DIY Electret Microphone for Video - HELP

[JDias]

Hello there,

I'm building a low cost mmicrophone, using an electret capsule. I intend to use Schoeps style circuit, similiar to the Alice mike. Here's the schematic:



I plan to also include 2 22pF caps paralel to the 100K resistors. Also, I changed the leftmost cap to 100uF. The trannies used are going to be 5087 in smd package.

I have some questions:

1) Where the two 100K resistors connect togheter, in some designs (e.g. Alice Mic) it's added a 6K8 resistor from there to the 12V rail. As I understand, it is used to limit the current provided by phantom power. Yet, it seems like most phantom powering devices include this resistor internally. So, should I include it in the circuit or not?

2) The two biasing resistors are usually 2K2 Ohms, yet, some designs show different, higher values, like 5K6 (in both cases it is used a 2K2 Out Impedence capsule). What value should I use for a 2K2 IMpedance, 9.7mm capsule?

3) There are two variants of the capsule I intend to use. One has a -46dB sensistivity and the other -36dB, if I''m not mistaken. I'm going to be using the mic mainly for dialog and ambient sound recording (for films) and, ocasionally, music recording (as ambient mics, also for films). What would be the best fit?

It would be most helpful if you could answer these quesitons.

JDias


P.S. If there is interest in these mikes, once they're finished I'll post some photos, and the BOM and board if anyone desires to build them.

[StillTrying]

It seems to work OK in LT with the values as in your schem.

I don't think you need to worry too much about really big smoothing caps. as the out of phase currents tend to cancel any ripple.
A 6k8 on the collectors doesn't seem to make much difference, I'd leave it out!
Increasing the two 2K2s towards 5k6s would increase the output level from the EMC.
The polarity of C4 and C5 are the wrong way.
If you're bothered about phase, I think the Hot and Cold are the wrong way around.

[JDias]

Thanks for your reply!

The polarity of C4 and C5 are the wrong way.

There's no problem, I'm using film caps for those.

For the 6K8 resistor I decided to dd it in. Someone made a really good point that without it the circuit would be consuming near-maximum current allowed by P48 systems. If using  battery operated phantom powering, I't could even not be able to dleiver that full current. I think it's good to at least include the footprint in the board.

If you're bothered about phase, I think the Hot and Cold are the wrong way around.

I hadn't really given much attention to that. I plan to use the Linkwitz Mod. Wouldn't that flip the phase and thus this wiring is correct? Or should I connect it differently?


Thanks again for all your support!

[StillTrying]

For the 6K8 resistor I decided to dd it in. Someone made a really good point that without it the circuit would be consuming near-maximum current allowed by P48 systems. If using  battery operated phantom powering, I't could even not be able to dleiver that full current.

You're right, the 6k8 on the collectors reduces each transistor's collector current down from 4mA to 1.3ma each. I thought they were a bit high, there's still a plenty 7V across each TRs emitter-collector.

I plan to use the Linkwitz Mod.
I don't think it's worth it in this circuit myself, but I've never tried it. They do it here but say, The upper drawing is for an electret capsule without an internal FET.
http://www.audioimprov.com/AudioImprov/Mics/Entries/2015/4/23_Basic_FET_Microphone_Circuits.html

It seems to have a very low frequency response down to about 2Hz, so will be effected by wind noise, rumble, and even the pressure change of a door opening/closing.

[JDias]

I got some ideas:

1) add to Hot and Cold a 47R resistor in series and a 22nF cap to ground

2) instead of using a zener, adding a regulator for loer noise

What o yout hink?

[StillTrying]

add to Hot and Cold a 47R resistor in series and a 22nF cap to ground

You can do that, it changes the high frequency response from the green to the red in the pic.

instead of using a zener, adding a regulator for loer noise

I don't think there's much point, some noise on the collectors 7V won't make much difference. I think 22uF to 47uF across the zenner and a similar value on the EMC's supply rail would do.

I plan to also include 2 22pF caps parallel to the 100K resistors
I don't think there's any point in these 22pF caps.


For a portable mic I still think you'll have to reduce the 2 1uF base coupling caps down to about 220n to cut the 2Hz frequency response a bit.

Have you simulated the circuit yourself?

[JDias]

I still think you'll have to reduce the 2 1uF

Yes, that schematic is just a loose reference.  I was planning to use 0.47uF.


Have you simulated the circuit yourself?

No, I haven't simulated it yet. I know how to draw schematics and PCBs, but I've never really learned how to do simulations. Once I get some free time I'll to learn it.

Also, that high fruequency cut doesn't seem bad at all, it could cut some high frequency harshness form the electrets, don't you think?


Thank you for all your help, once again!