[sdiy] Vactrols & Lopass Gates

[mark verbos]

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