The think tank at part15.us will know the answer to something that confuses me.
The output impedance of a condenser mic I’m building is described as “2.2k (maximum).”
The think tank at part15.us will know the answer to something that confuses me.
The output impedance of a condenser mic I’m building is described as “2.2k (maximum).”
That impedance is determined by a 2.2k resistor between the battery and the capsule.
Does “maximum” mean the NUMBER 2.2k shouldn’t be exceeded?
Or does it mean that the resistance in ohms shouldn’t be GREATER, which is an inverse number.
Totally unsure.
Mic Z
It is desirable to have a low impedance for an audio source so that noise may be reduced and the signal will not be lost due to the load impedance. The spec. on maximum impedance here means the mic. impedance will not exceed this number. This gives a predictable maximum signal loss for a given load.
Neil
Knowledge vs. Don’t Knowing
Niel:
See, I know you know what you mean, but you have simply repeated the problem.
Does that mean that 3k would exceed the MAXIMUM of 2.2k?
Or does it mean that 1k would exceed the MAXIMUM of 2.2k?
My neurology is compromised.
Mic Z
Carl,
Sorry I wasn’t too clear about this. The wife arrived just as I was posting and I cut it short to talk to her.
So, I’ll try again. This specification is to set the mic. output impedance and the lower the better but there is a tradeoff. Low Z is desirable to reduce noise and loading effects but as I understand your post the output Z is set by a series resistor. (Even if the R is not physically in series with the output it appears so in the AC equivalent circuit.) The output voltage from the mic. will be determined by this R. Too small, too little output. Too large and noise and loading effects (cable capacitance cutting high frequencies for example) are a problem.
So my answer to your query is that the 2.2K is the maximum R to use and a smaller one will work with reduced signal strength.
This is based on what I picture you are doing which could be wrong. Anyway, hope it helps.
Neil
Hmm..
Is the mic element an electret condenser?
Still Not
Yes the capsule is electret condenser microphone.
The question I originally asked is still not answered by the words “larger” or “smaller,”
since “50,000” is a larger number
but 1-ohm is a larger impedance.
Does the word MAXIMUM in my original note refer to the numeric description
or to the idea that 1-ohm is a large impedance.
Got It
Suddenly there is a sign of brain activity and I see that MAXIMUM applies to both the number AND the impedance.
Example: a MAXIMUM of 2.2k-ohms means that 5k-ohms would be MORE than the maximum, both numerically AND impedance-wise.
Therefore: values BELOW 2.2k-ohms would be within bounds numerically and impedance-wise.
But I don’t think I have dislexia.
Thank you for guiding me to the light.
Check This Out
With this new found knowledge here’s what I’ve done.
The spec sheet says the mic input impedance to the mixer is 600-ohms. So, I changed the 2.2k resistor to a 600-ohm resistor, which is the voltage from a 9VDC battery to the electret mic capsule.
Also, for the best low end possible, I changed the 1uF NP capacitor to a 100uF NP capacitor.
Does it all make sense, or are there better suggestions?
Mic Frequency Response
Carl,
I see your point of confusion and am glad it is resolved. It is true that for capacitance larger C means smaller Z but for resistors and inductors larger values mean larger Z.
Regarding your low frequency “breakpoint” this can be calculated as follows. Use the R (600 ohms) in the equation F = 1/(2*pi*R*C). Doing so with C = 1 uF yields F = 265 Hz. Using a 100 uF gives F = 2.65 Hz. I would use C = 10 uF for F = 26.5 Hz.
Alternatively you can solve the equation for C if given R and F as C = 1/(2*pi*F*R) and set the F anywhere you want.
Neil
Addendum for “Mic Frequency Response”
Let me add to the above post some information which may go beyond what you want. The R in the equation should include the R of the input to the amplifier since this will also affect the low frequency point. Using the values I did in the previous post gives the highest low frequency cutoff and since the input R of the amplifier is not zero the actual frequency cutoff due to the 600 ohms will be smaller than 26.5 Hz as calculated. The overall system frequency cutoff will be determined by both the mic. circuit cutoff and the amplifier cutoff frequencies, the higher of the two dominating.
Once again semantics of “highest of the lowest” can be misleading but I hope not.
Bottom line: You should be good to go with C = 10 uF.
Neil
Splendid
In goes a 10uF non-polarized and the circuit is declared DONE!
Thank you for the class room.
Series Resistor
If I’m not mistaken, the series resistor also provides a DC bias voltage to the microphone element. This voltage is most often called “phantom” voltage. It is used specifically to operate the condenser (capacitor) mic element. Phantom power supplies can run from 9 volts on the low end to 48 volts on the high end. Recording studios use 48 volts most often, hence the 2.2k ohm resistor. It sounds like you got it figured out. As a side note: never operate phantom power on a dynamic or ribbon microphone. This practice lets the smoke out of the microphone and it stops working.
About This Electret
Back in the 90s I became frustrated because the expensive condenser and ribbon microphones I had failed to record the spoken voice to my taste, although they were great on musical instruments. So, just for grins, I got some 1.97 cheap capsules from RS and doubted my sanity when I found that one part #, 270-085, gave me the sound I wanted! But that seems absurd. Yet I went into laboratory mode and developed what I now call the Alpmic, and am up to version 12.3.
At that time the part was discontinued, so I bought every remaining capsule in town and have maybe two dozen of them.
The station microphone is built into a chrome drain pipe with window screen grilling sticking out of the top, and consists of two capsules facing 180-degrees front and back, which gives a very open omni-directional pattern good for one-mic interviews. The capsules are wired in series.
I still try other mics, but they always need EQing and this does not.
This mic is also great for recording outdoor air sound, with birds, cars, planes and church bells.