[Boatanchors] Headphone Impedance Matching

[Bill Cromwell]

On 12/04/2013 03:35 PM, Richard Knoppow wrote:
> Not sure if this is OT here. I've measured a lot of old headphones, 
> impedances vary from around 50 ohms to around 100K ohms. Modern hi-fi 
> phones are often made to loundspeaker impedance, i.e. around 8 ohms to 
> perhsp 30 ohms. Old type magnetic phones were made to work with 
> crystal radios where minimum loading was desirable. The best of them, 
> like the Western Electric 509, have an impedance of around 20K to 25K. 
> The highest impedance are Brush crystal phones, typically around 100K 
> for a pair. The DC resistance is often given and is confused with AC 
> impedance. Impedance of magnetic type phones is usually about 5 to 7 
> times the DC resistance. Note that many early tube radios were made 
> without audio output transformers so the DC for the output tube plate 
> goes through the headphones. This is where the DC resistance can be 
> important. Its also a good idea to check very early radios to see if 
> there is DC on the output terminals. Using an output transformer can 
> be a safety factor. I don't want to wear any sort of phones with 200 
> volts going through them. Low impedance magnetic phones were also 
> made: for instance Trimm made phones from about 3.2 ohms up to 20K 
> ohms. Western Electric made many types of phones with impedances from 
> around 50 ohms to around 300 ohms. WE also made an early moving coil 
> headphone. These were supplied for motion picture monitoring and for 
> audiology and usually carry a D- spec number indicating something made 
> by Bell Labs. Usually, moving coils type phones are specified by 
> _impedance_ rather than DC resistance. Headphones in pairs are usually 
> connected in series and the resistance or impedance is that of the 
> pair, not the individual units. An exception is Brush crystal phones, 
> which are connected in parallel. The reason is that that the elements 
> look like low-value capacitors so connecting in series would not pass 
> low frequencies at all well. BTW, I have never been able to find a 
> pair of Brush phones that work. In fact out of several pair I have 
> only one good element. Evidently, the crystal elements just do not 
> survive well. 

Hi Richard,

The "boatanchors" are from another era just as the Hi-Z cans are so I 
think it's appropriate discussion for the list. Mine are all magnetic 
cans with one set from Baldwin and two sets from Trimm. One of my radios 
has the audio plate current routed through the phones and I do have a 
transformer in the circuit for that radio. I often use a low-Z 
loudspeaker with that radio so the cans are mismatched but it doesn't 
matter - just as has been mentioned. It's not only unpleasant to get 
tickled by the B+ but a steady DC current can weaken the magnets (or so 
I've heard).

The most attractive feature of the Hi-Z cans is their narrower passband. 
They aren't a good tool for listening to Montovani or Mozart from your 
stereo but they help a lot reducing the background "hiss" and wideband 
noise in communication (ham) bandwidths. Seem to lift weak CW signals up 
out of the noise. I can listen to them for hours but the 4 or 8 ohm ones 
are killing me in less than an hour.

YMMV.

73,

Bill  KU8H