[sdiy] Re: 4803 Project Idea

[Aaron Lanterman]

My student Brian had been banging his head on getting a varactor-based VCF 
working. Based on the advise from the list, we decided to abandon that 
idea. Brian's going to go ahead and write up his experience from his 
experiments in case someone else finds it useful.

Anyway, to fullfill his project obligation, I told him to try build 
something based on an existing proven design. Since he already spent all 
that time working on the varactor, I didn't demand that he make his own 
design. I suggested he try building a VCF/VCA combo based on the 
Memorymoog, using a LM13700 instead of the two 3080As.

http://www.matrixsynth.com/blog/media/Moog/MemorymoogSchematics (big scan, 
see upper right corner)

Smaller cutout of part of circuit:

http://users.ece.gatech.edu/~lanterma/ece4803/homework/memorymoog_vcf.png

He's having some trouble as you see below. To help with his last question 
I pointed to him a webpage about "instrumentation amps." But I'm curious 
that he's seeing the + and - signal at the 3080 as being in phase... I 
thought these should be out of phase... what could be causing the trouble?

Any thoughts?

- Aaron

On Sat, 16 Dec 2006, Brian Tippins wrote:

> Cutoff Frequency:
>  I was measuring the cutoff frequency by measuring the voltage across the 1K 
> resistor at the bottom of the ladder and dividing by 1K. By using the 
> equation of fc=19.2Icon/(2*pi*C), I figured that for a 1KHz cutoff, I need a 
> current of approx 7uA. By playing with the cutoff frequency pot, I was able 
> to get currents between  1uA and 10uA. These should provide a cutoff 
> frequency between 138Hz and 1388Hz (not the best audio range, but an audible 
> range...). So, through that I believe my control current circuity was working 
> properly.
>
> Output OTA:
>   Even with the control current corrected, there was no audio at the output. 
> I traced it through to the OTA at the output and what troubles me is that V+ 
> and V- at the OTA are identical (phase and magnitude). So the audio is being 
> zeroed at the output. What really confuses me is that by following through 
> the schematic I do not see how they would be different. I know this circuit 
> works but I don't really understand how.