Stereo Amplifier

Review: Hafler XL-280 Stereo Amplifier

On the face of it, the requirements of a power amplifier for audio use are easily specified and straightforwardly met. The classic definition of the perfect amplifier as the equivalent of a piece of straight wire with gain really says it all, but of course to realize that goal in a working model which has to drive a highly reactive load, the loudspeaker, is another matter. Many designers have argued that a bandwidth limited to between 20Hz and 20kHz is adequate for audio use and indeed some maintain that to go far beyond it is inviting trouble from all manner of spurious effects-as one writer has put it, "the wider you open the window the more muck flies in". However, filtering to roll off the gain at the frequency extremes does mean introducing phase shifts within the passband which, although they are widely reckoned to be innocuous, do mean that any test which compares such an amplifier's output with its input under dynamic conditions must take them into account; the 'straight wire' reference has to be modified by the addition of LF and HF components which match those of the amplifier under consideration.

The American amplifier designer David Hafler has for a long time been an advocate of wide bandwidth amplifiers, and in his latest stereo power amplifier, the XL-280, he has set out to produce a circuit capable of direct comparison with an unmodified 'straight wire'. He has coined the term Straight Wire Differential Test (SWDT) for his version of this procedure. XL stands for Excelinear and the new circuit is a development of those employed in his previous DH-200 and DH-220 models, but goes beyond them in its amplitude, phase and so-called transfer characteristic linearity.

Like its predecessors, the XL-280 uses a fully complementary circuit culminating in MOSFET (Metal Oxide Silicon Field Effect Transistor) output drivers. Most conventional transistor amplifiers employ a Zobel network at the speaker output to preserve stability into reactive loads, but here again the phase angle is compromised so the XL-280 has been designed to do without one without jeopardizing stability. FETs are used at the inputs too and the usual input coupling capacitor has been removed to eliminate the LF phase shift. The low-level stages operate in Class A and have a separately regulated power supply to that which feeds the Class AB output stages. Further to this, although the mains transformer is common to both channels, separate bridge rectifiers and reservoir capacitors are used to minimize crosstalk. Local and overall feedback is used, albeit at relatively low levels, both to enhance the linearity of an already very linear circuit and to stabilize the operating conditions and prevent drift.

The XL-280 offers 145 Watts per channel, continuous, into 8 Ohms, both channels driven, or it can be bridged via a rear panel switch to provide some 400 Watts of mono power, also into 8 Ohms. Its exceptionally wide bandwidth is illustrated by the quoted 1 Watt output performance of ± 3dB from 0.1Hz to 500kHz. Phase shift within the conventional audio band (20Hz to 20kHz) is specified at less than 0.5°. As supplied, the XL-280 is set up to perform optimally with a 'standard' loudspeaker load, but it incorporates user-accessible preset controls, one per channel, to allow fine 'tweaking' of the phase response to permit the best results to be obtained via the SWDT null test with any given loudspeaker. Thus the purist who is prepared to connect up a suitable 'passive' test box can optimize the amplifier's performance with his own loudspeakers by adjusting for the best null as described in detail in the instruction manual. No other test equipment nor a special test disc is needed; the interstation noise of an un-muted FM tuner is ideal for setting up purposes, but of course any audio signal can be employed to demonstrate the achieved performance when this is completed. Even without such fine tuning, though, the XL-280 comfortably exceeds the specification of most competing designs.

Physically the XL-280 is very similar to the DH-220 although it is actually 25mm (1 inch) wider. The black case is in four pieces, two pressed steel sections which form the chassis and lid and two large anodized heatsinks, one at each end, which carry the amplifier modules and output transistors (six of these are used per channel to provide the requisite drive current). The front panel carries an on/off mains rocker switch (which may be left permanently on if power is supplied from a switched mains outlet of the chosen preamplifier). At the rear are two gold-plated phono input sockets, a small slider switch for converting the amplifier to bridged mono operation, and four 4mm screw terminals for the loudspeaker connections (these will accept the appropriate plugs, spade connectors or bare wire ends). Loudspeaker protection is provided by fuses which are accessible from the rear of the unit, and the amplifier itself is protected by line fuses and by thermal cut-outs which break the mains supply in the event of overheating.

Like all Haller products the XL280 is available either ready built or in kit form. The latter is especially good value (in this case the saving is some £65) and anyone competent with a soldering iron who can follow a logically set out and very clear list of instructions will be guaranteed a finished amplifier which will match the performance of a factory assembled unit. No profound knowledge of electronics theory nor the ability to use test equipment is required because the amplifier modules themselves are ready-assembled and tested. All that is required of the kit builder is the interconnection of the various items and the mechanical assembly.

How it performed

The SWDT test is very sensitive indeed and the null which can be achieved with a carefully aligned XL280 is remarkable, the more so when it is realized that all that remains audible in this configuration is the total of all distortions. A null level of - 40dB across the audio band represents a good amplifier yet the fine-tuned XL-280 is capable of achieving - 60dB, which equates to 0.1% total (linear plus non-linear) distortion.

Set up for Quad ESL-63 loudspeakers the Hafler XL-280 gave a truly exemplary performance. The bass was absolutely firm and seemingly more extended than I am used to hearing, despite the fact that my preamplifier has a built-in LF roll-off. The treble was crystal clear and wide open sounding, with no trace of sibilant 'splash' and the overall stereo sound stage rock-solid regardless of the dynamic activity of the music used. Some of the more subtle subjective differences one hears in top-end hi-fi equipment are terribly difficult to quantify, sometimes even to express, but my overriding impression of the Haller XL-280 is that it neither adds to nor subtracts from the source material fed to it. The word that kept coming to mind as I listened over a period of weeks was 'authority' and I can't think of a power amplifier that I've used, or for that matter heard, which demonstrates that quality more convincingly.