Perhaps not as widely known as their later relative the 1176, the UA 175 and 176 Limiting Amplifiers are nonetheless still used and sought after in the audio community, and are rare on the vintage market. The design began as the 108 tube preamp, which was found in the original console at Bill Putnam’s first studio, Universal Recording in Chicago; It was also found early on at his Hollywood digs as well. This amp predates the first groundbreaking modular mic pre, the UA 610. The 175 was one of the first commercially available Universal Audio products, and was introduced in 1961. Like the Fairchild compressor line, the 175 and 176 were based around a variable-mu tube gain reduction design. This means that gain reduction is controlled by highly unique variable-mu dual triode tube, which is no longer manufactured. Variable-mu tubes have the ability to reduce their mu (gain), as the input signal increases.The 176 shortly followed the initial release of the 175 and were concurrently available models. Apparently, the original 175 was stripped, and had no Input or Output gain control. Other units such as the 175B/C and 176 had gain control for Input and Output. Later, in addition to these units there was also the 177, which was without metering.
