This article is reproduced from March 1972 issue of HI-FI SOUND by permission aI Haymarket Publishing Ltd., Gillow House, 5 Winaley Street, London, W.1. Printed by Clarion Photographic Services Ltd., Q1-437 5578.
This loudspeaker is a superior version of the Domestic Monitor transmission-line system on which I reported in December 1970. In fact the Professional came first in IMF history, for it was introduced in the USA in 1968. Also in the IMF range is the closely related but smaller Studio model, subject of a report June 1971.
This brief report can be regarded as a follow-up to what was written about the Domestic Monitor, since the transmissionline principle is again exploited and there are so many other similarities. Further information on the design and its advantages will be found in the manufacturer's literature.
Design differences in the Professional, individually small but all important, can be summarised as follows. To begin with, the enclosure is tin. higher, giving about bin. greater transmission-line path length and, consequently; a further extension of bass response-though this offers little clue to the difference in quality of bass.
.'The mid-range unit works in its own transmission line, of larger dimensions than that in other models, the main aims being to reduce colouration and improve the power , handling in this region. Level controls give a total 4dB variation over the operating range of the mid and hf drive units to accommodate slight differences arising in positioning the speakers in the room.
Crossover arrangements are an important feature in any high-grade system, and in the Professional the circuit is particularly complex. Further development has increased the power handling capability. The choice lies between ferrite-cored inductors for crossovers-commonly used in commercial systems though tending to saturate at high inputs and thus contribute to distortion--and air-cored coils which are often held to be superior yet pose a problem in that too high a resistance leads to an unwelcome loss. In the Professional there is a compromise involving some coils trimmed with ferrous inserts. 1 cannot emphasise too strongly that crossover design---and in particular the handling capacity-can hold the balance between success and failure in high-grade speakers.
Choice' of the transmission-line in the interest of tight, deep bass necessarily involves the manufacturer in considerable complication, not least in the positioning and grading of internal damping materials. In the Professional model these damping arrangements are even more complex than before, the aim being to improve stability on big transient inputs--the kind of signal the speaker will certainly receive in its monitoring applications.
How the damping is arranged must, the designer insists, remain a secret between him and me. Actually, Alice remains in Wonderland and the secret remains his, but I am happy to acknowledge the results of what may be described as the antithesis of mass-production. After taking a look at the assembly and check-out procedures I can vouch for the thoroughgoing way in which details are tended. Manufacture of the enclosure and control of the TLS path constitute a work of art, helped out by science, and any reasonable user will take it on trust.
Enclosure construction is even more robust than for the Domestic model. The carcass is of high-density particle board, stress-laminated with Formica, a construction that is an important feature of the loudspeaker for it makes another contribution to the reduction of colouration. Indeed, the study of causes of colouration, leading to several approaches to the control of such effects, is an essential part of high quality speaker design, and I have not encountered a manufacturer taking this more seriously than does IMF.
As with other investigations carried out at Hirst Research Centre, tests involved the use of both sine-wave inputs and pink noise and filters. The frequency response traces include off-axis response, and the effect of the level controls also is shown. Also reproduced is the impedance curve -it comes out as a 'reciprocal' characteristic-showing a variation between and 5 8 ohms over much of the range but a rise to about 30 ohms at 35Hz, similar to the Domestic tested under the same conditions. Distortion related to 2V input was 1 % at 100Hz.. Above 200Hz it was lower than could be measured on the B &K equipment.
Efficiency worked out to 100dB level for 25W input, which is much the same as for the Domestic. This of course is a very high level but one expects that a monitor system for studio and professional use will accept big inputs. The appropriate range of amplifier ratings is 25.100W per channel, and indeed a Radford SC24/ SPA50 was used for the domestic part of the test routines.
Asked to isolate some features of speakers of really high quality, I should mention these as examples-with a reminder to consider each point along with others : willingness to be driven to realistic output levels coupled with good attack on transients (minimal hangover and boominess) ; smooth response with the right overall balance (no intrusive features) ; well-tailored dispersion of output and detailed, stable stereo presentation; low colouration in the interests of a transparent quality that, almost by definition, must be characteristic of a reproducer to be used for monitoring programme material. A well-integrated output would naturally be a feature of a speaker with these merits; and I should add that a reproducer for professionals must be reasonably predictable in different environments.
Trials with the professional Monitor show how closely such requirements have been studied and how crafty has been the adjustment of compromises that are involved in the design of specialised transducers. The Domestic has its share of virtues, but the Professional shows an all-round refinement of the design-lower colouration, improved power handling, tighter bass with even more impact and 'breathy' quality down to the sub-sonic region (below 20Hz), and an even better analysis giving quite remarkably detailed results on the best stereo programmes, to mention the most memorable features. Checking good recent discs I have noted particularly the feeling of 'listening through' the audio chain, such is the open quality and revelation of detail.
Newcomers may still wonder whether such a product takes them into an atmosphere too rarefied for full appreciation by non-professional listeners. But they can be assured that here is a professional speaker that is house-trained. This loudspeaker faces little competition--there are few real monitors-and, with ancillary equipment of suitable quality, will provide performance of outstanding merit in the domestic setting.
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