This is a multi function module. I wanted to have a compact
and versatile addition to the basic VCS3 functions. So I decided for a
combined S&H and Glide circuit.
It only needs one ("Time") potentiometer (plus the obligatory
VCS3 Output Level pot for each module, of course), and one 2pole, on-off-on
toggle ("Mode") switch.
It has 3 basic modes of operation, selected by the switch:
(1) Sample & Hold with internal clock
Ordinary S&H with internal clock source. Clock rate
is set by the "Time" potentiometer. Because of the other functions, the
highest Rate (=minimum Time) is on the ccw end of the potentiometer. If
you don't like it, just reverse the potentiometer pins. (A linear potentiometer
+ expo converter is used, so it doessen't matter. You can even add Voltage
Control of Time / Rate here with a single resistor.)
(2) Portamento / Glide
Ordinary Portamento / Glide function. Again, fully voltage
controlled, so change a resistor value if you want to modify min/max range
or add external voltage control with one resistor.
(3) Glide & Hold
Here is where the new stuff begins. (Though I can imagine
that you can do similar things with certain Serge modules.)
Clock source in this case is Pulse wave from Osc. 3.
Remember the so called "Sample & Hold" circuit of
the Korg MS-20 ? Everybody who tries to use it for the first time runs
into the same problem: Sampling is not triggered by the edges of the clock
pulse, but it's gated by the pulse itself. When the clock is high, the
circuit is transparent and the output follows the input voltage, and when
the clock is low, the last input voltage is held. So it's really a "Track
& Hold", not a "Sample & Hold" circuit. If you wanted to have a
normal S&H operation, you had to set the clock pulse width to minimum.
But although this was annoying when you wanted to have
normal S&H operation, it still had its uses as a Track & Hold function.
So I wanted to include this function, too. I chose to use the Pulse wave
from Oscillator 3 as clock source in that case. (It only makes sense if
you have control over the pulse width of the clock source.)
With an external clock, the local "Time" potentiometer
of the module is free for additional features. So why not keep the Glide
function even in the Track & Hold mode. With "Time" set to minimum,
I have normal Track & Hold (as in the MS-20), and with larger "Time"
settings, I have a "Glide & Hold" function. When the gate (Osc 3) is
high, the output follows the input within a slew limit (i.e. steps become
ramps), and the hold phase does what it always does. I have made some experiments,
and you can indeed create useful modulations with such a function. Just
think that you can continuously change from ordinary Glide (when the Osc
3 pulsewidth is 100%) over certain trapezoid waves to ordinary staircase
Sample & Hold (with small pulse width) and even freeze the last voltage
(with 0% pulse width).
But there is more to come.
Remember the old Electronotes article about a S&H
with variable series resistor in the capacitor charge / discharge path
? Like a S&H with Glide "inside the loop" ? Unlike a Glide circuit
after a S&H, this will not smooth the edges of the stairsteps, but
change the distribution of the random output voltage steps. Voltage steps
closer to zero become more likely than larger values, so you can change
the charakter of your S&H modulation patterns. This works with my circuit
as well: Use an external (Osc 3) clock and set a small pulse width to get
S&H operation. Then play with the "Time" potentiometer to change the
S&H pattern.
DIY hints
I have designed this circuit for my Synthi Clone, so
it runs from +12V / -9V supplies. But you will find it easy to adapt it
to a standard +/-12V or +/-15V system.
There is an ordinary npn/pnp expo converter that you
could use for any OTA-based LFO or ENV or Portamento function. It's output
current feeds a pnp pair that will route it either to the internal clock
oscillator or to the slew limiter circuit.
The clock oscillator is a current controlled ota/opamp
integrator plus opamp hysteretic switch. The small pulse with required
for S&H is achieved by a direct 1kOhm path from the schmitt trigger
to the integration capacitor over a diode.
When this oscillator runs (in S&H mode), it pulses
short spikes of rather high current into the glide circuit. So the glide
switches between "almost immediate follower" and "almost infinite glide
time", which is in fact a S&H fuction.
In "Glide" mode, the glide circuit gets a variable control
current directly from the expo converter, and the local oscillator is tied
down to the negative rail to prevent it from interfering with the glide
function.
The "Glide & Hold" function is implemented very simple:
The external pulse wave will either short the control current to the negative
rail with a npn transistor, or will let it pass to the ota as in normal
Glide mode.
I have tested the circuit on the breadboard, using BC550C
/ BC560C transistors, TL074 opamps (and a 741 for the external clock input),
and an LM13700 for the two otas. But it should also work with other small
signal transistors, other JFET input opamps, and 3080 otas.
Feel free to build this circuit for non-commercial use.
