MATCH MAKING

By Martin Colloms

This remarkable 150 valve preamplifier has already been assessed by Ken Kessler (Nov p85), but a full review of a recent sample seemed appropriate. In case you might be thinking that for the price the unit must be built from old bits of Meccano, let me assure you straightaway that this is far from the case. Indeed, apart from the cooking-styled industrial instrument case, the circuitry, build and component quality themselves would all be creditable at five times the asking price.

Like most valve preamps, the Micro does not offer a high gain moving-coil input, the disc facility having been optimised for moving-magnet. In the case of low output m-cs, the HFN/RR Black Head transformer would make a useful addition, but where m-cs of healthy output are concerned, reasonable power levels are possible with direct connection to the m-m input when used in conjunction with a power amp of good sensitivity. More importantly, the level and character of the Croft input noise are much less annoying than usual, sounding unobtrusively low pitched.

Input facilities for disc, CD and tape are provided, the only other control, apart from the power switch, being for volume. Input sockets are phonos.

Technical details
Inside this compact enclosure the active circuitry is mounted well to the left, to give some separation from the mains transformer. Each channel uses a total of three triodes, the first two ECC83s, the third a higher current 12AU7. The disc input triode is followed by a high impedance RIAA equalisation stage using shunt feedback on the second triode. The final triode is a unity gain buffer comprising a simple non-inverting, cathode follower.

Not only are the heater supplies DC regulated, but regulation has also been applied to the HT supplies for each channel. Two-stage input reservoirs improve the overall performance still further.

Hardwired with single-strand conductors in traditional style, the amplifier uses 1 % metal film resistors and high quality polypropylene coupling capacitors. Note that with a unity gain linestage like this, the power amplifier sensitivity must be sufficient for reasonable sound levels with all chosen sources, including tuner and cassette. These may provide as little as 200mV, but of course the 1V plus of CD will not cause any problems.

Lab performance
Decently low distortion levels were obtained from the line stage, which offered a moderate output impedance of 500ohms with a maximum level of 11 V. Via the disc input our full level intermodulation test signal (200mV peak, 19/20kHz, 1:1) gave some distortion, around -40dB, but it is surprising that the figure was not poorer still in view of the 6dB IHF overload figure for 20kHz.

In fact, the overload was a 'soft' rather than hard clipping and was considered innocuous. At more realistic signal levels, the disc distortion results were fine, while the midband overload figures were quite satisfactory. Stereo channel separation was pretty good at a typical 60dB midband, this deteriorating to a still satisfactory 36dB at 20kHz via the disc input. (This is better than all moving-coil pickup cartridges.)

Good channel balance was achieved, except at the lowest volume settings (-60dB), but in view of the relatively low gain of this preamp that is unlikely to be a problem. The input characteristics were entirely standard while the RIAA equalisation was a model of accuracy between 30Hz and 15kHz (0.2dB). It also has an extended - 3dB response-these points being at 5Hz and 310kHz - although there was a slight ultrasonic rise of 2dB at 100kHz.

The fine input noise result was of particular interest, measuring - 73.5dB IHF with the most severe CCIR ARM weighting (1 kHz reference tone). It was just sufficient for direct use with some recent, medium output moving-coils, but if required Croft can provide some 3dB of additional gain.

Whether used in a costly system or a 'real world' package, the Micro showed soundstage capabilities well beyond any preamp I've tried up to twice its price. Depth, while not astounding, was acceptable even with the deeper-than-thou portrayal of the Scintillas; Stage width was wide enough to guarantee that the Micro will not a limiting factor in this area.

Sound quality
There is no need to make excuses for the Micro. As KK has already reported, this is a preamp which stands comparison with some of the industry's established references. Both input sections attained an equally high standard. Tonally it was considered marginally 'lightweight' but not sufficiently so to result in a 'thin' quality. Definition and clarity were highly rated throughout the frequency range, despite a hint of imprecision in the upper treble.

Open and transparent sounding, this preamplifier portrayed depth and ambience well, with good stage width and strong image focus. It sounded lively and well balanced, with good dynamics. These are the hallmarks of a high-ranking product, probably little removed from the legendary ARC SP8 Mk1, for all that model's elevated price.

Conclusion
Lest I get carried away, I should point out that the standard attained by two other recent models also reviewed here. The Preamp II and the Cambridge C75 was similarly fine, but then these products, too, are quite exceptional. The fact that the Croft managed to achieve this level of excellence at half the cost makes its sonic attainment remarkable, although the loss of facilities has to be taken into account here. With the help of a good dealer you should first ensure that the Croft will interface with your chosen system in terms of input sensitivities, but if this is the case, then go ahead and buy one of the preamp bargains of the year, if not the decade! A beautifully built product, it should provide a long, trouble-free life and is strongly recommended. Note: 250 will buy a 'flash' version of the Micro, with a more elegant case. Also, just before going to press, the 660 power amplifier became available for a quick assessment. Initial impressions are that its performance is actually superior to that of the preamp!
 
Croft Micro Amplifier

~  H1-FI NEWS & RECORD REVIEW MARCH 1986


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