Gyraf Audio G22 Dual/Stereo/MS Vari-MU Compressor

G22 is an elaborated and dual’ified version of our venerable G10 vari-mu compressor. This incarnation allows it to be dual-mono, while still keeping a backwards-compatibility “G10 mode” where one set of knobs controls real stereo compression, like you’d want most of the time in mastering.

The audio part of the circuit is a faithful reproduction of the G10 that has been our flagship for more than 15 years – if it works, don’t fix it. The only minor change is a re-bias of the output stage, which allows some 4-5dB more headroom while preserving the original stage’s soft transition into overload.

Some changes has been made to the sidechain functions, including a wider threshold range (to allow S compression in weak-S M/S-mode) and even faster fastest attack time (because we can, it’s a vari-mu you know…)

A simple sidechain high-pass filter was added (located on the bypass switch) which cuts off compressor keying below ca. 80Hz at a slope of 6dB/oct. This mainly to keep pumping artifacts under control when working fast in bass-heavy material.

M/S Mode
Input and output transformers can be switched between standard two-channel L/R mode and the M/S mode where both matrixing and dematrixing done is internally: M/S mode allows you to independently control dynamics (and levels) in mono and side parts of program material and still have the result on the output as standard stereo information.

Linking/dynamic interaction between channels in dual-mode can be set continuously from none to full by the Vari-link control knob – enabling you to dial in a difference in processing between channels, then varying the difference amount or compression “correlation” with a simple turn of a knob. This function is especially nice when dealing with M/S compression, where the amount of correlation gets very important.

Bypasses are, off course, hard and relay-based. You bypass, G22 acts only like an additional piece of cable. Note that some modes prohibits certain bypasses – like when working in M/S mode, you can’t separate and hard-bypass M or S alone (as we use both channels transformers for the matrixing) so when you bypass M, it’ll bypass S also and the other way around…

First of all, this is now a dual-channel unit - allowing you to use it as two compressors on two different signal sources. 'nuff Said.

Second, it's still a fully stereo-linked compressor with all the freedom and ease of control this gives you - flip the "CONTROLS" switch to L/M mode, and now only the left-hand group of knobs are controlling the action.

Third, for users that like the sound of dual individual compressors on a mix, there is a continuously variable stereo link "VARI-LINK" that is activated by the "VARI" setting of the "CONTROLS" switch. This allows for gradual cross-linking between the two compressors, from full link (i.e. Stereo) to none (Dual).

Fourth, the "MODE" switch allows using the G22 in either standard stereo L/R or in M/S mode, where the source signals are matrixed at the input transformers into M and S components - and then later de-matrixed from M/S back into L/R at the output transformers. This allows for different processing on mono and stereo components of the signal, and is a very powerful tool for stereo imagery processing.

The "OUT" steps at the "MODE" function switch bypasses the unit completely for reference. The bypass is “hard”, meaning that the inputs are simply shorted to outputs through a gold-plated relay. If you wish to use the unit as a straight line amp, but with bypassed compression, the compression can be disabled at the two "Bypass" switches.

The two "Bypass" switches are also where the sidechain high-pass filters are controlled on the G22. First step fully CCW is bypass, next is compressor switched in with no filtering, and the last step is highpass filtering the sidechain signal 6dB/oct at 85Hz. The purpose of this filter is to allow you to compress bassheavy material without getting too many artefacts from the lowfrequency content triggering compression (a.k.a. “pumping”).

The input level controls are positioned right after the input transformers, allowing you to control the input level for the first variable gain stage - the "drive" for the compressor. Unity gain is obtained when input and output gains are set roughly at their two o’clock positions.

The Threshold pots controls at what level the compression sets in. Turning counter-clockwise will select a lower start point, resulting in more compression. When turned fully clockwise, the compression is in effect deactivated.

The Ratio pots controls the amount of compression in relation to changes in the input level, that is, for the part of the signal that exceeds the threshold value set. The knee of this compressor is very soft at low thresholds, getting a bit stiffer towards higher thresholds. Setting Ratio fully counter-clockwise will set ratio to 1:1, and thus deactivate compression.

The Attack pots controls the time the compressors takes to react to a rising input level above threshold. Note that fastest attack time has become very fast in the G22 (some 20uS), which is even faster than the G10.

The Release pots controls the amount of time it takes from the input signal falling below threshold, to the gain reduction being returned to unity. Range is approximately 50mS to 3.2 Sec.

The Output level pots controls the signal levels to the output driver stages and the outputs. The output impedance is less than 1K Ohm, and signal is - like the input - fully floating transformer balanced.

The GR-meters are reflecting the ongoing gain reduction - monitoring the vari-mu stage currents - allowing you to check the state of things at a quick glance. At heat-up the meter slowly rises to a position somewhere near the right end of the scale, and gain reduction is read out as left-going needle movement.

The meter offset (at the top end of the scale) will be varying a bit, depending on heat-up and average signal level, meaning that the zero point will vary a bit depending on a variety of factors. The meter zero can (if needed) be adjusted by the trimmer at the bottom centre at the front of the meter frame.

This compressor is based on two closely-matched differential pair 7ES8/PCC189 remote-cutoff triodes, differentially feeding two 6922/ECC88 cathode follower output stages. No feedback is used in the signal path, and the topology is pure class-A all the way through. Lundahl audio transformers with internal electrostatic shielding are used for both in- and output interfacing, giving a true floating input impedance of about 10KOhm, and an output impedance of less than 1KOhm.

This unit was originally intended for use with our analogue tape recorders, which means that the optimum operating levels are around that of +4dBu – and at this point you still have some 10dB up to the point where the unit starts to get tired, which happens around 15Vpp AC output – and then some before it starts sounding bad. This means, however, that you should consider checking your levels if you’re running a modern-day DAW, which often comes factory set to extremely-high levels like +24 or +28 for 0dBfsd.

Those kinds of levels are aimed at keeping a good safety-margin before running into digital-clip, but at the same time it’s common practice (for a good reason) to try getting as close to clip as possible. A good level for use with the G10 (and for most analogue gear in general) is somewhere around +10dBu for 0dBfsd (full scale digital).

Our audio path consists of ONLY transformers, tubes, and passive components, and is true-differential (balanced) all the way from input to output.

The sidechains and power supply circuits are solid-state based though. The sidechain senses the incoming signal, rectifies and times the voltages, and controls the DC voltage applied to the differential input stage in order to set the quiescent current of the stage, reflecting on the ? - and thus on the stage gain. The quiescent current in the input stage is monitored by the front panel meter, which then reads stage gain.

We use two types of tubes in the G22: The vari-mu set are 7ES8/PCC189 remote cutoff dual triodes, and the output driver set are 6922/ECC88 medium-mu dual triodes. Both types are still fairly available today, so don’t worry too much about availability for the future. These tubes should last for at least a couple of years - and often a lot longer than that. If and when changing tubes, contact Gyraf Audio for instructions on proper adjustment of the unit.

Although semiconductors and op amps are used in this unit, they're confined to power supply and sidechain functions. At no time will your audio pass through anything but transformers.