[sdiy] Blackmer VCA cell

[Mike Bryant]

As Douglas Self says "For many years the 'standard' VCA was the DBX2150, of which I have deployed more than I care to contemplate in VCA sub-group systems and console automation", but he later agrees they are a very expensive solution when you need 100s or 1000s of them.  

Obviously nowadays most consoles are digital but later analogue designs were moving to digital step attenuators as they were cheaper as they avoided needing a DAC, gave less crosstalk and were actually more accurate.


-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Ben Bradley via Synth-diy
Sent: 23 October 2021 04:10
To: *SYNTH DIY
Subject: [sdiy] Blackmer VCA cell

I just posted this in a thread on Analog Heaven, and it reminds me to ask here. There's been a dbx "Blackmer cell" high-quality VCA circuit since the 1970s, but as far as I know its only use has been in recording studio equipment. Has this device ever been used in a music synthesizer or (guitar-type) effects pedal? I presume my explanation below (much high cost than CA3080) is why not, but does anyone know more?

Post to AH:

There's a "pro-audio quality" (much lower distortion and noise than OTA-based circuits) VCA based on a Blackmer cell (apparently Patent #
US3714462) - it's a different circuit from the long-tail-pair based OTA circuit, and I presume the reason it's not seen in music synthesizers is because of dbx (now THAT Corp) holding the patent, and only making and selling chips rather than licensing the patent to anyone else, and the much greater cost of the chips. As far as I know the chips have always been several dollars each, many times the CA3080 when it was in production. I would have thought the Blackmer patent would have run out by now, I don't know of any other manufacturer using the Blackmer cell design.

The article in the first link says "The voltage-controlled amplifiers found in most recording consoles and analog effects products are based on the "Blackmer gain cell" developed by David Blackmer of dbx, Inc."
- I've been familiar with their use in recording consoles, and the "analog effects" here surely refers to studio compressors, companders and noise gates. I've not heard of Blackmer cell VCAs ever being used in effects such as guitar pedals - I've only heard of CA3080 and similar chips (which when in production cost less than a dollar) used in such effects and music synthesizers. I'm a bit surprised no synth maker used these, as they're worth the extra price in some applications, such as setting the "final output" volume control for each patch. If I recall correctly, the Chroma Polaris used a CA3080 there.

http://www.thatcorp.com/History_of_VCAs.shtml
https://www.edn.com/gain-control/
https://patents.google.com/patent/US3714462
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