Rare tube compressor/limiter that adds a thick texture and euphonic halo around any signal passing through it. These are labor intensive units to restore, and our tech staff performs this work beautifully.
The Universal Audio 175 and 176 compressors were the brainchild of the legendary Bill Putnam Sr., whose genius spawned several products that were initially used in his Universal Recording studios in Chicago and LA, and later became commercially available. The original console in Chicago utilized the famous 108 tube preamp, which in turn spawned the 175 and 176 models of tube compressor, the 610 mic pre, and the 1176.
The 175 B is a variable-mu style of compressor, where the gain reduction is achieved by a variable-mu dual triode tube, which reduces gain when the input signal is increased. The 175 B uses a fixed compression ratio of 12:1, with attack time adjustable from 300 to 1000 microseconds and release time adjustable from 27 to 527 milliseconds. Both controls increase the speed as they are turned clockwise, and compression can be shut off using the attack knob.
In one respect, the 175 is similar to its cousin the Urei 1176; the Input control sets the Threshold while Output sets the output gain. Both Input and Output gain controls can be set from 0 to 40 dB, and are stepped at intervals of 2dB. Finer settings within these increments are obtained by using the "Vernier" controls.The meter can be switched to view Input, Output, or Gain Reduction (GR). The barrier strip also allows for wiring an external VU meter, which can be set for all functions, or just gain reduction (which allows the internal meter to read input and output level).
One reason the 175 B has a unique sound is the way the compressor responds. With signal at the threshold level, the compression ratio is 12:1, but the ratio increases as the signal goes higher above the threshold. It will subtly tame the peaks when hit softly, but behave like a brick wall limiter if it's hit harder.
The 175 B was manufactured with Double Tip Input and Output jacks; the original cables for these had two 1/4" jacks at both ends of the cable, (which are not standard phone plugs). These have been replaced with standard XLR pigtails. There are also two screwdriver holes for balancing the unit, while keeping the front panel closed so that the VU meter can be checked during this operation. The front panel is hinged, and drops down to access calibration pots for Gain Reduction Zero, Plate Balance and Cathode Balance, as well as a three-position test switch.
The 175 B employs six tubes: 6BC8; 12AX7; 12BH7; 6AL5; OB2; and GZ34, along with UTC output and input transformers.
From the original operating instructions:
"The Universal Model 175 B is an extremely versatile professional quality peak limiting amplifier designed for use in the recording, broadcasting and motion picture industries, as well as other high quality audio systems. the 175 b is a true limiting amplifier capable of high gain, low distortion, linear amplification at levels below the threshold of limiting and with excellent limiting characteristics and negligible increase in distortion at levels above....The highest quality military type printed circuitry is combined with 1% precision resistors and overrated components throughout to insure maximum performance and reliability."