Originally posted over at Gearslutz, edited slightly to add links and images:



Hi there,

I'm one half of 'Expat Audio', and for several years now I've been looking into the problems with the majority of GSSL's (SSL clone buss limiters).

One problem which most builders and users simply didn't understand is that the GSSL does NOT behave like a genuine SSL buss limiter. about 8 years ago I made a video trying to show the differences when you pan a mono signal around.

Since people already noticed that "it compresses the kick and bass to hard" -and with those usually being high-amplitude, low-frequency-heavy instruments, the "band-aid" solution was to add a high-pass filter in the sidechain.

This was entirely the wrong way to fix the problem. Now the unit STILL doesn't compress centrally-panned instruments correctly, and in addition, it stops working with the correct sensitivity at low frequencies... Two wrongs don't make a right.

The CORRECT solution is to add a SECOND sidechain detector, which corrects the stereo panning inaccuracies of the GSSL design.

We built it, we offered it up to the DIY community, people bought it, built it and installed it into their existing builds, and the result was that every single one of them LOVED the result. http://expataudio.myshopify.com/products/gssl-turbo-pcb

That corrects the compression misbehavior... then people started to ask us about correcting the other issues which they were having, in particular the high noise floor. (Usually 120Hz hum in the US, and 100Hz hum in the rest of the world.) We came up with the 'CRC' board, which just solves the problem absolutely. 


next came a metering board, so that people had the option of using a true VU meter instead of a milliamp meter, for those that wanted to view the gain reduction in the same manner that you would see GR displayed on an LA-2a, or an 1176 etc. -That of course doesn't actually affect the SOUND of the unit, but DOES significantly affect used perception... an interesting option, but not strictly 'sonic' on nature. 


Now you have to understand that the signal path on an SSL limiter is very simple. The signal comes in, is buffered by an op-amp chip, then pases through a VCA, then goes through one final op-amp chip and hits the output. That's it.
-If you want to affect "how the unit sounds", as opposed to 'how it modulates gain', then you have to take this into consideration.

Personally, I don't go for simple IC-swapping "magic", and despite the fact that people do things like strapping a transformer across the output and declaring it sonically "transformed" (get it?), this is not an approach I recommend. Not at all. You need to make some changes in order for transformers to work happily. Most of the sonic differences a user may notice probably come from the fact that a GSSL -as originally built- was never meant to feed a transformer.

If you want transformers in the signal path, you really do have to change the circuit in order to DRIVE the transformer correctly... And for that reason, we came up with another add-on board called "the Cavendish".


The Cavendish is a board which REPLACES the input and output op-amps with circuitry that essentially mimics an API signal path. The input (a differential receiver) stays largely in place, but the chip is fed through a discrete op-amp (2520 or your choice of compatible alternatives) instead. The output circuitry is basically completely bypassed. the signal is 'grabbed' coming out of the VCA, where it then goes to an API output driver circuit, with a 2520 (or compatible alternative) which then drives a 25030 output transformer, which then goes to the output XLR's.

So, basically we've looked at all the various comments which people have had about the GSSL, and made "bolt-on" boards to address its issues. We've never offered anything for sale which is "voodoo" or offers "imaginary improvement". All of our modifications are test-repeatable and significant in terms of their difference, with the possible exception of the Cavendish, which is more of a "preference" thing.

Our other offerings like the DIY mic preamp stuff have been more of a way to offer other things on the webstore, but -speaking personally here- my primary fascination was always looking for ways to improve what was initially a rather poorly-executed copy of an excellent original design (the G384 SSL limiter was actually vastly better than the 'GSSL' clone... for example, simply add a turbo board and a CRC, and marvel at the improvement in noise floor and stereo limiting detection!)

I posted a couple of videos on YouTube -in my horribly mid-Atlantic accent- (search "expat audio turbo") which show things like the metering board and the turbo board. I'm happy to answer any other questions.

Keith A.

Written by Keith Andrews — September 08, 2015