The UA 175 B is a Compressor / Limiter that adds a thick texture and euphonic halo around any signal passing through it.
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.
Without limiting: ±0.5 dB from 20Hz to 20kHz
With 5 dB limiting: ±0.5 dB from 20Hz to 20kHz
Maximum Power Output:
After output attenuator with limiting switch off to prevent limiting at +24 dBm less than 1% THD from 50Hz to 15kHz
Jumper in low gain position: 24 dB
Jumper in high gain position: 37 dB
Minimum Input Level To Accomplish Limiting:
Jumper in low gain position: -12 dBm
Jumper in high gain position: -24 dBm
Maximum Output at Threshold of Limiting: +13 dBm
Input and Output Impedance: 600 ohms, unbalanced
SNR: 80 db
Compression Ratio (above threshold of limiting): 12 to 1
Attack Time: Adjustable from 300 to 1000 microseconds
Release Time: Adjustable from 27 to 527 milliseconds
Tube Completent: One each, 6BC8, 12AX7, 12BH7, 6AL5, OB2, GZ34
Dimensions: W 19" ; H 3-1/2" ; D 9-1/4"
Weight: 17 Lbs
The Gear Rack