Why transceivers use low-Z dynamic microphones

[k4kyv]

Older tube type amateur transmitters like the DX-100, Valiant, Ranger, etc. nearly always were designed to use a high Z crystal microphone, and the microphone input at the jack usually went directly to the grid of the first speech amplifier stage with nothing in between except the grid leak resistor to ground.

Ever wonder why riceboxes and solid state CB radios use low-Z dynamic mics instead of crystal?  This also explains why Astatic put the "power mic" amplifier module in the base of the D-104 G-stand.

When solid state amplifiers first began to supplant hollow state, before FETs came into common use, the bipolar transistor had too low input resistance for a xtal mic, which typically needs to work into 5 megohms of load or more. So the 150 or 500 ohm dynamic microphone became the standard, and remained so even after the FET gained widespread use.

Paradoxically, the manufacturers of crystal mics recommend a load impedance of 5-10 megohms, but the RCA tube manual usually recommends a maximum of 1 megohm of resistance in the grid circuit, to prevent the reverse grid current, typically about 0.2 microamps with most tubes, from affecting the bias of the tube and shifting the operating point on the characteristic curve into a region that would produce distortion.

I use 10 megs into a 12AX7 on mine, and this doesn't seem to be a problem. Until  recently I used 5 megs.  But with the 10 meg  resistor, I do notice that with some tubes, the grid reads a tiny fraction of a volt, typically 0.17 v, negative with respect to ground when measured with a DVM.

[ke7trp]

I just picked up an Eico Resistor box for this purpose. I am going to put it inline and change the Drid resistor step by step and chart the results. It will a tedious job fast. I was suprised to find EVERY resistor EXACTLY to the 0 in the correct value in this old Eico box.  Unreal.. They must have used high quality resistors. 

I normaly use 2Meg for D104s.  Lots of Ham type transmitters run ALOT less.  Globe ran 47K.  They figured you might use a Dynamic type mic.  Lots of people could have better AM if they just replaced this resistor

I have never tried 5 or 10 Meg.  I will report back when I try this.

One thing I did noticed is that newer D104s with the board on the bottom are low impedance. I have had to modify them to work correctly with the older gear. Most of the time, I just bypass the Mic.  The Globe KING will not reach 100% mod with a standard D104. Even when modified. The old Hand wired power D140 works FB though.

C

[AB3FL]

I would also think that the lo-z mic inpoot would be less susceptible to picking up RF. 


Tom - AB3FL

[k4kyv]

The reason for the high grid leak value is that the xtal mic is equivalent to an ideal a.c. generator with about 500 pf in series.  The higher the load resistance, the better the lower frequency response.  But there is a limit, because with enough resistance in the circuit, the grid voltage begins to wander from its nominal value.

I'm not sure about FETs.  Maybe someone with more experience with solid state design could shed some light on this. There might be some info on some of the audiophile (as opposed to audiophool) sites.

[AB2EZ]

Don

I use a 10 Megohm input impedance FET-based preamplifier with my D-104... but as an alternative, please consider the three (3) attachments.

The first attachment is the simplified equivalent circuit of a D-104... which is the same as what you decribed (except I'm using 1000 pF as the series capacitor).

The second attachment shows a D-104 with a .01uF capacitor placed across the output (anywhere between the microphone cartridge and the input of the rig's preamplifier).

The third attachment shows the Thevenin equivalent circuit of the combination of the D-104 and the parallel capacitor.

As you can see, the parallel capacitor has two effects:

a. It reduces the output of the D-104 by (in this example) a factor of 11
b. It reduces the required load resistance of the preamplifier by a factor of 11 (to get the same low-frequency rolloff)

So... assuming you have some gain to spare (or are able to increase the gain of later stages)... you can operate a D-104 with a 1 Megohm grid leak resistor... and achieve the same low frequency cutoff that you previously would receive with an 11 Megohm resistor... if you add the parallel capacitor.

Stu