[sdiy] TomG's 9v diode ladder VCF

[Happy Harry]

Howdy Neighbor !
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>From: Plutonique9 <Plutonique9 at symbiocom.com>
>To: <synth-diy at node12b53.a2000.nl>
>Subject: [sdiy] TomG's 9v diode ladder VCF
>Date: Thu, 12 Apr 2001 16:30:17 GMT
>
>Hey there people,
>I'm going to build the 9v diode ladder from the Cookbook as my
>introduction in VCF construction. I have a couple of Q's regarding it
>though.
>
>Diode matching:: I'll be using 1n4148's for the diode ladder, I'm
>pretty sure these need to be well matched. On my multimeter, it has
>some sought of diode testing inbuilt, this is a radioshack digital
>multimeter BTW. Is this suffiecient to match diodes, or should i
>build a dedicated "diode matching" circuit, which runs a 9volt
>battery through the diode via a resistor, no difference?

Not sure YOUR meter... but the Fluke 8050A and Fluke 77 use about
600uA in the diode test function... which is probably OK for basic matching. 
Jim Patchell has a schematic on is website and he matches at
5uA and 500uA.

You might want to consider a chip like the MAD1108 which has 8 diodes
in it. Diode PAIRS need to be matched... not the diodes in one "leg"

TomG's pc board for the diode ladder filter is not layed out this way. I'm 
gonna hack it to be "improved" one day... but it works OK without
any matching as well...
>
>Also, using a LM394 should yield better results than two 2n3904's,
>right, $4.00 here, hope its worth it.

How about a 2SC1583 instead. The LM394 should be saved for a VCO where
the expo respinse is REALLY that critical. I'm using 2n3904 in the ladder 
with simple DVM matching... works OK.
>
>Also, I'm not sure where that "BIAS" circuit is supposed to link up
>to? At the input, it has a symbol, there are 4 other positions on the
>schematic which also has these symbols, does the "BIAS" go to all of
>these?

I think you are correct.

>For example, what does the 100k resistor going into the non-inverting
>input of the TL071 connect to?
>
>Hmmm, I think that covers all the shady areas, thank you for any
>insight.

A nice filter... you will like it !!!

H^) harry
>
>
>-- Plutonique9, Plutonique9 at symbiocom.com
>
>http://www.mp3.com/Plutonique9
>

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