LM3900 and bypassing

[RobA]

I've been mucking about with the LM3900 op amp to see what I could do with it and came across an interesting potential application for it in a pedal context. On page forty of the app note http://www.ti.com/lit/an/snoa653/snoa653.pdf, there's a circuit for using it as an audio mixer. The kinda interesting thing is that it has a switching mechanism on each channel of the mixer that can be used to mute the channel. I figured that this could be used to do buffered bypass switching so I tried it using a transistor for the switching and it works. There's another example circuit in there somewhere that shows how to use two of the op amps to setup a flip-flop circuit too. I haven't tried that part, but since there are four op amps on the IC, it might be possible to set up a flip-flop and the two switched sections using one LM3900.

I don't think that I'll be looking into this myself because I'm now pretty satisfied with using either the NJM2121 or a double latched relay for switching if I need something other than a 3PDT. But, I figured somebody else might want to play with it as a potential solution, so I figured I'd pass it along.

[alanp]

IIRC, the 3900 is a Norton op-amp. I'm not sure what that means, except that it gets used in synth-diy a bit. (I was broke at the time, so couldn't afford to actually solder anything.)

[RobA]

IIRC, the 3900 is a Norton op-amp. I'm not sure what that means, except that it gets used in synth-diy a bit. (I was broke at the time, so couldn't afford to actually solder anything.)

Yeah it is a Norton op amp. I've come across them in synth stuff too. The main difference from a normal op amp is that they work on the current difference at the inputs instead of the voltage difference that a normal op amp does. It changes the way they are biased with some interesting advantages in single-supply operation. It also means they can get some monster voltage swing -- which said to me "Hey, see what I sound like when I distort" . So far, the results have been pretty good sounding but noisy. I'm still playing at this point.

[Isaiah]

  http://www.cgs.synth.net/modules/cgs85_tws.html

[crashguitar]

Sorry to derail this, but I am unfamiliar with using the NJM2121 for bypass switching. Could you point me towards how to do this?

Thanks,
Chad

[RobA]

Sorry to derail this, but I am unfamiliar with using the NJM2121 for bypass switching. Could you point me towards how to do this?

Thanks,
Chad

No problem, it's all about switching ideas anyway. The basic idea for using the NJM2121D is in this thread http://www.madbeanpedals.com/forum/index.php?topic=10558.msg94192#msg94192. There are other ways to do it as well. I've tried it using op amp input buffers instead of the transistor buffer and it works as well if not better. Using an op amp also leaves you the free op amp half to buffer the Vref supply.

Another way to do this I think would be to use two of the NJM2121D's to form a DPDT switch. I'm going to breadboard up this idea in the next few days.

[crashguitar]

Thank you very much! Please let us know about your results if you breadboard a dpdt.
I don't have any in my inventory, so it may take a while before I can give it a go.

Chad