1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

Mic PreAmplifier

Discussion in 'General Electronics Chat' started by Ayne, Apr 17, 2007.

  1. Ayne

    Ayne New Member

    Joined:
    Apr 30, 2006
    Messages:
    415
    Likes:
    0
    Location:
    Multan, Pakistan.
    Q no 1:- why we listen a sound when we put mic infront of speaker????

    Can we minimize that sound.

    Please explain in detial.

    I found this circuit on net.
    ur reviews about circuit?
    [​IMG]

    Thanks.
     
  2. dch222

    dch222 Member

    Joined:
    Jan 16, 2007
    Messages:
    100
    Likes:
    0
    Location:
    UK
    Its called feedback - actually positive feedback.
    Its because the mic signal goes to an amplifier which makes the sound bigger, so it comes out of the speaker bigger, and goes into the mic bigger and gets amplified even bigger etc etc.
    It eventually settles down to some large value due to saturation and phase changes in the amplifier, and the frequency you hear is probably something to do with the time taken to go around the loop (most of which is the time sound takes to get from speaker to mic through the air).
     
  3. dch222

    dch222 Member

    Joined:
    Jan 16, 2007
    Messages:
    100
    Likes:
    0
    Location:
    UK
    You can minimize it by making sure (in a band situation) that the mics are behind the speakers and are not very sensitive and are reasonably directional.
    Some experts may use notch filters to tune out the frequency
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,331
    Likes:
    653
    Location:
    Derbyshire, UK
    ONLINE

    You can even get "feedback destroyers", I have a Behringer one I was given (but have never used) - it's essentially a multi-channel automatic notch filter - so it detects feedback and inserts a notch at that frequency.

    An older method was to frequency shift the audio slighty (a couple of hertz), these means that the feedback doesn't sustain and build at one frequency.

    But your first plan should be careful and sensible mike placement and useage - it's no coincidence that live singers have the mike touching their lips - the closer the mike, the less the chance of feedback. It's also essential to use proper mikes - don't be tempted to use an electret one - that way lies madness and death!!! :D
     
  6. Ayne

    Ayne New Member

    Joined:
    Apr 30, 2006
    Messages:
    415
    Likes:
    0
    Location:
    Multan, Pakistan.
    PCB compeleted

    I have complete my pcb.
    Final check remaining.

    Put ur comments about my pcb.
    Thanks.
    [​IMG]
     
  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,331
    Likes:
    653
    Location:
    Derbyshire, UK
    ONLINE
    What are you intending using it for?, and with what types of microphones?.
     
  8. mneary

    mneary New Member

    Joined:
    Feb 27, 2007
    Messages:
    4,502
    Likes:
    67
    Location:
    California USA
    Almost all PCBs need mounting holes. Do you plan to mount this?
    How do you cross over the signal on the base of Q104 and Q204? Should you have holes for jumpers?
    I would put a small (22R) resistor on the incoming +ve and -ve supplies.
    C3 and C4 look small; what is their value?
    [edit] The ground could pick up stray magnetic fields and induce interference. I would probably prefer to have no trace from R201 to C3, and no trace from R101 to C4.
    Unless you need earth-shaking bass (low frequencies) you can probably use 100uF for C101 and C201. This will save some space that you can allocate to larger capacitors for C3 and C4. [/edit]
     
    Last edited: Apr 18, 2007
  9. RadioRon

    RadioRon Well-Known Member

    Joined:
    Oct 7, 2005
    Messages:
    1,763
    Likes:
    53
    Location:
    BC, Canada

    The tracks that go between Q101 and Q102 are closer together than they need to be and should be spread out somewhat. Same issue with Q202/Q201.

    This layout looks pretty good in general. If I were doing it, I might have used a ground plane on the other side for ground reference rather than the ring ground that you use, but your approach should be fine as long as you are not working with extremely low levels from the mic, like less than 1 mV.

    I’m happy to see that you have maintained balance through the layout to support the electrical balance seen in the schematic. I also see that you have some power supply filtering on the layout that does not appear on the schematic. All good. This design should have some good power supply ripple rejection in any case. I also like to put RF interference rejection measures in place on wire connections to/from the board, but that's just me.
     
    Last edited: Apr 18, 2007
  10. mneary

    mneary New Member

    Joined:
    Feb 27, 2007
    Messages:
    4,502
    Likes:
    67
    Location:
    California USA
    If it helps, you could exchange Cxx2 and Rx12, and Cxx3 and Rx13 for an easier layout.
     
  11. Ayne

    Ayne New Member

    Joined:
    Apr 30, 2006
    Messages:
    415
    Likes:
    0
    Location:
    Multan, Pakistan.
    I will use moving coil(Dynamic Mic).


    My supply will come from LM317 Voltage regulator.

    What should be the value of C3 and C4??


    Plz explain grounf plain, and also explain how u add it??

    Plz explain this in detial??? can it improve the performance???

    I am using LM317 for power supply. what u say!! resistors needed???

    [​IMG]
    [​IMG]

    I want to make this circuit superb.. all type of comments are welcome.
    Thanks
     
  12. Ayne

    Ayne New Member

    Joined:
    Apr 30, 2006
    Messages:
    415
    Likes:
    0
    Location:
    Multan, Pakistan.
    Completed

    [​IMG]
    [​IMG]

    Finally PCB completed.
    Changes i have done.
    1. Holes added for mounting.
    2. Resistors of 10:eek:hm: are added in series of VE+ and VE-.
    3. C4 and C3 are 470uF or greater.
    4. Ground Plain added.

    Need improvement.

    Plz sugges me the Superb Dual OPAMp for Audio use... (waiting for audioguru).

    Thanks.
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

    Joined:
    Mar 16, 2004
    Messages:
    32,592
    Likes:
    952
    Location:
    Canada, of course!
    Hi Ayne,
    The OPA2134 dual opamp is low noise and has only 0.00008% distortion.
     

    Attached Files:

  14. RadioRon

    RadioRon Well-Known Member

    Joined:
    Oct 7, 2005
    Messages:
    1,763
    Likes:
    53
    Location:
    BC, Canada
    A ground plane is a large sheet of copper that spans your entire circuit and acts as the 0 volt reference (or circuit common) for all of your circuits. It is the simplest and lowest impedance reference that you can use in complex circuits where there are many signal loops. It can be implemented as you have done it, by flooding unused areas on the component side with copper as long as there is enough available area, or you can put a complete layer of copper on the other side which is the more ideal case. I prefer to flood the other side of the board as it makes a more complete ground plane for RF work, but in your case the way you have done it is probably fine.

    RF interference suppression is something that I always add because my designs often must live near transmitters. It will not help your performance at all unless you expect CB radios or VHF radios or GSM cellphones to be used near this unit. If you expect this sort of thing, the easiest method of protecting your circuit is to add a small value (eg. 100 pF) bypass capacitor from each connector pin to ground. The principle behind this is that most RF interference occurs as rectification of the RF carrier in your semiconductor components, and the RF usually comes into the circuit through the external wire connections because they are the best antennas. With correct placement of capacitors you make each connector pin have the same RF voltage so that there is no potential difference across any of your semiconductors and no rectification occurs. In your case, it is likely that you won't need these, so you may leave them out. If you do suffer any RF interference later on, it is relatively easy to tack-on some small ceramic capacitors at that time. I should warn that you have to think about what effect these capacitors may have on your circuit's function and carefully choose the lowest value of capacitance to eliminate the RF. You don't blindly put one value on everything without thinking about what effect this capacitor will have.
     
    Last edited: Apr 19, 2007
  15. mneary

    mneary New Member

    Joined:
    Feb 27, 2007
    Messages:
    4,502
    Likes:
    67
    Location:
    California USA
    Looks nice.
     
  16. Peter_wadley

    Peter_wadley New Member

    Joined:
    Mar 20, 2007
    Messages:
    376
    Likes:
    1
    I second that
     
  17. Ayne

    Ayne New Member

    Joined:
    Apr 30, 2006
    Messages:
    415
    Likes:
    0
    Location:
    Multan, Pakistan.
    Help needed.
    I am searching a summer circuit and want to sum those two signlas created by two different MICs.

    Using one opAMP and it should be none inverted.. And with unity gian.

    Thanks.
     
  18. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,331
    Likes:
    653
    Location:
    Derbyshire, UK
    ONLINE
    It has to be inverting, if you want non-inverting (why?), then add a second opamp.
     
  19. Ayne

    Ayne New Member

    Joined:
    Apr 30, 2006
    Messages:
    415
    Likes:
    0
    Location:
    Multan, Pakistan.
    Summer Circuit.
    [​IMG]
    Pot(VR1) is in series of audio signal, It will decrease audio quality???
    Can anyone have better circiut, or how can i improve this?

    Thanks.
     
  20. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

    Joined:
    Nov 17, 2003
    Messages:
    39,331
    Likes:
    653
    Location:
    Derbyshire, UK
    ONLINE
    It's not in series with the audio signal, it's setting the feedback - and it won't have any effect on quality anyway.

    What's R6 for?.

    Have you tried looking at existing mixer circuits on the net?, there are huge numbers of them available.
     
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

    Joined:
    Mar 16, 2004
    Messages:
    32,592
    Likes:
    952
    Location:
    Canada, of course!
    Isn't it a microphone preamp? It doesn't have any voltage gain.
    A microphone has an output of about 7mV peak, your circuit shows 200mV peak which is 29 times higher. A microphone might produce a peak voltage of 200mV inside a very loud drum or if you scream at it.
     

Share This Page