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Review: Magnet LS-621 2-way loudspeakers The magnet LS-621 loudspeakers are a compact bass reflex system which will fit well into most lounge rooms. Magnet will be a loudspeaker brand name new to most Austra­lians. It is a company based in Thailand which makes loudspeakers designed by engineers from Holland. The European link is claimed to give the speakers a natural sound which otherwise might not be there if they had been designed in the East. The LS-621 system we reviewed is a compact 2-way system based on a 165mm polypropylene woofer and a 25mm tweeter. The bass reflex enclosure is wedge-shaped, tapering from the bottom to the top to angle the front baffle in such a way as to give some time-correction to the tweeter. That and the small frontal dimensions of the speaker combine to make it quite unobtrusive in appearance. Its dimensions are as follows: 435mm high, 206mm wide, 312mm deep at the base and 275mm deep at the top, with the grille cloth frame. Actually, the cabinet is also slightly tapered at the front which makes it look slightly smaller than its measure­ments suggest. Internal volume is 15 litres. The enclosure is ported with a tube of 55mm internal dia­meter and surprisingly long at 220mm. That is probably part of the reason why the enclosure is tapered, to allow a long port without making the box too deep overall. The cabinets are fin­ished in simulated walnut veneer with a black grille cloth. We removed the woofer to have a look at the internal de­tails of the enclosure and found that, surprise, surprise, the woofer is not of Asian origin at all but made by Peerless of Denmark, although we don’t know the model number. It is a well-made unit with a large magnet and a neoprene rubber roll surround for the poly­prop­ y­lene cone. And having revealed that the woofer is of European origin, the ferrofluid-cooled 25mm soft dome tweeter is too, made by Philips of Belgium. The two drivers are coupled together via quite a complex crossover network and this has an air-cored inductor wound with heavy gauge enamelled copper wire and uses wirewound resistors and a mixture of plastic and non-polarised electrolytic capacitors. The system is bi-wired so you can drive the tweeter and woofer with separate amplifiers, if you wish. The enclosure The enclosure is lined with bonded acetate fibre or a similar material and interestingly, there is an internal sloped shelf which undoubtedly adds to cabinet rigidity but we don’t know if it serves any other purpose. The four terminal posts are deeply recessed in a panel at the rear of the enclosure. This makes it quite difficult to make wire connections to the termi­nals unless you have the wires fitted with jacks. The terminal panels are so deeply recessed that the terminal posts do not protrude at all and this means you could set the cabinets right up flush to a wall, if desired. Frequency response of the enclosure is quoted as being from 40Hz to 22kHz within 1dB and -6dB. We do not have access to an anechoic chamber so we continued on page 93 28  Silicon Chip Several other computation functions are provided, such as mean value of phase or line voltage and mean value of phase current. An integration option is available, allowing meas­urement of amp-hours or watt-hours to an accuracy of ±0.2% + 1 digit up to a period of 999 hours. A further op­tion allows frequency measurement over the range of 20Hz to 200kHz with an accuracy of ±0.1 % + 1 digit. GPIB and RS232C options are provided, allowing the 2533E to be re­motely controlled and output data to be transferred to a PC. For further information, contact Tony Richardson, Yokogawa Australia Pty Ltd, Centrecourt D3, 25-27 Paul St North, North Ryde, NSW 2113. Phone (02) 805 0699. Economy soldering irons from Scope Two new low cost 25W and 40W utility irons for electronic work have been released by Scope Laboratories. These mains voltage irons feature long-life iron-plated tips that operate at around 380°C, a stainless steel barrel, a non-rolling impact-resistant handle and four tip shapes. For further information, contact Scope Laboratories by phone on (03) 338 1566. LS621 Loudspeakers – continued from page 28 are unable to verify this claim although the response is quite smooth overall. At the bass end there is usable response down to below 45Hz although if pushed hard, the woofer does tend to frequency double. At the high end, the tweet­er is a little prominent in the region of 7- 8kHz and then tapers off a little above that although it is smooth right to the limits of audibility. Efficiency is quoted as 87.5dB at one watt and one metre and the unit is claimed to be suitable for amplifiers rated from 15 watts to 150 watts. Our impressions were that you would need an amplifier of at least 40 to 50 watts and that anything much over 100 watts on program would be too much. That is backed up by the stated maximum SPL (sound pressure level) of 106dB. On music, the Magnet LS-621s give a good account of them­selves although the tweeter seems a little muted for our tastes. We found that they sound rather better with the grille cloth frames off and we think most people would listen to them in this way. On voice, they sound very natural without any tendency to chestiness or emphasis of sibilants. Our overall impression was that they were very satisfying on classical music, especially chamber works, and they give a good account of themselves on jazz material. If you are a heavy rock fan, you will want bigger guns and it would not be fair to expect them to do the job. Recommended retail price of the Magnet LS-621s is $1150 a pair and they are available from A-One Electronics, 432-434 Kent St, Sydney, NSW 2000. They have recently fitted out a sound lounge and to introduce the Magnets they have them on sale at $950 a pair, so get in quickly. Phone A-One Electronics on (02) 267 4819. (L.D.S.) SC FM Wireless Microphone – continued from page 67 it and set the operating frequency. For this you need an FM radio. Connect the 9V battery and turn on your FM radio. Now tune across the band until the speaker squeals. The frequency on your dial is now the operating frequency of the circuit. Now if you want to adjust the frequency of operation, you reverse the process. Tune your radio to a vacant part of the band. Let’s say this frequency is 99MHz. All you should be get­ting is hiss from the loudspeaker of the radio. Now adjust the slug of coil L1 until you get a continuous squeal from the radio. That’s it, the job is complete. In more detail, the tuning range of the wireless microphone can be adjusted upwards by removing the 1pF capacitor. With this capacitor in circuit, the tuning range of L1 will be in the lower region of the FM band: from This close-up view shows how the on/ off switch is fitted to the end-plate at one end of the tube. below 88MHz to about 102MHz. With the 1pF capacitor in circuit, the tuning range will be from about 95MHz. You have to decide which portion of the band you want your cir­cuit to operate in and then pull the capacitor out or leave it in. You then adjust the slug of L1 as described above. After you have adjusted coil L1 to your satisfaction, move the microphone well away from the radio so that the acoustic feedback squeal and distortion is no longer apparent. You should now be able to speak into the microphone and your voice should come from the radio with clean reproduction. You can now complete the construction of your microphone by wiring up the on-off switch and then installing the board and battery inside the anodised aluminium tube. They are held in place by pieces of foam plastic. The PC board is positioned so that the electret protrudes slightly from the end of the tube, after which the foam plastic windshield is fitted. The slide switch is attached to an endplate with epoxy adhesive and then the end plate itself is glued into the tube with the same epoxy. SC October 1993  93