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This month a look at a simple home assembly speaker and the latest rash of noise reduction systems. First, the speakers. If you have £100 or so to spend on a speaker system you will have a reasonable amount of choice.

The best speakers in this price range are the mini speakers. If however your wallet will not stretch even that far, you will have problems obtaining anything approaching hi-fi.

One possible alternative is to build your own speakers. Unless you are a reasonable craftsman though your efforts might not be appreciated by the (female) members of your family. One way round this problem is to obtain a ready made cabinet in which to place your drive units. Unfortunately it is difficult, and usually expensive, to find what you want. Assuming that you have got this far, for good results a crossover unit specially designed for the speakers in hand must be obtained.

Now it is certainly true that there are a few complete speaker kits on the market that include the woodwork ready made, but not many. I had these problems myself recently when trying to produce a pair of high quality speakers with a total budget of £60.

After looking around I found a pair of drive units that sounded good together with a matching, ready built, 12dB/octave crossover, specially designed for use with the speakers. The next problem was the cabinets.

As I don't count woodworking as my greatest talent, I decided to see if I could find a suitable, ready built alternative. My luck was in, J. Bull of Haywards Heath (who advertise in this magazine) stock a very nice pair of cabinets. Coming complete with an attractive sculptured foam front to boot for only £6.90 a pair, plus £3 post and packing, the cabinets are ready cut for an 8 inch woofer and 4 inch tweeter. This was just what I required.

The Do-it-yourself speaker kit.

It took about an hour to complete the speaker system from scratch. Because all the parts were ready made, all I had to do was drill two 4BA clearance holes in the back baffle to take the crossover board and mark out the mounting holes for the drive units. The flying leads from the crossover are fitted with push-on terminations which fit the drive units. A roll of BAF wadding half a metre long was inserted into the cabinet and lastly the drive units were screwed into place with ½ inch, round-head, self-tapping screws. The resulting sound quality was a revelation for such an inexpensive pair of speakers and easily outperformed the £100 competition in my local Hi-Fi emporium.

Being curious, I measured the frequency response with a capacitor microphone out of doors and found it flat within 3dB between 60Hz and 20kHz. Power handling is some thirty watts continuous but plenty of SPL can be generated with a ten watt amplifier. These are 'fast' speakers, the transient response is extremely good. They will also transmit plenty of detail when the signal is of good quality. Interested readers can obtain the drive units from Bewbush Audio, (Contact Details).

A kit containing two woofers, dome tweeters, crossovers and wiring instructions for a pair of these speakers will set you back £40 plus £2 postage and packing. In total, including the BAF wadding and cabinets, a pair should cost about £55.

On to other matters. There seems to be a battle royal brewing on the noise reduction front. One of the thorns in the side of Japanese audio manufacturers is the fee they have to pay to Dolby for the use of their noise reduction system. Obviously, if a viable alternative could be produced a lot of money would be saved. Against this background several interesting alternative systems have been developed, each claiming to be the ultimate solution to cassette noise problems.

Most of the systems are based on the compander, the technical term for a compressor/expander system. On record, the dynamic range of the signal is compressed. In this way the low level passages which are most prone to noise are raised above the noise threshold. Similarly, on replay, the signal is expanded back to its original dynamic range. If this is done properly the signal is much improved. In fact the Dolby system works in this manner providing an 8dB improvement above 1kHz.

Unfortunately Dolby operates by detecting the signal amplitude and compressing accordingly. Thus when tapes of widely differing maximum output levels are played on the same machine, the system produces what can only be politely described as unsatisfactory reproduction.

Certainly the noise is reduced but the treble is massacred as well. Quite a few friends of my acquaintance prefer the sound from a cassette better with the Dolby left out. Try it some time!

One of the main drawbacks of the compander system is that when low level signals are played back a pumping sound can be heard. This is due to the fact that there isn't sufficient treble content to mask the hiss in some parts of the signal. The only commercial system that overcomes this problem is DBX. This literally cuts the dynamic range of the signal in half on record and doubles it on replay. Treble pre-emphasis is used on the input signal to ensure that this part of the signal is always above the noise level.

Telefunken's Hi Com system is the most successful of the new systems if the published information is to be relied on. Claimed noise reduction approaches 30dB above 1kHz. An advantage of the system is that it is obtainable on a single chip. If it were taken up by the large manufacturers mass consumption would reduce the cost to an acceptable level. Aurex, another compander system, claims a noise reduction of 20db. This is a system I have actually heard and it certainly works as described. However pumping effects are evident on some material and in my experience this makes the sound tiring to listen to over a long period.

As you may imagine, Dolby have not taken this potential competition lying down and have entered the fray with two new systems of their own.

Dolby C is the name of their latest offering which works by cascading two companders in series, each giving a 10dB improvement in signal to noise ratio. The frequency at which the system starts to operate has been lowered from about 1kHz to 375Hz. Otherwise the system is similar to, and completely compatible with, the normal Dolby B systems currently available. The difference is a 20dB improvement in S/N overall and a claimed reduction in midband distortion.

The other new noise reduction system from Dolby is the HX system. This operates in an entirely different way to the companders and only operates on record. All tape recorders need a bias voltage applied to the record head to prevent distortion due to the hysteresis of the tape. This bias takes the form of an ultrasonic signal and for a good recording its amplitude must be matched to the tape type used. Too little bias and the high signals are apparently boosted with respect to the bass and midrange. Too much and the bass and midrange will be recorded at a higher level than the top.

Now it so happens that the response to high frequency signals of normal tapes drops for large amplitude signals. Dolby's solution is to lower the bias on high level, high frequency passages of music so that these are recorded with their natural balance.

The NAD 6040 is the only cassette player available at the present time with this facility built in. It qualifies as a noise reduction system only in that it allows a signal to be recorded at a higher level.

Lastly, Tandberg have come up with a similar system to HX which alters the treble pre-emphasis on record and restores the balance on replay. Details of this system are scanty and it has yet to be incorporated on any deck available in the UK.

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Guide to Electronic Music Techniques

Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Jun 1981

Feature by Jeff Macaulay

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