A circuit model for a microphone?

[bubbamc119]

Hi people.

For my final year university project I will be designing a low noise microphone preamplifier. Now I need to find a model for a microphone that I can use for analysis and start the design.

I have found a circuit model for an electret condenser microphone here;
http://members.optusnet.com.au/~esey...amplifier.html

Now I need models for a condenser microphone and/or dynamic microphone. Can somebody help? If you have any references that would be great.

Now for the amplifier it will have a differential front end with gain control, that is all I know at this stage.

If anyone has designed a mic pre could you please chime in!

Thanks in advance.

Beau McMurray

[EC8010]

Try the "Microphone engineering handbook" by Michael Gayford. Yikes! I've just seen the price. "The Microphone Book" by John Eargle is good and much more affordable. Have a look through the semiconductor manufacturers' sites

Oh, and as a former project supervisor, you need more than just a low noise amplifier to make a project that will do any better than a bare pass. And you haven't long before it's due in...

[kelticwizard]

Dr. Marshall Leach has a paper on his website on SPICE modelling or various audio devices, one of which is a condenser microphone.

In that article, he has the electrical, mechanical and acoustical models for the condenser microphone, as well as a whole SPICE program written out, along with a glossary of values you can use and what those values actually are.

I'm messing around with the vented box program myself. I do not have a technical degree and even I am having success figuring it out, bit by bit.

http://users.ece.gatech.edu/~mleach/...ce_electro.pdf

[Minion]

As for Mic capsules and curcuits I don"t think the one you posted in your link is Ideal....If you read the whole Page you will have noticed that even the writer of that Page hasn"t tested the performance of the Curcuit ,Pluss it is Quite complex were it doesn"t have to be.....

I suggest useing the Schoeps Curcuit (also called the Alice Curcuit) with a TSB-165 Capsule.... A search on Google will come up with many Hits ....This is a Long used and reliable Phantom Powered Electret Mic curcuit.....

As for a Mic pre , I just finnished a 2 channel Mic pre based on the INA217 Chip....
The INA217 Provides Theoretical Lowerst Noise,Differential Ballanced Inputs, wide Bandwidth.....

My version also has Ballanced outputs and a DC Offset Controll Loop with 60db of gain and Phantom power and Has Hot Plugging Protection and runs off of a 2x15v +48v Power supply which is also an easy build.....

If you want me to Post the schematic for you I can do that.....



Cheers

PS: sorry for the Crappy Picture....

[bubbamc119]

Thanks for the help guys, appreciated.

Originally posted by EC8010
Oh, and as a former project supervisor, you need more than just a low noise amplifier to make a project that will do any better than a bare pass. And you haven't long before it's due in...

EC8010 what do you suggest I do in order to increase my grade? My supervisor would have been happy with a low noise general differential stage only, I suggested the mic pre to make it more complex.

My supervisor gave me the following project objectives;
"This project is a research project where the emphasis is on learning rather than achieving prior stated outcomes. The student is expected to systematically work through the relevant theory/analysis, translate this into appropriate circuitry and then undertake appropriated experimentation with a view to correlating the theoretical and experimental results."

It is due in November. Thanks for the links and ideas, keep 'em coming!

[unclejed613]

for simple circuit analysis, you will most likely find that measuring the resistance, inductance, etc of various microphone elements and making a relevant spice model for them, is much more useful than using models you might find on the web, unless you get an actual model from the manufacturer. the simple model for a dynamic element for instance will consist of a voltage source, a series resistance, the voice coil inductance and the parasitic capacitance in the voice coil. to get an actual frequency response curve of the element requires figuring in the electrical equivalents of the mechanical properties of the mic element, but for simple circuit design, you don't need this. you might need it if you plan on introducing an equalization curve into your preamp. then you can add the mechanical model, which can also be determined experimentally, through techniques such as generating frequency vs impedance curves of the element.

[EC8010]

Originally posted by bubbamc119
"This project is a research project where the emphasis is on learning rather than achieving prior stated outcomes. The student is expected to systematically work through the relevant theory/analysis, translate this into appropriate circuitry and then undertake appropriated experimentation with a view to correlating the theoretical and experimental results."

Ah, the joys of education-speak! Whilst it's true that academia views it that way, your future employer will place considerable emphasis on achieving prior stated outcomes within the time and budget available...

From a supervisor's point of view, undergraduate projects are pretty easy and they won't accept a project proposal unless they can solve the project in under ten minutes and therefore decide whether it's actually achievable. That's not to denigrate your project, but the supervisor has to know that your project is achievable because the learning process is (as stated by your supervisor) more important than the outcome. Undergraduate projects are not blue-sky research.

However, there's a difference between an achievable project with substance and a project that boils down to making a single low noise audio stage. What can you do?

You are concentrating on random noise generated within the amplifier which is micro-detail, and that's important. But why not look in the other direction and think about how your device fits into an overall system and therefore what the system requirements are? What range of signals is expected from the microphone? From what impedances? (You've already started on that one.) What about susceptibility to man-made electrical noise? What electrical environment will your device have to suffer? TV studio, recording studio, theatre, open-air festival? How might these affect your design? What about the mechanical design? Where will your device physically be in relation to the microphone?

[unclejed613]

i'm not going to outright give you the solution, but point you towards one or more solutions.
fet input op amps (both in chip form and discrete) are very low noise amplifiers capable of very high gain, and can be easily tailored for frequency response by modifying the feedback loop. an example of a discrete one with very low noise and a tailored frequency response (RIAA response for phono cartridges, like i said, i'm not handing you a ready-made solution) is shown here.

you have a fairly simple project. what your instructor most likely wants to see isn't just "boom, here's a mic preamp" but "here's a mic preamp, here's how it works, and here's why i designed it this way....."

anybody can go to an "op amp cookbook" and design a mic preamp, but i think your instructor wants you to develop a "design philosophy", a logical process for the "how and why" of circuit design. you may later encounter the "gimme a simple mic preamp" situation in the workplace, but having a logical process that you follow in designing things is more important than knowing where to "lift" ready made designs from. it's like the difference between real troubleshooting and "tube jockey" troubleshooting. the "tube jockey" will get stumped the first time he encounters his first open capacitor, because he doesn't have a logical method behind his troubleshooting process.

[bubbamc119]

You guys are great, I can't thank you enough. I will keep you updated with my progress.

[EC8010]

Excellent, we will watch with interest.