitance to ground which will decrease feedback at high frequencies, boosting high gain.  You will need to add or increase feedback capacitance across the feedback resistor to counteract it.  If the high-frequency gain is excessive and the rising gain characteristic crosses the falling open-loop gain of the amplifier, you may get oscillation.  Even if it doesn't oscillate, you may get ringing at high frequencies.

[rockhorst]

Watch for the buildup of capacitance at the non-inverting input.  Long leads here may cause excess capacitance to ground which will decrease feedback at high frequencies, boosting high gain.  You will need to add or increase feedback capacitance across the feedback resistor to counteract it.  If the high-frequency gain is excessive and the rising gain characteristic crosses the falling open-loop gain of the amplifier, you may get oscillation.

Thanks for pointing that out. Wiring up the switchess involves quite bit of wire, spatially separating the opamps from the board which may not be so ideal...we'll have to see how that goes. Still hope to hear from someone who's actually done something like this succesfully.

[Mark Hammer]

Why?

[rockhorst]

@Mark: why what? (not sure what you're referring to)