Jim

What a most fascinating and informative reply. I had no idea that it was 
possible to hear such weak signals "by ear".  I learn something new  every day. 

Thank you. 

73s
Roger G3XBM

-- Via my iPod Touch 4g --

On 28 Jan 2012, at 00:44, "James Moritz" <[email protected]> wrote:

> 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 
>