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Using AOUT with Buchla 1.2v/octave


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As you probably know, the AOUT module is set up to support 1v/octave scaling, and that has been fine for me when I was using it with a Serge modular system. But I have been wondering how I could use my AOUTs with my Buchla-ish modular synth, which uses 1.2v/octave scaling. I contemplated building an opamp based solution, boosting the voltage a bit. But this seemed a bit of a hack to me, so I spent a bit of time understanding the opamp based outputs from the MAX525 DACs. I realised that by tweaking a resistor value I could get the AOUT to put out 1.2v/octave. I've tested this for a bit with two different oscillators (258r and 259r) and it seems fine for my uses. I'm no electrical engineer though, so if this mod blows up your rather expensive equipment I'm afraid you're on your own...

That said, for any other Buchla users who want to use an AOUT to output 1.2v/octave, just change RY to 1.8K on any channels you want. That should give you the extra bit of gain required, and then you can trim it to 1.2v/octave. FWIW I usually use the MBSeqV4 app, then use the nice new calibration stuff to get things super accurate. I'm able to get submillivolt accuracy over the whole voltage range. Top voltage is obviously a bit higher in this case, almost 13v from my measurements. That seems to have been fine in my system, but YMMV.

Also, I have set my AOUT so that V1-V4 have this tweak (1.8K for RY), and I use them for controlling Buchla oscillators. V5-V8 use the 2.2K resistor for RY still, and I use those channels for modulation CVs and to control the Serge oscillators which run at 1v/octave. All in all I'm most happy with this system and am glad I don't have to mess around with adding more circuitry.

Hope that helps someone in the future!  

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I’ve noticed that you are talking about the original AOUT module here.

I would like to try this out with an AOUT_NG so I’ve tried to reverse engineer what you’ve done here to apply it to the AOUT_NG. I’m coming up a resistor value of 4.7k to replace the 15k resistor in the op-amp negative feedback loop to increase the max output voltage to 13.81V. I’ve based my calculations on Vref being set at 2.048V.

Any thoughts on this?

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Hi folks,

nice to know this info is of use to some!

@Rowan I'm afraid both of my attempts at building an AOUT_NG failed so I gave up on that path. But yes to what Zam wrote, basically all I did was recalculate the opamp resistors. Don't know why it didn't occur to me years ago, but that's life :)

Good luck with your builds!

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  • 2 months later...

I finally got around to trying this out last night with an AOUT_NG and got it working. 

After testing a few values I settled on 10k as the replacement values for R9-R16 and could achieve 1.2V/Oct tracking. 

I suspect if the values was a little higher e.g. 12k, I would have been able to calibrate the module to 1V/Oct and 1.2V/Oct with the trimmers.


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