Circuit design for M32-style crossfader

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tomatoKetchup
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Circuit design for M32-style crossfader

Post by tomatoKetchup »

Hello,

I'm diving into diy electronics for the 1st time. I'm currently studying Sound production and we have an audio electronics class, but being 1st year my knowledge is still fairly basic.

With help from my teacher I designed the following circuit, which is basically a crossfader as found on the Moog Mother 32 patchbay, with 2 inputs and 2 outputs (the 2nd out is an inverted signal of the 1st).

Image

The 2 switches (KEY 1 and KEY 2) on the left-hand side simulate the internal switch of the 2 INs mini-jack connectors, so that when nothing is plugged here, a variable DC signal is output (same as on the M32).

In theory (and on Multisim simulations), the following circuit works as it should, but my teacher doesn't know much about Eurorack standards in modular synth world, that's why I'm posting the design here to get some feedback from you guys.

If you spot anything wrong here (again by Eurorack standards), or if you need more explanation of what each part of the circuit does, please let me know. Any comments to improve design are also welcome.

Hopefully I should soon be able to test the circuit on breadboard, we'll see how it goes! :)
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Xfader1.png
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euromorcego
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Post by euromorcego »

here is some previous discussion about a crossfader: viewtopic.php?t=87903

Disclaimer: I have no clue. But conventionally an eurorack circuit would have an 100k input resistor. And if the first OpAmp is a buffer, wouldn't you go in the non-inverting input? Also people would usually use a TL074. Likewise it is common to put an 1K output resistor after the OpAmp (but I don't think this is really needed and might even cause some problems).
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pinkandbluenoise
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Post by pinkandbluenoise »

You might want to take this up in the sdiy forum?
sludgeflesh
art decadence
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Post by art decadence »

Just off the top of my head, and based on my experience with the mother 32, i dont think you want thosw input caps there (unless you want to mix audio only). It'll block dc (CV). Not sure, but I think the mother 32 vcmix is dc coupled.

Someone else might be able to help further.
tomatoKetchup
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Post by tomatoKetchup »

euromorcego wrote:here is some previous discussion about a crossfader: viewtopic.php?t=87903

Disclaimer: I have no clue. But conventionally an eurorack circuit would have an 100k input resistor. And if the first OpAmp is a buffer, wouldn't you go in the non-inverting input? Also people would usually use a TL074. Likewise it is common to put an 1K output resistor after the OpAmp (but I don't think this is really needed and might even cause some problems).
Thanks for the link, I had come across this thread before but couldn't find it again.

Also thanks for the advice, I'll talk to my teacher about it, see what he thinks.
pinkandbluenoise wrote:You might want to take this up in the sdiy forum?
Sorry, didn't realise there was a dedicated sub-section for DIY stuff. I'll try to contact an admin and have it moved there.
art decadence wrote:Just off the top of my head, and based on my experience with the mother 32, i dont think you want thosw input caps there (unless you want to mix audio only). It'll block dc (CV). Not sure, but I think the mother 32 vcmix is dc coupled.

Someone else might be able to help further.
Ok, I always thought it worked the other way around. You needed the capacitors if you wanted the ability to mix audio AS WELL AS cv. But you might be right, thinking about it. I'll try to get more info. Cheers for the heads up anyway. ;)
art decadence
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Post by art decadence »

no problem! It depends on where capacitors are placed, but here they're coupling caps. Capacitors look like an open circuit to "DC" voltages at steady state, so they'll be "blocked" from the circuit.

I also recommend checking out the DIY forum. It's full of knowledgeable people and you can learn a lot there.

Best of luck with the crossfader!
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