Comparison of D-104 microphone heads, and the implications

Hi

There has been some recent discussion of D-104 microphone heads (elements)... and various approaches to designing a preamplifier to work with a D-104.

Although there seems to be some disagreement regarding the electrical equivalent circuit of a D-104... it is my opinion... based on both the physics of how a crystal microphone operates, and measurements I have recently made... that the electrical equivalent circuit of a D-104 is a voltage source in series with a capacitor. There is no series resistance.

Recently, I measured the series capacitance of two (2) different D-104 microphone heads (elements) that I have: by placing a .01 uF capacitance across the microphone head, and using the resulting output as the input to a 10-megohm input-impedance instrumentation amplfier.

Note: I am NOT using an "amplified" D-104.

The results were as follows:

When I measured a recent-production D-104 microphone head (element), that I have been using on the air for the last 6 months, I found that placing a .01 uF capacitor across the output resulted in a 10 dB (approximately a factor of 3 in voltage) drop in the output of the microphone... with no significant effect on its frequency response or the way it sounds. This is exactly what one would expect if the series capacitance of that microphone (element) is approximately 5000 pF. The .01 uF capacitor in parallel with the microphone (element) forms a 1:3 capacitive voltage divider.

When I measured another D-104 microphone head (element), that I happen to have as a spare, I found that placing the .01 uF capacitor across the output resulted in a 20 dB (approximately a factor of 10 in voltage) drop in the output of the mcirophone... with no significant effect on its frequency response or the way it sounds. This is exactly what one would expect if the series capacitance of that microphone (element) is approximately 1100 pF. The .01 uF capacitor in parallel with the microphone (element) forms a 1:10 voltage divider. I believe that this 2nd microphone head (element) is an older one.

What are the implications of this with respect to driving a preamplifier with a D-104 microphone head (element)?

Well, for the case of a D-104 microphone head (element) that has a capacitance of around 1100 pF, i.e., the older-production units: if you drive a preamplifier whose input resistance is 1 megohm, then the input to the preamplifier will drop off by 3dB at a frequency of 145 Hz. If you want the response to extend down to 50 Hz, then you need to use a preamplfier with an input resistance of 2.9 Mohms. This is the advice that most people give with respect to interfacing a D-104 to something like a Ranger... i.e., "increase the input grid leak resistor value to at least 3 megohms".

But, for the case of my more recent-production D-104 microphone head (element), since it has a series capacitance of approximately 5000 pF... if you use it in conjunction with a preamplifier whose input resistance is 1 megohm... then the low frequency 3 dB cutoff will be 32 Hz. Thus, for that "flavor" of D-104 microphone head (element), a 1 megohm input resistance is fine.

Best regards
Stu