Tannoy HF Compression Drive Unit:
Dismantling and Re-assembly

If you really feel the need to remove the HF diaphragm from your Tannoy Dual Concentric driver, these notes may help you. Actually, removal is easy, but re-assembly can be done incorrectly unless some care is taken:

On re-assembly, it will be found that the lateral position of the HF unit can be tweaked sideways in the Reds, Golds and HPD's. The HPD's perhaps less so, but it can still be tweaked since there are always mechanical tolerances.

The effect of the lateral position can be easily heard when feeding white or pink noise into the speaker. However, not everyone knows what smooth noise sounds like, so while this provides a good indication that alignment affects the sound, it requires a skilled ear to use it as a way to get the correct alignment.

If the lateral alignment is incorrect, a pure tone may reveal this more easily to a less well trained ear. Actually, I have used tones in the range of 500Hz to 1kHz for this (fed direct to the HF voice coil - but be careful about levels!). The HF cone excursion is greater for low frequencies, so it more readily reveals misalignment (the voice coil contacting/scraping the magnet plate makes a rattling sound).

I feel that some skills are need to do this with confidence, so it may not be something that everyone can do.

I is somewhat disconcerting that such small differences in mechanical alignment make such big acoustic changes - not an ideal thing I feel!

Also, be aware that the correct number of packers or paper spacers are required between the skirt of the HF unit and the magnet plate. This packing thickness varies for each driver, due to manufacturing tolerances (count packers and type when dismantling a driver). See also "Tannoy HF Drive Unit" for a few more details. If the packers are too thin, the concave section of the HF cone can come into contact with the convex shape of the HF pepper-pot, again giving a rattling sound or subdued output.

Finally, I suspect that un-even tightening, or over tightening the four screws on the HF unit mounting can be bad. On the Reds/Golds the backing plate is rather soft metal (aluminium probably) so it can distort if clamped too tight. This effect make be less pronounced on the HPD which has a thick bakelite backing plate.

(Don't forget that the lateral position has two degrees of freedom, left to r ight (x-axis), and up and down (y-axis) ), making the alignment more tedious.


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