These are schematics I have never tried to build :>

These seem to be from one of the Octave Cat synths. I've read they really sounded good in their day. I wonder if full clones are ever built by enthusiasts.
This is a high pass filter from . I'm sure it has a nice sound from the way it is set up. If you're willing to invest in a large HPF take a look.
This is a good looking frequency divider from a synth. I imagine the parts are available, but I'm not sure.
This is supposed to be a popular phase pedal for guitar, so making a synth module might be harder. The small stone[]

Electronic Formulas and musical numbers

Amps times Volts equals watts

really here is something helpful

-The efficiency of an audio amplifier is expressed as power out divided by power in, but it can be expressed as a percent by multiplying power out by 100.
 
 

--the cutoff frequency of a lpf or hpf, but I refer here to an lpf. They are esentially the same in resistance capacitance constants.

Fo = 1 / (2 pi R C)
where
Fo = cutoff frequency in HZ
pi = you know, the number that makes a more accurate equation the more digits it uses.
r is the ohms of resistance prior to the capacitor
c is the farads of capacitance connected to ground

for example if you have a 100k resistor and a 10nf capacitor (same as .01 microfarad) connected to a buffer the cutoff will be approximately 159hz
2 X 3.14 X 100,000 X .00000001 = .00628
1/.00628 is close to 159

--To make a linear volume control exponential place a resistor to ground on  it's output path, the middle terminal. This works if there is no resistance prior to the potentiometer. The resistor must maintain a ratio of 100 to 15, if the full value of the control is 100k ohms the resistor must be 15k. This can actually be more accurate than a cheap audio taper but can also cause problems if done in a complex circuit.