[sdiy] Ladder filter analysis

[Tom Wiltshire]

Could we use the same trick to improve some other types of filter design? Isn't the discrete OTA basically a differential design? Could we build a fully-differential OTA filter? (diff in and diff out?) Wasn't the SSM2044 similarly a differential design? (it has handy differential inputs, I remember). Differential audio is a marvellous thing, and in these days of mixed-signal circuits with microprocessors and digital stuff on the same board as our analog audio, we should do more of it, I reckon. It seems like an obvious but ignored way to keep digital noise under control. Perhaps that's because the parts don't exist to build a fully differential signal path.

With the Moog ladder design, it was my understanding (limited as that is when it comes to the Moog filter) that it simply wouldn't work unless it was differential. The CV feedthrough would be awful, *except that* it's a differential design, so it gets cancelled. Is that right?

But I agree - it was a clever idea any way you slice it.

Tom

On 7 Jul 2014, at 18:19, Tim Ressel <timr at circuitabbey.com> 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.
> 
> Moog really was a clever guy.
> 
> 
> Tim Ressel
> Circuit Abbey
> 503-750-9331
> timr at circuitabbey.com 
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