Powering Electret Microphones

Discussion in 'The Projects Forum' started by MattP, Apr 19, 2015.

  1. MattP

    Thread Starter Member

    May 21, 2012
    54
    2
    So I'm planning on building this circuit to amplify an electret mic (WM-61A).

    As far as I can tell (which isn't much as I know very little about electronics) the circuit does not provide power for electret capsules.

    So, is this all that's required, and is it an efficient and reliable way of providing power?:

    (R7 and C4 have been added to the original circuit)


    circuit with electret power.gif

    The micrphone spec sheet says that it requires a 2.2k resistor at 2v, so I'm wondering if 2.2k and 12v is a high enough value.

    Also, c4 should be able to be quite a low uF value, correct? I have 22uF caps already, but it would be handy (size wise) if I could go with say 1uF caps.

    Thoughts anyone?
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    759
    Why don't you first built it and test the results. The we can modify the circuit
    That circuit is from ESP. Which is a proof that it would work
    I would not change a circuit from Rod. His work is well known amongst us.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Most electret microphones are biased with a current of 0.5 to 1.5 mA.
    The WM-61A uses 0.5 mA.
    I would use a value of ca 20KOhms for R7.

    Bertus
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    The problem with the posted circuit is that the Electret current is derived by having R7 connected directly to the +12V bus. Any noise on the 12V bus gets into the low-level audio. It needs a secondary filter between the 12V and where it goes into the Electret. Split R7 into two series resistors, and bypass the hell out of the tap.

    The downstream circuit is basically a home-made opamp. Why just not use an opamp suitable for audio?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    MikeML, I certainly agree that the supply needs to be bypassed, but there's a good possibility that our original poster may not know what "bypass" means in this context.

    MattP, MikeML means that the 2.2k resistor should be divided into two separate 1.1k resistors wired in series, and the junction of the two resistors should be connected to ground using an aluminum electrolytic capacitor, anywhere between 100uF and ~1000uF.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,781
    1,103
    I agree with Bertus that 2.2k for R7 is too low with a 12V supply.
    As for C4, reducing its value will reduce the bass response of the circuit.
     
  8. MattP

    Thread Starter Member

    May 21, 2012
    54
    2
    Thanks for the help guys!

    It would be great if the circuit could work with a wide variety of electrets (though still primarily the WM-61A), so if I use a resistor in the region of 6k, that should allow for acceptable voltages for most capsules, correct?

    The WM-61A is rated up to 10v, so with a 6k resistor it should receive 9v. If I were to hook up an electret that needs 1.5ma, then it should receive 3v, which does seem to be a nice compatible range.

    I'm learning here... slowly.

    I've updated the circuit diagram with the bypass filter you've described. As I've mentioned I know very little about electronics, so your description is much appreciated.

    If I were to power the circuit with batteries, does it mean that the bypass filter isn't required?

    circuit with electret power 2 (filtered power).gif
    (R7 and R8 also have higher values now as mentioned above)



    Ah, don't want that. Will leave it at 22uF.
     
    Last edited: Apr 19, 2015
  9. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    You could bring down the voltage using a zener:

    Circuit_with_electret_power_filtered.png

    That way it can still be used on 12 Volts and is safe for many electrets.

    Bertus
     
  10. MattP

    Thread Starter Member

    May 21, 2012
    54
    2
    Neat idea! I'm not that familiar with zeners, so does that configuration essentially cap the voltage at 6.8v between R8 and R7? So after R7, a 0.5ma electret will receive 3.4v?

    Might be a bit of a problem for electrets that draw over 1ma, so would changing R7 to be a 1k resistor work well? I presume not, as you've changed it to 6.8k, but I can only ask. :O)
     
  11. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    The zener will reduce the voltage on the junction R7,R8 to 6.8 Volts.
    The maximum current for the electret will be 6.8 Volts / 6.8 KOhm = 1 mA.

    If you want to know more about the zener, have a look at this page of the eBook:
    http://www.allaboutcircuits.com/vol_3/chpt_3/11.html

    Bertus
     
  12. MattP

    Thread Starter Member

    May 21, 2012
    54
    2
    Yeah, been reading up about them since your other post. You're a good teacher - thanks for being patient!

    Here's what I've come up with:
    (please note that this is the 30v version of the original circuit)
    circuit with electret power (30v with zeners).gif

    If my calculations are correct, this means that it'll be 8.2v for 0.5ma capsule, and 6.4v for 1.5ma capsule. Perhaps on the high side but the spec sheets do say they're fine up to 10v, so it should be ok.

    I'm more or less happy to try this out now, but let me know if there are any obvious errors.
     
  13. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    In your last schematic the maximum current will be 9.1 Volts / 1.8 KOhm = 5.1 mA.
    Use a 0.5 watt resistor for R8 as it will dissipate about 0.25 Watt.
    Better raise the value of R7 to 9.1 KOhm, to get a maximum current of 1 mA.

    Bertus
     
  14. MattP

    Thread Starter Member

    May 21, 2012
    54
    2
    Thanks Bertus. Will order the components and let you know how it turns out.
     
  15. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,781
    1,103
    Why such a low value for R8? You could increase that to, say, 4k7, thus avoiding the need for a 0.5W resistor.
     
Loading...