Choosing a speaker for an audio amplifier

Discussion in 'Analog & Mixed-Signal Design' started by ali_786, Oct 10, 2016.

  1. ali_786

    Thread Starter New Member

    Aug 5, 2016
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    hi everyone ! plz tell me how to choose a speaker for an audio amplifier. 4 ohm or 8 ohm?? and what is meant by the resistance that speaker has on its label. e.g 4 ohm speaker, 8 ohm spkr....either its the resistance of the coil or anything else?
    I have designed an MOSFET based audio amplifier. its no load gain is 56. but when i connect 8 ohm load (in proteus simulation) the output becomes zero. why?? plz help me
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,140
    1,790
    What does your load look like? A resistor? Is the output of your amplifier AC coupled to the speaker?

    A speaker is basically a moving coil of wire. It has a DC resistance, but it also has inductance. Any device that has both resistance and inductance has impedance. Impedance is represented by a complex number. The resistance is the real part and the inductive reactance is the imaginary part. The impedance depends on the frequency. The magnitude of the complex impedance is averaged over a portion of the audio frequency range and this number is used to match speakers to amplifiers.
     
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,289
    1,255
    You can get more power from a 4 ohm speaker using the same power supply voltage.
    Most of the speakers 4 ohms is resistance, but a small portion 10 - 15% is reactance.
    Can't see what might be wrong with your amplifier. Maybe a schematic would help.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    You neglected to list the part of the impedance that actually moves the cone and makes the sound.
    That appears as an added resistance to an AC signal, since it is doing work.
     
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  5. Papabravo

    Expert

    Feb 24, 2006
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    You're right, but I think the TS might need to swallow all of this in small chunks since this is apparently terra incognita.
     
  6. WombRaider

    New Member

    Oct 11, 2016
    1
    0
    This is probably happening because you need to connect a resistor in series with the speaker for proper impedance matching. Try to put a 500 ohm resistor in series with the 8 ohm and tell us the results.
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    That's okay for the test, but it will limit the speaker output to a very low level.

    What do you mean "proper impedance matching"?
    Modern solid-state amps have a very low output impedance which is not matched to the load.
     
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