Retro Instruments creates amazing redesigns of classic tube audio equipment. The Retro 176 Limiting Amplifier is a fantastic modern improvement on the Universal Audio 176. The same goes for the Retro Sta-Level Compressor, which draws influence from the classic Gates Sta-Level. While both of these Retro Instruments devices reduce dynamic range, they shouldn’t be seen as one device. Let’s take a look and compare the Retro 176 to the Retro Sta-Level.
The Retro Instruments 176 Limiting Amplifier is a single-channel compressor. It utilizes an authentic variable-mu (fancy word for gain) tube design that employs tubes on both the line stage and at the heart of the gain-reduction element. Six vacuum tubes make up the design. A 6BC8 dual-triode tube provides the variable gain stage, which is the actual compression element. The audio signal goes from the 6BC8 tube through the inter-stage transformer and is fed to the output amplifier. The output amplifier is based on a pair of 12AX7 and 12BH7 dual triodes. Although the other three valves aren’t directly in the audio signal path, they play significant roles in the Retro 176’s design. A 5Y3GT power rectifier is used in the power supply. This is significant because most modern power rectifiers found in power supplies are silicon diodes. A 6AL5 rectifier valve handles the sidechain signal. An OB2 voltage regulator valve feeds the 6BC8.
The Retro 176 has an inter-stage transformer between the gain-reduction element and the output stage, which means that the transformer can be switched in and out of the signal path. There are four compression ratio settings (2:1, 4:1, 8:1, 12:1) that allow for a range of soft compression to heavy limiting. A unique asymmetry tool determines whether the detector reacts to the full waveform or only to the positive or negative sides. This can help “open up” compression on signals with asymmetrical waveforms, like vocals. There’s also a wide-range sidechain High-Pass Filter! The front panel of the Retro 176 offers potentiometers for adjusting attack and release, a large backlit VU meter for gain reduction monitoring and I/O levels, a power switch with a red status indicator and a hard-wire bypass switch. The rear panel features the valves and transformers, the power receptacle, the fuse and mains-voltage selector, XLR connectors and an RCA jack that allows stereo coupling of two Retro 176 compressors with the optional stereo link panel.
The Retro Instruments Sta-Level is a single-channel compressor that has 40dB of available gain reduction. While the standard version ships with 6BJ6 tubes, there’s another option that comes with coveted 6386 tubes. Regardless, the Sta-Level comes with a 6386 socket in case you want to run a 6386 with the 6BJ6 tubes. There’s also an RCA connector so you can connect two or more Sta-Level compressors for stereo, surround sound or sidechain operation in tandem with the optional stereo link panel.
It’s possible to make internal and external adjustments to attack and release. There are three attack and release settings: Single, Double and Triple. Single creates slow attack and release and changes the operational feel of the release control. Double is program-controlled attack and release that builds density without any pumping. This is similar to how the original Gates Sta-Level compressor operated. Triple combines program-controlled release and fast attack. Triple quickly caps transients and is the least transparent out of the three modes. The front panel VU meter lets you monitor all three compression modes with a three-position toggle switch. The Recovery Knobs lets you set the recovery time of the compressor once the signal slips below the threshold. Other Retro Sta-Level features include XLR transformer-balanced I/O, an IEC power connector and an internal component safety cover.
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