ECE4803B: Theory and Design of Music Synthesizers
Due: Thursday, Sept. 14 at the start of class
Semiconductor Application Note 31
(or pretty much any textbook that has op amp circuits in it)
The main goal of this problem is to get you looking through some
synthesizer schematics. Since each solution will be different, grading
will be less boring.
Do some google searching for synth schematics. They could be commercial
or “homemade” designs; you’ll find a large community of
Synthesizer Do-It-Yourself enthusiasts.
(Some early synths, such as the Buchla 100 series and early Moog Modules and
EMS Synthi, are made entirely with discrete transistors. Those are extremely
difficult to analyze; I’d recommend staying away from them for now.)
Make sure each “subcircuit” you are asked to find is being fed by a
low-impedance output (for instance, another op amp; also you may assume
that any input to a module is coming from a module with a low-impedance
output). For variety, use a circuit by a different manufacturer or
DIY designer for each of the four problems below.
1) Find an instance of a simple voltage follower (one without resistors;
output directly tied to negative terminal.) Print out the schematic and
locate the instance.
2) Find an instance of an inverting amplifier (voltage in through a
resistor to negative terminal, feedback resistor from negative terminal
to output, positive terminal to ground). Print out the schematic, circle
the instance, and compute its gain. If it’s an inverting adder
with multiple inputs,
compute the gain for just one of the inputs and specify which input.
3) Find an instance of a noninverting amplifier (a resistor from negative
terminal to ground, feedback resistor from negative terminal to output,
input direct to positive terminal). Print out the schematic, locate
the instance, and compute its gain.
4) Find an instance of a inverting single-pole lowpass filter (voltage in
through a resistor to negative terminal, feedback resistor and capacitor
in parallel from negative terminal to output, positive terminal to ground).
Print out the schematic, circle the instance, and compute its 1/2 power
cutoff frequency (in Hertz) and its gain (at DC).
A few words of warning: In an inverting configuration, sometimes you’ll see
resistor between the positive terminal and ground. These resistors are designed
to compensate for non-ideal op amp effects and may be ignored in your analysis.
Moog (east coast) and Buchla (west coat) developed their ideas about
voltage controlled synthesizers independently. Moog used a pitch control
standof of 1 volt/octave, which works out to 0.08333… volts/semitone
(the pitch difference between to adjacent notes on the piano is a semitone;
there are twelve semitones per octave).
Buchla preferred to use 0.1 volts/semitone, which works out to
Hence, if to try to directly drive
a Moog oscillator from a Buchla pitch control source, or vice-versa,
everything will be horribly out of tune.
1) Design an op-amp circuit that will covert pitch control voltages from
Moog standard to the Buchla standard. You may assume that your conversion
module is given an input from a voltage source with zero output impedance
and is being fed to a module with an infinite input impedance; you also do not
worry about input and output protection (assume nobody will be abusing
your module). For this part
of the exercise, assume you have perfect “zero-tolerance” resistors.
2) Off-the-shelf resistors never exactly match their listed values.
Let’s do a “worst case” analysis for the case where your circuit is given
a one volt input. If you use 10% resistors, assuming the true resistance
is uniformly distributed, what is the highest voltage you might get out?
What is the lowest voltage? How many semitones above and below the desired
value are these voltages in the Buchla pitch standard?
3) Repeat the above analysis for 5% and 1% resistors.