Text OCR'd from scans of Briefing document.


The fundamental requirement is that the amplifier must be accurate under all conditions of use, No amplifier can satisfy this requirement if it has a poor frequency response or if it generates significant noise or distortion. The 732 is therefore designed to provide a flat response (+ 0, - O.2dB) across the entire audio band and to maintain inaudible levels of noise and distortion under any conditions of use short of voltage clipping.

In order to ensure that voltage clipping should not be a problem, and especially considering the present day combination of low efficiency loudspeakers and the increasing dynamic range becoming available from digital sources, it was decided that the amplifier must be capable of delivering very high power levels, without effort, into any loudspeaker load. A power capability of at least 200 watts per channel (RMS continuous, into 8 ohms) was considered essential for this purpose. In addition the amplifier is designed to operate safely without current limiting or protection since this would restrict the transient performance, producing "current clipping" effects, especially into more challenging loads. Bipolar output transistors were therefore chosen rather than FETS in order to avoid the inherent current limiting effects of these devices. Transient currents of 30 A per channel may be delivered without a significant increase in the distortion even when driving low impedances of arbitrary phase angle.

Te ensure that the amplifier's accuracy is maintained irrespective of the nature of the loudspeaker load the amplifier has been designed to be unconditionally stable and has a low output impedance. This ensures that the accuracy of the signal supplied to the output terminals of the 732 does not depend upon the current required by the loudspeaker. The ultimate musical performance is then dependent upon the loudspeaker (not to mention the sound sources and transducers which present the original signal to the pre-amplifier) and the extent to which this is capable of accurately reproducing the signals presented to it and of handling the high levels of dynamic power which are available.


732 POWER AMPLIFIER

A number of amplifiers have power supplies which employ massive smoothing capacitors or use two separate supplies, one for each channel, in order to reduce dynamic distortion effects. This reduces the symptom without necessarily effecting a cure. The 732 power amplifiers have a circuit topology which is designed to be insensitive to fluctuations in the power rail voltages. Each amplifier draws the current it requires from each rail, ignoring the rail voltage. In this way the problems of dynamic and crosstalk distortion are dealt with without recourse to massive capacitors or dual supplies. This design technique also permits the use of a higher standby rail voltage than would be safe with an excessively regulated supply. The amplifier's ability to drive large transients, particularly into higher impedances, is therefore enhanced.

The pre-amplifier has also been designed to meet the fundamental requirement of excellent performance under dynamic musical conditions. It goes without saying that it must have an accurate frequency response with negligible noise and distortion but as, or more, important is its ability to handle at the input, signals of wide dynamic range. To ensure that clipping shall never be a problem the 730 has a 40 dB overload margin on all inputs, including the moving coil cartridge input, and can drive in excess of 10 V RMS across the audio. band at its output before clipping occurs. The GOO ohms output impedance is sufficiently low that it will drive long runs of cable without loss of high frequencies or any increase in transient distortion.

The 730 power supply is of discrete design and has a reserve of at least 10 times the current requirement of the pre-amplifier under any conditions of use. This discrete design is markedly more stable and noise-free than the stabiliser integrated circuits presently available which were tried in the early stages of development and discarded. In addition each rail of each stage of amplification on each channel has its own passive filter network and each stage is of a design which is inherently insensitive to power rail fluctuations. As in the power amplifier this results in a very high degree of immunity to dynamic forms of distortion which might otherwise be communicated along the power rails.

Both the 730 and the 732 have integral power supplies. This physical proximity of the power supplies to the amplifier stages helps to minimise the common return impedance of the power rails and of the ground circuit. This aids the ability of the amplifier to deliver accurate transient signals.


730 PRE-AMPLIFIER

A number of recent amplifier designs have resorted to separately boxed power supplies, principally because of the problem of magnetic field radiation from the transformer, capacitors and associated wiring. This radiation can increase the noise level and produce dynamic distortion if unchecked. In dealing with this problem in development we, once again, decided to attack the problem itself rather than seeking to reduce the symptom. Over two years we worked in close co-operation with a specialist manufacturer to develop a toroidal transformer with exceptionally low external field coupling. As an added benefit this has also resulted in a more efficient type of transformer as the field coupling between primary and secondaries is thereby enhanced.

Looking inside the 730 or 732 it will be noticed that the wire looming is markedly more tidy than in many other amplifiers. This is more than a cosmetic feature as accurate looming is essential to control the noise and distortion performance of an amplifier, especially under the conditions which prevail when driving high power transient signals. In both the power amplifier and the preamplifier the power supplies are regarded as vitally important parts of the overall system. The very low common earth impedances and the careful arrangement of the ground return system ensures that the amplifier performance does not depend significantly upon the external mains/earthing arrangement chosen. For this reason the amplifier functions equally accurately when the 732 is connected via the 730 or when each has its own independent mains/earth lead.


Feb 1982