[sdiy] SSM2164 Phaser - another way?

[Tom Wiltshire]

On 18 Feb 2014, at 18:55, Donald Tillman <don at till.com> wrote:

> 
> On Feb 18, 2014, at 10:14 AM, Tom Wiltshire <tom at electricdruid.net> wrote:
> 
>> 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...
> 
> The 30K resistor in series with the 2164 input will limit the tuning range.  If the signal amplitude at the 2164 input goes over the roughly 52mV range of the input diff stage then it the tuning will dynamically distort.
> 
>> 
>>> 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.
> 
> What frequency is the notch at?

Neil said about 113Hz. I got something similar. Does that make a difference?

>>> 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.
> 
> Do you mean the notch?  That'll be there without the 2164's.

No, I mean all the other filters and VCAs and what-have-you that I've simulated with that model. It might be basic, but it's nonetheless useful. But here we've reached the limit.

To be honest, the model isn't the problem, and fixing some variable-impedance effect that the model doesn't have isn't the solution. The problem is that for a moment there I let what the circuit looked like it should do lead me into believing that's what it would do. Although in my defence, that's exactly why I posted it here - I knew you lot would soon put me straight if it was a lot of hat!
The solution is to go back to the drawing board and find a better way.

Having played with it quite a bit, it feels like there should be some single-op-amp-single-VCA 2164 phaser stage out there, but I can't find it.

Thanks,
Tom