Croft 4 / 4S / 4SA

First reviewed in 1986, the Series 4 stereo power amplifier is a modest 40W per channel device on a chassis of imposing physical size. No compromises are apparent in transformer ratings, and in fact the whole affair is conservatively rated and should provide a long working life (which is not always the case with some high performance valve equipment).

Finished in glossy black, and smelling of hot paint for the first few hours of operation, this amplifier has a high input sensitivity of typically 0.4V RMS for full power, which facilitates its use with the Croft pre-amplifiers that have unity gain auxiliary input terminals. (What goes for a Croft pre-amp also goes for other pre-amps with passive line stage options, or for those accessory passive line controllers that directly connect a CD player to a power amp.)

Test sockets are provided for adjusting output biasing, but an accessory voltmeter is needed to monitor the levels so this is probably best left to a service technician. Further bias adjustment may be necessary after the first few weeks of use. Speaker connections use 4mm socket/binding posts, and the audio input phono socket. A good length of mains cable is included, which eases a frequent source of frustration.

Built on the classic push-pull ultralinear configuration, an unusual feature is a fully regulated high voltage supply that uses valve regulator elements, a costly approach that reflects Croft's dedication to their own performance standards. Likewise, they eschew printed circuit boards on the grounds of sound quality; instead this amplifier is craftsman built with point-to-point airspaced construction and single-strand wiring.

For the more advanced SA variant, the reservoir capacitors are doubled, all resistors are selected from the Holco or Vishay ranges, with some wirewound devices, and the regulator circuitry is improved. Briefly outlining Series 4, the 5650 standard 40W model is still available; the 5730 48 has 60W per channel power output while the 5999 SA offers the improved componentry besides.

Note that all these amplifiers may also be used as higher power monobloks; the output impedance is halved, peak current doubled, output power increased, and the sound significantly improved.

LAB REPORT

The SA just met its 8ohm power specification at lkHz, but with a notable shortfall at higher frequencies (not uncommon amongst valve power amplifiers). The 4ohm both channels delivery was a modest 14dBW, so monoblok operation is clearly worthwhile if used with more awkward loudspeaker loads is envisaged. Peak output current was a satisfactory 5.5amps. Midband distortion was quite low at full level, but was rather poorer at the frequency extremes, measuring 8% at 20Hz and poorer than 10% at 20kHz (happily the harmonic content was comparatively benign), indicating some output transformer stress. However, some improvement was noted at lower powers, and the high frequency intermodulation results were quite satisfactory.

The SA has a slightly lower output impedance than the standard model. These fairly large values can be expected to modify the tonal balance of some loudspeakers subtly, usually in the direction of a brighter treble; electrostatics will scarcely be affected. Channel balance was fine, with pretty good separation especially in the midrange. Background noise and hum were satisfactorily low. Easy to drive, the high input sensitivity has already been mentioned, and might impose rather low volume control settings for non-Croft pre-amplifiers which have a normal gain line stage. The frequency response was nicely flat, with a mild rolloff at high frequencies. The supply intermodulation spectrum Was dominated by harmonic distortion, mains related components being nicely suppressed.

SOUND QUALITY

The standard 4 did well in earlier issues and remains a decent power amplifier for the money, with notably good focus and bass for its price. The 4S with its slightly higher power delivery shows greater authority and improvements in clarity and transparency again a good result for the price.

The 4SA represents a further significant improvement. The bass sounds remarkably solid - fast and tuneful with satisfactory extension. The midrange delivers a full and generous sound, very clear and detailed at most levels, albeit revealing a compressed quality with mild glare towards full power. The treble had a touch of `zing' and sibilance, but the overall rating was still fairly good, with strikingly well focused, wide and deep stereo images. Dynamics were very well conveyed.

CONCLUSIONS

This is a fine series of amplifiers. All deserve Recommendation, but the SA model showed touches of magic - given a little more mid dynamic openness and a tidier treble it could compare to some of the finest made today; as it stands, it is still a very worthy competitor.

Martin Colloms


Home page Reviews main page
More Reviews........
Previous review Next review
 

© Croft Acoustics 2008 (design by EnterpriseDesign) t: +44(0)1723 355111 e: glenn@croftacoustics.co.uk

 

Back to homepage Send email