Astatic D-104 Microphone Output Impedance Matcher

The venerable Astatic D-104 microphone has been a favorite among CB'ers and amateur radio operators for decades. The D-104 is appreciated for its clarity-emphasizing frequency response that rises in the presence band. This is due to the designs of both the crystal microphone element and the microphone head assembly with it's purposely placed acoustic padding. The D-104 is a very high impedance microphone (>1M Ohms) and it is commonly known that the low frequency response will drop off at a much higher frequency if it's terminated with a low impedance. Feeding a 600 Ohm amplifier input will demonstrate this dramatic effect.

Some D-104 microphones have a two stage transistor pre-amp in the base. It might be a surprise that even the manufacturer-designed circuit loads down the crystal element to the point where low frequency attentuation occurs.

Is this a bad thing? Some operators logically argue that only the 300-3,000 Hz voice range is necessary for maximum readibility, particularly during contests or challenging band conditions. The D-104 will probably sound fine regardless of the loading for those applications, but others might prefer a more "full" sound with a broader and flatter frequency response. Very good results can be achieved with proper impedance matching, and a properly loaded D-104 will sound like an entirely different (and much better to many) microphone.

Increase transmitter input impedance - This is the most common fix and it can be as simple as changing the value of a grid resistor. Does it increase the low frequency response? Yes, but...

The problem: This fix raises the impedance of the entire circuit from the microphone element through to the transmitter input, and that circuit includes the shielded microphone cable. As the impedance goes up, so does the effect of distributed capacitance along the mic cable, and that creates an attenuation that increases with audio frequency. More low end is noticed partially because of the rolloff at the high end. This long high impedance circuit is also very sensitive to RF pickup.

Of course, plugging the D-104 into a separate impedance matching amplifier has these same problems. I have to admit that this was my original plan until the distributed capacitance issue became apparent and thus the importance of locating an impedance matching circuit close to the microphone element.

Circuit Description:

To avoid the problems outlined above, this circuit must be located in the base of the D-104. That shouldn't be a problem because there is plenty of room for the small circuit and a 9V battery. The shielding of the D-104 base and the relatively short leads from the microphone head to the base keep RF pickup and distributed capacitance to a minimum. R1 should be 10M Ohms or higher for a light load as well as some gate protection if you switch head elements. R2 can be omitted unless you find that there is too much audio output. The end result is a very flat frequency response with 1.5 dB rolloffs at 20 Hz and 45kHz. In other words, you'll hear exactly what your D-104 really sounds like.

This circuit draws 0.5 mA at 9VDC. A power switch (not shown) can be used to turn off the circuit when not used. Some D-104 T/R switches have extra leads that can be used for this purpose. I used the spare blue (N.O.) / green/white (COM) / white (N.C.) with green/white grounded, blue connected to the negative (-) lead of the 9V battery, and white connected to the output. Doing so grounds the output when unkeyed and completes the power supply circuit when keyed. I've seen variations on this wire color scheme so you should use a test meter to determine what colors are used for your particular model. If you don't have spare leads, the negative lead of the battery could be connected to the keying lead via a blocking diode (with the anode connected to the keying lead). Be sure to use a diode of proper value, especially if you have a high voltage keying circuit!

The output is taken off the source of the FET for two reasons. First, it helps keep this circuit at unity gain to reduce the need for large changes of input level on your transmitter if you switch mics. That's why this circuit isn't called a "pre-amp". However, you might see an increase in level because the microphone element won't be loaded down by a low impedance anymore.

The other reason is so that the output phase is the same as the input. I have been surprised and frustrated by amplifiers that invert the audio phase. Proper phase is a BIG DEAL when it comes to AM or SSB transmission and any asymmetry in your voice should result in greater positive than negative modulation to maximize transmitted audio and minimize distortion from negative clipping or overmodulation. If you find that you are reaching 100% negative modulation (carrier cutoff) well before you reach 100% positive modulation then your audio phase could be reversed. The two simplest solutions are to either swap the leads on the back of the D-104 microphone element or have the option to select the proper phase by using this modified circuit:

This is basically the same circuit except that a polarity reversing DPDT switch is added to take audio from either the FET source (in phase) or the FET drain (phase reverse). The resistors on the source and drain are of different values to keep the circuit gain nearly the same regardless of switch position. In one position, positive sound pressure on the microphone element will result in a positive output increase. Switching this phase switch results in the opposite, which solves the problem if your transmitter inverts the phase between the mic jack and the modulator output.

There are two reasons for not putting this switch on the yellow and green mic leads. First, some D-104 models (mine included) were factory wired so that pins 1 (GND) and 2 (MIC LO) are jumpered at the three pin connector at the top of the mic stem where the head attaches. This makes the yellow lead always "hot" and the green lead ground, unbalancing the microphone circuit and defeating the ability to switch phase in the mic leads. So, to have a more universal solution for all D-104s and avoid rewiring the connector, the switch was placed at the circuit output. The second reason is to reduce noise by not running the highest impedance part of the circuit through switch contacts.

The only remaining issue is reducing the "thump" heard at turn-on. This is caused by the charging of the DC blocking cap on the output. Lowering the value affects low frequency response more than it helps. This may not even be an issue for most transmitters, but I want to figure something out. An ON/OFF switch could be used instead of using the T/R switch to avoid the pulse each time the mic is keyed.

This project is a work in progress and your input is always welcome! Stay tuned...