[sdiy] Filters

[James Patchell]

That is a really good question....thing is, the question should really be 
implementing diodes as current controlled resistors...but, that is a quibble.

I can only think of good examples.  The Neil Steiner mentioned below is a 
very good example.  Also, if you can find a circuit for a diode ladder 
filter (as opposed to a transistor ladder filter), that is another good 
example.  Also, the Paia 2700 series lowpass and band pass filters are two 
more examples of using diodes as current controlled resistors.

The thing you have to remember about a diode is that the dynamic resistance 
is proportional to the current flowing though it.  You also need to keep in 
mind that any changes in the diode voltage are going to make an exponential 
change in the diode current....this is why in general you would not want to 
use a diode as a current controlled resistor, because it is not very 
linear, but, hey, for us, that is generally a good thing.  You do have to 
be a lot more clever using diodes as control elements.

At 12:57 PM 1/3/2004 -0500, David Reichert wrote:
>Now that this has been mentioned, are there any good sources explaining
>how to implement diodes as transconductance devices?
>
>-----Original Message-----
>From: owner-synth-diy at dropmix.xs4all.nl
>[mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Grant Richter
>Sent: January 2, 2004 10:12 PM
>To: Linium; jbv
>Cc: synth-diy at dropmix.xs4all.nl
>Subject: Re: [sdiy] Filters
>
>on 1/2/04 5:58 PM, Linium at intent at netpratique.fr wrote:
>
> > On Vendredi 2 Janvier 2004 14:50, jbv wrote:
> >>>> Is there a (easy) way to modify this VC-BPF design :
> >>>> http://www.cgs.synth.net/modules/cgs35_syntha_vcf.html
> >>>> to add VC-Q to it ?
> >>>
> >>> Replace the Q pot with a LDR+LED (and 1k resistor parallel to the
>LDR) ?
> >>
> >> Mmh... I guess there my be a slighly more hi-tech, OTA-based
> >> solution...
>
>That is called a "Sallen-Key" filter. Buchla implemented it using a
>Vactrol
>for the transconductor. Neil Steiner used diodes as the transconductor.
>The
>Korg MS-20 implemented it using an OTA.

         -Jim
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