[sdiy] SSM2164 Phaser - another way?

[Neil Johnson]

David G Dixon  wrote:
> The 2164 does develop a measureable voltage on its input terminals when the
> gain is increased (I've measured it).  It is not large, but will create some
> error if the design absolutely depends on that voltage being exactly zero.

Which is probably most designs using a fixed resistor for the V-to-I
conversion.  This voltage will cause distortion since the simple
linear relationship between Vin and Iin no longer holds.  Looking at
the shape of the THD_N vs Amplitude curve (irrespective of the
horizontal axis *doh*) you can see the very definite break-point at
the far right at which point distortion shoots up at an alarming rate.

I suspect that for many synthesizer modules, where signal levels are
higher than the nominal 775mV used in the datasheet, few designs
attenuate the input to keep the input current within the sweetspot.
In the case of a filter it'll mean some distortion, which will be
noticable more on the HP output.  For an oscillator the waveshape
won't be so perfect at higher frequencies, and also the VCO will go
out of tune at the top end.  Certainly any VCA design should bear this
in mind.

I also wonder if there's anything special about the value of 30k with
respect to the input stage.  What is that value's significance on
noise/distortion/bandwidth/stability??  33k would have been a more
widely available value.  Or was it simply am odd decision on the part
of the person who got the (un)lucky job of writing the datasheet?
Informed opinion would be most welcome.

Just a wild thought: professional audio typically runs at +4dBu
nominal.  So to keep the 2164 in its optimal input range at 0dB gain
(normal operating point) we would need to scale that 30k by 4dB.
Which works out rather conveniently to 47k.

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