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My System (Peter Empson)

Before I painted them it was necessary to file the start of the cells at the throat so that they were as sharp as possible to prevent reflection back to the driver and to ease the air flow into the horn.

A slight blemish at the mouth was tidied using automotive filler and then both horns were sprayed with several coats of Aluminium primer and a few more of matt black paint.

My original plan was to purchase some cheap Monacor compression drivers but following a discussion with Kevin we decided that they wouldn’t really be suitable. Kevin suggested he may be able to source a used pair of S3’s from a pro supplier for a reasonable price. These were tatty but affordable (and apparently had new diaphragms) so I bit the bullet. Once installed they sounded very nice but quite shut in and I decided that I would have to look at a very wide range tweeter or go 4 way to get the results I wanted.

Above: S3 fitted to painted RH330

Fate intervened after I realised that one of the units was producing a slight vibration on certain notes. It turns out that this was being caused by an off centre diaphragm rubbing. This is apparently a common fault that can occur when diaphragms are slightly misaligned, and is hardly surprising given the very fine tolerances required.

Rather than have them rebuilt Kevin suggested that he had a new pair of S5’s and would take the S3’s back on a trade in.
I agreed and handed over yet more of my hard earned, but the second I connected them (even before they were anywhere near run in) I was thrilled to hear the sound I was after. They were more sensitive, had much better treble extension (I can almost use them without a tweeter) and sounded dramatically more detailed.

Pictures above and below show the S5 close up

To add a little sparkle I am currently using a pair of Sound lab slot tweeters (specs below) which are rather crude but they will have to do until I can afford to buy some JBL 2405’s. As these tweeters were very cheap and not terribly satisfying I started modifying them. I’ve removed the Ferrofluid (I was never going to need its power handling benefits anyway!) and I now have more extended highs and much greater efficiency. However I do believe that some of the drivers resonance's are now more pronounced but fine tuning of the crossover may be able to improve the integration.

Bass Horns

Not long after buying the S5’s I opened a Hi-fi magazine to be greeted by yet another temptation. Kevin had a pair of refurbished AK157 drive units in his advert. I couldn’t resist them. This was starting to get expensive!
As I don’t have any Thiele Small parameters I measured them by using the procedure detailed here however I’m not sure my figures are that accurate so haven’t published them on the site. I subsequently discovered Speaker Workshop which is what I’d use if I were doing it again.

I originally wanted to try to build a bass horn from Fibreglass or Carbon Fibre in order to keep them light enough to move and allow the horn path to follow the smoothest curve, however in the end I decided to try to use curved MDF to save time.

Sadly this was never to be. To save space I had tried to bend the MDF as much as it would take but when put into practice it became clear that this wasn’t going to work as the immense forces required to hold the tight radiuses were twisting the whole structure. Shortly after the picture below was taken the prototype was scrapped. Shame, as I really wanted to hear how these could sound...

Below inside mouth of curved horn

With only a few days of workshop time/holiday left I had to resort to using a much more conventional design. As a lot of 18mm MDF was left over I decided to use it.

I kept the same expansion as on the curved design above but saved an awful lot of space by being able to fold it much more tightly. I would have preferred to use the exact Thunderbolt design but the depth of them would have blocked a passageway!

The design is nothing daring, the folds are still inspired by the Thunderbolt/Air partner and uses a similar non-bifurcated Z shape profile.

I used a horn constant of 0.6 as recommended in the Edgar articles and the flare frequency frequency was set at 47 Hz (one eighth sized) for no reason other than it seemed to fit the mouth size I wanted. For the main calculations I used the excellent calculator on the Melhuish site here.
The horn is not foreshortened at all as I didn’t want to compromise too much with my first design. The length worked out to just under 2 metres. To try to maintain the highest upper roll off possible bends were kept to a minimum as I wanted them to mate with the 500 Hz roll off of the mid horn.

The area behind the driver has been left open at the front exactly as done in the Air partner. I would have preferred to have had the option to seal the back but as the speakers are used in the nearfield and the mid horns sit on top I wanted to make them as short as possible so they were close to being at ear level, even though this compromised the area available behind the driver somewhat.
I have retained the option to be able to add a rear chamber at a later date if required as the bottom simply unscrews and an extension could be fitted.

They are pretty big at 110 cm tall by 63.5 wide and 63 deep and are enormously heavy even without the drivers, so getting them up a flight of stairs was ‘entertaining’.

In the plain MDF above they look a little unsightly and one day they will be veneered, although I may experiment with some internal and external bracing before ‘finishing’ them. The hidden early part of the horn is varnished to try to harden up the MDF thus lowering its absorption of upper frequencies.

I’m very pleased with their sound which is very fast and dynamic with reasonable bass extension considering they are not really in proper corners at the moment. The big panels do lead to some colouration at times but it is not really intrusive.
I am very happy with their upper response which was my main aim and the integration with the mid horn is good as a result, although I am still experimenting with crossovers and may still try a ‘phase plug’ as used in the Thunderbolt.

If I were building them again I would probably change the first reflector as I was never very happy with it, I would also use good Plywood too as I prefer it for horn speakers, based on my experiments with cabinets for Lowther drive units in the past.

A box for the mid horns has been cut but not assembled yet and I will follow Kevin’s advice and damp the RH330 with expanding PU foam when I finally get around to putting it together.

Rest of my system as it stands at the moment: Manticore Mantra turntable, Manticore Musician tonearm and Denon DL304 cartridge running into Ortofon T3000 silver wired step up transformer. Digital comes from the amazing Audio Note DAC2.1 Balanced via a variety of transports.
All cables are home built unscreened (except the ones on the turntable) and are made from solid core pure silver with cotton insulation and Eichmann Bullet plugs (some gold, some silver).

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