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Dynamic Microphone preamp design help

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atwoz:
Hello,

I'm designing a preamp for a dynamic 600Ohm Microphone. I want to be able to connect the dynamic microphone directly to a computer via de 1/8inch input jack. So far I have tried 2 solutions and both gave me bad results.

1) I tried using the max4466  (http://datasheets.maximintegrated.com/en/ds/MAX4465-MAX4469.pdf). I followed the datasheet and built the sample circuit. I didn't put the bias resistors since those are intended for an electret mic. The rest is almost the same. I didn't get enough gain and the sound was distorted and it sounded too hollow.

2) I built this circuit (exactly as shown) http://www.maximintegrated.com/app-notes/index.mvp/id/1931 . I got better results but since I didn't get the gain I wanted I modified the feedback resistors a bit to increase the gain. I was able to get enough gain then but the distortion was too bad and the sound was also hollow.

Does anyone have experience with this? I could use some help. Does anyone have or know of any reference design? What parts do you recommend? Is it better to increase the gain in several steps? For example, 4 op amps, each with a gain of 6?

Thanks.

David_AVD:
You could use an INA217 IC with minimal extra components.

atwoz:
Thanks for your response!

I believe the INA217 is used for balanced Microphones (correct me if I'm wrong). I forgot to mention that the microphones that we will use will be unbalanced dynamic microphones.

fpliuzzi:
Hello atwoz,
Is the link that you supplied above for your #2 circuit correct? The circuit that the link leads to shows a phono cartridge preamp, complete with an RIAA playback curve filter (the filter's extreme response is shown in figure 3 of that page). This circuit would not be usable as a microphone preamp.

Rod Elliott has a page on his site that shows three versions of a mic preamp designed for unbalanced 600 ohm dynamic microphones. Maybe one of his circuits would work for your needs. He also has many other useful audio related circuits there.

http://sound.westhost.com/project13.htm

Regards,
Frank

EDIT: Rod lists the noise specs for his Class-A discrete mic preamp in the text of his article and they're quite respectable. Also, the INA217 that David_AVD mentioned has served me well in a few of my projects. Nice chip for circuits where you have the necessary voltage rails.

atwoz:
Frank,

I made a mistake, since I was in a rush today I did build that circuit, no wonder it didn't work as expected.

Thanks for the link. I did come by that site before, didn't try building the circuits for 2 reasons. 1) The supply voltages required are too high. I need 5V max since I will be connecting it to a computer. 2) I believe you need low noise op amps to achieve good results. But maybe I am wrong and those circuits give good results.

Thanks a lot for the suggestion.

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