SSL G-Bus Clone distortion measurements

[adalogue]

I searched the forums and didn’t find this question specifically addressed so posting here!

I’ve benched tested several g-bus style 500 series modules, two OEM and one kit, with vastly different distortion specs.

1. SSL G-Bus (THAT VCAs)
2. Stam 4k5mk2 (dbx 202C clone VCAs)
3. Sound Skulptor CP4500 (THAT VCAs)

SSL was by far the lowest at unity gain (0.003% IMD and 0.001% THD @ 500 Hz). Distortion values increased as compression increased (1.6% IMD and 0.22% THD @ 500 Hz with 12 dB of compression).

Stam was by far the highest at unity gain (3% IMD and 1.5% THD) and got a bit higher with compression, but not much. IMD plot also looks pretty crazy (attached).

SS was between the two at unity (0.05% IMD and 0.02% THD) and higher with compression (2.2% IMD and 0.3% THD with 12 dB of compression).

I’ve heard it said that the dbx 202C (cloned in the Stam unit) have higher distortion specs, but the cutsheet appears to disagree.

How does a vintage SSL console compare? I’ve tried to find one to test locally and haven’t been successful yet. The dbx 202C cut sheet does not show high levels of IMD or THD at unity, but rather pretty low values, close to those I measured for the SS CP4500:
https://www.gyraf.dk/schematics/Dbx202.pdf

 

[adalogue]

 

[gyraf]

..this shows IMO that the Stam "clones" of the DBX202 are basically uninformed, i.e. made without insight into the topology. This grade of "coloration" is what you would expect from building 202's from off-the-shelf components without bothering with matching parts at all. And note, part matching was and is the essence of the Blackmer-cell that these were based upon.

Original DBX202's were significantly better (more neutral than) the integrated DBX2150 - and almost, but not quite, on par with the later 2180

/Jakob E.

 

[adalogue]

Thank you Jakob, I appreciate your reply.

 

[adalogue]

Original DBX202's were significantly better (more neutral than) the integrated DBX2150 - and almost, but not quite, on par with the later 2180

/Jakob E.

Do you know how THD varied as a function of frequency with compression in the original 202C? I’m noticing quite a variation in THD between low (50), mid (500), and high (5k) frequencies for both the SSL G-Comp and SS CP4500 with compression. I tested 6, 12 and 18 dB compression with make-up gain to achieve unity out.

 

[gyraf]

When compressing, you are hearing sidechain modulation artefacts, not the vca. Those artefacts are off course very frequency dependent because of limited filtering in attack/ release circuit.

For evaluation of the vca under different gains, just use the makeup gain, not compression.