Side chain filtering for SSL bus comp

[alexdingley]

Hi! Just trying to wrap my head around how one might use one of those old grey-face SSL bus comps with an external filter (to reduce bass-induced zippering/etc.) It seems that a lot of the modern variants / emulations of these have some sort of built-in high-pass SC filter, yet the older generation SSL units don’t have that.

If I were to use one of these original G384 / Grey-Face SSL bus comps, I’m curious: Would one just mult the main mix to both the SSL-in and the Filter-in, then patch the filter output into the Key-in on the SSL? My presumption is that, in this configuration, varying the filter freq on the external filter unit would allow the SSL bus comp to ignore the lowest frequencies in its detect-circuit... just like the built in SC filters on modern variations.

[alexdingley]

Oh, and in this configuration... are there any ideal high-pass Filter units that might be the best choice ?

[Nick Sevilla]

With the older units:

TWO outputs from the Master buss,

One goes to the SSL comp, and you filter out up to whatever low end (maybe also some top end) you did not want going into the SSL.

Two goes into a second filter, we normally would use a Stereo EQ for this task, either a GML 8200, or similar. This channel,
you do the exact opposite, and filter out everything EXCEPT the low (and super top end if need be) you wanted the pass through.

Then you'd mix them through a group channel, into your tape machine. Your faders on the group would be the final print level,
post compressor.

Your mix would go to a total of four mono group buss channels, which had inserts, instead of the Master buss. Once they got processed,
you could mix them back into two last channels and off to the Master channel. You could EQ a little at the very end before tape, if need be.

[vvv]

The description reminds me of a multi-band compressor.

EDIT: oh, wait - the second group is without compressor?

So you are kinda doing a frequency-defined parallel thing?

[Nick Sevilla]

The description reminds me of a multi-band compressor.

EDIT: oh, wait - the second group is without compressor?

So you are kinda doing a frequency-defined parallel thing?

If that was for me, YES. Manual one band compression. LOL.

[vvv]

[ashcat_lt]

I feel like if you're going that far, in many cases you could just do a pre-emphasis/de-emphasis thing. One EQ before the comp to take the bass out and one after to put it back in. In analog, that might cause some compromise in the S/N, but only in that low frequency area where it might not even be noticeable, and it only really mattes if it matters. Course, if you've got a key/sidechain input, you might as well do it the "right" way and save an EQ.

[The Scum]

I had to track down a photo of the original to make sure I understood the question.



To use the key as a filtered sidechain:

• Sum the L and R signals to get mono (try an aux out from console).
• Patch them to filter/EQ of choice
• Patch that to the key in.

The compressor signal path is stereo, but the detector is mono, shared with both channels.

As for the filter, it's pretty non-critical. I might throw an API 560 at it...or even one of those Shure inline highpass guys...or build a little Sallen-Key for it.

[alexdingley]

As for the filter, it's pretty non-critical. I might throw an API 560 at it...or even one of those Shure inline highpass guys...or build a little Sallen-Key for it.

Okay The Scum, I’m reading you loud and clear... and it seems to be a different approach than what Nick is detailing in his post (where he seems to be suggesting that there’s some re-blending of the filtered and unfiltered signals after the compression)

So, if I’m understanding this function of the SC / Key input, then I’m basically gonna need to copy the signal and filter off the bottom (or in reality I can EQ this copy however I like, to shape the Key signal), and then I need to figure out how to sum it to mono to jam it into the SC/Key in of the SSL bus comp rack.

My question (based on the quotes section above) is does it really not matter how high-quality the filter is? I mean, if I put the “key copy” of my signal through a crappy EQ that allows me to filter (but also has a shitty overall signal path) might I be badly “blurring” the signal that’s supposed to be triggering the VCA circuit in the SSL rack? And if that’s the case, and I causing the compression that I’m getting to be less finely tuned to the non-key/non-filtered signal that’s actually traveling the audible path of the SSL?

Maybe I’m over-thinking it.

I was just sort of thinking it out and wondering if (presuming I got a used SSL FX g384 long term, I’d probably want to set up a good ‘low cut key filter’ for the unit...) the EQ used to filter the Ket signal needs to be balls-out fancy.

[The Scum]

The key input gets rectified & turned into a DC representation of the signal amplitude, which becomes the CV to the VCAs for the compressor. From an audio perspective, it gets mangled pretty severely, so audio quality of the filter probably doesn't matter much. As long as it's not so noisy as to cause compression when the input is silent, it's probably fine.

That's what the API 2500 does, and what the various SSL clone + filter combinations do. In fact, tracking down some schematics for the Gyraf sidechain filter mod, it's a reall simple first order filter. A Sallen-key would at least be second order...

I think Nick is describing some sort of patchwork multiband compressor.

[The Scum]

A couple more options hit me:

Standalone filters (as opposed to equalizers) aren't super common in hardware. An old-school trick was to use a Drawmer 201 gate in "key listen" mode, so the unit becomes a highpass/lowpass pair.

You could also use the high output of an active crossover, if you've got one handy.