SYNTHESIS TECHNOLOGY
PAGE
17
MOTM-110 ASSEMBLY
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Read the voltage on RING MOD OUT (right side of R24 or on the jack). Adjust
the X TRIM for as close to 0.0 Volts dc as you can.
Repeat the procedure for Y IN. Set X IN to 0, Y IN to 2. Apply +15 to Y IN.
Set Y IN pot to +5.0, looking at the left side of R13 and ground. Adjust the Y TRIM
for 0.0 volts dc out of the RING MOD OUT jack.
VERIFYING THE UNIT IS FUNCTIONAL
You will now verify the VCA and Ring Mod are fully functional.
Apply an audio signal to the VCA IN jack.
Feed the VCA OUT signal to an amp or scope.
Apply power. You should be able to turn the GAIN knob up and down and hear/see
the audio signal out of the VCA OUT. With the knob set to 1, the signal is basically
‘passed through’ the VCA with no control voltage (CV) applied. Note that the VCA
can apply gain (see the OPERATION section for more details).
Apply a sine wave to BOTH the X IN and Y IN jacks (a Y-cord is handy!). Set
the switches to AC, the UNBAL to 0, and the X IN and Y IN pots to 7. You should
be able to hear/see that the RING MOD out is frequency-doubled version of the
input. Turning the UNBAL pot should increase the signal level, and change the
timbre slightly.
If you do not get these results, carefully check over the unit before contacting us for
assistance.
ELECTRICAL THEORY OF OPERATION
The following discussion explains how the circuit operates, and suggests area of possible
modification. Refer to the schematic.
VCA
The VCA section uses half of a CEM3330 dual VCA. The VCA has a linear response. The
1% resistor R2 sets the bias current for the 3330, and that’s why it’s a 1% part. The audio
input is AC coupled through C8 into R4. The RC circuit of R3/C9 is frequency compensation
for the 3330. The current from R4 and the offset current of the CV REJECT trimmer TP1
and R6 are fed to the 3330.
The output of the 3330 is a current, so a high speed, low noise op amp U2A is used to
convert the current to a voltage, using R27. You can set the output voltage to cover
different ranges by adjusting the value of R27.
