Dear Mal, Roger, LF Group,
G3KEV wrote:
The DLR5 is nothing special the Z is only around 50 ohms whereas S G Brown
and similar are 2000Z.
Obviously there is no direct connection between efficiency/sensitivity and
impedance - the impedance varies with the number of turns/thickness of wire
in the electromagnet windings, and so can be chosen at will by the
manufacturer, but the sensitivity, i.e. the signal power required to produce
an audible signal, depends on the basic details of construction and
operating principle. All the balanced armature types I have personally
encountered were of the order of a few hundred ohms impedance, but no doubt
others have been made in a high impedance version with more turns of finer
wire.
Whether high Z is better than low Z depends on the output impedance of the
circuit they are connected to. For crystal sets and simple valve/tube
circuits, many kilohms are usually desired. For my electromechanical RX and
passive DC receiver projects, the low impedance 'phones were close to ideal.
In any case, as JB says, you can use a transformer to get what impedance you
want.
But balanced armature types are definitely more sensitive than others. The
"traditional "type has a steel diaphragm acted on by a U-shaped
electromagnet - due to the stiffness of the diaphragm, and the increased
force needed to move it, sensitivity is reduced. I have some high impedance
SG Browns "type A" that use a "reed" construction - this has the steel
diaphragm replaced with a magnetic reed coupled to an aluminium foil
diaphragm. There are adjusting knobs on each earpiece which vary the spacing
between the reed and the magnet - reducing the spacing increases
sensitivity, but go too far and the reed sticks to the magnet. This
construction is intermediate between traditional and balanced armature, and
sensitivity is intermediate also. I beleive the "DLR No 5" balanced armature
headphones were made for the military by SG Brown - the military favoured
low impedance because the thicker winding wire was more robust and less
susceptible to corrosion.
Cheers, Jim Moritz
73 de M0BMU
|
