Now pin 42 is particularly important: The output from each 82E26 card is the cathode* of a diode. Because of this, the channel with the highest input will dominate the response of the 82E27. Put another way the entire buss comp is controlled by "the greater of two peaks."
*On the actual 82E26 the diode polarity is reversed. The anodes are common. Please also see:
The GSSL is a different animal. Note that in the GSSL the audio is combined into a single sidechain path:
For the GSSL to function like an SSL buss compressor, that is "greater of two peaks," it requires two sidechain paths (the 82E26 card part) whose outputs are combined at point C here:
This is what ssltech, one of our new members here have written about over at Prodigy-Pro.
There is a significant difference in these two approaches. By combining outputs after peak detection, the effect of signal polarity/phase correlation is eliminated.
By summing audio prior to detection, as in the GSSL, compression is biased to L+R. To put another way, the mono component is compressed, the L-R is not.
It isn't the purpose of this post to say which is "right" or "wrong." The point is to explain what the differences are and why they matter.
But, because this difference is significant, it isn't really fair to say the GSSL is a clone of the SSL. It is not.
EDIT: Jakob, by the way openly says this on his website and mentions the "tilt" effect in the 4K as being the reason he used a combined detector.
I'm not sure what he means about mis-tracking and tilt. If tilt means one side becomes heavier creating a center-image L-R shift I don't see how that could occur with the VCAs driven by the same control voltage to both the left and right channels. The only situation I can see creating that would be control-voltage tracking within the VCA. A common detector wouldn't solve that problem as the mis-tracking would almost have to be post-detection. I'm unclear about that. I commissioned a lot of SSLs and never heard that happen.Another change is that the sidechain is common to both VCA's, making better tracking abilities - your mix wont' "tilt" so easy when you're compressing heavily. We did things like that to the SSL in the old days. This might seem like a big change, but actually the sonics are preserved very close to the original, and tracking errors are cancelled.
Now if the slope switches on the 82E26 cards don't track well one channel could dominate compression but the center image wouldn't shift. The compression just wouldn't be right.
Using dual detectors in a greater of two peak sidechain is identical to the original and looks at the overall peak level of the stereo pair.
Algebraic summation at the input, e.g. summing left and right, allows the compressor to only "hear" in mono. In the extreme example, two identical level signals input completely out of polarity are not detected at all.