[sdiy] HPFs and ADSRs

Tim Stinchcombe tim102 at tstinchcombe.freeserve.co.uk
Sun Jun 6 20:53:09 CEST 2010

> I've only seen this "Bach" 
> thing a few papers and I think one textbook. 

The original paper is from 1960: 'Selecting R-C values for Active Filters',
R.E.Bach, Electronics, May 13, 1960, pp82-85. The paper proposes a
generalised filter-section 'chaining' technique in order to realise
higher-order filters, and the second-order case is really the simplest,
non-trivial example (so 'n=2'). (The buffers are kind of 'leap frogged' in
pairs over every other one.)

> I've also seen it called something entirely different - named 
> after a person - but I can't remember where I saw that or 
> what the name was.

In the book 'Active Filters for Integrated Circuits', Heinlein & Holmes
(1974) introduce this topology as a derivative of the Sallen-Key, in a
section entitled 'Kundert Filters', and indeed the main reference there is:
'The RC Amplifier-type Active Filter: A Design Method for Optimum
Stability', W.R.Kundert, IEEE Trans Audio (AU-12), Jul-Aug 1964, pp66-71. It
is probably no surprise that Kundert starts with it as it is a nice simple
case to work with. A few pages later in the book, Bach is acknowledged with
generalising the idea with the n-stage chaining method.

> I wondered if the Korg folks knew about this "Bach" filter...

My guess would be erring on the 'probably not' side...

> or if they looked at a lowpass Sallen-Key, wondered what 
> happeded if you replaced the resistors with OTAs,

...that would be my guess too: 'hey, this Korg35 thing doesn't perform that
well, what happens if we simply take a Sallen-Key and stick some OTAs in
there instead?'...

> and then 
> worked out the frequency response.

...probably followed not too long after with 'Dang! that sounds OK, let's go
with that!' - no analysis required!

Well them's my guesses!

Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at tstinchcombe.freeserve.co.uk

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