The Alpmic Version 13.0 is going on display.
At the next ALPB meeting this information will be handed over to Chairman MRAM for inclusion in the How To Projects Category.
Thank you to everybody for all the helpful input.
While storms were flashing at 3:30 AM I paged through the Radio Handbook and became inspired by a Push-Push Doubler Circuit that showed me how to design and build a figure-8 microphone.
But will it work as a true figure-8 using these same capsules? That will be the answer until which we have a working question.
NOTE: The Alpmic Schematic Link above has been supplied with a better drawing for easier reading.
Judging the sound of a microphone is most commonly done subjectively using ears as the test instrument.
The other test instrument is the voice or whatever sound is being used to judge how well the mic is reproducing the sound. They write books about subjects like this.
As a judge of microphone quality I've noticed several weaknesses in the human ability to evaluate what he hears. The same equipment will seemingly sound different at different times. The microphone that sounds spectacular today might sound grungy tomorrow.
To account for this inconsistency in subjective judgement it helps to make several evaluations over time and reduce the result to its average.
Carl,
Good to learn that you have had great results from your microphone design. There is one detail about the schematic which raises a question. The electrets with FET included which I have seen use the FET in a source follower configuration.whereas your schematic shows the FET used as a common source amplifier.
The reason a source follower can be preferred is that it is linear over a wide signal amplitude range where the common source configuration gives a non-linear square law response. If the common source is operated with small signals it can approximate a linear response.
The combination of two elements as shown will give some interesting phase response with enhancement and cancellation possible depending on what each electret "hears".
Nonetheless, can''t and won't argue with success.
Neil
It's a thing they do in the military. When soldier trainees are really put under scrutiny is during a "full dress inspection" when everything gets criticized from how their weapon is kept to how the foot-locker is packed by a highly decorated ten-star general wearing arch-lifters in his shoes.
Neil's observations about the circuits I've patched together with these tiny mic capsules arrives at the same time that I think there's still something I am overlooking or possibly do not understand, which has opened a full "back to the drawing board" moment.
Two things being held as "proven by experience" are these:
1.) Yes, of course there is certainly a phase shift which takes place when mixed acoustic components are displaced-in-space by any amount, but because of the tiny size of the capsules the phase shift is able to be very minimized by placing the capsules as close-togteher as possible.
Side-Bar: The same phase-shift problem occurs in common loudspeakers between woofer/tweeter elements especially at the crossover frequencies;
2.) Comparing the difference between using a single capsule vs. using 2 capsules in-phase but facing opossite directions... the latter gives a more "open" omni-directional quality which is a matter of taste when it comes right down to it. It would be easy enough to install a switch to choose between directional and omni-directional.
Commercially produced condenser microphones have the same internal phase issues when they are the double-diaphragm type, but the physical closeness of the flat capsules makes the off-phaseness all-but undetectable as a performance factor.
Later this evening I will take the liberty of posting the original simple microphone design printed by Radio Shack with special instructions for Neil to re-design the circuit based on specifications which I will present.
Not trying to be critical of what you are doing rather just trying to learn. I did resolve my question about how the FET is connected. Reading some datasheets revealed to me that these are two wire microphone elements where the ones I used in the past were three wire units. The two wire units have FETs connected as you illustrated where the three wire units were connected source follower.
The three wire units were instrumentation grade microphones used for precision acoustic measurements so they likely needed the extra linearity given by the source follower.
Neil
Good sound on your mic, Carl, in that recording, the compression isn't biting down when you start sentences, that's really fast action for that amount of compression, it seems to me.
Neil, I have no pride of worksmanship in this project and welcome analysis and critical comment, since there are things going on that I don't completely understand. Therefore you can park the politeness and won't dent my fragility.
I've heard about "3-wire electrets" but haven't seen any living examples.
Linearty sounds like a good idea. Can it be achieved with 2-wire electrets?
I'll be posting a "Design This Mic" challenge either tonight or tomorrow...
My slogan "Slow But Better" requires not too much hurry.
Carl,
I learn more from asking than answering so my motive is not just to be polite (though there is merit in that).
"Linearty sounds like a good idea. Can it be achieved with 2-wire electrets?"
I suppose so. Common source FET amplifiers are non-linear for large signals but are much better for small signals. I would presume that the electret signals are small enough to not cause problems. This seems to be confirmed by your success in getting quality audio from your microphone.
Neil
Reading up on Common Source FET amplifiers, it appears that access to the Gate of the transister is necessary, and that's not possible with the 2-wire electrets.
I'll hunt around to find an example of what a 3-wire electret looks like so I can learn what that 3rd wire is for.
The charged diaphragm is present at the Gate of these mics.
The 2-wire electrets can be wired to output audio from either the Drain or Source.
In my studies I have also collected schematics of several commercially sold condenser mics which employ electrets, and they vary from each other in numerous ways.
In Post # 16 we enthusiastically declared success with Alpmic Version 13.0.
Today, however, all that has been withdrawn as we unveil Version 14.0b.
A full discussion of the change can be heard on the audio sample.
Blare OnAir Lite - Alpmic 14.0b
Schematic Diagram Alpmic 14.0b
It's been about a year since this thread saw any activity, and now we have something to report.
Looking back to Post # 19 Neil Radio8Z offered:
"There is one detail about the schematic which raises a question. The electrets with FET included which I have seen use the FET in a source follower configuration.whereas your schematic shows the FET used as a common source amplifier.
"The reason a source follower can be preferred is that it is linear over a wide signal amplitude range where the common source configuration gives a non-linear square law response. If the common source is operated with small signals it can approximate a linear response."
We've applied the suggestion into Version 16.0 of the Alpmic and we may be zeroing in on a best way of arranging the circuit components for the best of all features.
Alpmic V 16.0 will be in use at the ALPB Meeting on Saturday and I hope you are on board, Neil, for this gathering.
Later down the line I will A.) Repair the earlier links in this thread; B.) Provide a new audio sample and schematic of the newest configuration.