[sdiy] Ladder filter analysis

[Michael E Caloroso]

If you look at each stage of the ladder filter, it has differential
inputs *and* differential outputs.  The simple cascaded architecture
contributes to the common mode mechanism.  Combined with control
current and audio voltages, those are the keys to the performance of
the design.

The classic OTA-based filters have differential inputs but not
differential outputs, and they aren't capable of operating in true
differential mode.

MC

On 7/7/14, Tim Ressel <timr at circuitabbey.com> wrote:
> 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,
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