[Boatanchors] Headphone Impedance Matching

[David C. Hallam]

OK, next question about my headphone impedance matching.  I have 
listened to the audio from my 75S-3 in three different ways.

312B-4 Collins speaker
Telex 600 ohm impedance headphones
Lincoln/Telex 2000 ohm impedance headphones

When using the Telex 600 ohm headphones and the audio gain turned all 
the way down, I hear a distinct hum.  I do not hear this hum with either 
of the other two.  Any explanation of why I hear it only with the 600 
ohm headphones?

David
KW4DH


On 12/4/2013 5:56 PM, Richard Knoppow wrote:
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>      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.
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