Stereo to mono summing - would this work?

[cathexis]

Hi!
It's been very, very long since the tip of my iron was hot  - but things are clearing up and I wanna get back at it.

A friend of mine has a bass with separate output jacks per pickup. He has asked me for a summing box for it, in short a 2-in 2-out box that either bypasses a "stereo" signal or sums it to mono (identical output from both output jacks).

It seems simple, but I feel rusty and will humbly admit that I never really got my head around the concept of impedance... I found a few other posts on stereo summing here, but I'd be happy if someone could find the time to take a look at my schematic and see if there are any foreseeable problems with it.

Thanks!
Lars

[Seljer]

The summing circuit is ok. The input impedance however is a bit low (the 10k resistor from the input to the negative input of the first opamp), this isn't such an issue if his bass has got active pickups, otherwise just change all the resistors on the first opamp to a higher value (something over 220kiloohms).

Your switching circuit however wouldn't work. The "Y" connection between the jacks is there even when you've got the "stereo" setting selected


The quickest solution I can think of right now is to use two opamps as buffers, then a third one as the summing amplifier/mixer with apppropriate gain control. The buffers are always connected (so no true bypass here) and then you use half of a regular DPDT switch on each output, to choose between the output of the appropriate buffer or the output of the mixer. Use a 3PDT for an indicator. The mixer's output is out of phase but you could just add another inverting stage after it to fix that (and you could utilize a quad opamp and use up all the opamps in it).

If he wants true bypass you're going to have get a 4PDT switch

[PRR]

If pickups are passive and similar (not mag versus piezo), just short them together.

[cathexis]

Thanks for the input, especially for pointing out that glaring and rather embarrassing error on the output switching - I'm rusty indeed. I've got a 4pdt coming in the mail...

[Satchmoedie]

Craig Anderton has a mixer project that would do this. I think it is in his first book and used a 4739 chip. It is an 8 in one out mixer. Just build what is shown and do not do the other 6 inputs. You get a balanced XLR, or inverted and non inverted out. You can tweak the inputs to suit the mix. I suppose if you want to get crazy add an op amp and tone controls. It is in his first book. Projects for Musicians or something like that. I have used it on ES 355s, 345 Stereo and Gretsch stereo Chet Atkins guitars. I think it will work on active pups too. It works on Effects you wish to mix in. There may be a buffer 4000 series chip too. This is touted as an 8 in 1 out mixer. 1 out indicated mono in my experience. It is shown with XLR and 1/4" jacks.