[ARC5] Crystal Headphones

[Brooke Clarke]

Hi:

There are some LS-685/U Crystal Loudspeakers on the market.  But they have a non standard connector that would need to 
be replaced.
https://www.prc68.com/I/LS685.html

Sound powered phones are 10 to 20 dB more sensitive than conventional designs.  See the comparison chart at:
https://www.prc68.com/I/SoundPoweredTelephone.html#Background
Early loudspeakers made use of the sound powered phone type drivers.
Note sound powered receivers have cone movement at right angles to the magnetic center line, while conventional 
receivers have parallel or coaxial center lines.
See drawing for the TP-3-T1 at https://www.prc68.com/I/SoundPoweredTelephone.html#TP-3

Measuring and generating sound in order to test transmitters/micrphones, receivers/earphones or ears is complicated.
https://www.prc68.com/I/GRsound.shtml - many pieces of audio test equipment
https://www.prc68.com/I/Beltone12D.html - hearing tester + calibrator
https://www.prc68.com/I/TA-3Daim.html - for measuring the impedance of ears

-- 
Have Fun,

Brooke Clarke
https://www.PRC68.com
https://www.end2partygovernment.com/2012Issues.html
axioms:
1. The extent to which you can fix or improve something will be limited by how well you understand how it works.
2. Everybody, with no exceptions, holds false beliefs.

-------- Original Message --------
> One must be careful not to confuse sensitivity with the impedance. Sensitivity is a power measurement, that is 
> electrical power in to acoustical power out. It is possible to have high impedance but low power sensitivity although 
> I can't give  you an example of a commercial headphone. High impedance is desirable for sources that are high 
> impedance such as crystal sets and AC impedance bridges. However, where the source is low or medium impedance 
> headphones with matching impedance will be louder than the high impedance phones. Testing for sensitivity requires a 
> calibrated artificial ear if the results are to have any accuracy. These are rare. However, one can test with just  
> your ears or a sound level meter with the microphone held against the headphone. An interesting experiment is to take 
> an oscillator and feed various headphones from it through a very high value resistor, preferably at least five times 
> the expected headphone impedance. 100K is enough although more is better. Just connect the phones and see which is the 
> loudest. Usually, it will be the ones with the highest impedance but some with the same impedance will differ. In some 
> cases a lower impedance phone will be louder than a higher impedance one. The difference is probably in the magnet 
> strength but can also be due to differences in the diaphragm spacing from the pole pieces.
>    The loudest phones I have are: Western Electric 509W, WE Signal Corps P-11, Baldwin Type C.  However, if you make 
> the same test using a low impedance source, say 500 ohms, the results will be different. For instance WE type 528 (600 
> ohm) will be louder than the WE 509W. WE and Trimm made magnetic phones down to about 50 ohms per pair. For the same 
> reason you may find modern high-fidelity moving coil phones too loud when connected to a receiver where they are 
> across the loudspeaker output. Typically, this will be 4 to 8 ohms and most of these phones are are around 50 ohms 
> impedance. The receivers will often specify 500 ohms although 20K phones usually work just fine and are of reasonable 
> loudness.
>    Measuring efficiency is a matter of measuring power out to power in. That requires an impedance match on the 
> electrical side and, for best power efficiency, also on the acoustical side.
>     Most of the magnetic type phones have a strong resonance, typically in the vicinity of 1Khz. While some attempt 
> was made to broaden out the resonance (such as using a salt shaker type cap) the difference is not great. It is 
> possible to make quite non resonant magnetic phones but they become complex. The best example is the Western Electric 
> type HA-2 designed for the series 500 telephone. These are described in great detail in the Bell System Technical 
> Journal and elsewhere by Mott and Miner of Bell Labs who designed it. There are other types that fall broadly in the 
> "magnetic" category, such as the balanced armature type as typified by the Baldwin phones with mica diaphragms and 
> also used in some sound-powered phones.
>     While one can find oodles of information about moving coil speakers and microphones in the technical literature of 
> the last century there is not much about the plain magnetic phones although they were made by many different 
> companies. Frustrating.
>    FWIW, the highest impedance phones magnetic phones I've measured are the WE SC type P-11 (close to 30K), others 
> were the WE 509W (25K), Trimm Featherweight (24K) but the Featherweights are not as loud on a very high impedance 
> source as the WE phones or as the Trimm Commercial phones which measure only 17K (advertised value).
>     Measurements were made on: General Radio 650A bridge with external null detector, GR 1650A, GR Z-Y bridge, all at 1K
>    Note also that magnetic phones are mostly inductive so the impedance varies with the frequency. I measure at 1K. I 
> found the advertised values of Trimm phones are high at 1K but about right at 1200 Hz. However, the resonance is at 
> about 1K. I tried to measure the motional impedance of a couple of phones but gave it up because I can't separate it 
> from the overall impedance. i.e., you can't see a resonant rise in the impedance.
>     Enough, I am glad at least one or two people find this of some interest.
> On 9/18/2018 1:35 PM, Dennis Monticelli wrote:
>> More modern magnetic phones (still of the old style) can be quite good.
>>
>> I have a NIB set of H-43B/U headphones (datecode 1990) that were made for Geiger counters.  I measured an impedance 
>> of 21K at 1KHz so I presume the transducers are 10K wired in series. These cans produce plainly audible tones with 
>> only 1mV rms. That equates to 50 picowatts!  Impressive sensitivity and high enough Z for crystal radios.
>>
>> Dennis AE6C
>