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For several years now Hi-Fi enthusiasts have been experimenting with their turntables in the hope of getting a better sound. Turntable mats, record clamps and special headshells have appeared on the market in profusion, each claiming substantial improvements.

Metrosound clamp.

I must admit to being highly sceptical of these claims although some recent experimentation with my own equipment has justified some of them.

Before going on to describe these experiments in detail it would be as well to describe my own deck. Back in the heady days of '74, when the Hi-Fi boom was at its peak, most turntables were made by Garrard.

The only viable alternative to the ubiquitious SP25 was the 401 turntable fitted with a SME arm and V15III cartridge.

Around this time I swapped my SP25 for a BD1 turntable kit and an Acos Lustre arm fitted with a Shure M75ED. The arm is still available, albeit in a modified form, as the Rega arm for round £40. Since then only the cartridge has been changed foran Ortofon VMS20EII. This cartridge is one of the most neutral sounding moving magnet designs available at any price. The rest of the system consists of a home brew pre-amp and bi-amped speakers, the deep bass end being taken care of by an active woofer system. It is, by now, a well known fact that any record deck can be made to act as a microphone.

This can be easily demonstrated by placing the stylus on the stationary turntable and monitoring the output on a 'scope. Providing the 'scope is reasonably sensitive signals will be seen whenever sound is present in the room.

Turntable mats have a dual purpose. One of these is to support the disc. This may sound obvious but with the standard of flatness of modern records, this is no easy task. The other main purpose is to damp out platter resonances and acoustic pickup of the type described above. All turntable mats do both these jobs to a greater or lesser extent. The recent wave of accessory mats claim to do better and positively improve the sound.

The first of these mats of which I have personal experience is made by Griffin and retails for about £6. When tried it certainly improved the treble and to a lesser extent, the mid-range, but at the expense of the bass. The latter disappearing below 100Hz or so. Several other mats have since been tried with varying degrees of success. Although they all provide an improvement on certain aspects of the performance they all seem to do so at the expense of others.

Naturally the differences are in some ways extremely subtle. This applies especially to those which affect the stereo image. It is only possible to judge this properly on those recordings which have not been multi-miked. These albums tend to be something of a rarity these days! Record clamps as well seem to be somewhat problematical. Again several types are available. Most of them have a collet type fitting which is pushed over the spindle on top of the record. A disc of material in turn fits over this and is pressed onto the record and secured by a nut which tightens the collet.

A typical example is the metrosound clamp which costs around £4.50. Like the mats this is claimed to improve the separation between instruments, improve the bass and generally reduce coloration. In fact I found this device did fulfil some of the claims made for it. It seemed to make the instruments better defined and made low level information more intelligible.

However I found that the device tended to lift the record away from the turntable mat rather than bind it more closely.

On the Rega turntable, however, with its more compliant mat the clamp operated as intended. Surprisingly though the difference was not so marked as with the BD1. An improvement none the less.

Completed turntable mat.

Most turntables, if struck with a screwdriver blade across the rim, will ring like a bell. My BD1 was a particularly bad offender in this respect. If a note of the same frequency were to be picked up by the turntable it will react like a mechanical tuned circuit causing a peak in the response. My first experiment was to remove this resonance.

The obvious way to damp out a resonance of this kind is to add mass to the turntable. The most convenient material to use are Bostic car damping panels. These are basically bitumised felt pads and are self adhesive. They can be cut to size and shaped with scissors and are thus convenient to use. Taking the turntable out I stuck a strip of this material around the internal rim of the platter. This treatment lowered the frequency of the note produced and the decay time was reduced. Some squares of material were then added to the underside of the platter and this again reduced the decay time to a few milliseconds. It sounded dead when struck. After refitting the platter and replacing the existing mat some listening tests were tried. The main subjective result was that the music sounded much cleaner at high levels. Tapping the deck with the stylus resting on the turntable showed that the whole assembly was less prone to pickup problems.

All this leads on to this month's do it yourself project. The 'Macaulay' turntable mat! This is fabricated from four 'Bostic' car sound damping panels. As already mentioned these are self adhesive. The sticky side being protected by a paper sheet. Figure 1 shows the completed mat in plan and cross section view.

This material's self damping properties absorbs vibrations in the disc whilst damping platter resonances.

Fig. 1. The turntable mat fabricated from four 'Bostic' car sound-damping panels.

Construction starts by laying a pair of sheets side by side, paper side down. Take an LP record, preferably an old one, and lay it on the panels with the central hole over the join. Mark round the LP with a Stanley knife. Remove the album and cut out the semi-circles with a pair of scissors. Repeat this exercise with another two panels.

At this stage you should have four semi-circular cutouts. Take two of these and butt them together to produce a circle. Now find a circular object of about 4½" diameter. Place this on the centre of the disks, mark round it and cut out the circle. The cutout is required so that the record can lay flat and clears the raised portion in the centre.

Peel the backing paper from the two panels with the 4½" cutouts and press these into place on the other two panels as shown in Figure 1. Don't at this stage remove the backing paper from the other two panels until you have tried the mat out. The prototype improves the bass response and stereo imagery of my BD1 and imparts a dry quality to the sound. Preferably the mat should be used in conjunction with a record clamp to provide intimate contact between the disc and turntable.

As with all things audio the only sensible way to judge the difference is to hear it. If you like it the bottom panels can have their backing paper removed and the mat stuck to the turntable, further improving the damping.

One thing to watch when experimenting with different mats is to readjust the arm height for parallelism in order to keep the vertical tracking angle correct.

Other less obvious factors can affect the performance of record decks. The most common of these is the build-up of oxide on the pickup plugs and the cartridge pins. Even if these are clean they can still give trouble if they are loose fitting. The cure for these ailments are self evident and it does no harm to check and clean all connections periodically. The sound of a cartridge can be modified by adjusting the input impedance and, or, the capacitive loading. Most pre-amps offer an input impedance of 47k. A couple of metres of connecting lead between the cartridge and pre-amp will also produce a capacitive load in parallel with the resistance. Some cartridges, notably Shure and Ortofon models, require a load capacitance of 200-400pF. Often the capacitance of the cable is not sufficient to produce the optimum load. If your connections allow it 500pf trimmers can be soldered between the hot sides of the inputs and ground.

If a record is then played it is possible to adjust the sound to a certain extent to suit one's taste simply by adjusting the trimmers.

These comments only apply to moving magnet designs. Moving coils are low inductance and capacitive loading will have no effect.

The reason that capacitance actually alters the response may not be immediately obvious but it has to do with the internal inductance of moving magnet cartridges. The impedance of the inductance becomes larger with increasing frequency. In the extreme hf the response may go down by several dB. The capacitance forms a tuned circuit lifting the response.



Previous Article in this issue

Radiophonic Workshop

Next article in this issue

Looking at Microphones


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

 

Electronics & Music Maker - Mar 1981

Feature by Jeff Macaulay

Previous article in this issue:

> Radiophonic Workshop

Next article in this issue:

> Looking at Microphones


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