Help from someone who knows electronics...

timhulio · Post by **timhulio** » Sun Aug 03, 2008 6:57 pm

I need to build a mixer into a small pedal enclosure, and have all the inputs isolated - I just joined up two inputs with a simple A/Y box and it did some weird stuff to the synth input, like effects on the other input (a bunch of pedals) would effect the synth channel. I've got one of those cheap Soundlab 4 channel mono mixers and it works great, but I need a bypass stomper on it. A rehouse for that thing might be in order, it wasn't expensive, but I'd rather build one from scratch...
Anyway, hows this for the application I have in mind:
http://www.freeinfosociety.com/electron ... php?id=728

Will · Post by **Will** » Mon Aug 04, 2008 1:48 am

That schematic doesn't have all the inputs isolated. To do that, you'd need to use a buffer/amplifier stage immediately after each input, then connect the outputs of those buffer stages together. You could use 4 transistors (probably JFETs) or a quad opamp for neatness.
Can't find a schematic ATM unfortunately.

NickS · Post by **NickS** » Mon Aug 04, 2008 8:08 am

What do you mean by "isolated"? Do you mean just so they won't interact with each other? One way to do that cheaply is to use an op-amp in inverting mode. This shows a mixer using a dual op-amp, but you could use just the first stage. Positive input is effectively ground (for AC) all the time so all inputs look like they're connected to ground and don't interact.

timhulio · Post by **timhulio** » Mon Aug 04, 2008 9:09 am

Well, this is what I'm doing:

The main problem with a simple joiner box instead of a mixer is that when, for example, I turned on the mxr dist+ it would distort the synth output.

NickS · Post by **NickS** » Mon Aug 04, 2008 9:29 am

A simple virtual-earth mixer op-amp circuit should do the trick, then. You might find that the one-transistor circuit gives enough isolation for your needs compared to a joiner.

timhulio · Post by **timhulio** » Mon Aug 04, 2008 9:33 am

Ah great, thanks! I'll have a quick browse for an easy circuit...

timhulio · Post by **timhulio** » Mon Aug 04, 2008 11:22 am

Ah, the best I could find is this one:
http://www.all-electric.com/schematic/simp_mix.gif

The third ciruit, in the lower left. It effectively isolates all the inputs according to the text here:
http://www.all-electric.com/schematic/simp_mix.htm

However, the schematic isn't complete- it doesn't say what op-amp to use, or how to wire the thing in. Also how to power the thing.
Ideas?

NickS · Post by **NickS** » Mon Aug 04, 2008 1:24 pm

The link I posted earlier is better from that point of view.
http://www.elektroda.pl/cir/guitar/Simple%20Mixer.pdf
It shows how to power it and suggests an op-amp or two but leaves you with the problems of (i) working out which pin is which (from the data sheet) and (ii) wiring it up.

I'd draw it for you but I'm meant to be working at the moment...