PLAN B analog blog

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