I only ever associate a “mixer” with “audio” because my brain is wired that way. CV is a whole new concept for me despite it being around for ages. So I never thought about that being an issue. What would you have to change in the schematic to be able to have it recognize negative voltages for CV?
not sure but maybe with a bipolar led like in the schem for Sloth Chaos (Nonlinear Circuits)
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And you’ll have to drive it with an OpAmp because the transistor will only condut one way…
Somebody posted a schematic with values to get a nice proportional lighting of the LEDs, but I can’t find it right now… the one who did it, please stand up 
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It would be ideal if you could use a bi-polar LED so you can then have a visual reference for positive and negative voltages.
Would something like this work with a TL072?
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It works… kind of…
The LEDs don’t respond linearly to current (to be exact, it’s somewhere between the voltage at the input of the OpAmp and the brain that the non-linearity happens…)
So the schematics to get a nice proportional illumination is somewhat more complicated…
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Yeah. I recently tried that. It won’t work this way. For one there is a voltage gap where V is too low to light up LED. So there is blinking and non linear response after that.
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You can get around the LED forward voltage thresholds by putting the LEDs in the feedback loop:
but with this approach the signal source needs to provide the LED current, so depending on the circuit you may need a unity buffer on the way in.
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Or like this
and here the signal source does not supply the current. There is no current into the op amp inputs so there is none through R1, which means the voltage at the top of R2 is equal to the input Vin. Then all the current through the LEDs goes through R2 and is equal to Vin/R2. So the LED current is proportional to Vin, unlike what happens in that mixer circuit.
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It surprises me that people design mixers with a fixed gain in the mixing stage. Here there’s a gain control but it’s attenuating the mixing stage output. Why not put the pot in the mixing stage feedback loop? The issue is that when you sum multiple inputs it can push the op amp into clipping unless you confine yourself to the lowest parts of the input attenuators. But if you can reduce the gain, you can use the full range of the attenuators without clipping. Of course if you are only using one or two inputs you can turn the gain up.
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The schematic now shows an npn transistor as part of a circuit from the + rail to GND. Using a pnp transistor and a 2nd led one could make a circuit from the GND to the - rail which could be used to signal the presence of negative input voltages.
B.t.w. I believe that if in the original diagram the LED would be put on the other side of the transistor (so e.g. between the resistor and the collector), the transistor would start to conduct at lower input voltages, so showing some light for lower input voltages.
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You might be referring to something I’ve done and have written about in a thread. In Pedal-O-Matic I wanted to be able to see whether a pedal would output a positive or negative voltage. I designed a circuit to overcome the LED threshold voltage problem. I used an circuit where the led was connected to the output of an op-amp which would add the threshold voltage to the input signal using a trim pot, thus making it possible for the LED to shine as soon as the input voltage would just exceed 0V. One op-amp was used for positive, another for negative input voltages. Have a look at U1C and U1D in this schematic:
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I thing it was this
But now we have several options…
are you doing PCB panels for this , you know why I am asking …
I am, and for the VCO and VCA. It’ll be a couple weeks. Also for the LFO but there’s a sync input mod I want to try out and if I like it, that’ll be on the front panel, which will come later.
I’ll have a couple extras of each up on Tindie then. Or more than a couple — if you’re interested (or if anyone else is) I could order more than 5. But that order’s going in soon…
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cool let us / me know on the BST thread when you have them up.
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the cut off from your board that was a bit long for Kosmo format . it did come in handy
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I wish I could stay this organized
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