AMPLIFIERS OF ALL KINDS

Croft Series Four Power amp
by Martin Colloms

Croft promised me that they would be introducing a matching power amplifier while I was reviewing the Micro, and sure enough, here it is, the 40W/channel (15dBW) Series Four.

It costs 650, and comes in a large rectangular box finished in Croft's 'traditional' chocolate colored enamel with gold lettering. Michell binding posts are used for the speaker connection, with a single transformer tap, nominally matching 6ohms and suited to 4-8ohm loudspeakers. Bias adjustment is accessible from the front panel with the aid of a millivoltmeter.

Technical details
Anyone conversant with valve power amplifier design will find the Series Four something of a surprise, as it employs circuit techniques and features which are notfully realised in 3000+ gear. The HT rails are fully-regulated throughout, even the output stage; moreover, the regulators are valves, with selected neon voltage reference tubes. Very high isolation from the mains is assured, blocking any noise and similar interference. All the earlier stages also have separate regulated supplies, and apart from the massive, centrally-sited power transformer, the amplifier is built largely along double-mono lines.

The output stage uses EL34s in a conventional, essentially class-A, push-pull ultralinear configuration, with negative feedback taken from the transformer secondary. High quality components are much in evidence, including selected resistors and polypropylene coupling capacitors.

Glen Croft noted that it is permissible to wire the output terminals in parallel. If the same signal is fed to both inputs, the result is a monoblock with twice the output current capability, hence twice the power, into 4ohm loads, and about 60% more power into 8ohms. Sound quality is improved, and at 1300/pair, Series Four monoblocks are worth considering in their own right.

Lab results
With one channel driven, this amplifier just met the 16dBW 8ohm specification at 1 kHz. If 2.5% harmonic distortion is allowed, the 20Hz figure improves to 15dBW before clipping, but a 14dBW level at 20kHz resulted in 10% distortion. Losses of around 3dB occurred into 4ohm loads, resulting in an absolute power similar to that into 8ohms. The 2ohm pulsed output was a rather modest 8dBW, while . the peak current was unexceptional at around +/-4.5A. The output rose to 18.6dBW (70W) into 16ohms, meaning that as the paralleled monoblock would hold that level into 8ohms, around SOW will be available into 4ohms. Peak current in mono mode reached 8.7A, and this more expensive mode of operation must be preferred if high levels into difficult impedances are required.

As full power distortion levels were rather high, those in the table refer to half power, but these are still rather uninspiring. The 20kHz, 20W figure is 10%, but by 1 W (OdBW), midband distortion was approaching -70dB (0.03%), while the 20kHz value was fairly satisfactory at 1 %. At half-rated power, the twin-tone intermodulation was fine at -53.2dB, improving to -61.4dB at OdBW. The power spectrum at half power into 4ohms with a 40Hz signal showed high harmonic distortion, but with negligible power supply components. When the level was reduced to OdBW, the distortion harmonics also became negligible.

Apart from acoustic mains transformer hum, noise levels were fine, while stereo separation must be rated as 'very good' and channel balance was excellent. The output impedance was a moderate 0.46ohms over most of the range. while the input was easy to drive at 100k in parallel with 30pF. Overall frequency response was -0.2dB at 8Hz and 22kHz, with -3dB points at 5Hz and 72.5kHz. The above results are for a standard Four; a small improvement in HF performance has been made in current production.

Sound quality
The Series Four turned out to be a yet another remarkably good-sounding design, but with one problem, the higher-than-usual level of mechanical hum from the mains transformer. For me, the only sensible location was remote from my listening position, sited near the loudspeakers. Perhaps a mechanically-minded owner could isolate the transformer on rubber bushes? Meanwhile we must wait for Glen Croft to solve this problem and potential customers should check that this is satisfactory before purchase.

Fortunately, the amplifier was comprehensively auditioned prior to being measured. Given the rather dubious set of results, there is no way I could have foreseen the high standard of reproduction achieved by the Series Four. No excuses need be made, this is a genuine audiophile-quality product offering a fine subjective performance, competing with some highly regarded products costing up to 2000.

It produced volume levels higher than the rating would suggest, sounding more like a 60W model. Stereo imagery was unusually stable and well-focused, while in terms of tonal balance it was considered very neutral, more so in fact than a number of high ranked bipolar transistor models.

Bass articulation and definition met a very high standard; dry in nature, it also lacked a little in ultimate power and extension. The treble was also well-rated, if on occasion slightly 'dull' with a hint of a 'mechanical' quality. Despite this, vocals reproduced well, with a very natural 'feel' to breathiness and sibilance. Although the midband had alight' texture, the amplifier sounded transparent and gave a convincing impression of depth and ambience. In this respect it surpassed the Micro preamplifier and the SI Basic was felt to be a more appropriate partner in order to exploit its potential to the full. Overall, it gave a strong, coherent performance, musical without an identifiable 'valve' or 'transistor' character.

Bass articulation and definition met a very high standard; dry in nature, it also lacked a little in ultimate power and extension. The treble was also well-rated, if on occasion slightly 'dull' with a hint of a 'mechanical' quality. Despite this, vocals reproduced well, with a very natural 'feel' to breathiness and sibilance. Although the midband had alight' texture, the amplifier sounded transparent and gave a convincing impression of depth and ambience. In this respect it surpassed the Micro preamplifier and the SI Basic was felt to be a more appropriate partner in order to exploit its potential to the full. Overall, it gave a strong, coherent performance, musical without an identifiable 'valve' or 'transistor' character.

Conclusion
The double-mono system still actually represents good value in the power amplifier stakes and should be considered when maximum output and best load tolerance are required. The standard stereo Series Four concedes little in terms of real performance with normal loads, however, giving an excellent subjective performance for the price. When used with a Micro or SI Basic preamp, a fine amplifier system can be assembled which will bear comparison with models at two or three times the price.

Croft Series Four Power amp

~  H1-FI NEWS & RECORD REVIEW MAY 1986


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