G7 mic problems.

Hiya guys,

After nearly a year I've finally -almost- finished my G7 mic. I plugged it into my desk tonight and found that it was working. However, I noticed that at random intervals the diaphragm seems to get 'sucked' onto the backplate. It just randomly occours, with no correlation to what position the patter selector is in. What do you think could be causing this? The mic still has a few other minor problems but I reckon I ought to fix this first...

I've tried measuring the polarizing voltages but seem to get completely weird readings, like 2.6V and stuff. Basically, I have very little idea as to what I'm doing and need help... argh! :roll:

Oh yeah, I also get a really loud *BANG* whenever I change the pattern. Do I need a make-before-break switch or what?

Sounds like you have too much polarization voltage on capsule and you are measuring it after 1G resistor. You should get accurate voltage from junction of two 470K resistors. What is your B+ and what capsule are you using?

Me also..
what's the correct polarization current?
I'm using peluso 2X CK 12 and 2X CK67 capsules.
can be a problem?

Jakob's explanation at Gyraf site:

To act like a microphone, we need a voltage charge across the capsule. When the capsule changes capacitance (that is: the distance between the electrodes are changed by sound pressure) is changed, so does the amount of energy that can be held stored in this capacitor. But as we are charging/discharging with a hell of a small current - the 1GigaOhm resistor - the current really has nowhere to go, and so results in a varying voltage potential across the capsule.

The polarization scheme can be a little difficult to understand at first. To acheive simple remote switching of the polar patterns, a single variable voltage is used for this, only changing the charge of the back part of the capsule.

We want to keep the front electrode of the capsule at ground potential - 0V - at all times, both to act as a shield for incoming electrical disturbance and to avoid electrostatically attracting too much dust from the environment. So to keep a voltage charge across the capsule, we bias the center electrode by the means of two 470K resistors dividing our 160V supply voltage in half - resulting in +80V.

Now we have -80 volts at the front electrode, referred to the center electrode. If we now bias the back electrode with the same (-80V ref. Center = 0V polarization voltage), a positive sound pressure on the back capsule will have the same voltage-potential effect on the center as when applied on the front capsule. This sensitivity pattern is then OMNI directional.

If we polarize the back electrode at +80 Volts, no voltage difference will exist between this and the center electrode, already offset at +80V. This in effect mutes the back capsule, resulting in the CARDIOD directionality.

At last, if we polarize the back capsule at +160 Volts, we'll have a charge of +80 Volts relative to the center electrode. Now a positive sound pressure applied to the back electrode will produce an 180 degrees out-of-phase signal compared to the front capsule. When applying a sound pressure from the side of the microphone, so both capsules sees the same sound pressure, the signals coming from the two capsules will be in opposite phases, effectively canceling each other. This is the FIGURE-OF-EIGHT directionality.

The changing voltage potential on the center electrode of the capsule is picked up and amplified by the EF86 Pentode, which is wired in triode mode - just like the VF14 pentode in the U47. It's input resistance has to be kept VERY high not to disturb the charge/discharge of the capsule, so we use a 1G resistor to bias the grid to 0VDC.

Capiche?

:grin:
SI, but in the next days i would like to measure all the volts around in my microphones.

for a capsule without center screws you might want the voltage under 60V and over 40V depending on how the skin is tuned

[quote author="Nat"]Oh yeah, I also get a really loud *BANG* whenever I change the pattern. Do I need a make-before-break switch or what?[/quote]

Hi Nat
Think you need break before make, otherwise for a moment you connect points with 80V difference. I got sparks when I first wired mine up with make-before-break.

What capsules are you using? Look at Gus's suggestion.

Thanks for the response, guys. I'm using the Soundking SKEE102 or 106. I can't remember which is which, but it's an edge terminated capsule. I thought that someone had used this capsule without problems, but maybe not. I'll try fitting a center-terminated capsule and see what happens. Haven't had any time since I posted but I'll spend some time tonight measuring voltages before the 1G resistor and see what I come up with.

Hi
Many time I read about 50,60,80V on capsule. Example, on G7 after two 470K we can have this voltage but after 1G and after 33M connected to back we cant read high voltage.
Am I right?
Duka

Yeah, I think the impedance of the measuring equipment is too high to measure voltages after the 1G resistor. I don't really understand it though.

[quote author="Nat"]Yeah, I think the impedance of the measuring equipment is too high to measure voltages after the 1G resistor. [/quote]

You mean too low... The input impedance of the DMM loads voltage of such a Hi Z.