[ARC5] Crystal Headphones
Fuqua, Bill L
wlfuqu00 at uky.edu
Wed Sep 19 02:12:55 EDT 2018
Sorry that I have not been able to reply or even check email since Monday.
Having back surgery Thursday and will be out of commission for a while and been
distracted by other stuff.
I have a sound pressure meter (inexpensive Chinese version) and a GR sound pressure
calibrator. The sound pressure (level) meter was within 1.5db which I found amazing.
I have built a fixture to that has one port for the sound level meter and two for sources
such as earphones and calibrator. So I can easily compare two types of earphones or
one earphone and calibrator.
I also am building a fixture that will allow a signal source to produce an in and out of phase signals
so that I can neutralize the shunt capacitance of the earphone so true power to element may be
measured.
Some earphones, particularly the ceramic ones, have 1nF shunt capacitance or more across
the otherwise high real portion of the impedance.
I have found, by ear, that the Rochelle salt earphones appear to not only be the most sensitive but
have wider range of frequency response due to the very compliant foil diaphragm used. I have collected
a number of them and found plenty that have not been destroyed by humidity. The typical 2kOhm head phones
also lack low frequency response.
I also have ceramic earphones purchased from Mouser and other sources some years ago and
some also some of the more recently produced cheap cheap ones that come with current crystal
radio kits.
As far as impedance goes, my sensitivity test will be dBm in vs dB sound pressure out. Impedance
is not so important. Impedance can be compensated for by either transformers or by design of the
radio itself.
Since the radiation impedance of an electrically short longwire antenna will be only a few Ohms a
receiver could be designed that will have a low resistance output, using more modern diodes.
But I prefer Hi-Z myself.
The end result I hope is to come up with a crystal radio kit that is as sensitive and selective as the ones
100 years ago.
I can make some measurements of the Rochelle salt cantilever, mm/volt and compare it to current
ceramic bimorph cantilever designs and perhaps come up with a modern version (prototype) that
can be reproduced in quantity.
I also need to get back to my other projects. I have started redesigning my portable Transmitting Tube Tester and
want to get it working again and hopefully to 3-5 kW tubes. Currently it is limited to 1kW tubes and less.
And some amplifier projects.
73
Bill wa4lav
________________________________
From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> on behalf of Richard Knoppow <1oldlens1 at ix.netcom.com>
Sent: Tuesday, September 18, 2018 10:43 PM
To: arc5 at mailman.qth.net
Subject: Re: [ARC5] Crystal Headphones
I've written a little about this. I think most sound powered
phones are relatively low in impedance, not desirable for crystal
set radios. However, perhaps they work well anyway.
The problem with quoting this stuff is that I am responding
mostly to specific questions and don't want to be boring so tend
to simplify a bit. However, I have no objection to using my
stuff, with attribution. I make no claim to being an expert. My
knowledge, such as it is, is the result of plain old curiosity
and having the privilege of knowing some real experts who were
willing to teach me.
On 9/18/2018 3:34 PM, kn7sfz wrote:
> 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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--
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL
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