12
previously, the gain reduction is controlled by the photo-cell in the T4 el-op. In order to
track the operation of this cell and determine the gain reduction, a second photo-cell is
also illuminated by the same EL panel. This photo-cell is hand-selected to match the gain
reduction photocell and hence gives an accurate indication of the amount of compression.
Appendix
Creative Classics: The 1176 Solid State Limiting Amplifier and
the LA-2A Leveling Amplifier
The LA-2A and 1176 compressor/limiters long ago achieved classic status. They're a
given in almost any studio in the world — relied upon daily by engineers whose styles
range from rock to rap, classical to country and everything in between. With so many
newer products on the market to choose from, it's worth looking at the reasons why these
classics remain a necessary part of any professional studio's outboard equipment
collection.
The basic concept of a compressor/limiter, is of course, relatively simple. It's a device in
which the gain of a circuit is automatically adjusted using a predetermined ratio that acts
in response to the input signal level. A compressor/limiter "rides gain" like a recording
engineer does by hand with the fader of a console: it keeps the volume up during softer
sections and brings it down when the signal gets louder. The dynamic processing that
occurs at ratios below 10 or 12 to one is generally referred to as compression; above that
it's known as limiting.
Modern day compressors offer a great degree of programmability and flexibility while
older devices such as the 1176 and the LA-2A are more straightforward in their design.
Perhaps it is this fact that has contributed to their appealing sound and the longevity of
their popularity.
The LA-2A
The LA-2A leveling amplifier, a tube unit with hand wired components and three simple
controls, was introduced in the mid 1960s. It utilized a system of electro-luminescent
optical gain control that was quite revolutionary; gain reduction was controlled by
applying the audio voltage to a luminescent driver amplifier, with a second matched
photoconductive cell used to control the metering section. With its 0 to 40 dB of gain
limiting, a balanced stereo interconnection, flat frequency response of 0.1 dB from 30-
15,000 hz and a low noise level (better than 70 dB below plus 10 dBm output,) the LA-
2A quickly became a studio standard. Originally patented by Jim Lawrence, it was
produced by Teletronix in Pasadena, California, which became a division of Babcock
Electronics Corp. in 1965. In 1967 Babcock's broadcast division was acquired by the
