PLAN B analog blog

Wow, thanks for the epic reply, Peter.
After reading I've decided that I wouldn't want to change anything 
about the M13's sound just to get a shorter decay.  Now, If I had a 
bunch of them then I might go tinkering for the sake of variety.





--- In PLAN_B_analog_blog@yahoogroups.com, "(i think you can figure 
that out)" <peter@...> wrote:
>
> You can alter the decay - the ringing - of the Model 13.  This is 
due to circuitry I have 
> added to cut the bleeding down.  You can't do this with Doepfer 
LPGs, or Buchla 292's for 
> that matter, nor with the Wiard Borg which is very close to the 
Buchla 292 in design.
> 
> Looking at the PCB, you'll first notice it's basically two 
duplicate circuits (one each for the 
> two TG's) with  a slender line of circuitry in the middle (that's 
the crossfader section).
> 
> On each of the TG halves, at the bottom (edge) of their real estate 
you'll see four pads in a 
> line that are ellipically shaped.  In older M13s there are wires 
soldered to these pads which 
> go to the faceplate.  Newer models have a parallel connection via a 
12 (and now 14) pin in 
> line connector in the center of the board which lies right in front 
of the corssfader pot's 
> backside.  Holding the PCB so that the power connector is on the 
upper right, to the left of 
> those four pads you'll see two others labelled J1.  In older PCBS - 
those before rev. 2, there 
> a standup resistor soldered into J1.  In Rev. 2 and above, another 
resistor was added 
> directly to the left of those two pads - a 470 K ohm resistor. 
Those resistors, either 
> configured as a standup in the J1 pads, or from the added resistor 
next to them are the 
> current limiters that determines how much of the bleed killer is 
mixed into the the circuit.
> 
> J1 was configured as it was because it will allow for a two 
position jumper block to be 
> installed - like the little things that were on the back of SCSI 
disk drives to set their ID 
> number.  Inserting a jumper there puts the bleed killer on 
maximum.  removing the 
> jumper would remove it altogether.  I stopped installing those 
jumper blocks early on 
> because of availability issues with the jumpers.  I was not willing 
to slow delivery down to 
> to a G.D. part that for some reason (still surprised by that) - 
hard to get.  THey were listed 
> in supplier site, but constantly sold out.
> 
> The lower that resistance, the less bleed and slightly less ringing 
or response time to the 
> DECAY of the vactrol.  In earlier models I was shipping with a zero 
ohm resistor in there - 
> putting the bleed killer on maximum, which also cut back the 
decay.  Later I started 
> allowing less of that into the circuit, which slightly increased 
the bleeding, but made the 
> ring longer - that's the 470K resistor.
> 
> If you want to cut back the amount of ringing, increase the value 
of those resistors.  If you 
> want more, increase it or, to get the longest possible ring, remove 
those parts altogether.  
> Just cut them out.  This will make the vacs rings at their 
maximum.  Unfortunately, it also 
> effects the level of vac's inherent bleed.  WIth no resistor in 
there, the bleed is about 3mv - 
> meaning when the gate is fully closed, about 3mv of signal is still 
allowed through the 
> device at it's outputs.  On the other side of the possible 
variables - a zero ohm (a jumper 
> i.e. no resistance), there's only a third of a mv of bleed, but 
about half the ringing.
> 
> Don't shoot me about the bleeding - it's not my fault!  it's the 
vacs.  Their technology does 
> not allow them to close to zero ohms.  At best they will close down 
to only 80 ohms.  WIth 
> the bleed killer set with a zero ohm resistor, the bleed is pretty 
much inaudible, unless 
> you're gating very very harmonically rich signals with pure silence 
in between events.  With 
> the resistor removed (the bleeder then is taken out of the 
circuit).  the bleeding is a lot 
> more prevalent.
> 
> Another issue is no two vactrrols have the same amount of bleed.  
You will need to tune 
> each if you want them to ring the exact same amount.  Again this is 
due to vactrerol 
> technology.  If you look at the Perkin-Elmer spec sheet, you'll see 
a spec clled DARK 
> RESISTANCE AFTER TEN SECONDS.  THe After seconds part was not 
thrown in there to fill a 
> line - it's there because PE felt that was a safe amount of time to 
assure the vactrol was 
> fully closed - meaning some of them will take almost as long.  
Again, part of their charm!  
> THe ringing get to be more of an issue with the Model 12 vacsa - we 
actually have to bin 
> them before hand to cut it back to the point I think that filtwer 
sounds best.  THose which 
> don't make the mark are used in the M15 crossfader bacuaee bleed is 
less of an issue - it 
> actually makes for smoother crossfading.  The M13 usesa a slightly 
different vactrol than 
> the ones used in the model 12, 15 and 24, but they still don't go 
down to zero ohms.  No 
> vactrols do.
> 
> In summary - you can play around with alternatives if you want less 
(or more) bleeding.  A 
> zero ohm cuts the ringing (and bleeding) down to it's maximum.  
Removing the part 
> altogether is the other side of the scale - maximum ring and 
bleed.  If you really hate the 
> ring there's a little more you can do - either replace the vac with 
another type - one which 
> is by design much faster -or- get a Deopfer LPG.  He used an 
exteremely fast vac which 
> has little or no ringing. If you elect to change the vac in the 
M13 - make sure it's a dual 
> resistant element type.  These are the ones that have three leads 
onthe resistor side, not 
> just two.
> 
> I chose the vacs I did because I like the ringing as i feel it's 
part of the module's 
> personality.  They are really dead sounding in comparison without 
it. 
> 
> Hope this helps,
> 
> - P
>