Ground rules on this homework: You may verbally discuss approaches to the problems with each other while looking at the schematics, and are encouraged to do so; but you may not look at each other's written solutions or ask "what did you get on part XYZ of problem ABC." (In future homeworks, I will allow varying degrees of explicit collaboration on certain problems.)
Below, I will use underscores to indicate subscripting.
What is the "cutoff" frequency (i.e. the omega_c from lecture) of Rene Schmitz's MS-20 clone as a function of the current at the control pin of the CA3080s? (You should be able to do this by just using the formula for omega_c for the Sallen-Key in terms of the gains and capacitors; just include the gain of the resistive divider at the input of the OTA together with the gain of the OTA.)
a) Are the variable-gain integrator stages inverting or non-inverting? (Be sure to consider the combined effect of both the OTA and the op-amp, if an op-amp is being used as an integrator.)
b) What is the "cutoff" frequency as a function of the current at the control pins of the OTAs? (Again, just include the gain of the resistive divider at the input of the OTAs together with the gain of the OTA and you should be all set.)
0-3) Oberheim SEM VCF - you want to look at the page that says "VCF."
4-6) ASM-1 VCF - ignore the 30 pF caps.
7-9) PAiA 9730 VCF - either Filter A or Filter B (they have the same integrator structure.)
The patented Moog transistor ladder VCF contains a cascade of four one-pole lowpass filter sections. Find the cutoff frequency of one of those sections as a function of the control current being pulled from the tied emitters of the transistor pair that feeds the ladder. Two things to note: 1) Notice that when analyzing the Moog VCF, we don't include a resistive divider in the gain as we've done in other VCF cutoff computations; there is a resistive divider right at the first input, but it's not important for our frequency analysis. 2) I wrote expressions on the board for a one-sided ladder; for a real Moog two-sided ladder, the control current gets split between the two halves of the ladder, so you get a transconductance gain from each transistor pair that's like that of an OTA, and the formula for the cutoff is basically the same as for the OTA-C filters we looked at earlier (except we leave out the resistive divider).
If you don't see a specific unit on a capacitor, there's usually an implied "microfarads."
0) Monowave VCF - The Monowave was designed by Paul Maddox, a synth DIYer who hand-built and sold 25 as a limited edition; he's now declared the project "hardware open source." See the whole thing here!
1) Oberheim OB-Mx - Strangely, Tom Oberheim had nothing to do with this synth; Gibson had bought the rights to the Oberheim name. Don Buchla was called in to try to save the project, but it eventually wound up released before it was really ready against Buchla's wishes.)
2) Minimoog VCF
3) Moog Modular 904A VCF - assume the "Range 1" capacitor is switched in (notice the ladder is drawn "sideways," at the bottom of the page)
4) Moog Modular 904A VCF - assume the "Range 2" capacitor is switched in
5) Moog Modular 904A VCF - assume the "Range 3" capacitor is switched in
6) Moog Rogue (see last page for schematic)
7) Moog Prodigy (see last page for schematic)
8) Moog Source
9) Memorymoog (this is a reduced snippet of a much larger scan, Sheet1.tif, found here).
