Hi guys

I need to choose an electric microphone for an intercom. I usually google application notes for <component>. In this case, application notes for electric microphone. But I didn't find a good one.

Can someone give me some pointer please?

I am guessing the important specs I need is SNR and sensitive?

Thanks guys!!

An electret mic capsule is probably the most popular choice.

Back around the 70s or slightly later; Philips did various Electronic Engineer kits - most included an intercom among the projects. The kits used a 150 Ohm speaker to save the cost of a matching transformer in the output transistor collector load. In the intercom; both speakers served double duty as both mic and speaker. The input stage of that project was common base for its low input impedance for a good match to the speaker serving as a mic. telephone earpiece inserts are around 80 - 140R, so they would serve the purpose adequately. Or you could use a regular speaker with a matching transformer.

http://ee.old.no/library/EE20-colour-en.pdf gives the PNP germanium circuits - look on the main page for English EE1003 for NPN silicon. Not all the instruction manuals give component values in the back page schematics, so its worth checking through a few manuals for all the info you need.

 

R2, R3, and C1 form an RC lowpass filter. The corner freq needs to be a few octaves below your lowest frequency of interest, or the reference voltage will be modulated by the audio signal and possibly cause oscillation. Add an appropriate electrolytic cap in parallel with C1.

ak

 

R2, R3, and C1 form an RC lowpass filter. The corner freq needs to be a few octaves below your lowest frequency of interest, or the reference voltage will be modulated by the audio signal and possibly cause oscillation. Add an appropriate electrolytic cap in parallel with C1.

ak

Hi ak

How would the reference voltage will be modulated by the audio signal? I am guessing through the power supply??

My lowest frequency of interest is about 300Hz. If I add an 1u cap, my corner frequency is about 13Hz (2*pe*11K*1.1uF)^(-1) . Assuming my calculation is correct, will that be enough? I honestly don't know what a few octaves below my lowest frequency of interest mean. (I googled it, is it something about vocal?)

Thanks

Edit:
Will a ceramic cap OK? I would like to stick with smt part if possible.

 

An electret mic capsule is probably the most popular choice.

Back around the 70s or slightly later; Philips did various Electronic Engineer kits - most included an intercom among the projects. The kits used a 150 Ohm speaker to save the cost of a matching transformer in the output transistor collector load. In the intercom; both speakers served double duty as both mic and speaker. The input stage of that project was common base for its low input impedance for a good match to the speaker serving as a mic. telephone earpiece inserts are around 80 - 140R, so they would serve the purpose adequately. Or you could use a regular speaker with a matching transformer.

http://ee.old.no/library/EE20-colour-en.pdf gives the PNP germanium circuits - look on the main page for English EE1003 for NPN silicon. Not all the instruction manuals give component values in the back page schematics, so its worth checking through a few manuals for all the info you need.

Hi ian field

Thank you for your reply, the circuit you linked in is actually very interesting. I may make one to learn some analog stuff there. For my application, I simply need an mic preamp, so the I can pick up a conversation further away from the mic. At the same time to increase my SNR, because my mic will be a few meters away from the input of my embeded system.

 

Hi ian field

Thank you for your reply, the circuit you linked in is actually very interesting. I may make one to learn some analog stuff there. For my application, I simply need an mic preamp, so the I can pick up a conversation further away from the mic. At the same time to increase my SNR, because my mic will be a few meters away from the input of my embeded system.

Some years back I had an electret capsule booster published in Elektor magazine - someone on this forum has a link to a copy of it online.

The TL431 amplifier can be on the end of a twisted pair, all you have to do is put its load resistor local to the main equipment.

The technique I used to get maximum gain can also be applied to the B/C biased common emitter stage.