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Message-ID: <3D6129856D6944248B1B77637EE42962@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <CAHAQVWP_9KdqZ1NjzmJqB3O2mrb-GfJfZBf_D7CY0Y8FAasC3Q@mail.gmail.com>
In-Reply-To: <CAHAQVWP_9KdqZ1NjzmJqB3O2mrb-GfJfZBf_D7CY0Y8FAasC3Q@mail.gmail.com>
Date: Sat, 28 Jan 2012 00:44:13 -0000
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Subject: LF: Re: OT: Headphones
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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