+ Post New Thread
Results 1 to 12 of 12
  1. #1
    Newbie level 4
    Points: 38, Level: 1

    Join Date
    Feb 2020
    Posts
    7
    Helped
    0 / 0
    Points
    38
    Level
    1

    Adding a 3 band equalizer to a phantom powered electret mic amp

    Hi firstly I am new here and would like to say that I am more of a hobbyist than an expert in electronics, However right now I am working on a phantom powered electret microphone for use as a direcional mic. The design of the mic amp is not mine, and is in a way a balanced output and is powered via the mixer phantom power system. I have found how ever that I get a sort of ring at certain frequencies and I get feed back in the range of about 600 to 1.25 khz. I have reduced the mic sensitivity quite a bit but may have to go even lower. What I am thinking of is putting a pasive filter sytem to help reduce the feedback in the mid frequency range and obviously the hi range so as to make equalization on the mixer a lot easier. My question is where does one put the filters just after the mic capsule or on the output going to the mixer. I am not sure if I would be allowed to upload the schematic as according to your rules no copyrighted stuff to be uploaded. Maybe I can give the link to where I got the circuit for those who wish to see it in order to understand my plight. So will leave this post and wait for answers from Admin etc before I continue. Here is the link the the schematic. https://sound-au.com/project93.htm

    I have changed the values of R8 and R2. R8 is now 2.2 kilo ohm, and R2 is 4.7 Kilo Ohm. Looking at the schematic where would it be best to put the filters.

    Just a comment, as is it works really well but my mixer which is a Soundcraft Signature mtx22 is quite sensitive even though the max input on the balanced input for each channel is +12db. By the way the ring I hear is on the prefade listend and the channel fader and all other aux faders down. So it is due to the mic frequency response to high notes.

  2. #2
    Super Moderator
    Points: 54,159, Level: 56

    Join Date
    Apr 2011
    Location
    Minneapolis, Minnesota, USA
    Posts
    13,202
    Helped
    2628 / 2628
    Points
    54,159
    Level
    56

    Re: Adding a 3 band equalizer to a phantom powered electret mic amp

    When a mic produces 10 or 100mV amplitude it's wise to send it to a pre-amp right away so your desired signal stays dominant over any added noise further on.

    Your link is to Rod Elliott's site which has a wealth of free information and project articles. His site also probably lists a tone control which attenuates your desired frequency range. There is the Baxandall type, and it may simply need capacitors and resistors and potentiometers.

    With 48V ethernet power there's a chance your mic produces upward of 1/2 to 1 V amplitude. This is line level and you could get away without a preamp.



    •   AltAdvertisement

        
       

  3. #3
    Newbie level 4
    Points: 38, Level: 1

    Join Date
    Feb 2020
    Posts
    7
    Helped
    0 / 0
    Points
    38
    Level
    1

    Re: Adding a 3 band equalizer to a phantom powered electret mic amp

    Thanks for the feed back, Ok with R2 and R8 reduced in value it does attenuate the output signal wich gives me more head room on the mixer, however I need to be carefull not to go too low on the supply to the mic, which will reduce its noise rejection. I was thinking on a passive system less stress and also takes up less room. I remember working with AKG Mics in the 90´s which had a pasive filter system on them, The mics were just normal mics and did not need phantom power. I will check out the Baxandall type. and see how it behaves. Sadly simulating this kind of thing does not always give one the best resuslts. I need to play with it physicaly. My eda program is DEXPCB 2020. I have never really worked out how to do I can reduce R9 but there comes a point where the output starts to distort, so I will leave it as it is. simulations on it as I have never needed to. I do have a copy of CIRCUIT DESIGNER STUDENT which is the free version and the simultation does work albeit simply, it just gives me an idea of the output and what the wave form looks like.

    Rod is quite a reasonable guy and I have comunicated with him on this project and other issues a while back, however he has a policy that unless you are going to buy something from his site do not expect indepth feedback or help, which I fully agree with. His site is a gold mine of info and circuits.



  4. #4
    Super Moderator
    Points: 265,435, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,378
    Helped
    14115 / 14115
    Points
    265,435
    Level
    100

    Re: Adding a 3 band equalizer to a phantom powered electret mic amp

    It's rarely the microphone that's responsible for "ring at certain frequencies" and it's quite unusual to use an equalizer in front of the mixing desk. Electrete microphones have a very linear frequency response. More likely a specific microphone mount and acoustic resonators are causing the problems. Less likely a flawed amplifier circuit. It's not clear to me why you see at all a need for a preamplifier. Electrete microphone level is high enough to drive a mixing desk microphone channel.



  5. #5
    Newbie level 4
    Points: 38, Level: 1

    Join Date
    Feb 2020
    Posts
    7
    Helped
    0 / 0
    Points
    38
    Level
    1

    Re: Adding a 3 band equalizer to a phantom powered electret mic amp

    Just a question, or a comment. The eq would have to ref to earth, as pin 3 is the return from the mixer. Yes?

    - - - Updated - - -

    I have looked at many of the circuits available and many of them need a battery to power them. I do not want this as I do not want the mic to fail halfway through what ever is happening up front. I found that this circuit works well, it is just that I find it quite sensitive to those mid frequencies.

    In regard to an eq in front of the mixing desk I found that when working with feminine voices that are high pitch or even male voices that are soft I can eliminate some of the problem up front. The problem is the hall where i want to use these mics was not designed for acoustics and is not air conditioned so I can get various differences in my eq settings for the same person depending on the humidity of the air, the temperature etc. How ever this mic once I have got it working will be used to pick up ambient sound and fed to the videos streaming mixer but also I want to use it for when we do childrens presentations, so as to reduce the risk of them always using the normal mics and dropping them etc. I know it is possible to solve this and I will, it will just take time and patience as well as lots of ingenuity. I know too that I need to redo the ballance on the house equalizers, they are way out. That will help a lot.



    •   AltAdvertisement

        
       

  6. #6
    Advanced Member level 5
    Points: 54,221, Level: 56
    Achievements:
    7 years registered
    Audioguru's Avatar
    Join Date
    Jan 2008
    Location
    Toronto area of Canada
    Posts
    8,839
    Helped
    2074 / 2074
    Points
    54,221
    Level
    56

    Re: Adding a 3 band equalizer to a phantom powered electret mic amp

    You do not EQ an audio circuit to eliminate feedback. EQ is used to make good sounds. The mic probably already produces good sounds so do not EQ it which will mess up its sounds.
    To eliminate feedback you record the sounds then play them back with the mic turned off.
    You could turn down the mic preamp or power amp then place the mic closer to the sound source.



  7. #7
    Super Moderator
    Points: 265,435, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,378
    Helped
    14115 / 14115
    Points
    265,435
    Level
    100

    Re: Adding a 3 band equalizer to a phantom powered electret mic amp

    You don't necessarily need an amplifier to use phantom supply for an electret microphone. You can also make a simple pseudo-differential circuit, using resistors and capacitors.

    If you are using an amplifier similar to Fig. 6, I don't recognize how it should change the frequency response. R8 of 2k2 however reduces the gain almost unity, hence the amplifier could be reduced to a simple common collector stage.

    I believe that the mixing deck's parametric EQ is more effective in tuning a frequency response than the intended 3 band equalizer. If you feel a need to implement a fixed equalizer, you should record the empirically determined parametric EQ settings and copy the response in your circuit. Or use 27-band EQ to model the optimal characteristic.



  8. #8
    Newbie level 4
    Points: 38, Level: 1

    Join Date
    Feb 2020
    Posts
    7
    Helped
    0 / 0
    Points
    38
    Level
    1

    Re: Adding a 3 band equalizer to a phantom powered electret mic amp

    Ok, that I understand. Right let me try to explain, When I connect the mic to the mixer, my gain is on infinity, the channel slider is down as are the aux controls to the monitors etc. What I hear through the pfl is a sort of ring on high frequencies, and I picked it up on a recording I did.

    Now because the hall is not that large my main amps are set at about just over half the max volume. Now my equ for that channel is more or less like this. High -4db, High mid frequency sweep -1k cut, High mid frequency range -4 db, Low mid frequency sweep, 2khz, Lmf range, -10 and the Low frequency is at +4db . With the channel sliders down and the gain at infinity I am getting about 3-5 db on the vu meter. Now I bring up the main volume to -5 on the slider and as I move the channel slider up I can only go to between -3 and -5 db before I start getting feedback on the main speakers. The thing is re the settings on the channel eq were done like that to reduce the feedback and still give me as much natural sound as possible. I can move the gain up to about 1/4 of the way up but that makes my work even more difficult. So I want be able to still work in that range but have more reasonable settings on the mixer eq. Like I commented earlier I need to redo the settings on the main equalizer as since we changed our main voice mics from Shure to AKG and also the mixer from an Allen and heath to a SoundCraft everything changes.



  9. #9
    Advanced Member level 5
    Points: 54,221, Level: 56
    Achievements:
    7 years registered
    Audioguru's Avatar
    Join Date
    Jan 2008
    Location
    Toronto area of Canada
    Posts
    8,839
    Helped
    2074 / 2074
    Points
    54,221
    Level
    56

    Re: Adding a 3 band equalizer to a phantom powered electret mic amp

    Shure dynamic mics for vocals peak mid-high frequencies for "presence" that causes feedback when the gain is too high. But they are directional.
    Electret mics have a flat frequency response but many pickup sounds from all around (omni) which can cause feedback when the gain is too high.
    Turn down the gain and ask the performers to "eat" the mic.



  10. #10
    Newbie level 4
    Points: 38, Level: 1

    Join Date
    Feb 2020
    Posts
    7
    Helped
    0 / 0
    Points
    38
    Level
    1

    Re: Adding a 3 band equalizer to a phantom powered electret mic amp

    The Electret mic is not for direct voice use. I have AKG mics for that and have no issues with them. The electret mic is for ambient pickup, however It gives me a reading of about -5 db on the vu meter with the gain fully down. The thing is firstly the Soundcraft desk I am using is very sensitive. We are about to buy a calibrated measuring mic so we can re calibrate the main eqs. I will then see how this beastie behaves. It could be that due to bad room eq ballance etc I am getting this issue. Meanwhile I am going to do a few more experiments with the placement of the capsule and tube design.



    •   AltAdvertisement

        
       

  11. #11
    Super Moderator
    Points: 265,435, Level: 100
    Awards:
    1st Helpful Member

    Join Date
    Jan 2008
    Location
    Bochum, Germany
    Posts
    46,378
    Helped
    14115 / 14115
    Points
    265,435
    Level
    100

    Re: Adding a 3 band equalizer to a phantom powered electret mic amp

    This sounds all quite strange. If you don't have a suitable professional microphone for comparison, can't you loan a known good condenser microphone from a collegue?



  12. #12
    Newbie level 4
    Points: 38, Level: 1

    Join Date
    Feb 2020
    Posts
    7
    Helped
    0 / 0
    Points
    38
    Level
    1

    Re: Adding a 3 band equalizer to a phantom powered electret mic amp

    Not easy here in Mexico. We are buying one this week, and then once we have sorted out the room eq I can carry on with my own tests. This mike will work fine for general recording but for what I want I need it to not be so sensitive and allow me to do normal equalisation on the desk and not have to go all over the place.



--[[ ]]--