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Hi-Fi


Five years ago the ultimate in pickup cartridges were high compliance, high output devices. In order to track the groove properly, it was necessary to fit these into low mass arms of low friction in both vertical and horizontal planes.

Although the requirement for low friction is unchanged these arms are not capable of extracting the best performance from today's moving coil cartridges which require high mass arms. Unfortunately, most of the arms available at present are an uneasy design compromise between the requirements of both types of cartridges.

This problem was evident recently when I received an MC81 pickup cartridge to review. The MC81 is an excellent example of a moving coil cartridge and possesses all the features one comes to expect from these. The compliance is low, tracking weight is about 2 grms and the output is well below 1mV.

A T100 transformer.

Naturally a step up device is required to match the cartridge to the normal phono input of the amplifier. For my own listening I used a T100 transformer. Transformers have the advantage of introducing no noise but if badly designed they can influence the final sound quality.

Luckily none of these problems beset the T100 which has a bandwidth of 10Hz to 100kHz.

For an initial listening test the MC81 and T100 were used in a Rega 2 deck. It was noticed at the outset that the cartridge produced previously unheard detail from our favourite discs and we were left in no doubt about the abilities of the combination.

All was not 100% though. When the cartridge was mounted in the Rega arm mistracking could be heard on certain records. I hasten to add though that this mistracking was only apparent on the orchestral peaks of the Telarc digital recordings. As these are cut without any compression they represent the ultimate test for any system, not just the cartridge.

On all normal records tracking is very steady and this combination can be heartily recommended. When I took the cartridge home and mounted into my own reference system the cartridge sailed through the same digital recordings without any mistracking evident.

The reason is not hard to find. The pickup arm fitted to my system is the Acos Lustre GST 1. This arm was introduced in the early seventies and received a lot of criticism at the time for its high mass. When it was introduced the high compliance cartridges that were very popular suffered from a low frequency resonance when mounted in it.

All cartridge/arm combinations have a characteristic low frequency resonance because of the cartridge compliance and the effective arm mass. If the compliance and arm mass is high this resonance occurs below 10Hz and is excited by record warps. As a result the arm oscillates colouring the sound. If the resonance is too high in frequency the lower bass is accentuated also leading to colouration.

With the current crop of MC cartridges the general rule seems to be the higher the arm mass the better. If you are already using such a cartridge it would seem prudent to attach any extra weights that came with the arm, rebalance and try the sound again. Often the addition of a few grammes in the right place works wonders.

Having suffered this rather lengthy preamble the reader will be pleased to know that I am about to get down to the nitty gritty!

The MC81



The MC81 cartridge.

This cartridge will set you back £49.95. Add a T100 at £24.95 and you have spent £75.

At this kind of price level the value for money aspect must be considered as well as the absolute performance of the device. There are basically two points of view on this. If you are already an enthusiast with a large record collection it makes economic sense to protect your investment by using a high quality cartridge. A good part of my record collection is almost unplayable due to being played on a 'Dansette', back in the early sixties!

The other point of view is that any cartridge that costs more than say £50 must be hewn from solid gold to persuade people to part with their money.

I must admit some sympathy for those who hold the latter view but one tends to find that they have, as a rule not heard what such a cartridge can do for their listening pleasure. The MC81 is a case in point. It performs as well as if not better than most of the (more) expensive cartridges that I've heard. The sound balance is absolutely neutral, instruments and voices are well separated. The image has both depth and stability. What's more impressive though is the way the cartridge never seems to lose control and presents subtle detail even in the presence of more powerful material. In short if you're in the market for a cartridge in this price range give it a listen!

Both the MC81 and the T100 are obtainable from Videotone Ltd.

One of the more interesting aspects of audio is the way the old adage 'There's nothing new under the sun' rings true! Nearly all the 'new' ideas turn out to be rehashes of older designs. An example is the rash of super A amplifiers that have arrived this season from the orient. For those who never read a Hi-Fi mag a word of explanation will be in order. Most power amplifiers operate in class B. This means that under quiescent, idling conditions they draw very little current. Overall feedback ensures that the output acts like a voltage source and provides all the speakers drive current as and when required.

Unfortunately, to do this a pair of complementary transistors are required. One transistor handles the positive going half of the signal whilst the negative going portion is provided by the other. Because it is impossible to find transistors that are perfectly matched some distortion is inevitably created. This distortion is known as crossover distortion. Although the overall feedback reduces this to low levels it can usually be heard and is worse on low level signals.

The obvious way to avoid this distortion is to increase the quiescent current in the output stage and biasing it into class A. When this is done the use of large heatsinks and power supplies become necessary and in simple terms this means a more expensive and inconvenient product.

The Super A amplifiers utilise a different method of eliminating cross-over distortion. Essentially the idea is to raise the quiescent current in sympathy with the incoming signal. Thus the output stage will always operate in class A but with an efficiency closer to that of a class B.

Again this idea is far from new. In the late fifties and early sixties Mullard published several simple designs of this type. These circuits were far from Hi-Fi but they show, once again, what purports to be an innovation is in fact just a development.

Whilst on the subject of developments I promised to explain a bit more about active crossover systems. Anyone who reads this column regularly will know most of the advantages by now and it is probably about time to get down to specifics. In all speaker systems the most fundamental decisions are the crossover frequency chosen and the filter slope used. The frequency is chosen in precisely the same manner as a passive system. (See September issue).

The filter slope is more a matter of personal preference although if the ranges of the speakers overlap by an octave or so then the second order type is as good as any.

One thing to ensure is that the Q of the filter is 0.7 since this will ensure that the response will not suffer from 'ripples' in the pass band. That settled the filters can be simple Sallen and Key types built around an op-amp or two. Two matching filters for example are required for a two way system. Figure 1 shows the general layout and formulae for determining the component values. A good choice for 'R' is between 22 and 100k. It will not take long with a calculator to determine the other values.

Figure 1. Matching filters for a two way system.


Finally, a note on the op-amps used. The 741 suffers from slew rate problems. A glance through the Maplin catalogue will reveal several suitable alternatives.



Previous Article in this issue

Micromusic

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Starting Point


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

 

Electronics & Music Maker - Nov 1981

Feature by Jeff Macaulay

Previous article in this issue:

> Micromusic

Next article in this issue:

> Starting Point


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