Eleven years ago Peter Walker and Michael Albinson of Quad announced and described a new power amplifier circuit which they had christened after its major feature-"current dumping". Like most brilliant ideas it had a simple foundation, to use two separate and very differently conceived amplifiers working together to produce such advantageous results that they earned Quad the highly prestigious Queen's Award for Technology; and there are not many of those in our industry. For some time the hugely successful Quad 405 power amplifier (some 90,000 units sold so far) was the only representative of this technique, although a few foreign competitors had to have the patents drawn to their attention. Recently several near-copies which slip past the patents have appeared, from both East and West, as the virtues of the basic principles sink home.
In a current dumping amplifier, the lusty output transistors are only there as muscle; the quality is provided by a near-perfect low-power class A amplifier whose job it is to compensate exactly for the clumsy deficiencies of its big brother. Put simply, it is only necessary to aim the current dumpers at the dartboard and the low-power amplifier will make sure that you get a bullseye every time. There are other practical advantages: no crossover distortion, no adjustments, cheaper unmatched output transistors, less change in performance with temperature and consequently no 'warm up' time, improved reliability; the list is impressive.
Two versions of a professional rack-mounting amplifier using current dumping were added to Quad's product list a couple of years ago and one of them, the stereo Quad 520, was taken up by a number of domestic users who found it superior to the older Quad 405 (now in it s improved Mark II version). To those of us who study and prognosticate, the Quad 520 contained more than a few hint s at the way future Quad domestic amplifiers might progress and their Quad 306, which is the subject of this report, is an initial confirmation of these trends.
The first thing to be noted is the remarkably small size of this amplifier in relation to what it can accomplish in the way of performance. It achieves this by advancing Quad's oft -quoted theory that practical, cost-effective domestic amplifiers are for reproducing music and not test tones or other continuous duty signals. In fact it is exactly the same size as the Quad 34 control unit and FM4 tuner and the three can be stacked as a 'midi' system or even more handsomely 'floated' in their matching cabinet which tidily hides all the rear connect ions and leads. It is housed in the same stout steel case as the other two units and such multiple usage obviously helps to keep the price down. For some time, Quad has sported a different paint finish in their US market - charcoal grey instead of the familiar two-tone brown. This now becomes universal and, although existing models will be available in both finishes for some time to come, the Quad 306 is the first to be grey only.
Internally the Quad 306 has a deceptively simple construction on a single high-quality printed circuit board. This is dominated by the toroidal power transformer and the four squat electrolytic capacitors which smooth the two split power supplies, one for each channel (a significant change from the single supply of the 405 series-forecast in the 520). The die-cast front panel, a familiar feature of Quad products, forms the heat sink for the power transistors and the surface is vaned to assist heat dissipation; it becomes quite warm in heavy use, but never dangerously so as the safety devices incorporated prevent this happening. Bolted to the rear of this panel is a thick section of black anodized aluminium angle. This support s the front edge of the printed circuit board and the eight power transistors (two current dumpers and two class A per channel) are mounted through it in a row. All the small components, including the two bridge power rectifiers, are on the board and there is no internal wiring apart from the transformer lead-outs and the mains linkage. As in all Quad products, everything is to a high standard. The layout is first class and anyone would be pleased to show off this very British amplifier as an example of what we can do. There are some neat tricks in the circuit too which uses some of the less frequently found semiconductor devices, such as constant current diodes, to stabilize performance and simplify production.
Adherence to the requirements of some countries, who now dictate that all mains-driven components must have a separate on/off switch, means that this is the first Quad domestic power amplifier to sport one and it appears incorporated in a neat Quad logo a t the lower right corner of the front panel. There is also a green LED which shows when the unit is up and running. The same 'requirements' now prohibit the use of adjustable mains tappings and so there are 110-120 Volt and 230-240 Volt versions and owners must have the transformer changed if moving to a 110-Volt country or vice versa. Another change from Quad practice is the use of gold-plated phono sockets for the inputs in place of the four-pin DIN plug (which Quad still think superior, but failed to get adopted as an IEC standard). Standard 4mm sockets are provided for the loud speaker connections and there is a fused IEC mains socket and also a linked unswitched plug. The earth pins of the latter are connected to the chassis but the signal 'earths' are spaced off by 10 ohms to ward off possible loop problems.
Protection
In this amplifier, Quad have rethought their overload protection arrangements and incorporated a resettable circuit breaker in the mains supply. Unlike the ones which many people have in their domestic electricity supply panels instead of fuses, which operate magnetically, this is a thermally operated device which continuously monitors the current t a ken by the amplifier. If gross overloading is allowed to take place, or a short circuit occurs at the output, the trip will operate after a time decided by the seriousness of the overload and the amplifier will shut down. It can be restarted by pressing a red button on the rear panel. In normal operation, with regular programme material this will not happen; it has not done so in my three months of fairly constant and sometimes heavy usage. However, if continuous tones are applied, as when making measurements of performance, then frequent tripping will occur.
If these technical things put you off, I will attempt to summarize by saying that this is a 50 Watts-per- channel amplifier which sounds as if it were more than that' As to the quality of the sound, it seems to me to be eminently satisfactory. I personally rated the professional Quad 520 as sonically superior to even the latest version of the dear old Quad 405, and this £ 160 cheaper Quad 306 sounds to be its equal in every way (except, of course, power). It is better in one respect, that of background noise, which really is completely inaudible, even with loudspeakers of above average sensitivity. It was no surprise to find 'within the thickness of the pointer' agreement between the readings on my Radford measuring gear and Quad's own specification.
At £229 I have not the slightest hesitation in recommending this Quad 306 to anyone who listens at sensible levels and buys his or her equipment to enjoy music unfettered by the overshoots and exaggerations which some more macho souls associate with hi-fi.