Re Doepfer in keys on filters

Tom May ftom at
Thu Sep 19 19:11:43 CEST 1996

Tim Cockram <tim at> writes:

> Well, Mr Doepfer is sort of right. Bob Moogs filter uses the transistors as "transdiodes" which are for all intents and purposes 
> well behaved diodes.  Roland in the 303 and I belive EMS in the VCS3 etc used the plain old diode to get arround the Moog
> Patent which specificaly mentions the use of transistors ( Shame for Moog that his patent attorneys didn't just say Semiconductor
> Junction).
> Despite being sort of right in his analysis  Mr Doepfer should also  have said that for some inadequately explained reason the filters don't
> sound the same (don't take my word for it try it and hear).  To me the EMS filter sounds  er "mushy" whilst the 303 filter is just 
> not very refined or meaty(so much for quantative analysis).

As was pointed out to me recently on this list (thanks folks, and
hopefully I'll get this right this time), the transistors in the Moog
filter serve a dual purpose: 1) they provide a varying incremental
resistance by varying the bias current, just like the diode filter;
2) they are current buffers providing isolation between each of the
four stages (pretty damn clever!).  The isolation gives four poles all
in the same place for the Moog filter.  The poles of the diode filter
are quite different, and you can find a nice analysis of them by Don
Tillman in the archives.


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