[sdiy] Ladder filter analysis

[Tim Ressel]

Of course no circuit is perfect. I'm sure there is still some bleed. But credit where its due. Way smarter than I.

BTW How do you test for feed through?  Zero signal, square wave on the CV in?




Tim Ressel
Circuit Abbey
503-750-9331
timr at circuitabbey.com




________________________________
From: "rburnett at richieburnett.co.uk" <rburnett at richieburnett.co.uk>
To: Tim Ressel <timr at circuitabbey.com> 
Cc: Synth-Diy <synth-diy at dropmix.xs4all.nl> 
Sent: Monday, July 7, 2014 12:26 PM
Subject: Re: [sdiy] Ladder filter analysis


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,