The Mini SSL4000 project(diy dbx vca)

[Evillamp]

So i really wanted the sound of a ssl 4000 in my homestudio but iam no fan of plugins and record only the analog way.So i had this idea of cloning a singel channel of the ssl4000,complete with diy dbx202s and micpreamp , dynamics etc.
So for the eq part i got hold of one of caseys 4000eq,the dynamics will be provided by a original ssl4000 dynamics card that i found for a good price.Everything else including the vca fader will be recreated (somewhat) faithfully by me

 

[Evillamp]

The mic preamp was easy enough to clone.with that done the biggest part to figure out is the vca channel fader.ive been doing a deep dive into the dbx202 that is used there.got pcbs made for a gold can and blackcan and so far its successfull with minimal distortions.iam gonna update this thread with pictures and schems for what iam working on

 

[tommia 2]

Thanks for doing this

 

[Evillamp]

So for the Channel Fader ,the large one, I did solder this together. Just finished it yesterday. We can ommit the buffer for the Pc input and go straight from the Wiper of the large fader trough a 10 ohm resistor into R25 for the computer return

 

[Evillamp]

for the Fader i bought a few Penny&Giles Faders. Couldnt get hold of any vca faders so i used 10k Linear PGF3210 faders. I found this page in the manual for the Stereo Vca and image control circuit. Iam gonna take a few parts out of this for the channel vca ,for example the Fader Law adjust trimmer VR5 sounds like a good addition .Besides that the vca channel circuit doesnt have much difference to the stereo and image control.

 

[Evillamp]

So heres my 202 goldcan prototype ,the trimmers are for getting the correct voltages and didnt really fit the footprint i selected.yeah i also finished the control circuit ,i gutted a ADT audio vca fader for it and installed the pgf .so attenuation and boost seems to work great ,only thing i dont get is the infinite attenuation on the bottom of the fader,maybe there is a switch inside the ssl fader for disconnection or something? But so far the basics work.i have the vca running on a very simple test harness with a input resistor and cap as well as an output opamp and the symmetri adjust

 

[Evillamp]

So i spend the last few days testing the vca scaling pcb. As i said ,i cant get it to completley go silent at the bottom of the fader.so fearing the worst (my diy goldcan is a fail?!?) i solderd up a 202x from some dbx 2150s i had. Aaand well my goldcan is not to blame.they seem to respond pretty similar to the control voltage ,with the diy goldcan being a bit more crunchy at times. So ig ive proven to myself that the goldcans work. Iam gonna build a mute switch into the channel (because i can xd) so i dont really mind if it doesnt go completley silent at the faders end.So yeah a whole lot of words for a simpel update on the vcas but iam happy to report that they perform incredibly well for being made in my bedroom xd

 

[12afael]

Have you tested thermal drifting? just curious.

 

[Evillamp]

Have you tested thermal drifting? just curious.

sadly dont have much equipment to test the vcas, my approach was a more naive "throwing shit against the wall and see if it sticks" so this is in no way shape or form a 100 percent replica. and with basic hfe matched transistors it performs (in the circuit) close enogh for me to the 202x i tested. i also have the gerbers for it if anyone wants to have a go at it

 

[Evillamp]

so next up i modified the casey 4000 eq i build a few months ago.Pretty simple job.just getting rid of the balancing in and out circuit ,so it integrates into the channel iam building.input is now the junction point of R1 and C1 and out is the point of R56 and c28.yeah the balancing is not required up until the mixbus i think

 

[arthurdb]

Concerning the fader law, the Studer 928 console used P&G linear faders to control the VCA and have an interesting circuit to bend their law (IC34). IIRC the FADER CAL trimmer is there to set the unity gain point and the PTC resistors in the feedback loop to compensate for the temperature coefficient of the 2181.