Building better High sensitivity headphones

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?

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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.

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

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Or use a transformer.

John

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.

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.

Get Thee Behind Me, Satan!

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Yes, that's a variable reluctance headset.

Yes.

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

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

"John Larkin" wrote in = message news: snipped-for-privacy@4ax.com...

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

Tim

--=20 Deep Friar: a very philosophical monk. Website:

"amdx"

** 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:

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

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

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

Yup, it would. MikeK

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

and and

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

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)?

Maybe not, if some of the recovered carrier power was used for the drain..

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

Space.

I though crystal headphones were efficient.

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

Show me some specs to back that up.

Cheers

Ian

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

Cheers

Ian

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

greg