Active LS3/5 Clone - A HiFi Odyssey

All information and photographs supplied by Alan Cohen on Thursday 5 May 2005

Active LS3 - 5 Clone

A Hi-Fi Odyssey

I hope that some of you will be interested in this project, but first, some history.

I grew up with music and Hi Fi. One of my earliest recollections is of “fixing” my record player at the tender age of 6. My grandmother, babysitting at the time, was fortunately able to disconnect me from the live mains cable - I exhibited my wounds with great pride, and bear the scars to this day.

Dad (a schoolteacher, mad about music & especially opera, golden age of singing & all that) was never able to afford decent equipment, so DIY was the only answer, and it's much more fun than going to the shop. Other members of the family shared his passions, so Dad (a capable cabinetmaker) built the boxes, a cousin the electronics, and an uncle sourced the speaker drivers. We graduated from shellac 78's in the late 50's (the advent of LP's) to a good system for the time - Garrard 301 deck, Pickering Fluxvalve cartridge in an arm that weighed a ton, 20 Watt Mullard valve amplifier, with genuine Partridge Ultralinear output transformer, and a 15 cubic foot reflex speaker cabinet with 15” Wharfedale woofer and a Vitavox pressure horn midrange/tweeter (probably similar to the unit fitted to the Klipschorn at the time). Many friends visited for musical evenings and agreed that the quality was excellent, lots of deep clean bass, and enough power to entertain the neighbours 2 streets away.

In the late 60's I started on my own quest for perfect sound. The first transistor amplifier that I built (the 4 Watt design with germanium transistors from Hi Fi News) sounded TERRIBLE. Fortunately for all of us the first Linsley - Hood Class A design was published in Wireless World in 1968, and that changed all our lives for ever. It's still in use, albeit with a more modern regulated power supply - more than adequate to drive a Lowther horn, and sounds pretty good, even by today's standards. It was followed by quite a number of L-H 75 watt amplifiers; they sounded even better, and I'm still using them today. The local kits were so popular that they remained on the market until 1990. No exotic cables in those days, not at AF anyway. I'm an engineer - if you can't measure it, it isn't there, and I'm still not a believer. Hi Fi is all about smoke & mirrors anyway!

In the early 1970's Dad visited the UK, his first of many journeys outside of South Africa (he was then in his late 50's), and ordered the hardware to build a pair of Bailey t/lines (from Wilmslow, they were around in those days). The kits consisted of Radford (Goodmans) bass & midranges, Peerless tweeters & Radford crossovers. As it transpired, the container was damaged on the high seas, the parcel got wet, and only the crossovers & tweeters could be salvaged. We replaced the Radfords with KEF B139's, B110 SP1003's, and old type 6535 T27's (the horn loaded ones with the plastic face plates), sourced from Hi Fi Installations, the local KEF agent. We built up the ENORMOUS boxes, used the Radford crossovers, stuffed them with wool, and that kept him very happy until his death 30 years later. He also brought back an original 75 W L-H kit (the first that I built) for my amusement, it also served him well for many years.

I spent the next 30 years messing around mainly with horns (I own a few PM6's), and had a lot of enjoyment in the process. We listened to a great deal of music, and learned to live with the inevitable coloration of a single driver.

I inherited the KEF drivers and Radford crossovers (no place for the boxes), and thought that it might be fun to use the B110's & T27's to build a pair of LS3/5a clones - something totally different from the horns that I had tinkered with for so many years. Being an engineer, I was not satisfied with a copy, so introduced a few variations of my own. Others have done the same.

I decided to keep the box dimensions and driver orientation pretty much as original. The damped thin wall construction didn't appeal to me, it is an elegant design, but the characteristics of the materials must be known - impossible without instrumentation. Why not go for something "simple", as inert as possible? Lots of high end speakers are built this way (some more convincingly than others). I didn't like the idea of mounting the B110's on a compliant gasket to damp the well known ringing of the chassis, all my practical experience (based on solving vibration problems in very large machines, expensive bearings!) is that if it cannot be eliminated at source, use a massive structure to sink it. Pickups work the same way.

The following design principles were therefore used:
The B110's were damped by gluing a couple of layers of bitumen vehicle damping pad strategically to the baskets - obviously small pieces. I thought of stiffening the baskets with a polymer composite, say epoxy putty, but it doesn't seem necessary, and might have made it worse.
The boxes are 18mm solid syringa (violins are made of this), with dovetailed construction, battened and screwed in all corners;
All internal surfaces are lined and damped with a mixture of 6mm crushed stone and compliant polyurethane vehicle underbody sealant, which adds more than a kilogram to the mass of the boxes;
The cabinets are braced internally front to back, side to side, between the drivers (overkill?);
Baffles are glued & screwed in place;
Drivers are fitted from the front, rebated & flush mounted, screwed down, and sealed with silicone;
The interior stuffed with fiberglass;
12mm foam draught strip was applied around the tweeter (instead of the felt).

The photos give a good idea of the construction.

The boxes are pretty heavy, 7kg each, tap any of the panels and they sound dead - like concrete. But I don't have an accelerometer. There is no audible coloration from that source. Initially, as I am too lazy to wind inductors, I used the Radford crossovers - only the high pass tweeter and the low pass mid sections. The little speakers sounded very good, even driven by the 10W L-H, excellent imaging, and no coloration from the B110, but the tweeter resonance was clearly audible, especially with voice. They were obviously short of bass (remember the 15” Wharfedales in 15 cu ft reflex cabinets?), but a good starting point.

As I had the B139's, I thought of mounting them in closed boxes, a la Linkwitz. However, I also happened to have a pair of old (70's or early 80's vintage) 300 mm Celestion woofers with doped paper cones, 22Hz f3, ideal for closed boxes, and a pair of 100 liter boxes from a previous project. The measured parameters indicated that in a 100 litre closed box the Qt would be close to 0,7, i.e. a maximally flat alignment, so I tried them, and it worked out well.

Initially I used the full 3 way Radford crossover, and the system now sounded VERY good, with substantial bass down below 20Hz, 10Hz rattled the windows, and especially clean transients, so the bass alignment is good. f3 is at around 38 Hz and bass correction would be overkill - after all, the room does have boundaries! Very little coloration can be detected with sine waves, pink noise or white noise.

The intention was always to go active, and I already had the 6 amps and B4 phase linear crossovers. Out with the soldering iron! The -3dB points are set at 100 Hz and 3kHz. The B110 is unstressed and with the 4th order roll off the T27 resonance is now almost inaudible. One of the photos shows the electronics for one channel; the crossover is in the 3-way power amp. The box on the left is the regulated power supply.

After all this, what is the result?

The bass is impressive. Compared with the horns I was used to the system is very clean, detailed and uncolored, with excellent imaging and lots of detail, particularly noticeable with full orchestra but also chamber music and voice. A revelation. I've got the levels of the drivers more or less right - 1 or 2 dB is actually very critical, and final tuning has to be by ear, in the listening room. The system sounds OK to myself and others, the music sounds good, and that's the object of the exercise. Not comparable to a concert hall, we're still far from the real thing. I'll probably try out the B139's one day, also in a closed box, +-60 litres to get a similar alignment. I'm not in a hurry though, it's working well, so why fix it?

I'm still trying to find the best positions for the clones and subwoofers. My lounge is awkward - almost L shaped, with 2 doors near to the speaker positions, which obviously affects the bass. At present the subs are midway between the main speakers, which are mounted on heavy well-damped stands. The bottom end is not very well integrated, so I'll probably go back to the arrangement shown in the photos, with the main speakers on top of the subs (which must be kept out of the corners).

What have I learned? Good engineering doesn't age. The T-L's sounded very good in the 1970's, and the KEF units sound better in the new system. It's no longer possible to do an A/B comparison, but I think that the bass is a lot cleaner, as is the rest of the system. That doesn't mean that it can't be improved on. Of course more modern drivers will have advantages over these vintage units, but the law of diminishing returns is still valid. Speaker technology is no longer new, so it becomes more and more difficult, and expensive, to make progress.

I hope that you found this odyssey to be interesting, I enjoyed writing it, although at great length. At least I now have a record of the project.

Alan tells us that the rest of his system consists of:

"Thorens TD160 Mk 2 with Grace arm and Stanton 881S cartridge.
Marantz CD18 Mk 2 - vintage 2002, one of the best Marantz players & built like a tank.
Arcam delta preamp
Elektor 1987 3-way phase linear active crossovers, 4th order Butterworth, turnover frequencies 100Hz & 3kHz
Linsley Hood 75W power amps (3 per channel), downrated to 50W into 8 ohms, with L-H regulated power supplies."

He also reports:

"I still enjoy many of my LP's. The TD160 has never required even a drive belt replacement. The Grace is remarkably versatile - I've used it with Ortofon M/C's, Shure V15's, an AKG, & Stanton 681 SSS/881S/981 M/M's, it seems to be compatible with almost any pickup. The Stantons are all calibrated and reasonably high compliance, almost reference standard, if they were M/C's they would probably sell at 10x the price, & very robust. Replacement styli are still available for the 681 SSS & 881 S. The Arcam delta preamp I bought 2nd hand, in a hurry when most of my equipment was "liberated" a few years ago, ie burgled. I like it very much. No tone controls or filters, it has remote, & inputs for MC and MM pickups. It drives the active filters and power amps via 10m of microphone cable without any problems. It also has a digital input & D/A converter that I've tried, not nearly as good as the Marantz, they are generations apart."