Studer VCA. comp/lim modifications. need advise
« on: August 22, 2013, 11:38:52 AM »

I'm prototyping a comp/lim based around MSC (modulard-sub-card eurocard) Studer VCA

VCA schematic available here (1.914.528)]]]]

I build the "Limiter voltage processor" used to drive the VCA according to this schematic:

it's a basic 20:1, 1ms attack and programme dependant "auto release" designed to work with the VCA.
"input" is audio post VCA and "out" go to the VCA control voltage input

My idea is to modify the "voltage processor" to have more control.

I perform lot of test.
-right now i manage the threshold with a pot in place of the trim R6. Work fine
-I manage the release time with a 6 way switch, changing the R9,10,11/C2,3,4 network (i understand this is the auto release).
1 pos is the actual network, and other five position are different single R/C (fixed time). Work fine too
-ratio is set by changing R17, remplaced by voltage divider (resistor network on a 4 way selector), with output taped here. Work fine but need better adjustment

-NOW my problem, the attack time.

My understanding is, the attack time is set by R12-C5  and define the time needed to open Q3 (rectifier, peak detector and log amp send -15v instantaneously) DC go to output and also load R9,10,11/C2,3,4 (and will discharge "as slow as release time" when Q3 are close)
I change  R12 with different resistor in a rotary switch, but it do not really work
I have no other idea/option right now to manage the attack time, what i do look more like a delay in Q3 opening, but the attack stay fast.
I am missing something?

any suggestions?



Re: Studer VCA. comp/lim modifications. need advise
« Reply #1 on: August 22, 2013, 06:45:32 PM »
You could try changing the value of C5 instead?
Without spending too much time on analysing the circuit, if your issue is the threshold at which Q3 is switching, and when you change R12 you are changing that threshold, then just leave that as is and make the capacitor bigger.
The bigger the cap, the longer it will take to charge.

However if Q3 is somehow threshold-ing the switch on for the attack, it might still stay fast.

Re: Studer VCA. comp/lim modifications. need advise
« Reply #2 on: August 22, 2013, 09:05:02 PM »
ok, I will try bigger cap (and lower resistors) for C5

But i'm afraid you are right, this design don't allow long attack.
This "limiter voltage processor" are designed to act as a limiter coupled with the VCA.

global test are good and i'm surprised with the result I have, I want or imagine this could be a very flexible tool. But maybe i'm asking too much ::)
For note i'll will plug a studer "balancing unit" for line in/out, with pot make-up on output line amp. Stereo eurocard with trafo in and out bal/unbalancing, both way :)

I have 2 other thing I don't understand well
Q5 (diode mounted)  around IC2/1, also C10/R16 around IC2/2

Tkf for help etheory
i'll report back


Re: Studer VCA. comp/lim modifications. need advise
« Reply #3 on: August 23, 2013, 01:07:10 PM »
juts replace .1 to .47  for C5
Not sure if it's better, I have to perform more test
Maybe I will try to let  R12 fixed an swap capacitor with the rotary I put here (actually 3k/10k/30k/50k/100k/300k)


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