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Comments on a new cartridge and active speakers.

Last month I promised to look at the fundamentals of active cross-over design. First, though, a look at a new product which I'm sure will be of interest to most of you.

To begin at the beginning. For the last few years the cartridge market has been split between the proponents of moving coil and moving magnet systems. Certainly in the last couple of years the former type has gained an enviable reputation for sound quality. However, there is a new breed of moving coil cartridges that have an output voltage comparable to that of the moving magnets. Naturally I was interested to see if the sound quality of the old moving coils could be maintained and so I obtained a review sample of the new Coral MC-88E which is to be distributed by Videotone Ltd.

As with the previous R100 cartridge review the reference system used consisted of a Planar 2 deck feeding through a custom-built active speaker system. My own reference system with a BD1 turntable, Acos Lustre arm was also used. Unfortunately the Lustre is now obsolete but the Rega arm, the R200, was developed from it although the latter has considerably less mass.

The MC-88E is a heavy cartridge and requires a tracking weight of 2 grams to eliminate mistracking. Not that this proved to be a problem in use since the cartridge sailed through the most heavily modulated Telarc digital discs without problems.

One inconvenience from the consumer's point of view is that the stylus is not user replaceable. Those readers who already possess moving coils will be aware that the cartridge must be sent back to the makers for a replacement to be fitted. Another, albeit minor, problem was that the plastic stylus guard repeatedly fell off while the cartridge was being fitted to the headshell.

Enough of the preamble — how did it sound? In short I was pleasantly surprised. On first hearing, the sound was reminiscent of the VMS20E2 with which it was compared, with good detail transmission, neutrality and a feeling of immediacy.

It soon became apparent though that the '88' was delivering information that the '20' had completely missed. Subtle details, especially on cymbals, became noticeable without having to concentrate one's hearing. Bass also had a different character being better extended and more firmly controlled.

Playing my collection of direct cuts and digital records produced further revelations. It became possible not only to hear the musical notes but also the way in which the musician was playing them.

The new Coral MC-88E moving coil cartridge.

By now you will have gathered that I am enthusiastic about this particular newcomer to the audio scene. The output from the cartridge is 0.5mV/cm/s, about half that generated by the majority of moving magnet designs. As such the cartridge can simply be fed straight into a standard moving magnet amplifier input without problems.

The introduction of this type of cartridge raises some interesting implications for the preamplifier into which it is fed.

Moving magnet types have a large inductance and so the source impedance rises with frequency. So much so that the output impedance of the cartridge becomes comparable with the input impedance of the RIAA stage. Moving coil devices, including the MC-88E, have a much lower inductance and hence do not require special matching. Now the S/N ratio of any cartridge input stage is limited by the noise generated by the cartridge. This in turn is dependent upon its internal resistance. The larger the resistance the more noise is generated.

The MC-88E has an internal impedance some twenty times less than that of a moving magnet which means that with careful design a S/N ratio at least 10dB better than that normally achieved with moving magnets should be obtainable.

Another interesting point is the automatic extra 3dB of headroom obtained with this cartridge. This is quite noticeable when playing highly modulated passages. Often what was thought to be mistracking or poor pressing is revealed as unsuspected front end clipping.

All in all then highly recommended. The cartridge can be obtained from Videotone for £39.95.

Now I have some further comments on active crossovers. Potentially this technique promises a revolution in the perceived sound quality of speaker systems. However, in order for the technique to be fully exploited a change will be necessary in the general philosophy behind Hi-Fi.

Up to now most Hi-Fi systems that merit the name have been collections of separate amps, tuners, speakers and decks. The actual sound quality provided by these systems depends as much on their compatibility as their individual excellence. The reasoning behind this philosophy is not difficult to find. It does not follow that because a manufacturer makes a superb amplifier that their speakers are going to be as good.

Active speaker systems require a marriage between amplification and speaker systems to an unprecedented degree. Naturally this will not happen without a struggle between the various vested interests involved.

With the advent of transistor amplifiers, speakers systems started to get smaller. This pleased the housewife if nothing else since speakers became less of an eyesore in the lounge. However, in order to maintain good bass extension, efficiency had to be sacrificed. This in turn led to higher power amplifiers to maintain sound levels.

A cosy cartel of interests has thus arisen in which speaker manufacturers make low efficiency (expensive) speakers that are driven by equally high powered (expensive) amplifiers. One of the major contributors to this inefficiency is the passive crossover network. The more parts in this the more the insertion loss. A 6dB drop is normal.

Now 6dB is a ratio of 4:1. This means that a nominal 25W rated drive unit will require 100W when fed from that particular crossover. An active crossover has no insertion loss so only 25W would be required to reach the same sound levels!

Consider the situation from the viewpoint of the speaker manufacturer. Reputable manufacturers spend large amounts of money on developing high quality speaker systems. A lot of this design effort is directed towards the crossover. Indeed one of the arguments used to discourage the experimenter from building his own is that crossover design will be beyond his abilities. Apart from the obvious economic desirability of propagating such a belief it is, unfortunately, partially true.

It is not so much that crossover design is generally difficult, more that the information required is hard to find.

Another point — the checking of a speaker system's sound is done in an anechoic chamber. As such it is rarely found that the speaker's frequency response is flat in the average lounge.

Active speakers on the other hand require relatively little design work. A set of basic formulae and a calculator are all that is required for the job.

So far this situation has led to the inevitable tri-amped systems employing conventional amplifiers and have, in consequence cost the earth. The alternative self-contained models, Meridian M2's for example, offer higher levels of performance because the design effort can be directed towards fully integrating the individual parts rather than trying to manipulate existing designs.

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Working with Video

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Organ Talk

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


Electronics & Music Maker - Oct 1981

Feature by Jeff Macaulay

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> Working with Video

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> Organ Talk

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