Something like that. The switches are very good (and a bit expensive) MAX394CPP. They have an excellent break before make characteristic, and and brag a great deal about the absence of charge injection clicks you mentioned. The OP amps are really good chips too! LME49721s. Excellent low noise audio rail to rail op amps, that work well with 5V total.I buffer my main ins and outs with these, and yes I do have series resistance and some isolation capacitance wherever I deal with the signal in or out of the OP amps, for the very protection issues you mention. But, since I'm using 1/2V Virtual ground everywhere, including the ground references on all audio I/O, you don't get the usual sudden pulse/pop that often happens when inbound and outbound signals are suddenly referenced to a 0V grounds, and a capacitor needs to charge. Also, the incoming supply is from a regulated source, but your suggestion of adding some extra decoupling would not be a bad idea!
Yes, R11 through R18 are there to offer a virtual ground bias to all the returns from the pedals, as most of them seem to be internally capacitively coupled. So they need to be "pre biased" to vitrual ground voltage too. At least on the bench it works well, even with a very sloppy-jalopy version of the circuit. I've had a huge number of friends come over with all manner of pedals to test with. I think I owe much of the quiet switching operation to the quality of those MAXIM chips. Now if only i could find them Cheaper! Intercil makes a drop in replacement cheap, but you need to buy 1000 (Grrr!!! )
BTW, my first switching scheme involved opto isolators, which of course switch slowly so there are never any pops. But as you know, they create other problems during the brief period when mutually exclusive signal paths are momentarily both conducting (think "CHIRP 🙂) But anyway, thanks for the input.
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Hmm, 5V will clip some pickups, although this has the advantage that you won't exceed the rated input voltages for the analog switches. A lot of people think that clipping a pickup over 1~1.5V(Pk-Pk) is acceptable, and some don't even achieve that much output, but I don't think any clipping is acceptable in an audio switch.
Duh! Why do you think that might be?
Duh! Why do you think that might be?
I have an old Les Paul with Humbuckers that are pretty hot. And even when I really bang the crap out of it and look on a scope, I've never seen peaks beyond a volt P-P. But I guess its possible. I wouldn't want any clipping either. But these op amps are pretty tried and true rail to rail devices. So that's theoretically good to 5V P-P. . I've never seen any passive magnetic pickup put out anywhere near that. maybe if you put a ring of guitar picks on a chain saw.
Now Pizzo pickups might peak out, and acoustics with active preamp s might. But I think any clipping on those and I'll be in good company.. Hell I have some decent quality equipment by Bherenger and Sampson that will clip those inputs too.
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