The AM Forum

I decided to update my D-104, big mistake, my old element worked just fine.

So, I decided to put the old element back in.

Stupid me, I didn't write down which wire (Red or White) goes to (+) and (-).

Can anyone help this dufus ?

Tnx, Dick, W1KSZ

It doesn't matter.  A xtal microphone is an AC source.

--Shane
KD6VXI

If you enjoy ops on AM, then it could make a difference and the wiring will determine the phase of your typically asymmetrical voice energy.

Wire the microphone either way and observe your modulation on a scope or mod monitor. If you tend to hit the baseline (carrier cutoff at -100%) more than +100% on average then reverse the leads. If your positive energy is greater than +100% when the negative peaks are just reaching -100% then your microphone is optimized for that transmitter.

Wiring the microphone in this way will dramatically reduce distortion depending on your transmitter if you are using it on AM. Think of this as an opportunity to wire your microphone correctly for your transmitter.

The large metal back plate of the D-104 housing forms a capacitor with the side of the large crystal element that faces it. If the wires are connected to the element such that the grounded wire is connected to the side of the crystal element that faces the back plate... then the total capacitance across the two wires  (produced by the crystal element and the housing in parallel) will be significantly less than the other way around.

The equivalent circuit of the D-104 crystal element (not including the housing) is a time-varying source of electric charge (coulombs) whose value is proportional to the pressure produced by the incoming acoustic wave... in parallel with about 0.001 uF of capacitance.

The effect of the added, parallel capacitance of the housing, when the microphone is feeding a preamplifier with a high input resistance, is:

A) The voltage output, at audio frequencies higher than the low frequency rolloff frequency, is reduced by the ratio of the capacitance of the crystal element (i.e. 0.001 uF) to the total capacitance of the crystal element in parallel with the capacitance of the housing. With the non-grounded wire connected to the side of the crystal element the faces the rear housing cover plate... this will be roughly a factor of 2 reduction in output v. the opposite wiring.

B) The low frequency rolloff frequency will be reduced (i.e. improved) by the same factor as reduces the output in A).

I suggest that any desired phase reversal be introduced in the audio chain after the microphone  preamplifier

Stu

^^^^^ further proof that no matter how much you think you know, there's always more to learn :-)

Now I've got to check my D-104!

I agree - quite the explanation - I just learned a lot!

The very small planal area of the wire's capacitance at distance to the planal area of the mounting plate is quite small compared to the inherent 0.001uf of the crystal element in equivalent circuit,  a few pf at most vs. 1000pf.  How was the 2x reduction in gain determined?  Actual measurement?

Rick

It is not the capacitance between the wire and the back plate that is of importance here. It is the capacitance between the face of the crystal element that is closest to the back plate, and the (grounded) back plate.

If the wire that is connected to ground is attached to the side of the crystal element that faces the back plate... then both sides of that capacitor are connected to ground.

It the wire that is not connected to ground is attached to the side of the crystal element that faces the back plate... then that capacitor is connected across the wires (in parallel with the crystal element.

I discovered this effect when I tried to swap the connections to the crystal element of my D-104 in order to make its "phase" the same as my other microphones.

Stu

With all due respect (and not wishing to start a flame war), I'm not sure I buy the capacitance to back plate theory.  I use a D-104 crystal mic on a Collins 32v3.  I have had it for quite a while.  About a year ago I started to get poor audio and low positive peaks.  After research, I concluded that the little red drop of epoxy that connected the diaphragm to the plastic pushrod that warps the crystal was cracked in two.  A drop of special jewelry cyanoacrylate (bonds to metal) fixed the problem.

However, while looking for info is saw several pictures of crystal elements that had been opened for post-mortems.  I attach one I found on this forum.  The crystal itself is very small inside a very large (relatively) bakelite or plastic housing. The crystal is pretty much in the center of the housing (front to back) and the element assembly is inside the metal "lollipop" with metal on all sides separated by foam inserts on the back and sides and the fuzzy tan sound deadening material between the diaphragm and the heavy metal screen.  The entire metal lollipop is grounded and pretty much symmetrical around the crystal.  I would be surprised if the element itself had significant capacitance to ground compared to the several inches of wire running the length of the stand.  Further, I would be surprised if one face of the crystal had significantly different capacitance to the surrounding shell than the other face. 

At one point a few years ago (with a different antenna configuration) I thought I had found RF in the D104 element and after opening it found that the ground of the lollipop shell depended on the the connector screw ring. At least mine did not come from the factory with an actual ground wire, and it appeared that the screw ring wasn't doing a good job - when I got close to the head the RF in the audio increased.  Since it was a 3 pin connector, I used the spare pin to run a ground wire to the shell. The imagined issue sounds farfetched - but the RF went away when I added the ground. 

Found this in my D-104 files.  Maybe it will help.  Presume you are using a G stand.

While all the responses were interesting and worth reading, I still didn't know which wire went were.

So, I flipped a coin, soldered it up and got on the air. One of the locals said the audio sounded just
like my normal voice.

I declared victory and quit while I was ahead !!

Thanks to all who replied.

73, Dick, W1KSZ