Dear Roger, LF Group,
Is there a "modern" (i.e. can you buy it new today?) headphone set that is
*
extremely* sensitive in the same way that the old DLR5 headsets were in
terms of efficiency converting AF to sound pressure in the ear?
The DLR No. 5 headphones are "balanced armature" types, where a small,
pivoted iron armature between the poles of an audio energised electromagnet
is mechanically coupled to a lightweight, lightly sprung diaphragm. They
were invented in the days when amplification was hard to achieve, so high
efficiency headphones were very useful in increasing sensitivity. They were
used a lot in the WWII era for "sound powered" intercomm systems, i.e. at
one end there was a dynamic microphone which somebody talked into, and the
small signal generated by the microphone was connected directly to the
headphones at the receiving end, without any intevening amplifier, so they
had to be sensitive. The resulting passive system was much more reliable and
economical at that time than something using valve/tube electronics, or
carbon microphones requiring a DC power source. When I was working on the
"Electromechanical RX" I estimated that they were 10 dB or more higher
sensitivity than modern "dynamic" headphones, which use what are basically
small moving-coil loudspeakers, and try to optimise sound quality, often at
the expense of efficiency. The main drawback is that they are rather
"low-fi" with a restricted frequency response, although fine for
communications audio.
I think balanced armature transducers are still made for specialised
applications; sound-powered intercoms are apparently still found on
warships, I believe some hearing aid earpieces are balanced armature to
reduce power consumption. Telephone handsets, from the era when they were
supplied by the Post Office or BT and had rotary dials, were balanced
armature, although modern electronic ones are usually "dynamic", I believe.
I tried a couple of old telephone handset receivers, fitted into the casing
of some cheap stereo headphones, which were comparable in sensitivity to the
DLR No 5 headset. Communications headsets for "Clansman" military radios
used into the 90s use similar balanced armature inserts to those used in the
old telephones.
I believe Jim M0BMU claimed 2uV audio power could be heard using DLR5
headsets in his passive mechanical SAQ receiver. As DLR5s are more
difficult to find these days I was wondering if there is anything as
sensitive, or more so, around "new".
I was quite surprised at how little power was audible, around -100dBm audio
tone was perceptible using the DLR No. 5s. Naturally, this required very
quiet surroundings, keeping still and not breathing.... Frequencies around
1.0 - 1.5kHz had the best audibility, rather than the lower BFO pitch that
tends to be preferred for CW operating. I suffer from mild tinnitus which
might limit the lower threshold of my hearing; somebody with really low
noise figure ears perhaps could hear lower levels. To put this in
perspective, this 0.1 picowatts is well below the band noise level that
would be present using a reasonably larged tuned vertical antenna for 136kHz
reception., and typical CW bandwidth. Using the Electromechanical RX to
receive SAQ on 17.2kHz, it is always possible to hear the QRN in the
background, even though this circuit has considerable loss between antenna
and headphones. At around the same time as I was working on the
electromechanical RX, I made a "passive" receiver for 80m - basically an
antenna tuner and filter feeding an SBL-1 diode mixer module driven by a
3.5MHz VFO, and a matching network between the mixer output and the
headphones. With this gain-less direct conversion RX and a reasonably big
long-wire on 80m, it was possible to hear quite a lot of amateur SSB/CW
activity, even a couple of US stations on CW.
Cheers, Jim Moritz
73 de M0BMU
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