GSSL - DBX VCA's THD adjustment

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nextB

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Aug 13, 2015
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I have built 2 pairs of GSSL (type) compressors based on the original SSL 4KE 82E13 Rev3 VCA boards. Each board has SSL sealed Gold Can DBX 202C, 78L15/79L15 regulators and a pair of 5534 (+-18V). DBX's THD trim pots are sealed by SSL.

I measured those vintage SSL cards and I'm a little surprised to find that all 4 VCA's trim pots are NOT set at their lowest THD position. It seems like SSL intentionally leaves the second harmonics as high as 3rd harmonics. Something may have changed since it was calibrated 40 yeas ago though.

Those 2 pairs of SSL style comps sound very good to my ears at this setting, and they measure very well. Signal path is very simple, 5534 to unknown film cap to DBX to 5534 (unbalanced out, negative impedance compensated). No electrytic in signal path.

Measured at -1dB (RMAA)

20 Hz to 20 kHz -0.24, +0.02dB
Dynamic range (A-weighted) -115.7dB
THD + Noise (A-weighted) 0.11030%

I can make THD much lower if I move the trim pot at its lowest setting canceling out the even harmonics. I honestly can't hear much difference between lowest setting and this setting, but I wonder if anyone has a chance to measure a real SSL compressor, or I'm looking for an information about official SSL guidelines of calibrating VCA.

Anyway, Nelson Pass states many people think an amp sounds better with certain amount of 2nd harmonics left. https://www.firstwatt.com/articles.html
 

andow

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Would be really interesting how SSL calibrated these (although I doubt this info is available anywhere...).

I remember messing around with the calibration on a GSSL with 2181 and also found that it sounded best when harmonics are NOT set to the lowest level.
You'll often find that the theoretically best calibration is not the best sounding one.  ;)
 

andow

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gyraf said:
can confirm that at least one manufacturer mis-trims on purpose

Now I wonder who that could be...  ;D

Btw: congratulations on your 10000th post!
 

MaxSquare

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Dec 22, 2017
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Hey, guys and girls. Is there any way how to use Serpent Audio quad VCA (got mine pair as a gift) in regular GSSL build? What needs to be cut or changed? SOS
 

gyraf

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(please don't cross-post)

as we don't know what serpent put in there in the first place, we can't estimate that. Ask serpent perhaps?

/Jakob E.
 
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