DIY VCA Fader DBX 202 Question

[Rob Flinn]

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)

No they're not. If you want some VCA law faders to play with I have a bunch here you can have for postage.

 

[zamproject]

@Rob Flinn thanks for the offer, but no need here for this, already have my automation which is passive audio, not VCA.

any idea what is the taper then if it's not linear ? is that to mimic audio log over a constant dB/V of VCA

 

[Rob Flinn]

@Rob Flinn thanks for the offer, but no need here for this, already have my automation which is passive audio, not VCA.

any idea what is the taper then if it's not linear ? is that to mimic audio log over a constant dB/V of VCA

I believe it's not really log & it's not really linear. Maybe Google is a good way for you to find out more.

 

[zamproject]

For sure I do ... but find nothing like datasheet or curve drawing about -VCA fader taper-

 

[zamproject]

OK ! find a table at some P&G faders datasheet

As I suspect it compensate the dB/V (math log) to a more or less audio taper in a voltage divider potentiometer design for CV signal.
Usual (quality) audio log have ladder design with voltage ratio uncorrelated to resistance ratio.

 

[Rob Flinn]

OK ! find a table at some P&G faders datasheet

As I suspect it compensate the dB/V (math log) to a more or less audio taper in a voltage divider potentiometer design for CV signal.
Usual (quality) audio log have ladder design with voltage ratio uncorrelated to resistance ratio.

My understanding is that when MCI started fitting VCA automation to their consoles, they approached P&G to make a VCA law fader, but they wouldn't do it, so they went elsewhere & had them made. The resulting faders are Ok, but not P&G quality.

 

[Evillamp]

Yeah so i guess my 5k linear fader wont do the Trick ,any modifications /circuits that can make a Standart fader behave more Like a VCA law fader

 

[Evillamp]

Cant find Just the fader Part on eBay or elsewhere Just expensive jmh VCA cards

 

[zamproject]

Don't know for SSL about the actual fader law characteristic, but as there is computer involved on the fader reading (and send back), the non dB linear nor voltage linear characteristic of an -audio taper- may be extrapolated in software ?

Studer use Y=artanh X which involve differential pair in NFB topology

 

[Walrus]

You can run any 4000/6000 SSL without the computer, jumpers were supplied to loopback the VCA voltage when the desk was originally supplied.
So no software extrpolation is required.

 

[Evillamp]

Yeah i read the Part about bypassing the Computer too, ig the SSL Computer does some sort of passthrough and reads Out the fader CV as a analog Input and switches to Its own digitally controlled Pot/CV circuit for the Recall .Just my speculations.
So i got hold of a fader Pot on eBay from 1979 that seems to have the Same DB boost Cut range as the SSL faders.ill mess around with it a Bit when it arrives

 

[Evillamp]

My Goal is to create sort of a Mini SSL Channel anyone can build with all the harmonic and saturation goodness of the VCA card

 

[Evillamp]

If anyone IS interested in my Amateur ramblings and build process i can make it its own thread

 

[Evillamp]

So just a little update ,i looked a lot into vca faders .according to dougles self chapter on vcas those faders are linear with the the resistance on one side more spread out to achive a finer adjustment of the cv. I dont wanna pay 100 quit for a pgf3260 vca fader from the ssl4000. so i did some tests and loading the wiper of a pg 10k linear fader with a 2k7 resistor gives me a bit more of a log behaivor at a certain point so it works way better in the cv scaling circuit of the ssl

 

[Evillamp]

So for anyone interested j moved the whole project to its own thread https://groupdiy.com/threads/the-mini-ssl4000-project-diy-dbx-vca.88999/