andre tchmil

average to peak detector for SSL clone , schemo ?
« on: November 12, 2004, 07:34:14 PM »
Is there a way to incorporate an average/ peak detector in the ssl.
My Urei 7710 has this feature, variable with a pot and I love it.
I'm building a 3U racked one, so plenty of place to add an extra pot.
 :green:


SSLtech

average to peak detector for SSL clone , schemo ?
« Reply #1 on: November 12, 2004, 09:58:26 PM »
Well, the SSL is peak detecting as it stands. Analog devices and others make precision RMS-to-DC convertors. I think that you would have to employ one of each, but the problem lies in the fact that such RMS-to-DC convertors are usually deployed in feed-forward circuits. The SSL uses a relatively complicated rectifier circuit around the ratio selection, so it's not so easy to do.

Since what you would end up with would not be an SSL clone, the easiest way would be to send the signal to two compressors and blend the outputs... one would be RMS feed-forward and the other would be the SSL in feed-back.

The GML circuit is feed-forward and it also uses a 'crest factor' control which balances between RMS and peak sensitivity. It too is an all feed-forward device.

Since there was some disagreement about the merits of feed-back topology, including no less than George Massenburg saying that his opinions was that feed-forward just works better, and my observation that the majority of drooled over and coveted comperssors are feed-back (LA2a, SSL buss comp, 1176 etc) and sound better, then there was a need to agree to differ. Certainly the differences in operation are difficult to reconcile, so essentially, you'd have a hard time adding an RMS option to the SSL, and -while not impossible- the engineering changes would possibly have such an effect on the peak operation behavioiur that it might not have an 'SSL' mode any more.

DBX and certain later JBL-designed UREI compressors have been all-feed-forward, so crest factor or peak/RMS controls have been easy to integrate. Not so simple in the SSL I'm afraid...

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

ijr

average to peak detector for SSL clone , schemo ?
« Reply #2 on: November 13, 2004, 06:47:42 AM »
If you're building 2 detector _FB_ compressor, don't go for so-called rms
detector in "slow" section. First, rms is root-mean-square, and
in electronical therms, you cannot get here adequate response, except
heatting method; also, great qty of "ripple" if detector is fast and
very slow response if detector slow making use of this stuff
good only in log detector log time constant vca-based compressors;
 average detector more usable in non-linear (vc db) time constant
compressorwith linear detector like varymu or sslmixbus.
(this is btw theirs musicality)  :wink:
You can go for enchanced 7110 just make fast detector on kinda
balanced stuff like ua733 (possible with buffered outs too :-))
with two diodes and small (47-100n)cap and 1-4 megs release R
and for slow det. there' s tone of  appnotes you can play with;
then, you have to decide what way you want to mix them - like in 7110
 or like in whatcompressor or...something different ;-)
Crestfactor in 7110 is really not xfctor, it is threshold for fast detector.

I played with capacitance multiplyer sircuits, tryed opto's
in release controll....the best sound for me is voltage controlled
current source for release, and how do you controll it...
depends on what you have in your mind.
Hope it helps,
IJ.
"The ear is the only judge of tonal balance."
Radio Designer's Handbook, 1953, p.632.

matthias

average to peak detector for SSL clone , schemo ?
« Reply #3 on: November 13, 2004, 08:35:04 AM »
some time ago I thought of another method, to improve the G-SSL sidechain/detector...

actual the sidechain-signal gets summed and leads into the sidechain vca...

in the SSL FXG 384 Comp each signal gets compared and the loudest signal on each channel drives the sidechain...

so this should provide a better stereo-imaging, and avoid phase problems..

there's an interesting dbx note on that :

http://www.dbxpro.com/ftp_mirror/PDFs/WhitePapers/DetectionandDynamicsDuo.pdf


what do you think?

would it be worth to implement it..??
or would the difference be inaudible??

mat

ijr

average to peak detector for SSL clone , schemo ?
« Reply #4 on: November 13, 2004, 09:17:39 AM »
Yes, better is two (l and r) detectors, actually four for dualdetector sidechain in stereo,and then shared timeconstant circuits,one for
peak/one for avg, then summing(actually, comparator logic) circuit; difference beetween "mono before detector" and
 "two detector then timeconst." is very audible.
"The ear is the only judge of tonal balance."
Radio Designer's Handbook, 1953, p.632.

SSLtech

average to peak detector for SSL clone , schemo ?
« Reply #5 on: November 13, 2004, 09:18:03 AM »
The difference is audible, and leads to entirely different behaviour on mono and wide stereo signals between the two compressors.

I have a couple of mods to do this, but none fully documented sadly...

Essentially it can be done with two precision rectifiers feeding the same VCA, saving having to use the second VCA... and a further variation is to allow the two methods to be 'blended'. -Either way produces great distortion of a stereo signal in the sidechain, but it is the exact distortion needed to correct for the imaging error in Jakob's version as compared to the original.

There are also other mods... I'll post them when I've completely built them, they're on my bench at the moment, while everything is on hold with this Bloo stuff...

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

ijr

average to peak detector for SSL clone , schemo ?
« Reply #6 on: November 13, 2004, 09:31:34 AM »
Quote

Essentially it can be done with two precision rectifiers feeding the same VCA, saving having to use the second VCA...

It will work not very good on fast peaks:-))
I'll avoid using "diodes-in-feedback" method for fast detector too;
sidechain distortion and phaseshift in not something insignificant...
"The ear is the only judge of tonal balance."
Radio Designer's Handbook, 1953, p.632.

SSLtech

average to peak detector for SSL clone , schemo ?
« Reply #7 on: November 13, 2004, 12:40:46 PM »
Quote from: "ijr"
It will work not very good on fast peaks:-))
I'll avoid using "diodes-in-feedback" method for fast detector too;
sidechain distortion and phaseshift in not something insignificant...
Perhaps, but the result will be a lot more like the true SSL behaviour than the Gyraf version.

In this case the distortion is not a random matter, it's actually the same method that SSL use with the "diodes-in-feedback" as well. The only difference is summing before one VCA as opposed to summing after two (or four on the original!...but two are silent on stereo operation and are only there for quad) VCAs. The result should be identical whether you turn down then sum, or sum then turn down. In both cases the sampled signal at the junction of the detector outputs is a distorted version of the inputs where two conflicting signals meet.

The result is the same behaviour characteristic as the original SSL.

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.


 

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