Please note: The copyright of this article belongs to the BBC, and while I believe that the inclusion here constitutes fair usage given the historical value, please be aware that it may be removed at any time.
LOUDSPEAKERS
New amplifier for the LS5/8
Graham Whitehead reassures us that the L55/8 has not been pensioned off; it is back with a brand new drive amplifier.
For many years the LS5/8 has been the mainstay of BBC Grade 1 monitoring. There are some eighteen hundred in service, representing a capital investment in excess of a million and a half pounds.
Recently, two factors had combined to give users and project engineers the impression that they were no longer available: (i) they had been withdrawn as a stock item from the Central Stores catalogue, and (ii) Quad had ceased to manufacture the domestic amplifier which provides the power. However, contrary to that impression, the LS5/8 is very much available, is even better than before, and is in a form which could bring a new lease of life to older or problematic monitoring areas.
Two years ago, demand for the LS5/8 had fallen to such a low level that it was uneconomical to keep them in stores as an off-the-shelf item; they were demoted to being available to order only. By this means, price and delivery could be made to reflect demand accurately, a situation which carried on right up to the recent closure of Central Stores and Supplylink. But at no stage were they "not available" or "no longer manufactured".
Then, in late 1991, Quad announced that they were ceasing new supply - with effect from January 1992 - of the 405 amplifier which provides the core of the AM8/16 used with the LS5/8. This meant that, unless a replacement could be found, future orders for the LS5/8 could not be met.
(Existing users should have little to worry about, owing to Quad's policy of maintaining discontinued products for up to 25 years after withdrawal. Indeed, if you find yourself with a faulty AM8/16 output card, call Quad Service on 0480-52561 and they will be pleased to help.)
So came the difficult task of finding a worthy successor to the AM8/16 for future orders. Over the years, many new loudspeakers and amplifiers have been brought to the attention of the Loudspeaker Liaison Committee, and auditioned for possible use. Likewise, the Committee has been advised over the years of any shortcomings in our existing range of units. If available at the time, a commercial product has been substituted; otherwise, we have adapted our equipment to meet the customer's requirements.
One comment passed on to the Committee several times concerned the bass performance of the LS5/8 under some circumstances. These were most common from Radio Group 2 users who found the bass characteristics inadequate when dealing with bass drum and the like. Indeed Maida Vale engineers experimented in 1987 with modified Quad amplifiers, and later with Yamaha PC2002s, in an effort to address this. The results were promising but did not at the time provide the answer; partly because nobody had clearly defined the question!
Thus, a design brief for a revamped LS5/8 was drawn up by the Committee and included the following points:
- The sound balance from the new amplifier/loudspeaker combination should be the same as that of the existing LS5/8.
- Ideally the new amplifier should be more powerful than the Quad 405 (100 watts per channel), but not so much that the loudspeaker drive units became unreliable.
- It was desirable that only the amplifier be changed. Furthermore, it should be possible to retro fit the new amplifier to an existing LS5/8 system.
- It should have sufficient space within the box for the LS5/8 Active Crossover card to be fitted.
- The AM8/16 has a slight rolloff at the top end, due to the input transformer used. The replacement should not do this.
- The AM8/16 was considerably more expensive than the Quad 405, because of the extensive modifications required to the standard part. The replacement therefore should preferably be from a small British company which was willing to produce a modified version of a standard product, at little extra cost.
- No cooling fan should be fitted, as the amplifier is likely to be sited close to the loudspeakers.
- Build quality and safety should be to a suitably high standard.
- Any product should be available with the minimum of BBC development effort, as there was no specific budget line to support large investments of this type.
Needless to say, a replacement for the AM8/16 had to be available within a few weeks, as customer requests were already coming in.
Introducing The AM8/20
As already mentioned, many amplifiers have been auditioned over the years, both domestic and professional. The tendency for professional units is slanted towards the "sound reinforcement" industry, and hence fan cooling is common. Large areas of the domestic market are dominated by products which are either not gutsy enough for long-term studio use, or very esoteric and hence too expensive for our purpose.
The LS5/8 uses an RD-designed active crossover card, built into the amplifier. Initially, amplifier candidates were auditioned using a pair of more conventional loudspeakers. These have 500W power handling, 4 ohm impedance, and a very inductive crossover; so any amplifier able to handle them should be able to take the LS5/8 in its stride!
While some amplifiers showed distress when trying to drive these beasts, most met the bill satisfactorily. But one met it better - displaying superb dynamics and generating Sound Pressure Level (SPL) peaks usually only encountered from very large and heavy sound reinforcement amplifiers. All from a unit little larger than the Quad 405, it was the Chord SPM 800 amplifier which impressed us so much.
Chord Electronics is a small concern in Maidstone that makes a range of "high end" domestic amplifiers. The SPM 800 is the smallest of these, offering 160 watts per channel into an 8 ohm resistive load, but with a total dynamic reserve of almost 1kW for driving reactive and/or low impedance loads. This is due to an innovative switch-mode power supply design, and the amplifier configuration.
Two SPM 800s were each fitted with an LS5/8 active crossover for audition purposes. Outputs to the loudspeakers were via Neutrik Speakon, and similar connectors were added to the LS5/8. We expected a benefit in bass definition and overall transient response but gained a lot more, observing subtleties of definition, imaging, effortlessness, and greater control, while the balance remained the same.
The new amplifier was subsequently coded AM8/20 and licensed to Chord Electronics. Field trials of these prototypes in TC7 and MV1 evoked a similar response to that observed at Avenue House. The revamped LS5/8 displayed a marked improvement in bass control and definition over its predecessor: kick drum was sharpened up, even in the presence of synth or double bass; dynamics were dramatically improved, and the extreme top was smoother, as was the middle. According to Mike Lucock from MV1; "There has always been an edginess; a wiry quality to the upper strings and a lack of warmth to the sound. On loud, complex sounds, the speakers can sound very hard. Whilst some of the subjective effects can be attributed to the acoustics of the cubicle, these new amps have definitely made a difference. The upper strings sound smoother and there is more openness in the sound. The amps seem to handle the transient responses more cleanly. This benefits the lower strings as well. Please can they stay?" It rather looks as if the LS5/8 has suddenly come up to date!
This is not the forum to enter the "great amplifier sound debate". We could make comparative measurements of dynamic power, slew rate, TIM, output impedance, etc and find several perfectly good reasons why the new amp sounds better, but that investigation would not be cheap. That it does sound better is what is important.
Production versions of the AM8/20 have the hf channel current limited to around 2 Amps, to protect the tweeter, and power-up delay is ten seconds to allow the desk outputs to settle. The inputs are the same as on the AM8/16; balanced P0316 or XLR3. A 1.5 metre double-ended Speakon lead is supplied with each amplifier, and we would recommend that a Speakon connector is also fitted to the loudspeaker. That is the only modification to the loudspeaker proper, although we further recommend referring the LS5/8 to us at Avenue House for a Quality Control check while the connector mod is being done; are the drive units tired, are all screws tight, are the handles airtight, etc? Naturally, all new units undergo these Quality Control checks as a matter of routine. The AM8/20 - at just over £1000 each - is a bit more expensive than was the AM8/16 (£700) but represents much better value.
While the prototypes are still undergoing field trial, two pairs of new LS5/8 systems are already in full service. The photograph on page 5 shows one of the systems finished in black for a post-production suite in Manchester.
The AM8/20 and LS5/8 are now available to order from Development Group at Avenue House. For more information, or any other query about Grade 1 monitoring, ring the Hotline on Avenue House (036) 2500, or contact:
Graham Whitehead Development Group
Tel: Avenue House (036) 4273