[sdiy] SSM2164 Phaser - another way?

[Neil Johnson]

Mattias Rickardsson wrote:
> I use an extended version if that model, including the CV impedance etc:
> [snip]

The CV input impedance is useful indeed.  I wouldn't bother with the
lead-lag compensator, it has negligible effect on the external
circuitry in simulation.

> When connecting a 2164 according to the datasheet, to the
> virtual-ground summing node of an OP-amp, the 2164 *output* will look
> like a voltage-controlled resistor *from the OP-amp's point of view*.

Umm, from the op-amp's point of the view the output of the 2164, being
a current source, has an incredibly *high* internal impedance
(theoretically it is infinite).

> But the 2164 *input* looks like the virtual ground summing node, so
> the resistor you put there is the only resistor drawing any current,
> and it's constant.

The 2164 input *is* a virtual ground node, the same as for an
inverting op-amp's negative input.  So it will be close to GND only
while the internal feedback loop is operating in its linear region.
You can get a rough idea of where that limit is from the datasheet.

> Speaking of the SSM2164 datasheet, did you notice the error in Figure
> 6? It's supposed to show THD+N vs. amplitude - which would be a very
> useful graph! - but it has a 20-20k frequency scale on the X axis, so
> it's impossible to know what amplitudes the curves show.

Yes it is a bit of a disaster.  The performance table suggests a
couple of points of interest, and we can kind of guess that the bottom
of the Class A curve is around the 0dBu level (0.775Vrms).

> Funnily
> enough, Coolaudio just copied it to their V2164 datasheet... :-)

Copy-paste the product, copy-paste the datasheet.  At least they're consistent.

Neil
--
http://www.njohnson.co.uk