[sdiy] Suggestions for oscillator coarse control methods?

[David G Dixon]

Sorry, my last message got garbled by the Synth-DIY thing.  Let me repost:

I like five controls:

1 - Octave switch (8 positions)

2 - Coarse control -- linear pot (maybe with centre detent) through 30k

3 - Fine control -- linear pot through 430k

4 - Ultrafine control -- linear pot through 10M

5 - Reference frequency -- PCB Trimpot through 30k

The octave switch and ultrafine control are important for calibration.  The
ultrafine pot could be removable, plugging into the PCB, or even a minipot
on the PCB.



-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of
Quincas Moreira
Sent: Wednesday, January 22, 2020 7:47 AM
To: Spiros Makris
Cc: synth-diy mailing list
Subject: Re: [sdiy] Suggestions for oscillator coarse control methods?

personally i like linear pots over the entire range for playability. I don't
mind being careful not to bump into it, and I find octave switches
constrictive.

Sent from my iPhone

> On Jan 22, 2020, at 08:03, Spiros Makris <spirosmakris92 at gmail.com> 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.
> 
> Here is what I have thought of so far, and the pros/cons I can come up
with.
> 1. Linear potentiometer. It can sweep continuously, which is good, but
because of that is less precise. Cannot do octave switching, for ease of use
or playability. Easy to nudge and detune. Cheap. Temperature coefficient of
potentiometers is not always quoted. Can replace with a small potentiometer
for nudge-proofing but at the cost of playability.
> 2. Rotary switch. Octave switching at specific steps. Up to 5-6 steps
commonly available, tactile feel, hard to nudge, expensive, moderate to
large surface cost, can be made very precise with suitable resistors.
> 3. Toggle switch. Up to 3 steps, easier to find and cheaper than rotary,
the rest is the same.
> 4. A digital hardware switch system, where pressing two pushbuttons in
conjunction with a timer and suitably chosen resistors and opamp, LED
indication. Looks impressive, can be very precise, high usability, jellybean
components, high part count, moderate cost, large surface area, large panel
area.
> 5. A bare metal uC with a rotary encoder or buttons, LED feedback, same
DAC as (4). Same advantages as (4) apply. moderate cost, low-moderate
surface cost, noise considerations, might require additional 5V regulator,
has to be programmed in-circuit.
> 
> My intent is to use one of the above in conjunction with a fine pot, with
+/- 6 semitones (or 12, it's debatable).
> 
> So what do you like as far as coarse/octave controls go?  What else could
I use?
> 
> Spiros
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