Design aims

The system to be described was built for my own personal use and was to replace a pair of reflex cabinets 9 inches square by 36 inches tall and containing Morel MW166 160mm bass, Bandor 50mm mid-range, and of course HRL-1 tweeter. The upgrade would have to last me several years without further craving. Absolute efficiency was not a key issue as 320W RMS per channel is available to drive them.

Parts outlay was about 800 pounds including postal charges etc..

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Bass issues

I wanted to get rid of a somewhat muddled bass so decided that the new system would be transmission-line based, and that I could afford to increase the depth of the cabinets considerably without impinging on living-room floor space. I decided to build cabinets capable of taking an 8inch unit should the small Morel unit be found to be inadequate in it's air-moving abilities.

19mm MDF was used to construct the 33in x 9.5in x 15in bass cabinets. The line length is just over 7 feet.

Mid/treble issues

This time I would house the mid and treble units in 'satellite' cabinets for convenience and to facilitate further partial upgrades or modifications. As can be seen in the pictures, these were also constructed from 19mm MDF to the minimum achievable frontal area as defined by the drive units' dimensions. The finished size is 13.5in x 7.5in x 7.5in.

The existing speakers suffered from a wonderfully detailed but somewhat recessed midrange and some attention to this was needed.
To upgrade the mid-range I decided to go for the larger 100mm Bandor unit since my previous experience with the tiny 50mm unit had been good and one of the few cone speakers that can do justice to a ribbon treble. I was sure that any problems of 'recessed' midrange that I had with the 50mm units were simply down to insufficient cone area and a somewhat low 83dB quoted sensitivity for the 50mm unit. The 100mm unit would give a quadrupling of cone area and a 3dB efficiency boost to the midrange.

These aluminium-coned units have an excellent transient response and wider-than-average frequency response together with good fine detail retrieval and apparent total lack of any 'nasal' or 'shouting' qualities that so often seem to blight conventional cone speakers. The ribbon tweeter is added as 'icing on the cake' which is how it should be.

Crossovers

I placed the crossovers at 300Hz and 4.5kHz, well within the bounds of all three units, and ensuring that the critical mid-range was well-covered by a single competent unit. The bass/mid section would be mounted in the bass cabinets and the mid/treble section in the satellites such that the satellites could be used as full-range bookshelf speakers if required. To compensate for voice coil inductance of the Bandor unit at the upper crossover frequency, a Zobel network consisting of 10uF and 10 ohms in series were fitted across its terminals. Without this, there would be a nasty peak at the upper crossover. This unit was connected to the crossover in opposite phase to the other two units. 3dB resistive attenuation was used on the ribbon as the quoted efficiencies of the other two units is only 86dB.

Damping

Long fibre wool was used with a density of 0.75lb per cubic foot, added through the removable bottom panel. Strips of ordinary green plastic fencing matrix (holes about 2 inches square) were cut to hold the teased-out wool inside the ducts and prevent settling to the bottom with time.

Images

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Internal Transmission-Line Layout. Note that an oblique panel at rear top is not fitted at this stage. The cabinet bottom is made removable for packing of damping material into the lower sections of pipe. Edges of sides and top/bottom were set slightly proud so that they could be taken down flush later, thus avoiding any need for filler.

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Bare Bass Cabinets Before Sanding.The cabinet bottoms are not fitted at this stage. The proud edges were taken down flush using a car-body plane with 40-grit paper. A mask must be worn when working with MDF dust.

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Painted Cabinets Ready for Packing. Two coats of MDF sealer, lightly sand, wood primer, lightly sand, and finally some one-coat white gloss. The machined recesses on the bottom corners are for some protective aluminium corners to prevent damage when lifting or transporting. These were machined from some 19mm square aluminium bar, to be stuck in place with RS silicone rubber after painting.

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Rear View Showing Connectors. The lower connector is the main amplifier input while the upper is the output above 300Hz to drive the satellite mid/treble. The connector trays are standard gold-plated Monacor 4mm items from Wilmslow.

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300Hz Crossovers Mounted Inside Cabinets. The PCBs were wired with standard 79-strand OFC speaker wire (RS 368-946).

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Bare Satellite Cabinets After Sanding. The holes were cut after glueing together. Efforts to minimise satellite cabinet width have caused the front baffle to become fragmented below the midrange unit. The 100mm Bandor has an unusual pentagonal sub-chassis construction which necessitated some routing above and beyond the normal circular hole.

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4.5kHz Crossovers Mounted Inside Cabinets. Ready for fitting of drive units. A large piece of neoprene sheet (RS 245-9599) was stuck to the bottom of each of the satellites to prevent them skidding around on top of the bass cabinets (or even falling off - heaven forbid as they are VERY heavy for their size). This is very effective since the heavy satellites appear to create a vacuum seal between the two surfaces, making movement or accidental displacement very difficult.

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Crossover Assemblies - 300Hz/12dB on right, 4.5kHz/12dB on left.
The PCB's were made using photo-coated board from RS to a pattern produced on acetate transparency. Note the right-angles between the ferrite-rod inductors (to minimise stray coupling).

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Mid-Range Compensation Networks.The 10uF cap and 10R resistor were added to the Bandor mid-range units with DS tape. This compensates the unit's voice coil inductance which would otherwise cause a nasty peak at the upper x/o frequency.

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Finished System. The Morel bass unit shown during construction was finally ousted in favour of a Peerless 8 inch unit type 831709 from Wilmslow Audio. Bass is very clean and extended beyond my dreams and I have heard no complaints from them with a 2x320W amplifier. Currently I cannot readily see an upgrade path from this system. Perhaps a full active crossover........?