tube VCA for digital volume control?

Have you seen this thread?
http://www.groupdiy.com/index.php?topic=16375

;Matthias

rlackey wrote:

The idea I had last night, is to use a similar feedback from the output of the vactrol back into the microcontroller, so that the microcontroller knows exactly what the level is on the resistor side of the vactrol. Then matching levels and response across multiple channels will be much easier, and the tweaking can largely just be left to software.

Click to expand...

One of the multi-channel products I designed using Vactrols some years ago used feedback from the resistive side of the Vactrols to linearise the transfer function. You have to be very careful with a design for manufacturing production, as the variation between Vactrol parts is huge. A one-off hobbyist design is a simpler matter, and you can afford to adjust individual components to get it to track properly.

The reason I used and continue to use Vactrols rather than, say, multipliers or vari-mu valves is that they are really good distortion-free resistors. You get attenuation only, of course, but I have used them as the gain-settiing resistor in an instrumentation amp configuration with excellent results. The other great advantage is that the resistor side is completely isolated from the LED in the side-chain or other controller, so there are no ground loops and very little feedthrough.

Yes, it was Manley Labs who used the "Vactrol with a trick" slogan in one of their compressor designs.

Those are all good things to keep in mind, thanks. At the moment it's a one off, but I started it with the intention of producing something that I wanted to work with in a studio situation and then seeing if there is any kind of commercial market for it. Such a market would probably be quite small, and the cost of such a console quite high. I would imagine the fact that one might only sell a small number of units in a year would justify hand assembly of the console, and so taking some time to tweak things might not be a problem for such a specialized product.

A friend of mine who helped me start my current business but recently resigned as co-director, has expressed some interest in funding development of this particular project and engineering a commercial product.

It's very early days for that though, I just want to build one console in a overly-modular fasion so that I can replace different boards easily (like switch in and out different pre amp or EQ designs, different DAC and ADC designs etc.) Once the final combination settles down, a lot of these "modules" would be combined onto single, mass production oriented boards with high component packing density (also probably forced air cooling) and compact final layout in mind.

The challenge is in the combination of surface mount digital and discrete hi voltage analog curcuitry in very close proximity, even on the same physical curcuit boards in some cases. Obviously isolating any kind of noise getting into the audio path is the biggest concern. PSU design is also going to be crucial.

Should it all come together though, I don't think there is anything out there at the moment quite like it.

But, I digress from the actual topic of this thread.

Thanks for all the replies!

Rich