SSL mod: switch between feedforward and feedback compression

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matthias

Well-known member
Joined
Jun 4, 2004
Messages
768
Location
germany / frankfurt
hi,

I'm just thinking of another mod for the ssl comp.

the api2500 has a switch where you can select "old or new style"

new= the sidechain gets its signal from the inputs (as it is in the ssl comp)
old= the sidechain gets its signal from the output of the vca (feedback compression like the 1176)

this should be also easy to implement in the ssl comp.

can somebody tell me if this would work:

feedback.GIF



cheers,
mat
 
It won't fully work....or at least your makeup gain won't work. I thought about doing this before, but if you take a closer look at the way the SSL works you'll see that it's actually a feedback compressor, because the VCA in the sidechain is located before the detector.

If you try to connect the output from the main audio vca to the input of the sidechain you'll quickly find that your makeup gain pot now behaves as a threshold pot. In order to do makup gain in this arrangement you'd need to have a varaible gain stage following the VCA.

Cheers,

Kris
 
Matthias, the SSL is already a feedback compressor, even is it looks different. It uses the third VCA to make a feedback compression. You can try to cut the control voltage from that third VCA to make it a feed forward compressor.
 
I would say unsolder the 470Ω resistor that goes to pin 3 of the sidechain VCA.
But maybe it will totally change the gain structure of that compressor and you have to adjust some other thinks.
 
so I did it and it sounds really interesting!!
I really like the way the transient get shaped...

just listen yourself: http://www.cmaudio.net/sslfeedbackandforward.mp3

first one=normal ssl compression
second =forward compression with 470R lifted
(that goes to sidechain vca pin3)



there's only problem: threshold pot doesn't react anymore ... the gain reduction is very high in that mode it feels like the threshold would be turned completely down..
 
hehe... maybe.... but I want to make things perfect you know !! :grin: :grin:


so there has to be great to have a threshold or input pot...
otherwise this is too unflexible..
 
Try your makeup gain.....like I said in an earlier post, it will act like a threshold control. The downside, you lose makeup gain at the VCA, and need to 'make it up' somewhere else, usually in the amplifier stage immediately following the VCA.

This is how most feedback compressors work.

Kris
 
Try a different mod:
Solder that 470Ω in again. Unsolder the 100kΩ resistor right from the Threshold pot. It should have the same effect as before, but you will have the threshold pot working.
ssl_mod.gif
 
thank you, it really works great!!

but I will have to decrease the range of the threshold pot in that mode, otherwise it's too much compression and the meter goes wild...

try it yourself, it really sounds good!!
 
hi, i'm interested in that mod :

which 100R resistor you're talking about ?

on the schematic >
the one before CV to VCA or the one before CV to SC control ?

i suppose it's the one before CV to SC control.
and i also suppose "unsolder" means let the track open (and not "replace 100R by a wire")

but can you confirm before i screw up my GSSL

thanks ;)
 
matthias said:
thank you, it really works great!!

but I will have to decrease the range of the threshold pot in that mode, otherwise it's too much compression and the meter goes wild...

try it yourself, it really sounds good!!

This sounds like something I'd like to try.
To decrease the range of the threshold, would it simply be a matter of increasing the value of the 47K resistor that's added between the CCW side of the pot and the control pcb?  I think by using a dpdt you could simultaneously "remove" the 100K and "add" another resistor to the threshold.
 
which 100R resistor you're talking about ?

on the schematic >
the one before CV to VCA or the one before CV to SC control ?

i suppose it's the one before CV to SC control.
and i also suppose "unsolder" means let the track open (and not "replace 100R by a wire")

anyone ??  ???
 
He said 100K, not 100R...but I'm still trying to figure out if which one.  I think he means the one on pin2 of the tl072 (there's no 100K on the threshold pot, but that's the closest one), but I'm not sure as I haven't had a chance to try it out.  I'm building a gssl at the moment, so when I get to that point, I'll try it out and see what I can find.
 
Well.... I'm looking at the schematic, and...

I must say.... It doesn't make that much sense to me..... (I'm no expert).

Tomorrow I'll open mine, and unsolder that one, see what it does.

I'll let you know.

[Edit : Now I see, it has to be that one, yes]
 

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