SYNTHESIS TECHNOLOGY
PAGE
18
MOTM-110 ASSEMBLY
WWW.SYNTHTECH.COM
The VCA is controlled by the amount of current generated by the VCA MOD pot setting,
and the GAIN setting. If the GAIN is set to 0, and the input CV is +5, then the output
voltage equals the input voltage (unity gain). This is the ‘normal’ operating mode of the
VCA. Note that the GAIN voltage can vary from -5V to +15V. When the pot is set between
the -5V and 0 V, the CV is ‘buried’. This means that in order to get unity gain, the CV input
must be +5V plus the bias voltage’. If the GAIN pot is fully counter-clockwise, it takes a
+10V signal to achieve unity gain.
See the OPERATION section for interesting things you can do with different GAIN
settings.
RING MOD
The Ring Modulator design is based on a Bernie Hutchins
Electronotes
article, and was
designed by Thomas Henry. Essentially, the ring mod is a VCA circuit with one clever
resistor added. In our case, the clever resistor is R10. R10 couples the X input to the
output. The Y input drives the VCA’s CV input. If you look carefully at the design, it is
almost identical to the VCA with the exception of R10.
The output of the circuit is (-X)(-Y)/5. This is due to the 2 input stages (U3A and U2B)
being an inverting summer. The X and Y input stages are identical except the Y input has
the UNBALance pot VR3 as an additional DC voltage source. Since the ring mod is a DC
multiplier, the audio tone quality will vary with DC content. That is why each input has an
AC coupling cap that can be bypassed with the front panel switches.
Again, the output of the 3330 is a current, so R22 and U3B convert this to a voltage. An
additional small cap C14 is used to remove some high frequency overshoots in the op amp
due to pcb trace inductance. The 3330 and OP275s have very high slew rates, and can
easily generate harmonics out past 20Khz.
The ring modulator generates the sum AND the difference frequencies of the 2 inputs. For
example, if we feed in a 1Khz sine wave into BOTH inputs, we get the sum (2Khz) and the
difference (0Hz). What is 0Hz?, Why, DC, of course! If you were to look at the output on a
scope, you would see a scaled (remember, the circuit divides by 5) 2X frequency sine wave
with a DC offset.
Since the musical scale is exponential, the output of a ring mod sounds very ‘metallic’ or
‘clangy’ since the sum and difference frequencies are not harmonically related. But of
course, this is EXACTLY the effect we are looking for!
