This circuit converts the bipolar output of the MOTM 320 VC LFO into a single-sided
signal to be used with pedal inputs. It in no way modifies the MOTM 320. Consider it a
separate module. Thanks to Larry Hendry for his input. (You are welcome Mark) L.H.

Please note that the Saw output of the MOTM 320 already goes from just about ground
to a bit over 5V, so this circuit is unnecessary when using the Saw waveform with pedal
inputs.

This is a simple circuit consisting of two op-amps. The first op-amp is an inverting
summing amplifier. The summing amp divides the +15V supply by six (300/50 * 15 =
2.5), and the input voltage in half (100/50 * 5 = 2.5). So an input of -5V is summed to
zero. The second op-amp is an inverter. Without any input, the output of this circuit is
2.50V.

All resistors are 1/4W 1%.

Each op-amp is half a Texas Instruments LT1013. The first op-amp is run off of the -
15/+15V bipolar supply. The second op-amp is run single-sided (+15V and ground) to
prevent it from producing a negative voltage.

Bypass caps are not shown. I filtered the power supply on mine using a ferrite bead in
series with each rail with a 10uF (10 mfd) tantalum cap shunted to ground. Using
electrolytic caps is fine, perhaps even preferred, but tantalum caps are what I had on
hand. The second op-amp has additional .1uF (100nF) cap going from Vcc to ground.

I built two of these circuits on the same board using two LT1013's. One IC was used
for both summing amps, the other IC was used for both inverters. You can build the
entire circuit on one LT1013, but it will output a negative voltage if the input goes
below -5V.

The LT1013 was chosen primarily for two reasons: 1) it's DC accuracy 2) it will run
single-sided with inputs up to 300mV below ground. An OP213 has very similar
characteristics. The LT1013 is available from Linear Technologies, Texas Instruments,
and Maxim (MXL1013).

Here is the Schematic

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