Caractéristiques

par Le Grincheux  

Depuis l'annonce de l'ouverture de ce blog, vous avez été nombreux à me poser des questions. La première d'entre elle est sans nul doute pourquoi un amplificateur à tubes. Je ne fais pas partie de la secte des idiophiles capables d'acheter au prix de l'or un préamplificateur ou un amplificateur mal bricolé en Chine sous prétexte qu'il se trouve une double triode bien visible sur le châssis et quelques transformateurs qui rayonnent joyeusement pour faire joli. Le tube a ceci de supérieur au transistor : son impédance de sortie est plus forte, ce qui fait qu'il sature moins vite et de façon beaucoup plus agréable pour l'oreille.

La seconde est indéniablement pourquoi avoir choisi un Fisher et précisément un X-1000 ? La réponse à cette question est assez simple. C'est purement subjectif et motivé par un certain nombre d'articles sur ces amplificateurs. J'aurais très bien pu jeter mon dévolu sur un ReVoX, mais étant déjà l'heureux propriétaire d'un A77 sauvé des eaux avec amplificateur intégré, il me fallait passer à autre chose.

Quant aux autres questions, elles tournaient autour de la disponibilité des composants. Rassurez-vous, on fabrique à ce jour bien plus de tubes électroniques qu'il y a ne serait-ce que dix ans. Même des tubes comme les EM84 sont à nouveau fabriqués pour en faire des vu-mètres et, hier, en furetant sur des sites de fabricants, j'ai même trouvé des tubes nixies. Tout n'est pas perdu et l'histoire est un éternel recommencement.

Pour clore ce billet, voici ce que publiait à propos du X-1000 le magazine 'High Fidelity Magazine' en novembre 1961.

AT A GLANCE: The Fisher X-1000 is the highest-powered and most elaborate in a new line of Fisher integrated stereo control amplifiers. It provides a multitude of operating features and controls (for stereo and mono) as well as two power output channels of 55 watts each, with an optional "centre channel" output without the need for an additional power amplifier. The unit is very well built and, despite its many facilities and high power, is relatively compact and handsomely designed. In tests conducted at United States Testing Company, Inc., it met its specifications in all important respects and, in fact, proved to be an outstanding performer in its class. Dimensions are 16 15/16 in. wide by 5 13/16 in. high by 13¾ in. deep (exclusive of knobs). Weight is 44 pounds. Price: $329.50. Cabinet extra.

IN DETAIL: The X-1000 has 8 pairs of inputs for microphone, tape head (with equalization for either 3¾ ips or 7½ ips tape speed), magnetic phono (two inputs, one of which is equalized for RIAA and the other for either RIAA or Columbia), tuner, tape monitor, and two separate auxiliary inputs, one pair of which has individual channel level controls located on the rear panel.

After passing through the input selector, low level input signals are fed to a twin triode (type ECC83/12AX7) for initial preamplification and equalization. These signals then are fed, as are high level input signals, to the function selector. Provisions are made for normal stereo, reverse stereo, left input to left amplifier alone, right input to right amplifier alone, both inputs to both amplifiers monophonically, or either input to both amplifiers.

A second ECC83/12AX7 is used for the next stage, which furnishes the tape monitor inputs, recorder outputs and inputs, and outputs for the Fisher "Spacexpander" reverberation unit. Also associated with this stage are a centre channel (monophonic) recorder output, low frequency cut-off filter, and phase reversal switch. Bass, treble, volume, balance, and variable channel separation controls, as well as high frequency cut-off filters come next, with each amplifier using a 7247 twin triode, half an ECC83/12AX7, an EF86/6267 pentode, half of an ECC82/12AU7, and two EL34/ 6CA7 pentodes in push-pull outputs. Output impedances of each channel are either 4, 8, or 16 ohms, and provision for connecting a "centre channel" speaker directly to the amplifier is provided, as well as a centre channel high impedance output jack and a low impedance stereo earphone jack.

Tests indicate that the Fisher X-1000 would make an excellent choice for an all-in-one stereophonic amplifier and preamplifier. Each channel will deliver approximately 55 watts of clean signal at 1,000 cps with a total harmonic distortion of only 0.5%. At half power, or 3 db down from maximum power output, the total harmonic distortion is less than 1% from 50 cps to well above 20 kc.

At full rated power output (55 watts per channel), the frequency response of the amplifier was measured as flat within 2 db from 24 cps to 20 kc. Response at half-power level was essentially the same.

All frequency response measurements were made with the tone controls in the mechanically flat position, as indicated by the dot or arrow on the control knob. However, manufacturers rate their amplifiers and preamplifiers with the tone controls in the electrically flat condition, but since most owners of high-fidelity equipment do not have the facilities, to check accurately the frequency response of their amplifiers, USTC operates the amplifiers as the consumer would. Slight variations between mechanically centered and electrically flat are normal in most amplifiers, and many users eventually find the exact control setting that suits their listening needs. In any case, the high frequency response of the X-1000 could be greatly improved by setting the treble control at "1 o'clock," and a slight improvement also could be made in the low frequency response with a small amount of bass boost.

The bass control provides a maximum of 14.4 db of bass boost and cut at 50 cps, and the treble control provides up to 15 db of treble boost and cut at 10 kc. The low frequency rumble filter operates at a slope of 10 db/octave below 80 cps and the scratch filter operates at the rate of 16 db/octave above 5 kc. These are desirable filter characteristics since they permit the filter to suppress most of the noise with virtually no loss of musical quality.

The equalization provided for RIAA recordings and NAB tapes (7½ ips) is very good, being quite close to professional standards for those media. Similarly, the equalization for 3¾ ips tape resembles the EIA standard for that speed.

The intermodulation distortion was extremely low, less than 0.2% up to 10 watts, less than 0.5% up to 40 watts, and 2.9% at full power.

The channel separation of the X-1000 was better than 56 db at 1 kc, and was down to 38 db at 10 kc, which is more than adequate for all stereo program sources. Channel balance, once adjusted, remained excellent at all volume levels, with a measured variation in balance from full power to minimum power of only 0.6 db.

The amplifier's signal-to-noise ratio, measured at maximum gain, was 78 db on the high level inputs, 52 db on the RIAA phono input, and 48 db on the tape head inputs. The amplifier sensitivity (for 55 watts output at 1 kc) was measured at 170 millivolts at the high level inputs, 2.2 millivolts at the RIAA inputs, 0.85 millivolts for microphone, and 1.25 and 1.4 millivolts for tape head inputs, 3¾ and 7½ ips respectively. A speaker-damping factor of 10 was measured at both the 8 and 16-ohm speaker taps. The amplifier, incidentally, appears to be quite stable and should encounter no difficulties in driving electrostatic speakers. With its clean sound, operating versatility, and fine workmanship and appearance, the Fisher X-1000, in sum, would make a very suitable foundation block for those people who are just getting into stereo or high fidelity, as well as for many others who are considering up-dating their system.

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