[sdiy] Transistor questions

[Spiros Makris]

An interesting observation in regards to the sawtooth core:
Its operation has an inherent non ideality that can be optimised but never
eliminated (you will never switch "instantaneously" no matter what); the
apparent simplicity comes at the cost of balancing transistor capacitance,
conduction resistance, trace inductance, comparator hysterisis and delay
etc. The reset portion of the waveform will last a constant amount of time,
which eventually becomes comparable to the adjustable ramp portion, and
that messes your tracking, as well as the waveform itself (although I'd
argue you won't be able to hear that difference in texture).
In contrast, the triangle core oscillator does not have this inherently
unachievable feature, since the discharge/charge currents are not required
to ever become infinite. Of course, the comparator propagation delay,
frequency dependence of hysteresis, capacitor leakage will create a set of
constraints in operation. However, it is admittedly easier to make a very
fast accurate comparator than making an adequate reset circuit for a
"wideband" tracking sawtooth core. The cost you have to pay is a
significantly more complex bidirectional current source, which cannot be
realised easily with discrete components.

On Wed, 5 Mar 2025 at 21:51, David G Dixon via Synth-diy <
synth-diy at synth-diy.org> wrote:

> My Yamaha practice amp has a tuner built in, and that is what I use for my
> guitar (when not just using my ears, like most of the musicians I listen to
> do).
>
> For the VCOs, I just hooked them up to my CV keyboard and played octaves
> and
> scales while twiddling the trimpot until it sounded good.  Then I switched
> up several octaves with my precision 1V-step rotary switch voltage source,
> and it sounded good at every octave.
>
> I'll do it with more precision when I actually build all 12 VCOs for the
> final build, but for now, at least, I am confident that the VCOs track.
> Given how simple they are (a matched pair of 3904s for CV and one cap, two
> transistors, two diodes, and one resistor in the core), there are actually
> very few reasons why this VCO shouldn't track very well.  It doesn't matter
> how old the design is -- good design is timeless.
>
> I attached a picture of my layout of the core -- this is the heart of the
> VCO -- the purple trace is the current from the matched pair, the yellow
> trace is +5V, and the dark green trace is the sawtooth output.  It is
> simplicity itself.
>
>
> -----Original Message-----
> From: Roman Sowa [mailto:modular at go2.pl]
>
> I can't believe that being a guitar player you still don't have any
> tuner app installed on your phone yet. I always use tuner app, the one
> that looks like Boss TU-3, to tune everything musical and it works
> great. Despite having various options to measure frequency with lab
> equipment.
>
> Kind of sad to admit that the simplest, low parts count VCO designed 50
> years ago is still one of the best tracking oscillators
>
> Roman
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