Building better High sensitivity headphones

[Rich Grise]

...
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?

Well, there's cheating and then there's cheating. If there's a powerful
AM or (analog) TV station nearby, you could tune one Xtal set to that,
and just derive DC from the detected RF, then power the headphone for
your selected station by the DC that's generated by detecting ambient
RF.

Cheers!
Rich

 

[amdx]

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?

 

[Scott Dorsey]

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

 

[Scott Dorsey]

I think some were magnetically biased, with an iron diaphragm, a
permanent magnet, and the variations in the coil acting to vary the
force on the diaphragm.

Yes, that's a variable reluctance headset.

Come to think of it, a lot of the old amateur regenerative receivers
called out the head phone connection in the plate circuit of the final
audio amp (that's right! Wrap the plate supply around your head!).
This would have provided the necessary bias right there, albeit in a
manner that Ralph Nader probably would shake his head at.
Yes.


Get Thee Behind Me, Satan!

Look for "infinite impedance detector" in the ARRL Handbook if you want to
go one step farther. You can always use a triode if you want.
--scott

 

[Tim Williams]

Or use a transformer.

I have a few 50kohm microphone transformers, if you're interested. IIRC, the primary is 50-300-600 ohm.

Tim

 

[Phil Allison]

"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

 

[amdx]

--
MikeK

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

 

[Graeme Zimmer]

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

 

[Ian Bell]

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?

The sensitivity depends only on the electromagnetic circuit of the
phones so you will not be able to make one that is 'more sensitive' than
any currently available i.e. circa 110dB SPL per mW input. Your only
real option is a transformer to match current ones to the crystal set.

Cheers

Ian

 

[amdx]

The sensitivity depends only on the electromagnetic circuit of the phones
so you will not be able to make one that is 'more sensitive' than any
currently available i.e. circa 110dB SPL per mW input. Your only real
option is a transformer to match current ones to the crystal set.

Cheers

Ian

What is the efficiency of a speaker with 110db SPL with 1mW input?
What is the cause of the losses?
Mike

 

[GregS]

What is the efficiency of a speaker with 110db SPL with 1mW input?
What is the cause of the losses?
Mike

Space.

 

[GregS]

The sensitivity depends only on the electromagnetic circuit of the
phones so you will not be able to make one that is 'more sensitive' than
any currently available i.e. circa 110dB SPL per mW input. Your only
real option is a transformer to match current ones to the crystal set.

Cheers

I though crystal headphones were efficient.

 

[[email protected]]

The sensitivity depends only on the electromagnetic circuit of the
phones so you will not be able to make one that is 'more sensitive' than
any currently available i.e. circa 110dB SPL per mW input. Your only
real option is a transformer to match current ones to the crystal set.

That makes sense. Professional headsets tend to spec 100dBA/mW.

 

[Ian Bell]

I though crystal headphones were efficient.

Show me some specs to back that up.

Cheers

Ian

 

[Ian Bell]

That makes sense. Professional headsets tend to spec 100dBA/mW.

Yup, and the audiophile ones come in around 90dBA/mW

Cheers

Ian

 

[GregS]

I though crystal headphones were efficient.

Show me some specs to back that up.[/QUOTE]


Only that its pure capacitive, and small at that. Very
little energy required to drive.

greg

 

[Ian Bell]

Only that its pure capacitive, and small at that. Very
little energy required to drive.

greg

That says nothing about efficiency. The only way real power can be
transferred is into the resistive part of the device. The fact that it
is 'largely capacitive' says nothing about the energy required to drive it.

Cheers

Ian

 

[amdx]

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?

Here's a page with info using piezoelectric elements, specifically Kyocera
KBT-33-RB-2CN.
Not a lot of info on the build but does seem to say these would be quite a
bit more sensitive than
even the best sound powered balanced-armature headphones.
http://wiki.waggy.org/dokuwiki/piezo_headphone
and the piezo element page.
http://www.avx.com/docs/masterpubs/piezo.pdf
MikeK

 

[Angelo Campanella]

Here's a page with info using piezoelectric elements, specifically
Kyocera KBT-33-RB-2CN.

The Kyocera data book is

http://www.avx.com/docs/masterpubs/piezo.pdf

KBT-33-RB-2CN data is found on page 20. It's about 35mm in diameter.

The impedance is capacitive, stated as 3k at 1 kHz, and less at higher
frequencies, being about 1 k at 3 kHz.

Where is the 107dB SPL measured? Perhaps not at a distance of 1m. Perhaps at
the transducer surface?

I have not seen reports of crystal receiver rectifier (CR) output
voltages... perhaps millivolts?

It's still practical to test CR circuits with the radiation from local AM
broadcast stations.

My last venture with a CR such worked OK on WOSU AM radiation, that
transmitter antenna being about two miles east of me. [I used an old 2k-ohm
magnetic headphone set, which also produces enough sound from my old B&K
2203 SLM ac-out port signal to show me whether that SLM was working OK in
the field...]

Ange

 

[Scott Dorsey]

The Kyocera data book is

http://www.avx.com/docs/masterpubs/piezo.pdf

KBT-33-RB-2CN data is found on page 20. It's about 35mm in diameter.

The impedance is capacitive, stated as 3k at 1 kHz, and less at higher
frequencies, being about 1 k at 3 kHz.

ALL of those devices have narrowband resonances all over the place. The
response looks like a hedgehog. And they have to be driven with a high-Z
source or it gets worse.

Where is the 107dB SPL measured? Perhaps not at a distance of 1m. Perhaps at
the transducer surface?

Probably, if the transducer is not necessarily intended to couple into air
anyway.

It's still practical to test CR circuits with the radiation from local AM
broadcast stations.

My last venture with a CR such worked OK on WOSU AM radiation, that
transmitter antenna being about two miles east of me. [I used an old 2k-ohm
magnetic headphone set, which also produces enough sound from my old B&K
2203 SLM ac-out port signal to show me whether that SLM was working OK in
the field...]

So what's wrong with using a JFET and a low-efficiency moving coil driver?
It's hard to get efficiency and flat response at the same time.
--scott