[sdiy] Ladder filter analysis

[rburnett at richieburnett.co.uk]

On 2014-07-07 18:19, Tim Ressel wrote:
> Hi,
> 
> I was talking to a guy who was trying to eliminate control voltage
> feed through on a filter when it dawned on me: the differential nature
> of the Moog transistor ladder filter design eliminates CV feed
> through. The audio is balanced on the two sides of the ladder but the
> current is common mode, so it gets cancelled at the differential
> amplifier.

That's the theory anyway.  In practice small mismatches between the 
components on the two sides of the ladder, and finite common-mode 
rejection in the diff amp can conspire to make the control signal 
cancellation less than perfect at the audio output.  It's a clever bit 
of design with a handful of transistors though!

-Richie,