Pro Audio Visie: (compressortest december 2009 door Wessel Oltheten): Wat me aanspreekt aan de Gyratec is dat  het signaal heel mooi intact blijft het stereobeeld wordt niet smaller, het spectrum blijft vol en open. Dit zijn goede eigenschappen voor een masteringcompressor. Ook voor instrumenten die wel compacter moeten zijn in een mix, maar daarbij iets van hun natuurlijke klank mogen inleveren is de Gyratec een goede keus.
 Gyratec 2 tube recording channel

The Gyratec II is Gyraf’s most popular product to date. It is basically an all-tube recording channel, combining a microphone/line preamp, an equalizer and an optical compressor. Although solid state electronics is used - in power supply and side-chain - the audio signal never gets touched by anything but tubes, transformers and passives. Very different from most "tubed" consumer-type studio gear.

The signal flow is simple, all functions are realized with use of only four double triodes - the 5814 military type. This tube can be substituted with standard ECC82 or 12AU7A types, so you dont have to worry about tube availability for the near future.

The input switch selects between "Instrument" (DI for e.g Bass or Guitar) "Line", "Mic" and "Mic 48V". The input impedance is 1K Ohm for the mic input, 10K Ohm for the Line input, and 1M Ohm for the Instrument input.

The Microphone and line inputs are of cause floating and transformer balanced - an interface standard used in all serious audio designs.

The gain switch sets the level of the incoming signal, before it is passed to the selectable Low-cut filter - a soft 6dB filter at either 15 (off), 80 or 160 Hz. From here we drive the Equalizer circuit.

The Equalizer is a bypassable, three band, variable frequency boost/cut type build around fixed-Q, passive 6dB filters. The filters are optimized for gentle corrections, and you shouldn’t expect them to do anything bizarre like telephone-type of sound or the like; they're merely for sweetening the tone of your input signal. Each band has six switchable frequencies, set at spots that we worked quite a bit on getting right. The low and the high bands are "shelving" type controls, the mid is a "bell" curve.

Next in line is the Compressor. This is based on the optical principle, that is, the gain reduction element is a light dependent resistor, controlled by a light that changes in proportion to the audio signal. This is also how classics like UA's LA2, LA3 and LA4 - among many others - work. The controlling input for the compressor is taken either from the input or the output of the equalizer section, in reality making it up to you to decide if you want the compressor before the EQ, or the EQ before the compressor. This is selected with the "Comp" pre/post switch, which also acts as compressor bypass in it's middle position.

The Compressor is fully adjustable, giving you control over Threshold (at what input level the compressor starts working), Ratio (how much it will do once it starts), Attack time (how fast it will attenuate loud signals) and Release (the amount of time it waits before releasing the attenuation).

The working of the compressor - actually the gain reduction introduced in this circuit - is monitored on the large VU-meter. This is switchable, allowing you to monitor input level, output level, or gain reduction.

For most recording tasks this compressor is excellent, but again - like the EQ - you shouldn’t expect it to do something dramatic. It is designed with signal integrity in mind, not as an effect.

This more or less concludes the Gyratec-II tour, we are now arriving at the output level control, setting the level for the final output driver stage, responsible for driving your further line of equipment. The output is - like the input - floating transformer balanced to get rid of potentially problematic hum loops. The output resistance is less than 1K Ohm, so driving all sorts of modern equipment - or even long cable runs - will never be a problem.

 
 Gyratec 3 Electro-Optical stereo compressor

The Gyratec III is a true tube stereo compressor based on the opto VCA principle. This method - controlling the gain by the means of a light dependent resistor - is not as fast as the vari-mu method, but much more subtle sonically. This principle is known from eg. the classic Universal audio LA2, LA3 and LA4 compressors..

We have recently updated the front panel on the G3 - that's why it doesn’t look like the image in the Resolution review above. The inside has had minor updates, but roughly speaking it's kept the same as the old one.

It is based on SRPP gain stages with no feedback, using selected 5814 dual triodes for both input and output stages. These tubes are still fairly easily available today.

Although semiconductors and op-amps are used in this unit, they're confined to power supply and side-chain functions. At no time will your audio pass through anything but transformers, tubes and passives. So - as with the rest of our product range - we're talking REAL tube audio here.
 
 
 Gyratec 9 Dual Tube Mic Pre-Amplifier

The G9 is a dual true tube microphone preamp - made like our mother did it. It's a simple, yet powerful unit, based on a variable-gain input tube stage driving a tube SRPP output stage. No silicon or other crap in signal path.

The controls on the G9 are as follows:

The input level switch - in eleven steps - sets the raw gain of the first stage, thus allowing you to control the input level, the "drive" for the preamp.

The output level pot attenuates signal to the output stage, allowing you to fine-trim the output level going to the next stage in your signal chain

The input switch is used to select either Line, Mic, or Mic-with-P48 as input for the unit. If you insert a jack at the front panel socket marked "Inst.", this will break the signal selected on the input switch, and in stead have a very-high impedance instrument input - perfect for DI of e.g. bass guitar.

A gentle high-pass filter of either 80 or 160Hz is activated by the "Low-cut" switch, and the unit's absolute phase can be reversed using the "Phase" switch.

The G9 is floating, transformer balanced at line and mic inputs, as well as on output. Input impedance for the mic input is some 1KOhm, line is some 10KOhm, Inst. >2MOhm, and output impedance is <1K Ohm.

Although semiconductors are used in this unit, they're confined to power supply functions. At no time will your audio pass through anything but transformers, tubes and passives. So we're talking REAL tube audio here.

Conclusie review Interface Magazine
 Gyratec 10 vari-mu stereo compressor

The G10 is a true tube stereo compressor based on the classic variable-mu principle. This method - controlling the gain by altering quiescent current in differential tube pairs - is much faster than the optical method. The speed is why it was widely used in early limiter devices such as the Fairchild 670 or the Universal Audio 176, but the sonic by-products of this topology are the main reasons why it is so popular today.

It is based on a very short true-differential signal path from input to output, using very, very tightly selected PCC189 (7ES8) remote cut-off dual triodes for the VCA stages, and E88CC (6DJ8) low-noise triodes as cathode followers for the output stage drivers. Both tubes are still fairly easily available throughout the world.

Although semiconductors and op-amps are used in this unit, they're confined to power supply and side-chain functions. At no time will your audio pass through anything but transformers, tubes and passives. So we're talking REAL tube audio here.
 
 Gyratec 14 Parallel-passive Stereo EQ

The G14 is a true tube, all passive, stereo equalizer. It consist of five bands - each with 11 switchable frequencies, variable "Q", and boost/cut selection. It also has an output level trim, and a "hard" relay bypass control.

The two channels are linked - controlled with one set of knobs. The "Q"s (filter sharpnes'es) are rather low when boosting, but higher at cut - approaching a notch on extreme cut settings. This is both a side-effect of the applied passive technology - as well as quite desirable in situations where sound integrity is the paramount factor.

The frequencies are selected with special attention to controllability of the midrange area, and with half an frequency-range overlap to the adjacent bands.

This equalizer is a very extended version of the classic passive inductor/capacitor type filters, the most famous being the "Pultec" units. Even though the classic versions are useful tools in most situations, they are somewhat limited in use due to their simple construction. Our engineers wanted - as engineers tend to do - more of everything.

So we came up with the parallel-passive design, the G14.

It is a truly passive design - based on switching a set of inductors and capacitors in and out of circuit - combined with an E88CC (6DJ8) tube-based makeup gain/output stage. Not a single IC's or semiconductor in the signal path - this is a real tube unit.

The inputs and outputs are - off course - transformer balanced, for trouble-free interfacing to the outer world.
Gyraf Audio website