Doepfer A-178 User Manual
doepfer
A-178
Theremin Volt. Source
Antenna
CV Out
Offset
+
-
System A - 100
1. Introduction
Module A-178 (Theremin Voltage Source) produces
a
variable control voltage
closer your hand gets to its antenna.
You can use this control voltage in any modulation or
control process, and thus have access to an extra
system of real-time control in the synthesis process.
You use the Offset control to set the null point (zero)
of the control voltage output. Two LEDs give a visual
indication of the voltages produced.
The module also produces a gate signal at the
output: the signal goes "high" as soon as a voltage is
sensed which is above the threshold set with the
Threshold control
of the presence of a gate signal.
Theremin
which gets bigger the
. An LED gives a visual indication
A-178
gate
Gate Out
Threshold
This gives you the ability to produce a gate signal
simply by moving your hand.
1
A-178
Theremin
System A - 100
doepfer
2. Overview
A-178
Theremin Volt. Source
Antenna
CV Out
Gate Out
THER
Offset
10
0
+-
10
0
Threshold
➀
➁
➂
➃
Controls:
Offset : control for setting the null (zero) point
1
2 LEDs : LEDs to give a visual indication of the
voltage present at output
LED : LED to give a visual indication of the
3
presence of a gate signal at output $
Threshold : control for setting the gate threshold
4
"
In / Outputs:
! Antenna : antenna input
CV Out
", §
Gate Out : gate output
$
: CV outputs (internally linked)
2
doepfer
System A - 100
Theremin
A-178
3. Basic principles
The theremin acts as one plate of a capacitor and a
human body as the other plate. Moving the hand
towards and away from the antenna produces tiny
changes in capacitance (so tiny that they’re measured
in the picoFarad range). The electronics within the
theremin measure this change in the following way:-
The signal from an oscillator (whose frequency can be
subtly adjusted with the offset control 1) is compared
with the signal in another oscillating circuit, whose
capacitor is made up of the antenna and an external
object like the human body. If the capacitance is
changed (by, e.g., moving your hand) the circuit’s
resonant frequency changes. This is measured, and
converted into a control voltage.
Depending on the oscillator frequency, we may hit
either the rising or the falling edge of the oscillating
circuit’s resonance - and thus produce respectively
either rising or falling voltage as the hand gets closer
to the antenna. The module is factory adjusted so that
as the hand gets closer to the antenna, the voltage
rises. This can be reversed if required, though.
4. Controls
1 Offset
Control 1 is used to adjust the null point, so that the
control voltage at output " is at 0 V, when the hand is
some distance away from the antenna (more than
about 30 cm).
H Because this module, like all theremins, is
very sensitive to fluctuations in humidity,
temperature changes, etc., it’s necessary to
check adjustment each time you use it.
In addition to the offset control on the front
panel, there’s also an internal trim inductor
for
coarse adjustment
quency - see appendix on p. 8.
Using this trim pot for coarse adjustment may
be necessary if, for instance, you install
another antenna, and it’s then no longer
possible to set the null point with the front
panel offset control; or you want to use the
falling edge of the circuit resonance, so that
the control voltage generated gets lower as
you approach the antenna.
of this offset fre-
3
Loading...