[sdiy] SSM2164 Phaser - another way?

[Tom Wiltshire]

Hi Don,

On 18 Feb 2014, at 17:26, Donald Tillman <don at till.com> wrote:
> 1. I think this circuit will work, it just won't work well, as the range will be limited and it will clip weirdly.

Interesting, do go on...

> 2. Where did this circuit come from?  Any background on it?  The Electric Druid image suggests it's been simulated.  And "It looks ok in the sim" suggests that it works.  What does "ok in the sim" mean exactly?

I drew it. It occurred to me, so I drew it in LTSpice. "Ok in the sim" means it at least makes a notch like it's supposed to. However, the notch doesn't move like it's supposed to.

> 3. Even though it might look like the SSM2164 is acting like a variable resistance to ground in the diagram, that's not what's happening.

More's the pity. If that worked, other interesting possibilities might be open to us.

> To understand how it works you need to look at the simplified schematic on the SSM2164 data sheet and note that the input impedance of the 2164 changes with the control voltage.

..which would give a (limited) way for it to behave like I wanted, even if not by the method I had in mind.

> 4. The way the 2164 operates is that it's a fixed gain transconductance amp that steers its output current between two directions: to the output for high gains and feeding back to the input for low gains.  Advantages over a traditional OTA are that the low gain settings are linearized and the 0 V setting is balanced to unity gain.
> 
> 5. Tom, that Spice "model" is way too abstracted and simplified to work in this application.

Yes, that's what I'm discovering! Still, what's amazing is how much it will do, not what it won't.

Thanks,
Tom