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From: Phil Allison on 22 Jul 2010 19:37

"amdx"
> I'm interested in a discussion about building a high sensitivity set of
> headphones.
> The intended use would with a crystal radio.
> The usual thing is to find an old set of Brandes headphones with 2000 ohms
> DC resistance.
> It seems like the collective intellect here could design or modify
> existing headphones
> to provide a better impedance match and increase the sensitivity of
> headphones.
> Maybe there are some better materials now then there were 80 years ago.
> Any ideas about the ideal characteristics for a set of crystal radio
> headphones?


** Standard 32 to 100 ohm headphones can be used with a small "100 volt
line" matching transformer to increase the effective impedance to a much
higher number.

Eg: http://www.altronics.com.au/index.asp?area=item&id=M1109


Using the 0.5 watt input and with 32 ohms connected to the 8 ohm secondary
tap, the nominal input impedance is over 50 kohms.


..... Phil




From: amdx on 22 Jul 2010 19:51


--
MikeK
"Scott Dorsey" <kludge(a)panix.com> wrote in message
news:i2a205$8ou$1(a)panix2.panix.com...
> amdx <amdx(a)knology.net> wrote:
>> Hi Guys,
>> I'm interested in a discussion about building a high sensitivity set of
>>headphones.
>>The intended use would with a crystal radio.
>>The usual thing is to find an old set of Brandes headphones with 2000 ohms
>>DC resistance.
>> It seems like the collective intellect here could design or modify
>>existing headphones
>>to provide a better impedance match and increase the sensitivity of
>>headphones.
>>Maybe there are some better materials now then there were 80 years ago.
>> Any ideas about the ideal characteristics for a set of crystal radio
>>headphones?
>
> You want an infinitely high input impedance and no shunt capacitance.
>
> The old method was to use the finest possible wire, and as many turns of
> it as possible. This results in some design issues; because your coil is
> now extremely massy, you can't use a moving coil transducer design and
> get good high frequency response. So most of those high-Z headphones were
> variable reluctance types.
>
Variable Reluctance... my inspection of old headphones leads me to
believe
this would basically be an iron cored electromagnet with a ferous plate near
it
to vibrate. The electromagnet would have a magnet on the core to bias the
ferous plate.
I don't doubt that this is called a variable reluctance headphone (could be
wrong)
but where is the rulactance varying?
I guess the forous plate changing it's distance from the electromagnet would
change the reluctance.
Hmm... any advantage to completeing more of the magnetic path inside
the headphone? It's a long air path from the backside of the electromagnet
to the edges of the ferous plate


> Another old method is to use a piezoelectric transducer. I think that is
> still a viable method, maybe using some of the Motorola piezo horn
> elements.
>

I think that is driving a capacitor, any circuit changes to optimize for
the piezo.

> The easiest way to do it today would be to use a modern low-z headphone
> with a JFET follower in front of it. Gives you as high an input Z as you
> would ever want. But, really, that would be cheating, wouldn't it?
> --scott

Yup, it would.
MikeK


From: Graeme Zimmer on 22 Jul 2010 20:10
It's generaly considered that the most sensitive headphones ever made are
the "Sound Powered" Navy types. These are of "Rocking Armature" type
construction.

Google on "RCA Big Cans"

see
http://www.crystalradio.net/soundpowered/
and
http://www.crystalradio.net/soundpowered/repair/index.shtml
and
http://www.antiquewireless.org/otb/dxxtal.htm

............. Zim


From: Robert Baer on 23 Jul 2010 03:27
amdx wrote:
> Hi Guys,
> I'm interested in a discussion about building a high sensitivity set of
> headphones.
> The intended use would with a crystal radio.
> The usual thing is to find an old set of Brandes headphones with 2000 ohms
> DC resistance.
> It seems like the collective intellect here could design or modify
> existing headphones
> to provide a better impedance match and increase the sensitivity of
> headphones.
> Maybe there are some better materials now then there were 80 years ago.
> Any ideas about the ideal characteristics for a set of crystal radio
> headphones?
>
Maybe re-design the magnetic circuit some to allow Neodium(sp?) or
other super-strong magnet with space for more wire (say in 10K region)?
From: Robert Baer on 23 Jul 2010 03:29
Scott Dorsey wrote:
> amdx <amdx(a)knology.net> wrote:
>> Hi Guys,
>> I'm interested in a discussion about building a high sensitivity set of
>> headphones.
>> The intended use would with a crystal radio.
>> The usual thing is to find an old set of Brandes headphones with 2000 ohms
>> DC resistance.
>> It seems like the collective intellect here could design or modify
>> existing headphones
>> to provide a better impedance match and increase the sensitivity of
>> headphones.
>> Maybe there are some better materials now then there were 80 years ago.
>> Any ideas about the ideal characteristics for a set of crystal radio
>> headphones?
>
> You want an infinitely high input impedance and no shunt capacitance.
>
> The old method was to use the finest possible wire, and as many turns of
> it as possible. This results in some design issues; because your coil is
> now extremely massy, you can't use a moving coil transducer design and
> get good high frequency response. So most of those high-Z headphones were
> variable reluctance types.
>
> Another old method is to use a piezoelectric transducer. I think that is
> still a viable method, maybe using some of the Motorola piezo horn elements.
>
> The easiest way to do it today would be to use a modern low-z headphone
> with a JFET follower in front of it. Gives you as high an input Z as you
> would ever want. But, really, that would be cheating, wouldn't it?
> --scott
>
Maybe not, if some of the recovered carrier power was used for the
drain..
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