Maximum Impedance

The microphone is a signal source and a spec. for maximum impedance likely refers to the maximum output impedance of the microphone.  This would describe its ability to drive a load.

True, the resistor sets the impedance of the microphone, but I wonder what happens with values less or more than "maximum"...

What does the use of the word maximum imply... it's not the same as would be the word "recommended".

It sounds to me like it says, "Don't go above this impedance (for some reason) but you may go below this impedance.

In the context of a source impedance, let's say that you want to drive a load with a Z of 2.2k ohms.  The source Z is in series with the load Z and the voltage produced at the load will be 1/2 the source voltage.  Now, let's use a load Z of 47k ohms and we find that the voltage developed at the load is almost 95% of the source voltage (here the source voltage is the voltage internal to the source before the source Z).

The maximum source Z is specified so that you can predict the minimum voltage across the load.  If the source Z is made higher then the load voltage will be smaller.  By specifying the maximum source Z we are also specifying the maximum signal loss when connected to a load.

The 2.2k resistor you mentioned for the microphone most likely sets the source Z and was probably chosen to provide the microphone element with a proper load with which it can function.  Generally, you would want the load Z to be 10 times greater so as not to disturb the Z seen by the microphone element.

Yes, everything you're saying compares to what I've been doing... I just didn't comprehend what the word "maximum" was telling me.

To go over things using my numbers...

My LoadZ is 600-ohms, the rated microphone input to a Radio Shack Mixer.

The guideline I'm using comes from a Neumann Whitepaper for microphone voltage amplifier input impedance to pre-amp inputs..   "The source impedance should look into a load of a magnitude at least 5-times its value"...

Therefore I want sourceZ to be 120-ohms.

Your guideline quoted 10-times the magnitude stepping up from source to load, but it's the same idea.

120-ohms is well below the "maximum" of 2.2k, but it works and the level in practice is just right for the input to the mixer.

Unless I continue to misunderstand I may be doing it right. The fact that it works isn't enough to convince me.

Thank you.

It's fortunate this isn't a job, because my management style is to goof off first before not finishing a job.

In a rare and unheard of mistake I installed an electrolytic backwords with reversed polarity and ran it like that for about 5-minutes. Nothing flamed up and the circuit worked, but now I'm worried about that capacitor. It was receiving 4 VDC.

A further concern is the unknown age of the 50 uF, 16 V capacitor.

That's why today's endless search was first for my capacitor checking instrument and then for the manual that tells how to use it.

The capacitor reads a solid 68.5 uF.

Electrolytics connected backwards fail due to heating to the point of outgassing (exploding).  The damage to your capacitor would be due to heat and fortunately not enough was produced to raise the pressure enough to outgas.

Is there any swelling of the cap case or rubber seal?  In any case it would be wise to replace it even though it tests OK for capacitance since the seal may have been broken and it can eventually dry out.

Neil

Three newer ones are already standing by, will make the replacement tomorrow.

Thank you for the good practice suggestion.

Hi Carl

You can use a lower value resistor but the resister is used to set up a low DC current

to the microphone. Those microphones have a small built in op amp and that why

a DC vlotage is required to turn the microphone element on.

The impedance on those microphones is usually very high in value.

I like using the older microphones that have a impedance of about 500 ohms.

Some are also around 650 to 1000 ohms and have such a better Bass response.

Well they sound great over AM broadcasting.

I saw your post on the capacitor. Change it out. eventually it would dry out or just

short and cause you bigger problems.

I finally bought a Range Master transmitter and plan to buy four more of them

and link them all together. This way I have maximum coverage over my area.

As you know I do not do FM it really a waiste of my time. I tried the Decade MS-100S

transmitters and they have such a limitted range. I have to cover such a large area

to reach an audience.  I live in the middle of no where and homes are spread out very

far. So linking 5 transmitters would at least give me an audience of a decent size,

or I would just be wasting my time.

FM would not even reach one house since homes out here are spead about 1400ft to 2000ft apart ,and some are even a mile a part.

Now if my signal would go out about 4 miles west of me I could hit a housing area,and

hit more than a 100 homes there but that be a true challenge. I think the transmitters can be spaced only 2000 to 3000 ft apart. I was going to try and space

them in a circle format to get the most omni directional signal I could get.

Even if I spaced two transmitters going west it would be a very weak signal 4 miles

out west of me . If it make even makes it that far. I have notice that the maximum range on the range master seems to be 2miles  and if I space the second transmitter

2500 ft west of the first one chances are very slime that the signal would be crystal

clear 4 to 4 1/2 miles out from the first transmitter. I will still cover alot of homes

before the housing area but reaching that housing area is slim. Now everything north,south and east of me is homes very far and spaced out.

Yes I live in the country and the closest city to me is about 50 miles out.

It strange but some one built a housing area out in the middle of no where.

but it could be closer to 5 miles west from me,and once you pass that housing area

it goes back to homes very far spaced apart.

Anyways I am just going to place the five transmitters up and hope for the best.

I figure placing them in a circle is probably the best way.

Your Friend

Sean

Hello Seanw40!

You said: "It strange but some one built a housing area out in the middle of no where."

Not many years ago the place where I live was way outside the city limits and had about two farms and dense woods. Now it's a cram packed sprawling "metropolis" extending way off in all directions.

Listening closely to what you are saying I am guessing that you have access to a lot of land and can place transmitters in the distance to get them close to the housing? That is a dream most of us will never know... you are very lucky.

It is very interesting to think of ways to get power and audio to far away transmitters, and that will be part of the fun for you.

I'm putting a transmitter 100-feet behind my building and even that is a challenge that I think about.

Appreciate knowing the experience you shared with electret microphones. I'll be posting more about that this week.

Judging a microphone is more complicated than judging dogs at the animal shelter.

Objective scientific measurements can be made up to a point, but ultimately it comes down to "sounding good" or "sounding bad".

The impetus to fiddle with electret capsules began as a curiosity experiment in 1992 to find out how much worse these $1.68 capsules might sound compared to relatively expensive microphones. Unexpectedly the microphone that resulted struck me as sounding BETTER for voice recording than anything I had in a closet filled with big brand microphones. An audio-book reader that used my studio chose the electret experimental mic for her voice recordings after testing several choices.

The present version which was demonstrated on the Blare OnAir show found in another thread leaves me hearing too much bad studio acoustic, but the magic potion left out this time is compression.

Previously I've always added my compression recipe to voice recordings which mashes the corn flakes into the milk.

Let's go try the compression and be as subjective as possible.

I'm right when I include the compression factor in the judgement of how a microphone sounds on the radio.

We live in a noisy world, indoors and out, and to raise the level of our audio choices above the background noise we absolutely need compression for the recording and processing for the transmission.

Ignore for the time being any conflict between the recorded compression and the broadcast processing. No need to induce a headache.

Within the last 15-minutes I've come to several realizations which re-write the way in which broadcasting should be done, and during the next seven years I will be posting details here at part15(dot)us.

Let's get started.

By compressing the test audio from the new version of the home brew microphone, both the qualities and deficiencies of the mic are magnified, bringing better clarity to judgemental analysis.

Found: this current version of the microphone, V 13.0, is distorted.

Next step: figure out why it's distorting and remedy.

Up above Seanwk40 stated that electret microphones contain an op amp.

However, according to everything I reference the electrets actually contain an FET (Field Effect Transistor) which is different than an operational amplifier.

FETs are distinquished by very high input impedances and low output impedance.

Of course the input to an electret's FET is the charged diaphragm, with a resistor added to the output whose function is to set output impedance and gain.

A first design difficulty arrises when trying to set impedance and gain separately from each other.

My particular project has the compound difficulty of combining two electret capsules intended to act as a single microphone with two diaphragms in an omni-directional pattern. It was discovered that this approach gives a more "open" sound.

Looking back over the various circuits tried since 1992 some of them worked quite well, and I went forward owing to unanswered questions in my limited understanding of the electronics... when I used a part I wanted to know why it was needed or if it was needed or if there was a better part... if something worked well I wanted to know why it worked well...

You can see a symptom of my curiosity in the confusion over the word Maximum printed on the data sheet.

Following a careful review of everything done in this recent re-design of the Alpmic, we found that we were totally right about this design except for one vital adjustment that was not fully understood until now.

Everything is exactly according to the design objectives.

The microphone sounds smooth, clear, wide-range;

The output impedance is on the money (120-Ohms);

The DC Voltage applied to the capsules is 4.5 V (which the data sheet stated as "optimum");

Noise floor is below room ambience;

Discernible distortion is 0 %.

Stay tuned for the publication of this new design which will replace the one posted at the ALPB.