[sdiy] SSM2040 'clone' question
rburnett at richieburnett.co.uk
Tue Jun 18 22:49:16 CEST 2013
> 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,
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