«The double-bass sounds absolutely natural; each string produces a wealth of over- and undertones, and all of them are clearly heard.»
Listening to GFS-07 obviously has to do with a synergetic effect: SITs, germanium, passive correction and silver in the output transformers — all these ingredients, taken together, form an exquisite dish, which has every chance to satisfy serious music lovers. Try it yourself — especially as during the quarantine the S.A.Lab company will be pleased to bring to you any of their products for testing it at your home.
To assess the bass, I used headphones connected to TEAC BX-550 amplifier with DC circuit design (with working frequency band beginning from 0 Hz). The lower range of GFS-07 seems to be limitless; the bass produced by the cartridge goes to the monitors without any losses. I have more than once mentioned the 'velvet low range' provided by germanium transistors; the same is true for this device. The double-bass sounds absolutely natural; each string produces a wealth of over- and undertones, and all of them are clearly heard. To this, one can add the rich spectrum of the vibrations of the body: the instrument's real size is felt even in the headphones. The bass does not sound artificially strained, as is the case with correctors with deep general feedback. Worth mentioning is also the complete absence of 'veiling' in the upper band: the sound is, so to speak, 'cleaned up' to full transparency.
To begin with, I decided to compare this corrector with the one incorporated into the preamplifier Technics SU-C1000 Mk2 – a top-class device produced by a famous company and, by the way, powered from batteries. Five minutes were enough: the Japanese device was obviously inferior in terms of stage depth, the subtlety of sound colours, and the plasticity of sound delivery. To put it simply, its sound in the upper range was plain, expressionless and rather rude. As regards GFS-07, it gave a highly balanced picture according to all criteria: nothing special at first hearing, but I did not want to switch it off and kept listening to records, one after another. The sound is not attached to monitors; the large three-dimensional stage exists as if all on its own. The dynamic contrasts are rendered lightly and freely on both low and high levels — perhaps a bit more stiffly than with a vacuum-tube corrector (I have tested also one such device on C3g pentodes), but more correctly in terms of sound colour: each sound is heard perfectly clearly.
«GFS-07 gave a highly balanced picture according to all criteria: nothing special at first hearing, but I did not want to switch it off.»
The printed circuit board is placed in a thick-wall aluminium case with a nice lacquer coating. The corrector is completed with three damping supports with steel balls, two power cords, a support installation template, and white gloves. The corrector is delivered in aluminium suitcase with porous filling.
The manufacturer recommends the GFS-07 model for medium and high-level vinyl tracks.
The К504УН2Б chip is followed by the cascade on germanium complementary pair МП37/МП26, and then the driver on ГТ402/ГТ404 – and all this with bipolar power supply and an immediate, capacitor-free connection. The driver 'rocks' the step-up(!) output transformer with silver winding, which doubles the amplitude of the output signal. It also allows to organize a balanced linear output, though this has not been realized in the present model.
GFS-07 is powered by +/- 24 V power unit; in the corrector itself the voltage is stabilized to +/- 15 V. According to my measurements, the limitation of signals on linear outputs begins under 12 V RMS; under the nominal sensitivity of 5 mV this gives an overload capability of around 22 dB.
«The driver 'rocks' the step-up (!) output transformer with silver winding, which doubles the amplitude of the output signal.»
The weighted noise level of -96 dB, declared by the manufacturer, at first seemed to me unlikely, but I had a chance to measure it myself: due to the quarantine the corrector remained at my home during several weeks.
In short-circuited input, a microvoltmeter without weighting filter has showed around -88 dB in relation to the maximum output level, which is close to the declared value. If GFS-07 is plugged in the sound system, noises are not heard at all – at least I could hear nothing, though I strained my ears.
The S.A.Lab staff has found a beautiful engineering solution: the corrector's input stages are based on К504УН2Б integrated circuits, containing two differential stages on static-induction field transistors. They are made from silicon, with vertical structure and p-n junction as gate. Their advantages include an extremely low level of noise and the characteristics of a vacuum triode, i. e. a short spectrum of harmonics dominated (in this particular scheme) by even partials. These chips provide the corrector's primary amplification (around 200), which thereafter is reduced ten times by means of passive RIAA chains.
«The corrector's input stages are based on К504УН2Б integrated circuits, containing two differential stages on SIT transistors.»
We have already dedicated a number of publications to amplifiers based on germanium transistors, especially as Aleksey Sëmin, with his S.A.Lab company, has been constantly supporting this theme for several years. Until recently, however, all the development works have been conducted in the field of integrated or terminal amplifiers, where the advantages of germanium were revealed first of all in the powerful cascades working in the AB class. The situation with preamplifiers and especially with phonocorrectors is more complex, since in terms of noise characteristics germanium transistors are inferior not only to their silicon 'cousins', but also to vacuum tubes – triodes, pentodes and nuvistors. The technologies of the 1960s were incapable to provide good amplification in microcurrent regimes with minimum noise; besides, the reverse currents in the transistors of those times were too strong.