DIY VCA Fader DBX 202 Question

[Evillamp]

Hi,
so i cloned a dbx 2020 (early black can design) and got pretty decent results(can make a thread about that if anyone is intereted).
I ran into some problems , i want to make a diy vca fader design for my mixing console ,i looked through the ssl 4000 schematics but i dont really see where the control voltage gets scaled / connected for the dbx 202 vca.
so how would i go about designing a fader that outputs CV for the dbx vca.
i have a 5K fader and a 5V supply connected to it.
any help would be appreciatet.
cheers Elias

 

[gyraf]

..the fader input for the SSL clone compressor would be the virtual-ground summing node on the TL072(B), inverting pin6, where the 120KOhms from sidechain and the 620KOhms from makeup gain joins.

Just add your fader voltage to this summing node through an appropriate resistor..

/Jakob E.

 

[Evillamp]

..the fader input for the SSL clone compressor would be the virtual-ground summing node on the TL072(B), inverting pin6, where the 120KOhms from sidechain and the 620KOhms from makeup gain joins.

Just add your fader voltage to this summing node through an appropriate resistor..

/Jakob E.

iam sorry iam a bit stupid on that one ,so would i only need one opamp for the scaling ? just this part(tl072) or would i need additional buffering/scaling circuitry.
i couldnt find a example on dbxs schematics for the cv fader part

 

[Evillamp]

what iam trying is staying fairly accurate to how the ssl 4k does it but ommiting the compressor/sidechain part

 

[gyraf]

..so just omit sidechain vca, rectifier and timing?

For simplicity, you'll probably need the rest of the GSSL anyway: balanced input, voltage-to-current into VCA, current-to-voltage out of the VCA, output balancing

If you want to invent the wheel all over again, do just that?

/Jakob E.

 

[Evillamp]

..so omit sidechain vca and rectifier?

yeah so in theory i just want the audio to go through this part of the mixbus with a fader on the controll port to allow me to boost up to 10 db or to lower down to -40 db (excuse the crappy drawing xd)
so basically just the vca fader functionality of the ssl 4k no compression or sidechaining just a vca fader if that makes any sense.

 

[zamproject]

I suspect a misleading discussion here ?!?

Are we talking about SSL4k channel VCA fader or SSL4k VCA bus comp ?

In case, have a look at 82E13 schemo (VCA card)
There is computer involved in the CV chain, but this should not be a problem to considerer a wire in place (except if you want recall/automation)

 

[gyraf]

aah yes, I was thinking of the stereo bus..

 

[Evillamp]

I suspect a misleading discussion here ?!?

Are we talking about SSL4k channel VCA fader or SSL4k VCA bus comp ?

In case, have a look at 82E13 schemo (VCA card)
There is computer involved in the CV chain, but this should not be a problem to considerer a wire in place (except if you want recall/automation)

Yeah iam talking about the Channel VCA fader sorry If i didnt make that clear,so i Just want a way to Control the VCA via a fader so Not the compressor. I thought the Part in the schematic is right beside the sidechain and compressor time constants.i tried the Part where the Computer got involved and bridged it in my circuit but IT didnt really give me any useable CV so idk what iam missing

 

[zamproject]

i tried the Part where the Computer got involved and bridged it in my circuit but IT didnt really give me any useable CV so idk what iam missing

So you have that SSL VCA card in hand ?
I won't take the time to get the whole digital and logic behaviour across the CV handling, but it's possible a wire between computer send and computer return is not enough with all mute and group involved that may or may not hold the signal before the last buffer ?!?
Maybe try to just replicate the two last buffer (ic CA324, T2 A and B) which seem to handle offset/gain/scale/voltage limiter.

 

[Evillamp]

Well the Thing is i dont have any cards ON(beides a dynamics comp from the 4000) Hand xd i build the whole Thing myself,yeah so i want only the crucial parts for the voltage scaling and the Channel fader ,i looked ON every schematic but didnt really understood where the compressor ends and where the fader Part starts

 

[zamproject]

AFAIK (only scroll the 4000 service manual few time for self educating and bus comp value check), the channel fader and dynamic don't share the same VCA and are completely independent, the quad bus compressor is also another bloc, so you may stick on the fader vca shemo.

Basically you just need a single opamp from your wiper to drive your VCA CV input, which may request a gain/scale and offset.
Dedicated V+(and V- ?) and clean 0V ref for the fader and offset.
Plus, a little filtering (small time constant for smoothing the CV ramp) passive between fader and buffer or active in the FB may be mandatory for optimal operation.

Again, stick for what's around T2-A and T2-B (to be clear my ref is drg T82013-71)

 

[Evillamp]

AFAIK (only scroll the 4000 service manual few time for self educating and bus comp value check), the channel fader and dynamic don't share the same VCA and are completely independent, the quad bus compressor is also another bloc, so you may stick on the fader vca shemo.

Basically you just need a single opamp from your wiper to drive your VCA CV input, which may request a gain/scale and offset.
Dedicated V+(and V- ?) and clean 0V ref for the fader and offset.
Plus, a little filtering (small time constant for smoothing the CV ramp) passive between fader and buffer or active in the FB may be mandatory for optimal operation.

Again, stick for what's around T2-A and T2-B (to be clear my ref is drg T82013-71)

Ah OK ,i See what i can come Up with.
Huge Thanks for explaining it.
Iam fairly new to the whole Mixer stuff ,did only preamps and drummachines before

 

[Evillamp]

So i simulated the part of the schematic , i followed the schematic exactly ,and correct zener diodes but the cv is still very off. i get way too fast to the 4 v mark on the faderiam already maxed at the halway fader position ,so 4,57 v would be max attenuation for the vca .am i missing something or is the simulation just trash

 

[Evillamp]

i used the schematic u recommended @zamproject

 

[Rob Flinn]

Are you simulating using VCA law faders ? If not things will look a bit weird.

 

[zamproject]

@Evillamp... sorry the right to left schemo hurt my eyes too much... I can't read clear

 

[zamproject]

iam already maxed at the halway fader position ,so 4,57 v would be max attenuation for the vca

-Are we sure the computer return from some DA is a 0/5V signal ?!?
-Did you adjust range and offset ? also you may adjust some gain
-why adding an opamp ?
-4,57V is -90dB, not that bad...if you want more adjust for -5v max (100dB), more is overkill and will coarse the accuracy at top of the fader

Are you simulating using VCA law faders

aren't VCA fader linear taper ?!? which end in a math log scale label on most VCA fader ?
The only curve bending I know is from studer 900 series that electronically scale the linear CV for the VCA math log to end an audio log at fader travel with typical 20dB attenuation @ mid travel (where math log have 30dB IIRC)

 

[Evillamp]

-Are we sure the computer return from some DA is a 0/5V signal ?!?
-Did you adjust range and offset ? also you may adjust some gain
-why adding an opamp ?
-4,57V is -90dB, not that bad...if you want more adjust for -5v max (100dB), more is overkill and will coarse the accuracy at top of the fader

aren't VCA fader linear taper ?!? which end in a math log scale label on most VCA fader ?
The only curve bending I know is from studer 900 series that electronically scale the linear CV for the VCA math log to end an audio log at fader travel with typical 20dB attenuation @ mid travel (where math log have 30dB IIRC)

I did the schemo quick and dirty at Work during a Break didnt have time to place the components in the correct direction Just wanted to simulate sorry xd yeah OK so As i Said i am very new to this so ig my math regarding the attentuation was wrong ,thank you and the Others in this threat Here. I guess i Just try the simulated circuit without the Buffer for the Computer send ,-90db seems good and yeah didnt think of the fader type. Hope i wasnt too much of a pain

 

[Evillamp]

Yeah so thank you all ,thats why iam Here to learn some stuff,also Just orderd small Signal audio Design so i will study the Part about vcas