[Boatanchors] Mic's / RED FLAG

[Richard Knoppow]

----- Original Message ----- 
From: "Glen Zook" <gzook at yahoo.com>
To: "Rob Atkinson" <ranchorobbo at gmail.com>; 
<telegrapher at q.com>; "Greg Mijal" 
<bluebirdtele at embarqmail.com>
Cc: <boatanchors at mailman.qth.net>
Sent: Monday, July 18, 2011 11:20 AM
Subject: Re: [Boatanchors] Mic's / RED FLAG


The problems with the original D-104 cartridge are

They are very susceptible to humidity.  That is, the salts 
dissolve when exposed to humidity for a long time.

They are very susceptible to shock.  That is, drop the 
microphone, or hit it very hard, and the cartridge goes bad.

Finally, and this is the most important, Astatic no longer 
makes a replacement cartridge!

Glen, K9STH

Website:  http://k9sth.com

     These are problems with all crystal devices, 
microphones, headphones, phonograph pickups, and record 
cutter heads. Also some other devices such as pressure and 
vibration pickups. The Rochelle salt of which they are made 
is very hygroscopic, i.e., they readily absorb moisture as 
you state and turn into a sort of paste. The crystals can 
fracture, either from external shock or, if the crystal is 
driven from a voltage source, from being driven too hard. 
This used to be a problem when crystal cutter heads were 
common for home disc recording. Phonograph pickups could 
also be fractured by dropping the needle or scraping it 
across the record. The crystals are also sensitive to 
temperature. The maximum safe temperature is about 115F and 
at 135F the crystal is permanently damaged. Also, the 
moisture absorption increases rapidly with temperature so 
that if the seal of the crystal element is damaged the 
element will be quickly destroyed by the cobination of high 
temperature and high humidity. Also, the capacitance of the 
crystal changes with temperature so that the effect of a 
long cable becomes greater as the temperaure is increased.
    Much of the technology of crystal elements has to do 
with methods of sealing the element. Some methods were 
obviously more successful than others but its hard to make 
comparisons of old equipment unless the history is known.
    One of the important developments in making crystal 
elements practical was the invention of "bender" and 
"twister" type elements. Both are made by combining two 
crystals in such a way that the driving force either bends 
or twists the two elements. The output is the result of the 
differential voltage, because of the way the elements are 
mechanically fastened and electrically connected the output 
is much larger than it would be for a single element.
     Ceramic elements are very similar to Rochelle salt ones 
but are substantially less sensitive to moisture and 
temperature. They are still sensitive to shock and 
fracturing. The output is much lower than a Rochelle salt 
element, perhaps 10db for a similar working element. Since 
electronic amplification has developed to the point where 
the very high output of a Rochelle salt element is seldom 
necessary ceramic elements pretty much replaced them.
     Crystal or ceramic microphones, pickups, etc., are 
quite simple and cheap to produce in comparison to other 
generating elements such as moving coil or magnetic types 
but modern electrets are probably even cheaper and do not 
have the drawbacks. They require an electronic impedance 
matching circuit so do require a small amount of power. As a 
result they are nor direct replacements for other types of 
microphones (except carbon mics in some applications).
     The use of Rochelle salt crystals for microphones, 
etc., was first described by C. B. Sawyer in the Proceedings 
of the IRE in 1931 and he also held the original patent. The 
patent was licensed to the Brush Development Company who 
both developed practical applications and licensed other 
manuacturers such as Astatic and Shure. The Astatic D-104 
was the first practical crystal microphone to be offered 
commercially.
     BTW, nearly all Brush crystal headphones are dead or 
weak so they are likely to be only display items. They are 
not repairable.
     When working they are capable of quite high fidelity 
and have virtually infinite input impedance (more than 100K) 
so are useful for audio frequency bridges.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com