This post describes the design and construction of a 3-phase loudspeaker intended for Leslie Speaker emulation. The unit is intended to be driven by a 3-channel audio amplifier The Brute, which in turn is controlled by LEMS, a micro-controller based control system, the construction of which both are described in companion posts.
Companion posts are as follows:
Updated 29th January 2014.
You may also be interested in this: MIDIGEN – A (belated) LEMS postscript
Well, I once knew a girl with three heads.. No, you wouldn’t swallow that. But the name I’ve adopted is apparently quite common in the USA as a girl’s name. In fact it isn’t hard and you’ve probably already guessed that’s it’s a conflation of tres - meaning 3 (in spanish etc.) , and lie, the last part of Leslie. Whimsy? Probably, but I needed to call it something.
I’ve lifted a small part of text from the LEMS post which I hope clarifies what is being attempted, and this is given below:
Each phase is fed with the modulated audio signal so:
Note that the phase diagram shows the amplitude of the signals, not the actual signal itself, so that the lowest point on each waveform is zero output, with the highest full ouput.
The net (rough) result is that an audio signal comprising the averaged power input into the 3 speakers appears to have it’s point source rotated through 360 degrees. The speed of the rotation will depend on the frequency with which the signal is modulated.
From the above, I decided that the top and bottom of the loudspeaker system, should resemble a ’squashed’ hexagon thus:
with a loudspeaker facing out on each ‘long’ side.
I made the long sides ~12 inches (300mm), and the short sides ~6 inches (150mm). This gives an external angle of 120 degrees between the polar plane of each loudspeaker chassis. With an external height (on my cabinets) of around 27.5 inches (698mm), this gives a total volume of around 2.5 cu ft. Internal volume will be a little less than this, and of course each loudspeaker has only 1/3 of the volume, because of the 3 central partitions.
Each compartment should be made as airtight as possible – mainly due to the use of high-compliance (long-throw) loudspeaker units. These give good bass response in small cabinets, but require near-sealed enclosures, if damaging over-excursion of the loudspeaker cone is to be avoided. I found that screwed joints and the liberal use of a bathroom sealant that remains plastic when dry is the best solution. I advise against the use of glue. Access for maintenance would necessitate damaging a glued cabinet. With the sealant used, panels are easily disassembled, and the sealant can be simply replaced.
As this unit was intended to be experimental, and for use mainly at home with a small 70’s organ, and my guitars, I decided to cater for no more than about 30watts per channel, this keeps costs and size down, and allowed for the purchase of 3 reasonably-priced high-compliance units from my local stockist.
The loudspeakers are complimented with 3 cheap piezo horn units. Information on these suggested the use of series connected 47ohm resistors – I ignored this for the input powers I was using, and connected these up without the series resistor, to no ill effect.
Despite the loudspeaker units being cheap, it is worthwhile incorporating a suitable fuse in series. I used a 20mm, 1.6amp quick-blow item, that will protect the loudspeaker against gross DC flow. Too many loudspeaker designs omit this simple protection, with dire results in the case of accidents. The screw-in fuse-holders were sited on a small panel alongside the loudspeaker 4mm connector sockets.
The sides were all made from reclaimed chipboard, mostly melamine-covered, of the type used for kitchen cabinet carcasses and cheap 70’s fitted wardrobes. The top and bottom were made from .. .. new chipboard, and the loudspeaker grilles from … new MDF.
Internal bracing was all new wood – mostly offcuts and scraps 3/4 X 3/4 inch (20mm X 20mm).
The sound absorbent material filling each speaker compartment has one important criteria – it should contain bubbles. Bubbles are just great at absorbing (converting to heat) the sound energy that would otherwise bounce around the hard reflective inner surfaces of the cabinet. Don’t let merchants sell you grossly over-priced ‘wadding’. My experiments have shown good results with: a) The remains of old hollow-fill pillows or cushions; b) Bubble-wrap; and c) Expanded polystyrene. All of these can be hoarded until needed – the latter two are usually used as packing materials for stuff delivered to your door.
Photographs of construction and the completed unit.
Unlike previous posts, I’ve confined most of the photos taken during construction to an album on my photo gallery site. If you are considering building something similar, a glance through these should clarify my remarks in this post.
I’ve embedded a Shockwave viewer to my photo collection for the loudspeaker system below, or you can go direct to the site here: http://cullercoats.joebrown.org.uk/#37
I started by marking out and cutting the top and bottom hexagons, before rebating these on each edge with a small high-speed router. I next cut a loudspeaker-bearing panel, marked and cut out apertures for the loudspeaker and tweeter, and also 2 small side panels to size, finishing the long edges with a 30 degree bevel. These were then offered up to the top and bottom and screwed in place using slim screws from top and bottom into the panel ends. (see photos)
The loudspeaker units were mounted, wired and tested, before bracing was added and the next loudspeaker panel fitted. The rest of the panels were cut and fitted loosely with screws, ensuring everything was a good fit before removing and adding the 3 central dividers. These should have a double-sided bevel (making a 60 deg. point) on one side where they meet in the middle. Then everything is reassembled, sealing with flexible bathroom sealant. Each compartment should have some acoustic treatment (see Materials) and wired up before finally sealing. Note that I used no glue whatsoever. The cabinet is strong enough without glue, which would make eventual maintenance a nightmare.
The sharpish corners of the unit were trimmed and sanded, and a coat of paint given to each panel, followed by polyurethane varnish, before iron-on strips were cut to size and fitted up each panel corner. Masking tape was used to protect the previous paint as a contrasting colour was painted on the corner trims.
The top was similarly painted, handles fitted, and 3 wheels fitted to the base. The three grilles were marked out. I used paper templates and spray-on adhesive to stick these to the MDF before cutting on a small scrollsaw. After sanding the grilles were painted and varnished, fixing holes drilled and back-cloths fitted before fixing each grille in place on the unit. Finally,I fitted Aluminium angle around both top and bottom edges, to protect these from bumps and bangs.
Always an interesting exercise for me, having the artistic ability of a broken toy robot with bad batteries, but because of using scrap wood, the cabinet as built, although functional, is distinctly unsightly. After less than a microsecond of thought, I decided to carry on the brutalistic theme I had adopted with the amplifier and deck the cabinet in cheap and cheerful 60’s-type colours. A foray around local shops revealed very little choice and I ended up in B & Q, where I pulled 3 small cans of paint from the promising Crown ‘fashion walls’ ® range as follows: ‘double-denim’ – a blue; ‘eveningwear’ – a purple; and ‘celebrity’ – an orange.
After painting around the loudspeaker apertures with matt-black I finished 2 sides of the hexagon (adjacent fat and thin) in each colour, and left these to dry. Next morning I discovered that these shades had finished a little darker than the label on the tins suggested, despite being painted directly onto the highly-reflective melamine finish of the cabinet walls. Undaunted, I applied another coat, assuming this would ‘lighten’ it up a little – but no. Progressive coats only seemed to darken the result even more. Even the orange had a subdued, somewhat sombre, hue. The blue and purple promised floods of tears just walking in the room and catching sight of them.
I had intended providing 3 loudspeaker grilles, painted in a muted gold, but on fitting the 1st, decided that things needed brightening up, so finished the other two grilles in different colours: one of them silver, the other in a mixture of blue and silver paint. Also I varied the design of the other two grilles, so that no face of the loudspeaker is the same as another. Reflecting on the ad-hoc method of construction, and cheap materials used, I decided that the unit merited being embellished with the logo of the famous ‘Hackit & Bodge’ company, and a version of this adorns each speaker grille.
The top surface of the unit was covered in matt-black metal paint, then finished off with a sprayed-on black enamel. Each of the 6 plastic corner strips was painted to contrast with the panels either side of it, and the top and bottom edges of the cabinet were completed in 22mm X 22mm unpainted aluminium angle – cut and bent to size/shape in one piece.
Finally I added 2 swivel wheels just under the ‘front’ edge of the cabinet (phase 1) and a single wheel at the centre of the small side immediately opposite. Two heavy duty sprung-loaded handles were fitted to the top of the cabinet, making moving the cabinet easier to wheel/lug around.
Most surfaces were finally finished off with a coat or two of quick-drying polyurethane varnish.
Despite my efforts, the cabinet still has that ‘heavy’ look – something I was accused of being in the ’60’s. Also, the attempts to brighten things up, had made finishing off necessitate the use of fine brushes and masking tape.
A warning. If you have toyed with the idea of using the Crown ‘fashion-walls’ range of paints in your home, my advice is to cover no more that the surface area of a coaster with any of these colours, otherwise you are tempting an attack of depression in either yourself, or in your significant other.
The connector panel was made from plain thin steel sheet, painted grey, and I’ve supplied a suitable FrontDesigner project panel/drill guide here: http://joebrown.org.uk/images/ThreePhase/TreslieConnectionLabel.FPL. A high definition export of this is here: http://joebrown.org.uk/images/ThreePhase/TRESLIE_Connector_panel.JPG
The Crown ‘fashion-walls’®. paints I used, plus the Crown gold metallic paint, are available at B & Q. (here in the UK)
The black paint was obtained from Wickes, as was the 4ft X 2ft MDF, and 4ft X 2ft chipboard.
The high compliance 8inch loudspeaker chassis were purchased at my local friendly electronics store ESR and are designated: Part No. 203-031, Description 8 Ohm 25W Loudspeaker 204mmØ, (8 inch) and are currently available at £8.95 (GBP) plus VAT.
The piezoelectric units are also available from ESR, designated: Part No. 200-681, Description: Piezo Horn Tweeter 2″ x 5″, and these are priced at £1.71 (GBP) plus VAT.
ESR can also supply aluminium angle, and the heavy-duty handles I used, together with 4mm sockets, fuse-holders, 1.6amp 20mm fuses, and high-quality speaker cable.
The small swivel wheels I used are available from Wilkinsons, here in the UK, who can also supply both the Aluminium silver, and the spray-on black enamel paints I used.