[Boatanchors] D-104 Replacement Element

[Richard Knoppow]

----- Original Message ----- 
From: "Rob Atkinson" <ranchorobbo at gmail.com>
To: "Mark K3MSB" <mark.k3msb at gmail.com>
Cc: "Boatanchors List" <boatanchors at mailman.qth.net>
Sent: Tuesday, June 17, 2014 6:38 AM
Subject: Re: [Boatanchors] D-104 Replacement Element


> Not me, but it is interesting.  I think the original 
> element had a
> bigger diaphragm.  Not sure about the Z.  I had some idea 
> the original
> impedance was several megohms.  I see this new one is 25K. 
> I'd say
> that if you get one and it works out okay, order 2 or 3 
> more.  These
> small companies have a way of suddenly vanishing.
>
> 73
> Rob
> K5UJ

     Crystal microphones (and phono pickups) are peculiar 
because the impedance is mostly capacitive. The reason they 
need such a high load impedance is that the capacitance acts 
with the resistance of the load to form a high pass RC 
filter.  If the load is too low the lows are rolled off. 
Also, any parallel capacitance from the connecting cable 
acts to lower the level.
     Crystal microphones and phono pickups were popular 
because they had very high output level, thus saving the 
need for more electronic amplification, were relatively 
cheap to make, and could have quite good fidelity.  The 
D-104 has a rather steep peak at around 3500 hz and rolls 
off below about 200 hz. This was done delibrately to 
accentuate the speech range that contributes to 
intelligbility.  Astatic knew how to make flat frequency 
response mics, for example the T-3. The characteristic of 
the D-104 is partly from the diaphragm resonance and partly 
from the rather large case which causes a rise in response 
due to diffraction.  Any element installed in the case will 
be affected by the diffraction rise but may not have the 
same response as the original.
    Unfortunately, crystal elements are vulnerable to heat, 
moisture, and mechanical shock. Astatic was pretty good 
about their method of sealing against moisture but the heat 
problem is inherent in the Rochelle salt material. I've 
forgotten the limit but its pretty low, say around 105F. The 
crystals can be fractured by mechanical shock, say from 
dropping, or from a strong electical signal or direct 
action. The latter two are not important for microphones but 
crystal elements used for phono pickups didn't like being 
scraped across a record and those used for cutting records 
could be damaged by overdriving.
    Ceramic elements began to replace crystals when they 
became available. The output level is signifacantly lower 
(for the C-104 is about ten db lower than the D-104) but 
they are a lot more rugged.
    All of these have been pretty much replaced by other 
types of elements since the cost of amplification is now 
insignificant.
    Most crystal mics were intended to be worked into an 
"unloaded grid", meaning something like a tube amp with a 
grid leak and input resistance of at least a megohm.
    Somewhere in my e-mail I have the name of some outfit 
that makes a replacement kit for the D-104 including some 
shock mounting material, I will try to find it and will post 
it here if I do. I have no idea how close this kit is to the 
original.


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