[sdiy] SSM2040 'clone' question

[Richie Burnett]

> Justin Owen wrote:
>
> ...So is it 4x integrators or 4x low-pass filters... or something else?

Both :-)  In each section you've got a "long-tailed pair" differential 
amplifier that outputs a current proportional to the difference of the 
transistors' base voltages scaled by the current pulled down through the 1k 
"tail" resistor.  This is like a VCA with a current output.  The 1nF 
capacitor integrates this output current, so you now have a current 
controlled integrator.  The darlington transistors buffer the voltage across 
the capacitor, and finally the 10k resistor next to the capacitor wraps the 
whole integrator up inside a negative feedback loop.  Negative feedback 
around an integrator turns it into a first-order (1-pole) low-pass filter.

So in total, you have four current-controlled integrators, each with their 
own local negative feedback around them.  This makes a cascade of four 1st 
order lowpass filters, and then global negative feedback is usually applied 
around the whole lot to make it resonate.

I hope this helps,

-Richie,