The mic I use for my Drakes is an EV638, so I don't happen to have an Astatic
D-104 available.
I've been asked to build a universal mic preamp, and it has to be compatible
with the D-104. Statements of the D-104 output Z on the web are highly
conflicting. Is it 2 Megohms, is it 200K? Many published mic preamps for the
D-104 have input Z of 100k, but many D-104 owners say that's way too low, it'll
kill the bass response. The published specs for the D-104 said simply "high
impedance". Which can mean anything - my EV 638, for instance, is spec'd as
"high impedance", but in fact, I measured it at 8kilohms over most of the audio
range.
In order to predict what my preamp is going to do to a D-104 frequency
response, I need to know the actual output impedance of the mic, at two or more
frequencies.
But you can't simply feed a signal into a mic and measure current...that's
likely to damage the mic. Instead, you have to perform a "load pull"
measurement, which requires you to get the microphone delivering a signal, then
load the mic. Here's how I do it with high Z mics.
1) Have a way of creating both a 500Hz and 2kHz tone into a loudspeaker. A
modern radio, zero-beat to a carrier, then tuned off by 500Hz or 2kHz will do
the job nicely. A test oscillator into an amplifier will work, too.
2) Set the mic to be tested about an inch from the speaker driven with the
tone. The correct loudness is "sort of loud". As loud as if you were
listening to a person speaking, but only an inch from their mouth.
3) Use an oscilloscope with a 10:1 probe (so that the input Z of the scope is
10 Megohms or more) to measure the output voltage of the mic.
4) Start placing resistors across the output of the microphone until you find
the value at which the mic output voltage divides by two. That resistor is the
mic's output impedance at that frequency.
I expect a genuine crystal mic will have an output Z that rises as frequency
goes down. Just two frequencies is adequate to build a first-order model.
I borrowed two D-104s locally - turns out that later production used either a
ceramic or a high-Z dynamic element, and both were quite insensitive to load.
So, for this measurement, the older the better, and if there's a preamp in the
base, it should be turned off or bypassed.
Thanks Drake-o-philes,
Dave W8NF
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