The Universal Audio 175B Compressor

Pro Audio Hall of Fame / Universal Audio 175B

"The Universal Model 175B 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.”

So begins the operating manual for the Universal Audio 175B compressor/limiter. Versatile? Indeed. Professional quality? Undoubtedly. But as for applications, nowadays that’s mostly limited to the recording industry, which has discovered in the 175B one of those audio gems of the early 1960s that remains unrivaled to this day.

That gem is but one of many in the crown of Bill Putnam, Sr; the man who gave us artificial reverb, the modular recording console, multi-band equalizers, half-speed mastering, and of course the 1176 compressor limiter. And then he put those tools to work in studios he helped design, within whose confines the likes of Duke Ellington, Ray Charles and Chuck Berry churned out dozens of classics and hits.

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Bill Putnam Sr., founder of Universal Audio

Bill Putnam created gear that solved problems, and in the immediate post-war era, there were plenty of audio problems to solve. In 1946, Bill founded Universal Recording in Evanston, Illinois, and moved to Chicago the following year to set up the now-legendary Universal Recording studio. By 1949 he was beginning to modify existing equipment such as broadcast consoles, which led to his firm manufacturing gear that didn’t exist.

One such thing that didn’t exist in 1961 was a limiting amplifier that was designed for recording, as opposed to the handful that existed to protect radio towers in the broadcast industry. There was also the Fairchild, which reigned supreme in the cutting rooms of mastering studios. With his tube-based 108 mic preamp as a starting point, Bill and his team created the model 175 compressor. This was soon refined and became the 175B, one of the first pieces of gear commercially available from Universal Audio.

The operating manual goes on to tell exactly what UA had created: “The 175B 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.” And the means to achieve this was by going with a variable-mu® design, similar to the Fairchild, and for a similar reason: the warmth of sound that this method produces.

Like its descendant the 1176, the 175B Input control is what sets the Threshold, while the Output control sets the Output Gain. Both of these large Bakelite knobs are adjustable from 0 to 40 dB in stepped 2 dB increments. Finer adjustments can be made with the Vernier controls. An optional Langevin attenuator provided an extra 6 dB of output.

The vari-mu® design achieves gain reduction by a dual-triode tube, which drops the gain when the input signal is increased. This signal-dependent characteristic allows softer compression at lower input levels and harder, brick-wall style limiting at high input levels. The ratio also increases in this scenario; being a fixed 12:1 at the threshold level, and increasing as the signal comes in hotter. This is one of the reasons for the unique sound of the 175B.

Although the compression ratio is fixed, the attack time is adjustable from 300 to 1000 microseconds and release time is adjustable from 27 to 527 milliseconds. The attack knob can also be used to shut off the compression circuit completely, while still passing audio and imparting a warmth and character to it.

The meter is switchable between Input, Output, or Gain Reduction (GR). An external VU meter can also be wired in, either for all three functions or just GR (leaving the internal meter for Input or Output level).

The 175B was manufactured with “Double Tip” Input and Output jacks; the original cables for these were used by the telephone industry, and had two 1/4” jacks at both ends of the cable, (NB: these are not standard phone plugs!). There are also two holes for balancing the unit via screwdriver adjustment; this allows the VU meter to be read, since the front panel is kept closed (but can be flipped downward to open it with a turn of the thumbscrews). A heavy gauge steel chassis encloses all the circuitry, with the tubes and UTC transformers living on the back of the unit.

The operating instructions tell us what a close tech inspection will uncover: “The highest quality military type printed circuitry is combined with 1% precision resistors and overrated components throughout to insure maximum performance and reliability.” Undoubtedly true, considering how many are still in operation almost 60 years later. Part of that is due to exceptional build quality, but most of it is due to the fact that these units are not throwaways - their tonal character and life in the studio (as opposed to the broadcast tower) protected them somewhat from the purging of tube gear when the solid-state era arrived.

While BA6As (and even Fairchilds!) were tossed in the dumpster, many a 175B were spared. Their smaller size may have helped. While its bigger brethren were typically 6 rack units high, eating up valuable real estate, the 175B is a mere 2U in height. This made it one of the most compact compressor available at the time. And while it served a different purpose than the famous Fairchild 660/670 compressor, the 175B had a similar tonality. This is the main reason it is still so beloved today: full, fat, thick tones - especially from vocals, guitars, and bass.

For Vintage King’s 25th anniversary, we’ve teamed up with Retro Instruments to create an accurate reproduction of the 175B compressor. This new version of the compressor utilizes the brand’s 176 chassis and components, but returns the circuitry to the original Bill Putnam design from 1961. If you’ve been looking to add the sonic characteristics of the classic 175B to your studio, without the maintenance concerns and extremely high price of excellent condition vintage examples, then this new Retro Instruments/Vintage King collaboration could be just the thing you are looking for.