[sdiy] Suggestions for oscillator coarse control methods?

[Quincas Moreira]

A note on the center detent idea. I personally tune with the fine tune while the coarse pot is all the way down. Then I can use my controller/sequencer/whatever to set the octave and send notes. If I want to go crazy I can grab the coarse pot and have fun with it, knowing when I go back to 0 it’ll be in A=440 and will sound in tune when using my controllers again. works a charm, no center detent needed. An additional octave switch can be nice, but IMO not at the expense of the wide coarse por. The concentric push-pull switch pot solution sounded great, though Inimagine it’s not a cheap part.
Cheers!

Sent from my iPhone

> On Jan 23, 2020, at 13:45, Mike Beauchamp <list at mikebeauchamp.com> wrote:
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>> On 1/22/20 9:03 AM, Spiros Makris wrote:
>> Hello,
>> I am looking for ideas on how to implement a coarse tune control on an oscillator. I am trying to get the most stable performance, based on my understanding of the topology (let's see how well I do), so the control method has to be drift resistant or, at least, not be susceptible to it. PCB surface is important, monetary cost less so (let's keep it under 5 euros for single quantities?). SMD when possible is preferred.
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> Interesting concern about potentiometer temperature dependency. Is it the same across all types (Carbon, conductive plastic, etc?) and I guess more importantly does a temperature dependent change in resistance even matter if the potentiometer is set-up as a voltage divider? (maybe only if it's feeding a relatively low-impedance input? if so then could you buffer the pots output before hitting the summing op-amp on a standard vco setup?)
> 
> You mentioned wanting the coarse tune to be stable.. so if you're doing it as a pot, are you using reference voltage sources to feed the pot instead of the rails?
> 
> My go-to setup is a 6 position rotary switch as an octave switch, but that's because I like larger chonky controls. For tuning I use a pot with a +/- ~7 semitone range so all frequencies are obtainable. The need for a further "FINE TUNE" knob isn't required if the diameter of this knob is large enough. The ability to finely tune a control depends on it's range but just as importantly, the diameter of the knob which dictates how many useable finite values you can actually get out of it.
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> Once I collaborated with someone whose eurorack setup included off the shelf oscillators that had a single tune control that covered the entire audio range and was just a 1/4" plastic shaft. He spent 10 minutes getting it "close" until everyone else decided to just tune to him.
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