The Retro 176 is based on a Bill Putnam classic, the UA 176. The Retro 176 faithfully reproduces the sound of the 1960s original with new features for the modern recording studio. The Retro 176 provides one of the most desirable compression signatures in recording history.

By nature, the 176 is a fairly fast compressor, with attack values ranging from 0.1-2 ms and release values between 27(!) and 572 ms. Like the 1176, the 176 was originally named 'Limiting Amplifier', and it's a description that's right on the money: it illustrates the muscular, forceful action of the compression much better than the word 'compressor'. The attack/release values given above, referred to by Retro Instruments as 'single mode', are very similar to those on the Universal Audio original, and may be used when the Attack push/pull potentiometer is in the 'out' position. When it is depressed, the 'double' mode is engaged, in which the release is more program-dependent and the compression is smoother and slower overall, backing off from the pretty grabby and brutal gain-reduction that can be achieved in the 'single' mode. The Retro 176 has a side-chain high-pass filter with a fairly wide range, as high as 2.2 kHz (-3dB) in its maximum setting. In the minimum position, it is deactivated with a switch.

For sonic variety, the inter-stage transformer, which is located between the gain-reduction element and the output stage, may be switched in and out of the signal path, and there's also the very interesting 'asymmetry' function, which can be toggled off or into the '+' and '–' positions. The switch determines whether the detector reacts on the full waveform, or only on the positive or negative sides. As the comprehensive manual explains, it "determines which edge of the wave the compression will ride on”. This may help to 'open up' the compression on signals with asymmetrical waveforms, such as certain vocal or horn tracks.

The Retro 176 employs six vacuum tubes, just like Putnam's original. At the heart of the design, a 6BC8 dual-triode tube provides the variable gain stage, the actual compression element. A negative bias (the detector or side-chain signal) is applied to the grids of the triode elements, determining the output gain of the valve. The signal is then, via the inter-stage transformer, fed to the output amplifier, which is based on a pair of 12AX7 and 12BH7 dual triodes, operating in a push-pull configuration. The other three valves are not placed directly in the signal path, but they perform some very important duties nonetheless. Firstly, a 5Y3GT power rectifier valve is used in the power supply, a task that nowadays (apart from in certain guitar amps) is most commonly handled by silicon diodes. There's also a second, smaller 6AL5 rectifier valve for the side-chain signal and, finally, an OB2 voltage regulator valve, which feeds the 6BC8.

Type Tube
Number of Channels 1
Controls Ratio, Attack, Release
Ratio 2:1 to 12:1
Frequency Response 20Hz-20kHz
Inputs 1 x XLR
Outputs 1 x XLR

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