View Single Post
Old 20th November 2009
Lives for gear
JohnRoberts's Avatar

OK, the part is not a dc servo for the primary audio path. If it were it would be worth replacing because parts used in servos do matter.. but this isn't so that is not the concern.

It is as I speculated in the VCA side chain so somewhat less critical than primary path, especially for older VCAs. But you are not using the older VCA.

Regarding noise for adding N to the THD+N, from your schematic there is a passive pad in the CV (80/50), and an RC filter so the opamp's noise will be attenuated more than 6dB but still present.

AFAIK the general design advice is to keep noise at the control ports down in low single digit nV/rt Hz. Noise on these control ports shows up as a multiplication with the audio signal, so perhaps measurable on the new parts with high level THD+N testing, while not very audible (IMO).

But yes, do use modern precision opamps in the side chain at least for the VCA control port driver, and look at updating the circuit design for the newer VCAs. Note: eliminating the resistors in series with the CV buffer will require re-scaling side chain math (to make up for pad you remove) and could cause some unintended consequences if you ever use the CV slave input for stereo linking and two opamps try to drive each other.

I would check the THAT corp website, they may have some applications note about swapping new VCAs for old, while they probably don't get down to individual models of old DBX designs.

There is an AES paper written by Gary Hebert (THAT Corp) talking about the new VCAs , but perhaps more math than you are interested in. For the record they do drive the VCAs control ports directly from a precision opamp (AD797 with RC to ground for low Z at HF) to showcase them in the paper.

I have a sense that there is also a distortion mechanism associated with too high source impedance driving the control ports but can't find any discussion about that in his paper. Maybe I'm imagining it?

The new VCAs are notably cleaner... not sure the lovers of classic coloration will approve.