[sdiy] Vactrols & Lopass Gates

[megaohm]

Keep it coming. This thread is interesting to me and ,I'm sure, others.

peng


On 11/21/05, Peter Grenader <peter at buzzclick-music.com> wrote:
>
> Mark et al.
>
> First, et al: If this seems 'too much information-ish' for the lot of the
> list, please let me know, I'll take it off line.
>
> Comments on the schematic.  Not to say Don hasn't been known to change
> things once or twice, but it's weird - the 212 schemo I have (i only have
> the last page, but it does have the most of the LPG, the panner, reverb
> preamp and headphone amp) shows a completely different 292 , closer to the
> rev C. No fet. This is interesting because the Dodeca used a +/- 24
> supply,
> at least the one in this print did, (Randy Silgren's dodeca does, i've
> seen
> it) so you'd think Don would have used the REV 292 A in this but he
> doesn't.
> The one on my print here has the drive from the Rev B.
>
> Mark -  the LED driver isn't shown here in your print - does that use 24
> volts?
>
> In any event, this section of the 2120 print i have does not show the
> friggin mode switch nor the audio input buffer, that's on another page,
> but
> it does have a 4.7 mf decoupler - presumably AFTER the input buffer, not
> before.  A trival note to this, in my Model 13,  i had to add a 10uf
> before
> the vactrol resistors to get rid of a DC offset which was causing a major
> thump when the gate opened. Never noticed Don had done the same thing
> before
> today.  This cap isn't in any other incarnation that I am aware
> of.  Hmm...I
> did somehting Don did. Don't know if that's a good thing or....
>
> But that's not important.  What IS important is diff this three way switch
> manifests.  It seems to  keep the input attenuation alive (the divider)
> when
> the switch is set to the middle position when it would be removed with a
> standard switch.  that looks to be about a 3dB difference, somehere around
> there. OK, I see - it's cutting the amplitude down on the combined and
> filter modes and yes, the gate does tend to be a lot quieter than the
> other
> two settings -- as i remember Mark had commented on in the previous round
> of
> this discussion.
>
> So without a three position switch, filter only operation would  be
> attenuated through the switch, not the combined mode  There's a workaround
> for this by\ messing with the 15K's value after the filter.
>
> It looks like i could induce this three position switch by simply shorting
> a
> 12K to ground with the input signal externally to my 292 clone with the
> 7103
> in it.  Hold a second...
>
> Alreet - 12K may have done something on a scope (it's on the other side of
> the house and not portable), but it didn't do squat for my ears.  So much
> for my 3dB theory. I had to bring that pulldown more in the 1K - 500 ohm
> range to hear any noticeable attenuation.   Tell ya what, tomorrow I'll
> pull
> my scope over and give y'all some signal level stats before and after.
>
> It's just bizarre that the 292 rev B print doesn't show this center
> position....
>
> - P
>
>
>
>
>
>
> mark verbos wrote:
>
> > with a continuity meter, test which tabs are connected when in the
> > middle position. You will then see that there is one throw on each side
> > that happens in the middle as well as where you'd expect.
> >
> > look at this
> >
> http://machines.hyperreal.org/manufacturers/Buchla/Modular/schematics/buch292a
> > .gif
> >
> > this schematic was reverse engineered from a 212 module. The 12k
> > resistor connecting to the middle tab is actually just showing that it
> > is wired to the side that is active in the middle as well. The .0047 cap
> > is on the sape throw side of the other pole. Since that pole will have
> > the other throw active in the middle, it will only be engaged in one
> > position. I don't have the 292B parts list, but try it and I think
> > you'll hear the difference.
> >
> > incidentally, the way to wire a C7K 7211 to be a three position selector
> > switch is like this http://www.simple-answer.com/3_pos_switch.jpg
> > Buchla did that all over the music easel to select the sources on the
> > modules. The middle position in this diagram shows 1-2 and 5-6 connected
> > in the center position. I believe this is looking from the solder side,
> > but don't quote me on that. So for the lopass gate, wire the 12k
> > resistor to pin 1, ground to 2, 15k resistor from the FET to 3, .0047
> > cap to 4 and the output of the FET to 5.
> >
> > rock on.
> >
> > Mark
> >
> >
> >
> >
> >
> > Peter Grenader wrote:
> >> Sorry to beat a dead horse, but. I've been thinking about this for a
> couple
> >> of weeks (see the stream below - listed in reverse order
> chronologically),
> >> actually ordered a couple of 7211's, got them, was then trying to
> figure out
> >> how it may need to be wired to effect the change Mark spoke of.  The
> thing
> >> that was pulling on me is that with my analysis, it seemed all the
> critical
> >> functions inherent in all the other 292 rev levels required to short
> out the
> >> filter in amp mode, etc.  could be handled without the addition of the
> >> center on function of the 7211.
> >>
> >> This is not to say all the switches in all rev levels of the 292 are
> the
> >> same - the rev C has a triple pole , but it also has an entirely
> different
> >> back end to deal with (no external FET but an FET op amp instead,
> computer
> >> interfacing, a bunch of stuff).  But as far as the basic functions
> required
> >> to manifest the three modes, it seemed to me that a *normal* double
> through
> >> was the ticket.
> >>
> >> So I phoned JL,  and from his copy of the original Buchla 292 rev. B
> parts
> >> list he tells me the switch was listed as a... 7103 - a standard  on
> off on
> >> dual position, not a 7211 on-on-on three way.  This comes form from the
> >> ITT/Canon switch datasheet,  page 2 of 15.
> >>
> >> - P
> >>
> >>
> >>
> >> , mark verbos wrote:
> >>
> >>
> >>>
> >>> Peter Grenader wrote:
> >>>
> >>>
> >>>> The 7211 I've used, when wired with their suggested jumper
> configuration
> >>>> allows for three separate poles to connect to the wiper.    Possibly
> there
> >>>> are other jumper configurations possible that I'm not aware of.  My
> >>>> question
> >>>> to that in this instance though it that there is no indication on the
> print
> >>>> on this adaptation.
> >>>
> >>> that's true. I assume that the assembly drawing called specifically
> for
> >>> a 7211 because that's what's in there.
> >>>
> >>>
> >>>> The amplitude drop you speak of is a real issue, but I've been able
> to
> >>>> compensated for by putzing with the value the 15K divider that's
> switched
> >>>> on
> >>>> the top half.
> >>
> >>
> >> mark verbos wrote:
> >>
> >> a C&K 7211 doesn't have a third contact, the middle position leaves one
> >> throw of each pole connected. In the 292B the middle position leaves
> the
> >> resistor and the cap connected, or "both". In the 292c the input
> >> resistor makes the gain greater, so the gain is lower for 2 of the 3
> >> positions, just like the 292c. Without this, the lopass gate will work,
> >> it will just be louder in the middle position than in the other 2.
> >>
> >> mark verbos wrote:
> >>
> >> <<The Buchla 292B uses a C&K 7211 On-ON-ON switch. I am sorry, but it's
> a
> >> fact. I have built them all as well. This is the switch that looks like
> >> a regular DPDT but in the middle position on throw of each pole is on.
> >> If you don't use one the Lopass gate will work, it just will not
> >> compensate for the level difference between modes. In the 292c Don
> moved
> >> the switched resistor to the feedback loop on the input OP-Amp so that
> >> closing the switch made the signal louder rather than quieter. This
> made
> >> the input op-amp's gain follow the same logic as the resistor that is
> >> switched in after the vactrol, making it simpler to remote control.
> >>
> >> Peter Grenader wrote:
> >>
> >>
> >>> Mark -
> >>>
> >>> Have a look at the Buchla's schematic B2420B.  It's the one from the
> CBS
> >>> system.  Note the switch symbol - two contacts -- dual switch.  The
> top one
> >>> shorts out the filter, the bottom one kills the feedback.  There is no
> >>> connection shown for a third center position.  The off (center) keeps
> the
> >>> filter going but kills the feedback thru the FET.  There is a
> marketable
> >>> sonic difference between the center and down (LP) positions.
> >>>
> >>> Try it with a standard double pole triple throw.
> >>
> >>
> >>
> >> mark verbos wrote:
> >>
> >> <<as far as schematic versions go....
> >>
> >> 292 B uses an ON-ON-ON switch that is very expensive
> >> 292 C uses LF13331 switch chips that are rare and expensive
> >> My mod at www.simple-answer.com/DIY uses 4016 analog switches and has a
> >> tested board layout available.
> >>
> >> no pressure. ;)
> >>
> >>
> >>
> >>>> - P
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>> a C&K 7211 doesn't have a third contact, the middle position leaves
> one
> >>>>> throw of each pole connected. In the 292B the middle position leaves
> the
> >>>>> resistor and the cap connected, or "both". In the 292c the input
> >>>>> resistor makes the gain greater, so the gain is lower for 2 of the 3
> >>>>> positions, just like the 292c. Without this, the lopass gate will
> work,
> >>>>> it will just be louder in the middle position than in the other 2.
> >>>>>
> >>>>> Mark
> >>>>>
> >>>>> Peter Grenader wrote:
> >>>>>
> >>>>>
> >>>>>
> >>>>>> Mark -
> >>>>>>
> >>>>>> Have a look at the Buchla's schematic B2420B.  It's the one from
> the CBS
> >>>>>> system.  Note the switch symbol - two contacts -- dual switch.  The
> top
> >>>>>> one
> >>>>>> shorts out the filter, the bottom one kills the feedback.  There is
> no
> >>>>>> connection shown for a third center position.  The off (center)
> keeps the
> >>>>>> filter going but kills the feedback thru the FET.  There is a
> marketable
> >>>>>> sonic difference between the center and down (LP) positions.
> >>>>>>
> >>>>>> Try it with a standard double pole triple throw.
> >>>>>>
> >>>>>>
> >>>>>> mark verbos wrote:
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>>> The Buchla 292B uses a C&K 7211 On-ON-ON switch. I am sorry, but
> it's a
> >>>>>>> fact. I have built them all as well.
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>
> >>
> >>
>
>
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