Some Components of the Tannoy Monitor HPD 385
- Including some details on Re-Coning -

 

 Left:
The HF horn development through the magnet pole piece for the LF cone. The gap for the LF voice coil is easily seen.

 

Below:
Typical roll-surround failure. On this occassion after 20 years service.

 

 

Left:
Rear of the HPD cone, showing the so-called "Girdacoustic" ribs designed to reduce the effects of cone break-up in the mid-range.

Below:
The input connector to the speaker (as seen from the rear) shows somewhat flimsy construction, and minimal-section wire.

 

Left:
Inside the cast aluminium Tannoy chassis with the LF cone removed, showing the protuding HF horn. The remants of the old LF-cone surround have not been cleaned from the chasiss yet.

 

Below:
Mouth of the HF horn, and the LF voice coil gap. The four metal-thread posts are the mounting points for the spider of the LF cone. This feature makes the LF cone easy to remove and replace without the use of glue, and allows for alignment adjustment to some degree.

 

 

Left:
The HPD385 LF cone from the rear. The voice coil lead out wires can be seen sheathed in black PVC. The voice coil appears to be four-layers deep. Total force actuating on the voice coil is B*I*L (= gap flux * signal curren t* conductor length).

The HPD385, like the Monitor Gold has the same B*L factor of 19 Newtons per Amp. At full power, this corresponds to about 80 Newtons force, or 8kg of weight acting on the cone apex.

Right: 
The front gasket on the basket is attached by screws (not glue) and therefore is easily removed and replaced.

 

 
Above:
In this case, a new surround is being fitted. To allow for the components glued to the rear of the cone, small cut-outs are made in the surround prior to gluing it to the rear edge of the LF-cone.

Another technique used is to remove the "girdacoustic" cone stiffening, glue the surround to the rear face, then re-glue the cone stiffeners into position. Although this re-assmebles the cone as it was in the factory, I am not attracted to the idea of removing the cone stiffeners.

Right:
The new surround, showing the neat fit of the cut-outs around the cone stiffener.

Note: Before fitting the surround, the ends of the cone stiffeners are also glued down to the edge of the cone. (Originally the end of the stiffener was glued to the surround, which in turn was glued to the cone).

 

Above:
The genuine Tannoy surround is of the correct diameter (the Tannoy 15" cone is smaller than most 15" drivers). Also, most generic surrounds have a lower compliance than the Tannoy surround, which raises the cone resonance slightly. Generic surrounds can be fitted, although with some difficulty usually.

 

 Left:
The surround has been glued to the rear of the cone. The cone is fitted initially to mark the placement of the holes and cut-out required around the perimeter. The holes were made using a leather punch.

This surround had a very high compliance, and resulted in an ultra-low free air resonance of 12Hz. This is before treatment of the surround (Tannoy used "Tannoyplas". At this stage I am still descided whether to pursue treating the surround. It would raise the resonance. Silicone-sealant, diluted with mimeral turps has been proposed.)

 Left:
Tannoy HPD 385 LF cone voice coil - copper wound in four layers.
  Left:
HF horn-mouth from the side.

 

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