[sdiy] Suggestions for oscillator coarse control methods?

[Michael E Caloroso]

Somewhere I saw a tuning pot design that was clever.  It only works if
your coarse tuning pot is zero volts at center rotation.  I forget the
exact circuit, but it involved diodes with resistors in series such
that around center rotation the tuning resolution was finer.  That
would be a lot cheaper than a center detent pot.

I'm getting ready to do some radical modifications to a fixer-upper
ARP Avatar.  I have NOS Schade momentary switches (like those used on
OB-X/Xa and early Sequential stuff) and I'm implementing them for
octave switches.  I'm including one to disable out the coarse tune
slidepot, but you still want that control when using ring modulation
or hard sync.

MC

On 1/23/20, Quincas Moreira <quincas at gmail.com> wrote:
> 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:
>>
>>
>>
>>> 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.
>>
>>
>> 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.
>>
>> 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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