Tannoy Gold HF Unit: Voice Coil, Lead-out Wires and Crossover

The following information is regarding the Monitor-Gold HF unit, but is probably applicable to other drivers. ("Aluminum" is spelt "aluminium" in
this note).

HF unit voice coil

The voice coil is aluminium wire. It looks like about 40-50 turns of circular-section wire, double layered, wound on a fibre-former. The fibre-former is bonded (glued) to the duralumin diaphragm (an alloy of aluminium). The aluminium wire terminates on the voice coil former, where there is a join to another wire - the copper lead-out wire.

HF unit voice coil lead-out wire

The copper lead-out wire traverse the flexible surround of the HF diaphragm. I don't understand why Tannoy chose to make the lead out wire from copper perhaps it is more ductile, and less likely to fail from the minute flexing required in this wire (?). Another option would to have been to continue the aluminium voice coil wire, so that it traversed the flexible surround of the HF diaphragm, and then to make the aluminium/copper join on the body (non-moving part) of the HF unit, rather than on the moving voice coil.

HF unit lead-out wire join

The copper lead-out wire is tinned (with what appears to be normal tin/lead solder) at the end where it joins the aluminium voice coil wire. The bared voice coil wire is wrapped around this tinned-copper in a spiral, which appears to be well tensioned - with the lead out wire being glued firmly down to the voice coil former. My conclusion at this stage is that the join is actually a form of compression-joint - with the spiralled wires tightly pressured against each other.

HF Unit Gaskets

Under the outer ring of the HF unit, there are a number of gaskets-spacers that position the diaphragm at the correct working distance from the rear surface of the HF-unit pepper-pot. It has been mentioned before that the HF unit output can be increased by removing one or more gaskets. In my case, I had three gaskets (one thick, two thin). When I removed just one thin gasket, the HF-diaphragm just came into contact with the rear surface of the HF pepper-pot (so I had to leave all three gaskets in place). Close or near contact of the diaphragm with the rear pepper-pot surface causes distortion/rattling when the driver is fed even small signals at the lower end of its working range (e.g. 700 Hz). I conclude that the spacing between the HF-unit diaphragm and the rear surface of the HF pepper pot is very small indeed, and in my case it was about the thickness of some thick writing paper.

HF Unit Crossover - that 5O Ohm resistor

The 50 Ohm resistor in the HF-section of the crossover for Monitor HPD and Monitor Gold drivers has been questioned before. My understanding of the function of this resistor is to maintain a more-constant impedance looking into the input terminals of the HF-transformer, regardless of the setting of the "Energy" control. If the impedance was to vary when the Energy control was altered, then the crossover frequency would also shift slightly. When the HF unit is fed more power, by connecting it to the "top" of the auto-transformer, then the resistor is fed less power by moving its connection point towards the bottom of the transformer windings (and in the last position - disconnected completely). Like-wise, when less power is fed to the HF units, more is fed into the resistor. The effect giving roughly constant impedance into the HF-section of the crossover.

(PS: Thanks to Peter Campbell for his nice turn-of phrase: "The copper lead-out wire traverse the flexible surround of the HF diaphragm" - I could not think of a better way to describe this, so I quoted you direct.)


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