Help needed understanding SSL Mixbus RATIO circuit

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Just a quick view , but maybe try lower R9 (lower down to 20K you would get unity gain at 2nd rectifier) and maybe D2 just across to R9 and not to R4/R5 junction. I think R8 is just for biasing the diodes, you should use 10M at least for them. If you let it open they will get bias somewhere but just in case 10M is big enogh to not hurt anybody.

I think R9 should work as ratio, see:
10:1 1M (between A and B, always connected)
4:1  1M||510K ≈ 340K
2:1  1M||270K ≈ 210K and 1M||68K ≈ 64K across D1
So 1M for 10, 340K for 4 and 210K (and "strange" 64K) for 2 seems a close relation, but less the ratio lesser the resistor (less than 400K for 4:1)

So I would go down 150K for 1.5:1 (maybe 68K is a good place to start)
Also I would try D2 directly from A to B at least to see what happens.

If you want higher ratio change 1M for something bigger, but take care about stability. There must be a way to make it squash like 1176 all-buttons-in, it would be so intresting, I would try 1M from A to -12, 1M from B to +12 and opening the existing 1M from A to B.

I've never build any SSL buss comp, it's really hard to find any THAT218X around (maybe at U$20 each, but also hard) but I've order some to USA, if they arrive some day I'm realy excited doing this kind of mods...

Regards.

JS
 
If I change R9 to a 68K the ratio goes up slightly to about 2.1:1

I tried moving D2 to A-B and I could hardly get 5 db of compression total... it looked like the ratio was 1:1 or very close to that...
 
jwhmca said:
...I tried moving D2 to A-B and I could hardly get 5 db of compression total... it looked like the ratio was 1:1 or very close to that...

Maybe that is the way to go, try a little resistor, something smaller than the 68K in the original 2:1 you could just put a parallel resistor to the original 68K (another 68K or maybe a 100K or 50K pot and play a little, there you can find the ratio you are looking for)

JS
 
...is it at all possible - in a predictable way - to have ratios below 2:1 for feedback-compressors? I have a faint memory of a problem here...

Jakob E.
 
gyraf said:
...is it at all possible - in a predictable way - to have ratios below 2:1 for feedback-compressors? I have a faint memory of a problem here...

Jakob E.

Really wow... That might make sense from what I'm seeing. I wonder what the problem or issue is?
 
In reading the THAT design sheet (http://www.thatcorp.com/datashts/dn01A.pdf) on Comps ratios... it says "High ratios are difficult..." wouldn't that be lie 20:1 etc?
 
With open loop you would have about infinite:1 ratio, but it won't be stabe... for ratios lower than 2 (ratio=1+G) you would need G<1 also unstabe for an opamp, but you could get it from an attenuator... maybe you could try an attenuator after the detector but there is a problem, low Z atten will be a heavy load to the opamp and output diode, hi Z will affect the attack time (820Ω fastest attack)

2kΩ atten to get 0.5 gain (recomended load for TL07X) but will have an OPZ of 500Ω wich will affect the fastest attack time, if it's ok for you you could try it. A better way could be to add the atten at "D" in the schem, there will affect longer release time and offset of the next opamp (but low because of low bias current of jfet. (1M will half the longer release but get good dc offset, maybe 2M or 4M would work fine) Just gessing, is what I would try, but if you try something tell me what you get.

JS
 
Hi!,

I know this thread is old, but very useful to understand how the ssl bus compressor works. In that sense, i would like to know if the sidechain detector and ratio circuits are single or double?. I mean, there should be two sidechain vca, but two detectors? This is the doubt i have. Because the output of the rectifier goes to the attack circuit...

Merry Christmas!!

Jay x
 
i would like to know if the sidechain detector and ratio circuits are single or double?. I mean, there should be two sidechain vca, but two detectors?
SSL have chosen to use a mono'ed version of the L & R inputs to drive the detector.
It has the advantage of simplicity, but has the disadvantage that input signals that are out-of-phase result in too low compression.
I believe they relied on the capacity of SE's to avoid such mistakes.
OTOH, separate detectors have the disadvantage of steering the stereo image; in order to avoid this, there needs to be added a circuit that will take the highest control voltage, an analog OR.
There are DIY variants of the SSL bus comp that use two detectors.
If you intend to build one, it's a choice you can make.


EDIT: According to member moapms, the genuine SSL comp has two separate detectors. However, the attack and release controls are common to both channels.
My answer was based on many internet comments (i knew I shouldn't have trusted inerent!) and the belief that the SSL bus comp was similar to the GSSL.
 
Last edited:
Hi!

Ok!. Two detectors. But still, the output of the two sidechain detectors go to the attack section. And an opamp based OR circuit may still help select the higher value?. I also have seen diode based OR gates, but the reversed diode at the output of the sidechain rectifier made me doubt.🤔

Jay x
 
The schematic shows a part of the SSL type of compressor from one console (I can't send the whole schematic) where it is clear how the DC control voltage that is common to both compressor channels is obtained.

1640560988998.png

Don't be confused by the fact that the diodes (DC14, 15) are directed towards the sources, the control voltage is negative. Far more information can be found in threads dedicated to GSSL. There, these modes of operation (turbo, etc.) are pretty well explained.
 
In the SSL 4040E, the analogue OR was after rectifier, into the timing circuits

Background story:

In that desk, we had an intermittent error in the GR balance across the 4bus for a very long time, so we ended up mono'ing the whole sidechain in order to get it running for daily use, and it was left like this for several years. This was the state of things when I was commissioned to build the twin for our calrec studio - we needed comparable mix comps in the studios, and at that time nothing like this was available off-the-shelf.. The mono'ed sidechain behavior (mid-triggered compression) kinda appealed to most of our engineers, so we never changed it back..

/Jakob E.
 
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