[sdiy] Vactrols & Lopass Gates

[harrybissell]

The 7211 is the most versatile switch ever made imho. It can be used for
so many MORE things than just a three-way selector !!!

H^) harry

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.
> >>>>>
> >>>>>
> >>>>>
> >>>
> >
> >