[ARC5] Crystal Headphones

[kn7sfz]

Hi Richard,

Where do the 'sound powered' headphones fit on your chart.  I have heard 
they're pretty good for xtal sets.

Thanks,

Richard kn7sfz in Orygun



On 9/18/2018 2:33 PM, Richard Knoppow wrote:
> 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
>

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