What happens when you apply a LFO in this circuit?

[tiago razera]

Is this some kind of phase shifter? I'm trying to figure it out.



It's from the H & K Tube Rotosphere. Just curious about what this part of the circuit do.

[CynicalMan]

IC10A is a gyrator, it acts as an inductor to ground. The FET is wired as a variable resistor that changes both the simulated inductance and series resistance of the gyrator. So the first part is probably a variable band-stop filter. IC10D is an active low-pass filter. I'll take a look in Spice later and see what kind of frequency response that gets.

Just a note for others: in the Rotosphere, this stage is preceded by a 15k series resistor.

[CynicalMan]

Okay, this is what I get as the output, using a 2N5457 Spice model for the JFET, and stepping the LFO input from -1.8V to -1V.



The dip is caused by the gyrator filter, and the roll-off in the higher frequencies is caused by C52 and the op amp filter.

[ElectricDruid]

That filter at the top (IC10D) is a sallen-key lowpass. Putting the values into the AD filter tool at:

http://designtools.analog.com/dt/filter/filterW.html

..gives a cutoff of 489Hz, and a Q of 2.3. So it's a pretty low, pretty resonant filter.

Incidentally, are you sure about the 4n7 value? If it was 47n, it'd be a virtually textbook butterworth response. Just a thought - I've no idea what the whole thing does or is intending.

HTH,
Tom