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Message-ID: <4C07A9438C5145FABCB79D95FCF457BA@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Sat, 28 Jan 2012 22:21:16 -0000
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Subject: LF: Re: Re: OT: Headphones
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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