amplifying an electret microphone

Discussion in 'General Electronics Chat' started by MikeA, Jan 12, 2015.

  1. MikeA

    Thread Starter Member

    Jan 20, 2013
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    I'm looking at the spec sheet for the popular Panasonic WM-61A, and their hookup schematic includes a 2.2K resistor powering the mic from +Vs, and a 1uF capacitor on the output.

    I also found a schematic of a preamp that seems well regarded. Image attached.

    So if the WM-61A is attached to this schematic, including the aforementioned 2.2K resistor and the 1uF capacitor, isn't there a high pass filter (C1+R3) now in place right before the op-amp?

    Does the very flat response curve of the WM-61A take into account that this high pass filter will be in place? I modelled this in Ltspice and get -6db @ 20hz.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    That is true, that the capacitor introduces a high-pass filter. If you wish to lower the cut-off frequency you can increase the value of the capacitor to 10μF, for example.
     
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  3. MikeA

    Thread Starter Member

    Jan 20, 2013
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    At least I understand the circuit. :D But since they say to use 0.1uF in their spec sheet, will it still have a flat response from the mic?

    Maybe it naturally has a bump down low and that capacitor smooths it out?o_O
     
  4. MagicMatt

    Member

    Sep 30, 2013
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    I wouldn't really call the response of the WM-61A flat... it bums out at fairly low SPLs and has quite the bump appearing as you go below 150Hz. I would think the highpass filter is to compensate for that, since it would introduce a drop in output as you get into the low frequencies - it's not a sharp cut-off.

    Google the Linkwitz Mod for that capsule... it's a bit like taking the speed limiter off a sports car... makes a good capsule into an outstanding one.
     
  5. MikeA

    Thread Starter Member

    Jan 20, 2013
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  6. MagicMatt

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    Sep 30, 2013
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    Hence I can only assume their graph is WITH the highpass filter, to smooth out the bump... that and the fact the vertical axis scale is crazy - you need the scale to go from about -12dB to +6dB to see any detail... they've used -30dB to +20dB, and the only motivation I can see for that is to make the line look far flatter. I think there's something like 6dB difference between 20Hz and 150Hz under test, without any filters, and there's a peak of about 3dB around 12kHz. When you think that a 3dB difference sounds about twice as loud... suddenly the idea of it being "flat" goes out the window.

    It's those characteristics that make it such a great choice for recording without spending a lot on capsules though. The extra boost above 10kHz makes it sound clearer, and the boost below 150Hz gives it a bit more overall depth. It's a curious phenomenon, but we don't generally like the sound of recordings that are completely flat response.
     
  7. MikeA

    Thread Starter Member

    Jan 20, 2013
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    Just one of several tests I've seen online. It's fairly flat for the most important range of tuning room acoustics in the 20hz-10khz range.
    http://www.johncon.com/john/wm61a/

    Unrelated question about the circuit above. What is the purpose of R5 and C3?o_O
     
  8. #12

    Expert

    Nov 30, 2010
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    R5 and C3 keep the voltage at the microphone steady while U1 uses current from C4.
    Otherwise, the power needs would be felt at the mic and cause oscillation.
     
  9. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Yes, It is a filter. Since the output voltage of the mic is very low so any noise on it's line gets amplified along with the sound. So if the power supply has say some 60~ ripple you would hear a hum.
     
  10. MikeA

    Thread Starter Member

    Jan 20, 2013
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    I guess that makes sense, the power is split up into 2 through the resistor (R5), with a 2 caps keeping reserve (C3 & C4).

    But in theory, if the capacitor (C4) was perfect (large enough and fast enough) to smooth out any voltage drop from U1, would R5 and C3 still serve a purpose?
     
  11. #12

    Expert

    Nov 30, 2010
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    Yebbut...it's cheaper to achieve effective isolation if you do it in a 2 part arrangement.
    Think: Why else would anybody add two parts they "don't need"?
    Because it works better for less money.
     
  12. MikeA

    Thread Starter Member

    Jan 20, 2013
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    Well the "why would anybody" argument doesn't work that well when the schematic comes from an unknown source.

    "Don't trust everything you read on the internet." -- George Washington :D

    I understand the isolation benefit, it's just not clear why it would be an issue, since the op-amp and mic are such tiny current drains. Why not isolate the voltage divider (R2 + R3) from (R1) that is powering the mic? Just a few cents in parts. If the mic current brings down the voltage, the divider can shift too?;)
     
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  13. #12

    Expert

    Nov 30, 2010
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    You mean move R5 to the left one place?
    Yeah. That will work.

    Oh well. The proof is in the pudding. Build one (and I don't mean "simulate" one) without isolating the mic supply and measure the band pass response. Electrons can't lie.
     
  14. MikeA

    Thread Starter Member

    Jan 20, 2013
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    I meant putting another resistor to the left of R5. And an additional cap. Sort of like this. So the op-amp, the voltage divider, and the mic are all separate in a sense.

    The proof is in the pudding indeed. I'm just trying to understand the theory behind the design. :D
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    The basic design for the electret amp has been around for many years. AudioGuru, who no longer posts here, originally posted the circuit. I drew up that particular schematic from the one Audioguru posted, but I decreased the value of R2, as the TL07x has a common-mode input range of -V +3V to +V -1.5v; and so R2/R3 bias the input near to the middle of the common mode range; around 5.14v. The common mode range center will start out being 6v, but will drop as the battery drains.

    R5/C3 is a low-pass filter; it helps to prevent self-oscillation and keeps things quiet. Adding another low-pass filter stage wouldn't be as beneficial as changing the TL07x opamp for a more modern, quieter opamp.
     
  16. MagicMatt

    Member

    Sep 30, 2013
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    Yup... an excellent page, which explains a lot of the modifications you can make to greatly improve the SPL handing and get the response far more linear... (see the graph and the comments about it not being flat response)...
     
  17. MagicMatt

    Member

    Sep 30, 2013
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    As long as it does what you need it to do though, that's what really matters. You probably wont get a better capsule without spending a LOT more money.
     
  18. #12

    Expert

    Nov 30, 2010
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    That certainly clears up the unknown source! :p

    And thanks for the reference page. (That guy writes more than Wookie!)
    This is archival quality stuff.
    Bookmarked!
     
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  19. MikeA

    Thread Starter Member

    Jan 20, 2013
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    Change to something like a MUSES01? :D

    Any component recommendations for this circuit? Such as R1 being a low noise metal film resistor? Film C1 cap?
     
  20. ronv

    AAC Fanatic!

    Nov 12, 2008
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    :eek:You could buy a couple of tubes of these for the same price.

    http://www.mouser.com/ds/2/405/opa134-443846.pdf

    People say film is much better, but before you look at that put 25mv or so of 120 Hz ripple on your power supply and look at the output.
     
    Last edited: Jan 14, 2015
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