Taking on a classic - the Urei/Teletronix/UA LA-2A optical compressor circuit

All right, here's a first project for the blog. 

 I've been commissioned by the Richmond, VA-based branch of the Black Iris music collective to (re)create a studio compressor/limiter with circuitry substantially similar to that found in the venerable Teletronix LA-2A.   Let me note right off the bat that I am NOT here describing how to make an LA-2A. LA-2A is a trademark owned by Universal Audio. If you want an LA-2A, please plunk down $3,000 and buy a re-issue from them, or many thousands more for an original.  Rather, I am describing a project (see safety sidebar) which uses circuits and principles that have been widely understood since the 1960s and earlier, and which merely provide functionality similar to that of an LA-2A.

Introduced around 1964 or 1965, the LA-2A "leveling amplifier" was a breakthrough in its day, and is a still a mainstay of studios all over the world. Invented by the late, great Bill Putnam, the key advancement that enabled the LA-2A was the development of cadmium-sulfide photocells with a fast attack and gentle release characteristics. The resulting response is very non-linear! Fortuitously, this leads to a pleasing sounding compression, and one which is exceedingly tricky to duplicate by non-analog means.

The basic idea is pretty straightforward.  The strength of the incoming audio signal is used to variably control a side chain (with 12AX7 amplifier and 6AQ5A driver tubes) that determines the intensity of light given off by an electro-luminescent panel. This panel  in turn shines light on a photoelectric cell which is setup as a gain-controlling voltage divider right at the input of the unit.  Compression therefore occur right at the input, which is immediately followed by a voltage amplifier (12AX7 tube) to recover the signal, and a totem-pole style cathode follower (12BH7 tube) to provide a low-impedance output to drive the output transformer. In this manner, the unit offers about 40dB of gain reduction.  Aside from the compression unit, the side chain, and the output section, there are also circuits that facilitate matching when running two units in parallel for stereo, and a VU metering circuit with output level monitoring at +4dBu, +10dBu, and a gain reduction mode.  The primary user controls are fairly basic, with a gain reduction control pot that dials in the desired amount of compression, and a gain control that dials in the desired output level.  The stock unit also has a Limit / Compress switch which allows the user to select either a low compression ratio for soft compression, or a high compression ratio for hard limiting.  This switch will almost certainly end up being replaced  in our Wombat Amps version with an additional pot to facilitate dialing in variable compression ratios anywhere from hard to soft.   

More about the theory of operation is available here. A good quality schematic dating to 1968 is widely available on the web :