[Re: [Synthacon VCF]]

[M.Ruberto]

Magnus Danielson <cfmd at swipnet.se> wrote:
> From: "tomg" <efm3 at mediaone.net>
> Subject: Re: [Synthacon VCF]
> Date: Mon, 4 Dec 2000 19:30:09 -0800
> 
> > Thank you Mike and Romeo...hummmm...Diode ring?
> 
> Yes, thanks to your both...
> 
> Um, the bottom 2N5172s obviously looks like a standard expo-curcuit.
> I assume that the signal-in three-position switch will select the
> characteristic. It is not too uncommon to do such a thing by inserting
> the signal at diffrent places. I assume that this filter is a 12 db/Oct
> filter, right?
> 
it certainly sounds like a 12db/oct, bright, kinda nasal. looking at the
diagram i do believe it's 2 pole.

> The diode directions of the right 2N5172 seems kind of wrong at first,
> but if you redraw the schematics with +10 Vdc at top, 1 k (splitt off
> for a 2k2 to the left 2N5172), a 2k2, two diodes, a cap tap, one
> diode, a cap tap, one diode, a cap, two diodes and then into the
> collector of the left 2N5172. Now, the right 2N5172 will pull current
> out of it's 2k2 resistor in parallell with the diode ladder (which is
> really a single sided ladder and not a double sided as the EMS).

yes this is where i got confused and started thinking of the EMS ladder. that
is until i found the diodes were all connected in series, but by that time i
had already started drawing a 2 sided ladder.

 Thus,
> the current out of that 2k2 is being divided into the diode ladder and
> the expo transistors collector. The middle capacitor tap is really the
> output of the diode-core and the 2N5172 and 2N5138 is wired up as a
> amplifier.

i agree, the .1 uf cap is the buffer for the core output.

 The output of that is then feeded into the diode-core
> through two 2nF caps at the two other cap-taps. You select
> characteristics by inserting signal either at the middle cap-tap
> (really the input of the amp), at the lower cap tap or at the top of
> the common current feed.

the way i drew it, the taps are HP, BP, LP from top to bottom.
> 
> This initial analysis could possibly help people to get some grip of
> it. It's just my random thougths as I look over the schematic.
> 
> Cheers,
> Magnus
> 
-<mike>-


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