Compressor geeks! Please tell me what I have here... (a dbx 160 variation?)

[Big_Bang]

Hi geeksluts!

Its my first time in this part of the forums, so please be gentle

I am trying to to figure out a few facts on a couple compressors I ended up with, and am falling in love with!! Never thought much of them (used live) but I brought them back to the studio and they're blowing my mind silly!!

Its the Symetrix 425, dual comp/lim/expander.

I've read in numerous places it uses the same VCA as the dbx160, but that alone is not enough to grant it transparency and quality (though my use in the studio does!)

Also that it is basically 2x 501 into one unit. The 501 has a great history and reputation!

I found out this about it, but this is a strange and alien language to me

"It uses a CA3046 transistor array as a detector with both log peak and log average outputs. It's very similar to the Blackmer RMS topology (without squaring) and uses a 2150 VCA. "

So what does this say about these units?

Anyone have info on them?

Thank you!

[Big_Bang]

tech specs for 2150 VCA and CA3046 transistor array


THAT Corporation 2162 Dual Blackmer Voltage Controlled Amplifier ICs

[mexicola]

Alot of comps used the 2150 series vca (everything from various SSL clones down to the lowly Alesis 3630, IIRC).
The newer version of the 2150 series is the 2180 series made by THATCorp, which took over the design from DBX.

The VCA is only one small part of the circuit, and how it's implemented in the circuit is everything in terms of how it performs. If two different comps use the same VCA there is zero guarantee they will sound or perform the same. Saying "X" comp has the same VCA as "Y" comp so they should sound the same is like saying Elizabeth Hurley has the same hair color as Susan Boyle so they should be equally hot. Ain't so!

Those VCA's are available to anyone who wants to purchase them, btw.

[dualflip]

Agreed with mexicola!!!, specially cuz i think Elizabeth Hurley is the hottest woman on earth. Now, about the VCAs, as mexicola said, a lot of gear had the same VCAs and they were very different monsters, for example take MCI and SSL, SSL early versions had the dbx202c VCAs which were also found on MCI desks, however both consoles sound very different.

[Big_Bang]

So basically what you are saying is that you want me to strip search Lizzy and see whats under her clothes....

I can do that !! (I'll crack open one of them and take some pics when I get home)

Thank you all for your responses.

I've done some more research, and DBX was founded by David Blackmer. The quote I posted above talks about "...similar to blackmer rms topology..." - I would like to know what the first half of that sentence means. Could it be because there are no attack/release controls on this unit, so it works based on RMS? If so, that is a definition of program dependant, which also suggests it could work well on a 2bus... or am I leaping waaaay over the top here?

Now please dont beat me up for all this I am in no way WANTING to have a dbx 160 clone at all... just curious and wanting to expand my knowledge.

[JohnRoberts]

Originally Posted by Big_Bang

Hi geeksluts!

Its my first time in this part of the forums, so please be gentle

I am trying to to figure out a few facts on a couple compressors I ended up with, and am falling in love with!! Never thought much of them (used live) but I brought them back to the studio and they're blowing my mind silly!!

Its the Symetrix 425, dual comp/lim/expander.

I've read in numerous places it uses the same VCA as the dbx160, but that alone is not enough to grant it transparency and quality (though my use in the studio does!)

Also that it is basically 2x 501 into one unit. The 501 has a great history and reputation!

I found out this about it, but this is a strange and alien language to me

"It uses a CA3046 transistor array as a detector with both log peak and log average outputs. It's very similar to the Blackmer RMS topology (without squaring) and uses a 2150 VCA. "

So what does this say about these units?

Anyone have info on them?

Thank you!

Symmetrix was one of several small companies making rack mount signal processing gear for live SR and studio use.

Back in the not so good old days there were more than today but only a handful of VCA companies, and nobody selling detector chips like THAT does now.

We had to roll our own log conversions to manage the dB math to control VCAs for variable compression ratios and the like. Transistor Vbe junctions were used for this conversion because of their logarithmic relationship between forward current and voltage. However trying to perform a log conversion using discrete transistors would suffer large errors due to device matching and temperature differences. The 3046 (and 3086) were popular simple transistor arrays with a handful of transistors on a common substrate that will track well for temperature and be closely matched.

This is all ancient history, as dedicated ICs are now available, and the low cost transistor arrays have fallen out of favor.

While some advocate keeping the sound of legacy (read old) VCAs, you could probably drop a newer VCA into your comp and make it even more transparent, while a large part of a compressor's "sound" is how they manage the side chain manipulations.

Of course with any old soldier like that, you may want to look for tired marginal electrolytic caps.

JR

[Big_Bang]

John,

Thank you very much, I truely believe that was very informative but I have zero electronics/circuitry education. Would you mind refrasing that for, say, a 5 year old?

I truely hope that in years to come I will be able to interpret what you wrote, as soon as a friend clears up some stuff in his life, he promissed to do a couple of DIY projects, I'll probably start by cloning an 1176 as a hand-held kindergarten project

I live to learn

thank you!

[JohnRoberts]

Originally Posted by Big_Bang

John,

Thank you very much, I truely believe that was very informative but I have zero electronics/circuitry education. Would you mind refrasing that for, say, a 5 year old?

I truely hope that in years to come I will be able to interpret what you wrote, as soon as a friend clears up some stuff in his life, he promissed to do a couple of DIY projects, I'll probably start by cloning an 1176 as a hand-held kindergarten project

I live to learn

thank you!

I'm not sure a 5 YO could understand compressor design no matter what vocabulary was used.

In broad strokes.. If you consider the Symmetrix "transparent" sounding that is likely because of the side chain design (attack/release time constants, and smoothing tricks if used). It doesn't sound transparent because it is using old VCAs and old log conversion circuitry. The log conversion is necessary because VCAs use log control voltages not linear. Note: some of the magic attributed to RMS may be from applying attack/release smoothing after the log conversion.

If you are happy with the sound because of transparency, not coloration, you might be even happier upgrading to a newer generation VCA that is even cleaner (some other posts referenced the THAT corp website). Depending on how old that unit is you may see some benefit from upgrading opamps in the critical path. If it is already using TL07x and/or 553x improvements there will be modest at best.

Electrolytic capacitors that are a few decades old might need inspection for age related deterioration.

JR

[mexicola]

Originally Posted by JohnRoberts

If you are happy with the sound because of transparency, not coloration, you might be even happier upgrading to a newer generation VCA that is even cleaner (some other posts referenced the THAT corp website).

If you're upgrading the 2150 series to 2180 series, while they are pin-for-pin compatible, and you can simply drop them in if you want; in order to gain maximum performance from the 2180 series, you will need to make changes to the circuit. The reason is that 2180 incorporates into its internal circuitry some of the resistance values that were required externally with the 2150.
You'll want to have a look at this design note: http://www.thatcorp.com/datashts/dn137.pdf
page 3 begins several examples of a typical 2150 circuit vs a typical 2180 circuit.

Originally Posted by dualflip

specially cuz i think Elizabeth Hurley is the hottest woman on earth.

agreed!