VCO Calibration Tool

[LetterBeacon]

Now I've got my ARP oscillators built and working I now want to scale them properly. I was thinking of building the MFOS Calibration Tool below but I have a few questions:

- I should be able to power this off my +/-15V modular supply, right?

- If I add more 10K resistors and adjust Point TPA so it reads 7V, I should be able to calibrate from 0-7V, right?

Thanks!

Schematic came from here:

http://www.musicfromouterspace.com/analogsynth/August2003VCO.html

[slacker]

Yeah that should work fine, except that, with the original values at 15 volts the pot will only let you adjust between about 5.6 and 6.8 volts. If you change the 68k for a 51k then the pot will have a range of about 6.5 - 8 volts.
You might get away with a 47k instead but that puts the bottom end quite close to 7 volts.

[LetterBeacon]

Brilliant, thanks very much for that!

What equation did you use to work out the voltage range?

[Strazdas]

Maths are here: http://www.calculatoredge.com/electronics/voltage%20divide.htm (or google "voltage divider calculator" for more and info)

[forbin]

An approach that I use (but I do access to the hardware) is to use a PC and a MIDI feed from a Soundblaster compatible GamePort to drive my MIDI->CV and then get either an Oscilloscope or DFM to measure the frequency. I then get the script to read in the frequency value and work out the slope of the curve and hence it's calibration. I am thinking of getting the soundcard to pull in the audio and do a zero crossing measurement to work out the frequency to avoid the CRO/DFM. It is relatively slow as it takes ~ 3 min to do a sweep but you do get a pretty accurate cal! I have also run the script over a weekend to see how the Tempco is coping... I really need a digital temperature sensor... I would like to clean it all up and make a PC application to do it... Not sure if there is any interest? One thing that it did show up though is that my MIDI->CV DAC was fairly wobbly and then I ended up putting in a DMM to take out that variable... all got a bit like a Science Fair experiment at the end...

[LetterBeacon]

That sounds interesting! I've yet to build my Midi to CV convertor though (the panel is on its way) so in the meantime I'm going to go with Ray's circuit.

I've made a stripboard diagram which I'm hopefully going to build on the weekend. I'm going to solder the resistor ladder straight onto the switch using 0.1% matched resistors.

Is it worth me putting some 10uf caps between the power rails and ground to smooth the power out a bit?

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[emdot_ambient]

Power conditioning is always nice...not sure if it's needed but I'd do it anyway.

Did you build it and did it work for +/-15V? I'm in need of the same thing.

[LetterBeacon]

Sorry for the extremely late reply!

I've literally just got round to building the stripboard layout and it all seems to work!

I didn't put power conditioning on as I thought I'd do as Ray suggested and put crocodile clips on the power wires so I can piggy back off the VCO's power.

[emdot_ambient]

Cool. Sounds like something I need to build.

[emdot_ambient]

Oh...did you actually build yours to do 7 octaves? Your stripboard diagram shows an output to Position 8, but the MFOS schematic doesn't actually use that position, it maxes out at Position 6. Adding 2 more 10K resistors to Postion 7 and 8 would give you 7 octaves total.

[LetterBeacon]

I actually built it to go up to 8 octaves in the end. I soldered all the resistors on the back of the rotary switch.