[Boatanchors] Headphone Impedance Matching

[Richard Knoppow]

-----Original Message-----
>From: Bill Cromwell <wrcromwell at gmail.com>
>Sent: Dec 4, 2013 1:36 PM
>To: boatanchors at mailman.qth.net
>Subject: Re: [Boatanchors] Headphone Impedance Matching
>
>On 12/04/2013 04:27 PM, D C _Mac_ Macdonald wrote:
>> I suspect that crystal headphones suffer from the same temperature and humidity woes that affect crystal microphones.  I.E., extremely low tolerance for such.
>> * * * * * * * * * * *
>> * 73 - Mac, K2GKK/5 *
>> * (Since 30 Nov 53) *
>> * Oklahoma City, OK *
>> * USAF, Ret (61-81) *
>> * * * * * * * * * * *
>>   
>>   
>>
>Hi,
>
>I don't have any of those at the moment but never know what might follow 
>me home from a hamfest. Does some kind of a "bake" to drive out the 
>moisture have a chance of restoring crystal elements? Should I walk 
>right on by if I see those on a table for sale?
>
>73,
>
>Bill  KU8H


    Unfortunately, once a crystal element goes its gone permanently. The material is potassium sodium tartarate better known as Rochelle Salt.  If exposed to moisture it crystalizes into a sort of mush. It also melts at a fairly low temperature. It is also vulnerable to fracture so dropping an element, scraping the needle of a crystal phonograph pick-up or overdriving a crystal headphone or disk recording cutter will ruin it.  The makers of crystal elements went through a lot of trouble to try to protect them from all the above, especially sealing them against moisture.  Nonetheless, all to often crystal microphones or other items are bad.  
     A good crystal element looks like a low value capacitor.  If measured with an ohm meter they will look like an open circuit but will "click".  When they go bad they start to show some resistance although of course some can fail completely open. 
     Crystal elements were widely used because, as phono pick-ups or microphones, they have very high output so don't need much amplification, can have relatively good fidelity, and are relatively cheap.  After about the late 1940s ceramic elements began to be used in place of crystals because they are not so subject to moisture or heat damage. However, the output level is significantly lower.  For instance, the ceramic version of the Astatic D-104 (model C-104) has about ten db lower output.  
     Some important patents applying to practical crystal elements were issued about the early 1930s and initial development was done by Brush Development company.  However it was Astatic who got a license and made the first crystal microphone in 1933. This was the familiar D-104 which was made for decades with little change. The somewhat shrill sound of the D-104 is partly deliberate.  Originally two versions of this microphone were made: a flat version for general use and a rising response version for communications use.  The rising response version is the one that was popular and was continued.  Astatic made many other models some of which had quite flat and wide-range response and were used for music recording.  They had somewhat lower level than the D-104 but still higher than other types of microphones. 
     There were many methods devised for sealing crystal elements but the one used by Astatic seems to have been one of the more successful considering the number of their microphones which have survived. 
     Modern practice has pretty much replaced crystal and carbon microphones with electret types where high output is desired. These are very rugged and reliable and can have excellent fidelity.  However, like carbon and condenser microphones are NOT generators and require some associated electronics with attendant power supply.