Synthesis Technology MOTM-850, Owner'S Manual

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SYNTHESIS TECHNOLOGY PAGE 12
MOTM-850 ASSEMBLY 10/25/01
WWW.SYNTHTECH.COM
THEORY OF OPERATION
The MOTM-851 consists of 2 separate circuits: the voltage drive to the pedal and the
scaling/offset, and the Gate extractor.
The MOTM-850 can interface to 2 basic types of pedals: resistive and voltage-out. Resistive
pedals use a 3-conductor (stereo) plug, because 3 signals are needed: ground, voltage IN and
voltage OUT. CV Out pedals use a 2-conductor plug (mono) and only needs ground and CV
OUT.
The pedals we will use as examples for ‘resistive’ types are the Roland EV-5 and the
Yamaha FC7/FC9 series. They are probably others, but these two cover the kinds you are
likely to use.
In general, the resistive pedals are nothing more than a simple potentiometer (pot) in a
fancy case. The pot is used as a voltage divider to output 0 to +5V. This in turn feeds the
range/offset circuits and the Gate extractor.
This all sounds simple enough, but to make matters...errrrr....’interesting’ Yamaha and
Roland, on
purpose , wired their pedals up differently, and use different pot values. But,
being the clever people that we are, theses differences are compensated for in the design of
the MOTM-850.
Before we go into how the 3 types of pedals are dealt with, let’s look ‘downstream’ at the
other circuitry.
Dual precision op amp U4 (MXL1013 type) is used to ‘condition’ the output voltage from the
pedal to a wide range of possible outputs. U4A is a ‘reversible attenuator’ with unity gain.
The RANGE pot VR1 splits the pedal voltage into an inverter (U4A) to ‘straight through’ to
inverting summer U4B. We can also add in an external signal MIX IN through VR4 and
R15. Note that in the U4A stage, the MIX IN signal is attenuated by the ratio of R12/R15,
but the gain is ‘made up’ by the ratio in U4B of R21/R23 to have unity gain end-to-end.
U4B can also add in DC offset via pot VR2. Resistors R9 and R10, along with some gain
provided by the ratio of R21/R11, limit the DC offset to +-2.5V.
Resistors R14 and R19 are used to balance the DC offset voltage at the op amp’s + and –
terminals, due to input bias currents. This greatly reduces DC temperature drift.
OK, that’s pretty straightforward, but what about resistor R24? What is it doing? This is
the ‘clever’ part of the circuit! This is called a ‘warping’ resistor, because the pedal output is
non-linear with pedal angle. We will talk about this resistor in the next section.
The Gate extractor is comparator U1. GATE pot VR3, along with resistors R4 and R5, set
the trip point to +-5V. Hysteresis is set by R2 and R6. The ‘width’ of the hysteresis is set
“wide” (about 0.5V) on purpose. It makes false triggering of the Gate output greatly reduced
as you wail away on the pedal. If you think it’s too wide, increase R2 to 560K.