I want to get my mic preamplifier sound to a higher level.

Discussion in 'General Electronics Chat' started by dikidera, Sep 8, 2015.

  1. dikidera

    Thread Starter New Member

    Sep 6, 2015
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    Hi all, I am very new to this board, and new to electronics in general.

    I set out to make a simple analog intercom system between two locations, distance is 20 meters or so, I've bought the appropriate cables, tested them and saw they have low resistance(like 2.2 ohms at most) and no detectable voltage drop over this distance with a 9v battery. The speakers I chose actually are not powerful enough, they are 4 Ohm 0.15w speakers, so I will have to buy at least 1-2w speakers.

    I have built a preamplifier circuit from a single NPN transistor in common emitter configuration, it's a BC413B model with an hFE of 100. I admit when it comes to amplifiers from transistors I am still not familiar with the math, except the RC time constant and cutoff frequency math.

    The microphone I am using is just an ordinary electret microphone, I don't have any specs but I believe it's range is 10-30 or at most, 60mV, which with a gain of 100 is 100-300-600mV peak to peak I believe.

    Here is a working simulation of the circuit(you will need Chrome to view it) http://everycircuit.com/circuit/5814088627126272/what-is-wrong-with-this-preamp-circuit

    The RC circuit on the collector actually gives me a cutoff frequency of 33Hz, but I don't have a ~80uF capacitor for ~20Hz, and the bypass capacitor at the emitter should give me the maximum gain of this transistor, or 100(I have not done the math, but some tutorials I watched said it should). The volume of the mic is still very low, I can barely hear the sound when rubbing the microphone or blowing at it.

    If you are wondering why I didn't just buy an LM386, it was because I wanted to learn electronics and build this from scratch, however if need be I do have an AN7312 amplifier I pulled off from an old radio & cassette player.

    What the simulated circuit doesn't show is that I hooked up the output of the first transistor to another transistor in a common collector configuration for current amplification, the volume was higher, but still could not produce any audible sound when I speak.

    So I turn to you guys for help on how to make this circuit amplify more, and yes I do have more transistor of the same model, albeit they are all NPN.
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Please post your circuit as an image file or PDF.

    There are a lot of preamp circuits on the innergoogle. Try searching for "transistor microphone preamp circuit". But these circuits are not intended to drive a speaker directly, which might be your problem. If the 100 ohm resistor is overheating, that's because is is passing way too much current.

    Without a complete schematic of your intercom it is difficult to determine what the problem is. There are simple intercom circuits designed to drive a speaker without a lot of complexity if ICs. Search for intercom circuit or intercom circuit schematic to see many examples.

    ak
     
  3. dikidera

    Thread Starter New Member

    Sep 6, 2015
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    Here is a screenshot of it from the simulator. The speaker was hooked to the output of the 47uF capacitor. The simulator cannot yet depict microphones, so an AC signal is there instead.

    http://i.imgur.com/59R9CzW.png
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    951
    Sacrifice a headphone cable or ear bud and input the signal from a headphone port on your phone or computer and see if you can get audible sound from the speaker. I think you are over estimating the output power of your microphone. You will probably need a second stage of amplification to get an audible output from it.
     
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  5. dikidera

    Thread Starter New Member

    Sep 6, 2015
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    Seconds stage of what kind? Voltage or current amplification?

    Also, computers have built-in amplifiers, regardless of whether or not I amplify, I will still hear loud audio on the PC.
     
  6. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
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    A second stage of voltage amplification, of course.

    You require at least another voltage amplification stage identical to the first to bring the output of the microphone up to a high enough voltage to drive the loudspeaker to sufficient volume. But, a voltage amplifier stage cannot deliver the necessary current, so then you add a current amplifier stage to provide the current needed to power a small loudspeaker; A simple push-pull class-B stage is a good starting point.
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Without getting into driving a 4 ohm speaker through a 100 ohm resistor, your circuit is not an amplifier. If you disconnect the speaker and look at the signal voltage on the collector vs. the signal voltage at the input, you will see that there is very little gain, if any. This is because all four of the transistor biasing resistors are incorrect. There are many websites devoted to the design of a common-emitter stage (your circuit), and some of them have calculators to make it very easy to try various resistor combinations to get the performance you want.

    Bottom line, you can't get enough amplification out of one transistor to go from microphone-level to speaker-level audio unless you use a Darlington, and even then you will have significant distortion. Here is a link to a simple intercom:
    http://www.matni.com/Arabic/Elec-Info/INTERPHONE/intercom.html

    ak
     
  8. #12

    Expert

    Nov 30, 2010
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    Yeah, what he said. That's a really awful design and there is no such thing as a single amplifier stage that is optimized for both a millivolt level input and the ability to drive a speaker. That's like entering a turtle in a horse race.
     
  9. dikidera

    Thread Starter New Member

    Sep 6, 2015
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    Everybody that I've showed this circuit to have said that the gain is 100, nor have they said it's wrong. If there is no gain, or even small gain, I wouldn't hear any sound at all.

    And considering I am really bad at math, I've so far not seen a very simple dumbed down equation where I can simply substitute values to get what I need.
     
  10. #12

    Expert

    Nov 30, 2010
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    The impedance in the emitter circuit is the resistor in parallel with the capacitor. As frequency goes higher, Xc becomes less, and voltage gain happens at the collector resistor. (Xc = 100 ohms at 34 Hz) Then you look at the capacitor in series with the speaker and realize it's going to cut off a lot of low frequencies until its impedance goes down to 4 ohms (846 Hz).

    So, there is voltage gain. It's just a really awful design.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    The first hit for electret microphone datasheet, listed operating voltage 3V at 500uA. You can probably take that as being more or less typical.

    A grounded base front end stage would give you mostly lots of voltage gain, you can add a PNP emitter follower in the manner of a Szicklai pair to get some current drive capability - after that, pretty much any textbook pre-amp can be selected for whatever amplification you ultimately need.

    The operating voltage for electrets seems to be somewhere in the region 2 - 3V, so calculate the base bias to put about 3V on the base (don't forget to decouple the base to GND). The electret goes from emitter to GND. The collector current will depend on the electret tolerances, so you may have to select the collector resistor rather than calculate it, but it shouldn't be all that far from about 500uA to work out the resistance for the correct operating point.
     
  12. Odiesdad

    New Member

    Aug 7, 2015
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    Remember the power formula? The best bet is to use two stages, as mentioned, and use an emitter follower to drive a low impedance device such as a speaker.. . ..
     
  13. AnalogKid

    Distinguished Member

    Aug 1, 2013
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