[sdiy] Vactrols & Lopass Gates

[mark verbos]

Peter Grenader 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.

I assume you are referring to B2120-1 which is the schematic of the 
output section of the 212. I don't think there was any other schematic 
for the 212. Since the output section is the only thing unique in that 
thing. The 212 I have seen inside has 280 for envelopes, 291 for 
bandpass, part of 265 for randoms, part of 285 for ring mod.... and 292B 
for LPG. You are right, it makes no sense that the LPG uses the FET and 
the "voltage controlled gate" in the output mixer does not. go figure. 
Oddly enough, it is like a LPG stuck in the "gate" position, but he 
included the caps. Does that mean that even in the gate position it is 
lopass filtering to some degree?

> 
> Mark -  the LED driver isn't shown here in your print - does that use 24
> volts?

yeah, it's the same as the 292B schematic. +5 and -24 for the control 
side opamp.  However. it does NOT have the revisions done to it. The 
resitor change and additional cap around the control opamp, that is. Of 
course Don revised designs on each one he made so there may be changes 
that were only done to ONE UNIT!


> 
> 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....

nah, there's a cap in all the versions. In the older one it's a .47uF in 
the 292c it's up to a 1uF, the 212 output section has a huge 4.7uF cap, 
but no buffering, I guess it's not neaded when only gating. (if in fact 
it iIS only gating!! ;)

> 
> 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....  
> 

that is wierd.

Mark



> - 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.
>>>>>>>
>>>>>>>
>>>>>>>
>>>
> 
>