References: <CAHAQVWP_9KdqZ1NjzmJqB3O2mrb-GfJfZBf_D7CY0Y8FAasC3Q@mail.gmail.com> <3D6129856D6944248B1B77637EE42962@JimPC>
From: Roger Lapthorn <rogerlapthorn@gmail.com>
In-Reply-To: <3D6129856D6944248B1B77637EE42962@JimPC>
Message-Id: <1354933D-48C4-414F-8FFB-12B188C8ED33@gmail.com>
Date: Sat, 28 Jan 2012 08:05:05 +0000
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
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Subject: Re: LF: Re: OT: Headphones
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Jim

What a most fascinating and informative reply. I had no idea that it was pos=
sible to hear such weak signals "by ear".  I learn something new  every day.=
=20

Thank you.=20

73s
Roger G3XBM

-- Via my iPod Touch 4g --

On 28 Jan 2012, at 00:44, "James Moritz" <james.moritz@btopenworld.com> wrot=
e:

> Dear Roger, LF Group,
>=20
>> Is there a "modern" (i.e. can you buy it new today?) headphone set that i=
s *
>> extremely* sensitive in the same way that the old DLR5 headsets were in
>> terms of efficiency converting AF to sound pressure in the ear?
>>=20
>=20
> The DLR No. 5 headphones are "balanced armature" types, where a small, piv=
oted iron armature between the poles of an audio energised electromagnet is m=
echanically coupled to a lightweight, lightly sprung diaphragm.  They were i=
nvented in the days when amplification was hard to achieve, so high efficien=
cy headphones were very useful in increasing sensitivity. They were used a l=
ot in the WWII era for "sound powered" intercomm systems, i.e. at one end th=
ere was a dynamic microphone which somebody talked into, and the small signa=
l generated by the microphone was connected directly to the headphones at th=
e receiving end, without any intevening amplifier, so they had to be sensiti=
ve. The resulting passive system was much more reliable and economical at th=
at time than something using valve/tube electronics, or carbon microphones r=
equiring 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 "dy=
namic" headphones, which use what are basically small moving-coil loudspeake=
rs, and try to optimise sound quality, often at the expense of efficiency. T=
he main drawback is that they are rather "low-fi" with a restricted frequenc=
y response, although fine for communications audio.
>=20
> I think balanced armature transducers are still made for specialised appli=
cations; sound-powered intercoms are apparently still found on warships, I b=
elieve some hearing aid earpieces are balanced armature to reduce power cons=
umption. Telephone handsets, from the era when they were supplied by the Pos=
t 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 t=
elephone handset receivers, fitted into the casing of some cheap stereo head=
phones, which were comparable in sensitivity to the DLR No 5 headset. Commun=
ications headsets for "Clansman" military radios used into the 90s use simil=
ar balanced armature inserts to those used in the old telephones.
>=20
>> 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".
>=20
> I was quite surprised at how little power was audible, around -100dBm audi=
o tone was perceptible using the DLR No. 5s.  Naturally, this required very q=
uiet surroundings, keeping still and not breathing.... Frequencies around 1.=
0 - 1.5kHz had the best audibility, rather than the lower BFO pitch that ten=
ds 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 fig=
ure 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 i=
s always possible to hear the QRN in the background, even though this circui=
t has considerable loss between antenna and headphones. At around the same t=
ime as I was working on the electromechanical RX, I made a "passive" receive=
r for 80m - basically an antenna tuner and filter feeding an SBL-1 diode mix=
er module driven by a 3.5MHz VFO, and a matching network between the mixer o=
utput and the headphones. With this gain-less direct conversion RX and a rea=
sonably 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.
>=20
> Cheers, Jim Moritz
> 73 de M0BMU=20
>=20