JH's VCO

[jhaible]

Well, I'm not really competent to comment on the theory of thyristors, but
as I brought this (Korg) circuit up on the list, allow me a few remarks
nevertheless:

(1) I only tried the origininal transistors from various Korg instruments.
They are
cheap, and still available from Reichelt Elektronik in Germany, so I didn't
care
much about them. I did plug in other transistors (BC550 / BC560, my general
purpose
selection), and the VCO did work with them as well, but I didn't check for
performance with these transistors. So all the following applies to the
version with
original transistors.

(2) 30kHz is not that bad, isn't it ? In fact, this was the first time I was
perfectly
satisfied with a DIY VCO in the 2' Footage, and that without a HFT trimmer !
(And remember, the VCO core always runs in the 2' range, because the other
octaves are generated with dividers.)
Ultrasonic applications are a different story, of course. Did you choose a
smaller
capacitor, or a higher current, to push it to ultrasonic, btw ?

(3) Saturation / BE capacitance:
Wouldn't this capacitance be more or less just in parallel to the (desired)
integration
cap of the VCO ? (Have to check again - just "gut feeling" so far  ...)

(4) Accurate tuning
My circuit (and the the VCOs of some Korg synths) only need a few octaves
range
for the VCO core (divider ...), BUT the MS-20 uses the same circuit without
frequency
divider, and it goes over 6 octaves. How well, I don't know. (MS-20 owners
??)

JH.





----- Original Message -----
From: Martin Czech <martin.czech at intermetall.de>
To: <synth-diy at mailhost.bpa.nl>; <mbartkow at ET.PUT.Poznan.PL>
Sent: Thursday, July 01, 1999 4:10 PM
Subject: Re: JH's VCO


>
> :::I have been just playing with the thyristor-based VCO Juergen posted
> :::some time ago (well, just the core Q1-Q8, J1,J2 and U3, I don't need
> :::the interclock prevention circuitry yet). This circuit promises a high
> :::scale range due to the high speed of the thyristor switching. I
intended
> :::to use this VCO in a wavetable oscilator, so I needed a good ultrasonic
> :::range operation (to be scaled down by the sample counter).
> :::
> :::I observed accurate tuning within only about 4-5 octaves, and my VCO
> :::goes significantly flat over about 30kHz. I reckon this might be due to
> :::the general purpose transistors I used.
> :::
> :::My question is whether anybody tried this out and achieved wider
tracking
> :::range, especially in the high frequency range ? Do you use some fast
swi-
> :::tching transistors ?
>
> My nose says that thyristors may turn on quite fast,
> but may need considerable time to turn off.
> Total saturation....
> I don't have the design handy, iare there means to
> discharge the (saturated) emitter-base junctions
> to turn off?
>
> I rember how slow darlingtons turn off, if the big npn has no path
> from base to emitter. A 100 Ohm resistor made it 10x faster !
>
>
> Just my nose.
>
> m.c.
>
>
>