GSSL - DBX VCA's THD adjustment

[nextB]

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]

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. 

 

[gyraf]

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

 

[andow]

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]

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]

(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.

 

[nextB]

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

According to SOS article about SSL's new BUS+, it seems SSL officially admits they intentionally added second harmonics for their 4K VCA.

https://www.soundonsound.com/news/bus-ssl

 

[living sounds]

Interesting. I have found the harmonics of the THAT218x VCAs in the GSSL to be very level- and - to a degree also - frequency dependent. What is meant by this is that you can set the pot for minimum THD at a certain amplitude, but at the expense of more THD at higher and/or lower amplitudes. This is with no GR, of course.

 

[MiamiBoy]

I just bought 2 gold cans, the seller couldn't test 'em so we don't know if they're working or not.
So I got them a lil cheaper.

Is there a way for me to test them, before putting it in my GSSL?
all I have is my DMM and audio unit.

 

[gyraf]

best probably to try them in-circuit

not many pins, perhaps connect with alligator clip wires to test before committing solder?

 

[MiamiBoy]

perhaps connect with alligator clip wires to test before committing solder?

Great. That sounds doable, Jakob.

I’m still looking for what needs to be changed in order to use dbx instead of THAT vca’s. Anybody?

 

[Script]

Is it that you haven't found it yet because you keep cross-posting ?

https://groupdiy.com/threads/instal...-instead-of-the-that-2180.63189/#post-1074532

 

[MiamiBoy]

Is it that you haven't found it yet because you keep cross-posting ?

Maybe
I've been looking for months, can't believe you found it so quickly. Thanks, appreciate it!

 

[MiamiBoy]

Any idea where to find read material on those empty % spots on main pcb?
Nothings related comes up when searching %

 

[gyraf]

the schematic should explain these - it was left out when updating to trimfree vca's. Also explained a couple of times earlier in the big thread..

 

[MiamiBoy]

Ok thanks Jakob, i'vd tried but I haven't been able to find the explanation here when searching.
I've looked at the schematic, but it's not clear to me because I can't read schematics, so I have to find the explanation in plain English.

If you have a link, please send it. Or if you remember what it means by the top of your head, please advise.
Or even better, a screenshot of the schematic where the "answer" is and put a big red circle around it for me

So to be clear, they have nothing to do with using 202C's instead of THAT as I suspected, correct?