Thinking about making a super simple mixer - I would love to see some of the ones you all have made. Particularly interested in the faceplate layout. Show 'em to me!
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I did a combo mixer and multiple.
It looks very jank, and it is, but it gets both jobs done!
I also made a thread a while back about the super simple mixer if you are interested.
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I have been working on and off on my standalone mixer for…well…awhile. With many thanks to many forum members for helpful insight
The design is built around the case of this radio shack mixer (This one is mint, the other 4 I have are definitely not). I picked them up over the last 6 months or so on eBay for $15-30 each. Originally I bought one and figured I’d spruce it up a bit and use it, then I would add led faders, then I would swap the outputs to all 1/4, now I’m throwing away the original pcbs and just using the cases…it’s a slippery slope🤣
It’s a little late in the night for me now to go over alllll the features but basically it’s going to be a 4 channel mixer to stereo out. Mute, solo, Bass, treble, and fade controls on each channel and a stereo swap on the output. The swap, mute, and solo are cv/gate controllable using relays. And of course, an independent headphone output volume and and amp section
Pcb and overlay on case. Eventually wooden sides and fancy paintjob
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Oh man, great hack! I have that exact same model, must have bought it back in the 90’s back before RS became a full time phone accessories and batteries shop. Built my first guitar pedal from one of their little black plastic hobby boxes too…
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All I’ve got right now is this skeezy photo of my little mixer in-situ. It’s four channels with an attenuverter on the output. It’s DC coupled, with left channel normalled to -12V and the right to +12V, the idea being I could do some weird CV biasing/mixing if I wanted with nothing plugged into one of those channels.
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I made several Mixer module from N Ronneberg
with switch add
and also a mix of Sam simple mixer with a Cross fader from La roue tourne
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I picked up this mixer yesterday. It says Dynamix sound mixing system. It’s made in England but i can’t find much information.
Haven’t had time to play with it but everything is working and no scratchy pots at all 
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Sorry i didn’t realise this was for Kosmo mixers 
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It’s an interesting mixer though, I think it’s fine! Is it working all good?
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It’s working great and there are no noises or scratchy pots at all
I dont mind at all! That is a super good find.
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I am currently building a mixer with two routable outputs like this:
And now I am wondering if I should add another 100k to ground to each input sum to make sure that there is a defined level (0V) when nothing else is routed/connected to one channel. What do you think?
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I think that the opamp configuration with negative feedback and the noninverting input tied to ground should suffice. You should end up with a virtual ground at the inverting input when the circuit is operating normally (i.e. when inputs aren’t out of range) and even if there’s nothing connected.
That’s the theory anyway, I’d be tempted to bread board it just to check.
Cheers
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Not that you asked, and I risk sounding like a broken record (with the resurgence of vinyl I don’t have to explain that analogy any more), but the 100k input and feedback resistors in the second output stages can be reduced to 10k for a lower noise floor.
My understanding is the stabilization caps are not likely to be needed unless there’s the possibility of a capacitive load, so they could be left off the first stages and, with the series 1k out of loop, maybe from the second stages as well. If kept, your 100k and 100 pF gives a cutoff of 16 kHz, which is a bit low if this is for use with audio signals. I’d aim for more like 50 kHz. See Understanding stabilization capacitors - North Coast Synthesis Ltd..
I don’t think you do… if there is nothing connected to the inverting input except the feedback, then the voltage on that input would be equal to the output, not 0 V. So yeah, I think this would be unstable? I think? Maybe not, since it’s inverting? Breadboard it!
Which they might well be. I mean, you have six inputs and they can all go into the same mixer. It could be trying to put out 30 V or more. Rather than restricting myself to the first 1/6 of all the input pots when there are six signals, my preference would be to make the first output stage’s feedback resistor a front panel pot, so I can lower the gain as needed to deal with many inputs. Not that you have room for two more pots, of course. (But if it were me I’d make the module wider anyway, things are tighter than I like horizontally.)
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Sound advice. I did the second best thing and simulated it:
Says you’ll get around 10mV on the output and 450uV on the non-inverting input with this LM741 model.
Some small current is going to flow, even though we all pretend that op-amp inputs are ideal. On the BJT input LM741 it’s going to be nano amps, on the JFET input TL072 it’s more like a pico amp, but it’s still there.
Cheers
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Looks right. Similar in LSpice. Whereas if you float the + input it’s unstable.
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Thanks for your input! I changed the resistors on the second stage to 10k and removed the capacitors I also added resistors to ground from the pot wipers, to give it more of a log response, hopefully helping the issue you mentioned with only the lower part being usable when all (most) of the inputs are connected and the fixed gain. Probably something between 20k and 47k. Let’s see how that works!
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It is always good to have defined input values.
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