AMPLIFIERS OF ALL KINDS

Croft Stereo Integrated Preamp
By Martin Colloms

Small Midlands-based company Croft Acoustics have entered one of those periods of intense creativity, making it difficult for a busy reviewer to keep up. Hard on the heels of the amazingly cheap, utilitarian but good Micro preamplifier reviewed in March, we are presented with a new series of preamplifiers called the 'Stereo Integrated' - what about a better name Glen? - the least expensive version of which was originally billed as the 'flash' Micro.

The Stereo Integrated series (I'll call them SI for short) comprises a full-size, low-profile chassis built largely to double-mono principles, even down to the use of dual volume controls and selector switches. Consequently, the fascia is impressive, belying the modest base price of 250, for which the customer gets a preamp with the same specification and facilities as the Micro, apart from the control of balance made possible by the dual controls. The disc input is moving-magnet-only, but is suitable for high-output 'coils. Another 100 buys the next version, which offers tape facilities, the simple circuitry of the 'Basic' being augmented byline buffers to drive the ; tape outputs and isolate the tape loop from the main signal path. (It is worth leaving the tape-out leads of the Micro and the SI Basic disconnected when not in use, as the reduced loading improves the sound quality.) 
 
Finally, a modest 50 gives 400 for the top SI preamp, which has a valve m-c input as well as tape buffering. (A virtually identical predecessor was reviewed by Ken Kessler in March 1984.) The m-c sensitivity is quoted as 0.15mV for 650mV out, an overall gain of 74dB: ample for all but the very lowest output cartridges. Proper termination is provided by the 100ohm input resistance in parallel with a 3.9nF of capacitance.

Returning to the SI Basic, the m-m sensitivity is 2mV in for 650mV out, a little higher than the 3mV of the Micro, and in my opinion, quite a number of moving-coils will give this input satisfactorily. However, one important assumption must be made, relevant to all the Croft preamplifiers; that the power amplifier be relatively sensitive, requiring 0.5-0.7V for full output. Not only are the disc input gains set on this basis, but also the line-level inputs, CD, aux and tape having zero gain, the line stage simply being a unity-gain buffer. This does not present a problem with CD since typical outputs are 700mV, rising to an absolute maximum of 2V RMS. Some tuners and tape decks generally provide rather less output than this and compatibility in a given system should be investigated before buying a Croft preamp. (When discussing the 'passive' preamp idea three years ago I proposed the idea of an IHF standardised level of 500mV for line-level signals, with power amplifiers conforming to the same level for full output, regardless of power rating. This would greatly simplify component and source matching.)

We chose to assess the SI Basic in this review, and my comments should be read in conjunction with my review of the Micro (HFN/RR March p69). Technically, the two are quite similar, bar the inclusion of some extra power supplies and the double mono construction of the more expensive model. We were not prepared for the substantial improvement in sound quality that was heard!

Lab results
The measurements were essentially identical to the Micro which are reproduced in the Table on p81. Note, however, that the m-m input sensitivity is now 2mV and that stereo separation of the SI Basic is better.

Sound quality
Highly rated, the budget Micro was considered as unassailable at the price. Possessing an essentially neutral tonal balance, it provided a coherent and believable sound-stage, with very good focus, definition, and depth. Transparency was thought highly of, and the only perceptible weakness concerned a hint of bass softness and very mild treble grain. Dynamics were fine, especially at the price. In truth, I would have been content even if the SI Basic didn't improve on this; nevertheless it did so. Both focus and stage width were better, the sound-stage now possessing more depth and sense of ambience, while a greater transparency revealed more fine detail. The treble was a mite tidier and the bass, especially from CD, was firmer as well as more dynamic. The notably improved SI Basic preamplifier gave a sound full of life and sparkle which was tonally convincing yet low low in listening fatigue.

Conclusion
Croft's Stereo Integrated series of preamps are as much of a bargain in their price categories as the Micro is in its budget position. I can unhesitatingly recommend them, despite my personal dislike of separate volume controls. The build quality is high, far above what is usual at the price, while the sound quality is exceptional, perhaps threatening established models costing upwards of 1000. The Micro is an important model for impecunious enthusiasts, but for those intending to use it with a power amplifier of comparable quality, an Audiolab 8000P, for example, or a valve model in the 650-1550 region, then the additional cost of the SI Basic becomes relatively insignificant and I would be sorely tempted to pay the extra, except at the very lowest price break.

Integrated Valve Stereo Preamplifier

~  H1-FI NEWS & RECORD REVIEW MAY 1986


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