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The Gyraf Audio Gyratec II 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 sidechain - 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 should’nt 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 should’nt expect it to do something dramatic. It is designed with signal integrety 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.
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