old power amp output impedance missing

Discussion in 'General Electronics Chat' started by veenife, Aug 20, 2014.

  1. veenife

    Thread Starter Member

    Jan 16, 2014
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    hey, i found this old power amp at my cellar i tried it very quick with a speaker and the thing works... but the output impedance is not written any where.... the thing is so old that i not even know from which trademark that is from.....

    now i want to use it but im afraid the impedance from both doenst match....

    is there a way to find out the output impedance from a power amp?

    cheers
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Is this solid state power amplifier? If so, you don't have to worry about impedance matching. We only care about matching the impedance in RF circuit.
     
  3. veenife

    Thread Starter Member

    Jan 16, 2014
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    its sure not a tube amp.. more than that i cant know...
     
  4. Jony130

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    And your speaker impedance is ?
     
  5. veenife

    Thread Starter Member

    Jan 16, 2014
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    the amp is 4 ohms
     
  6. MrChips

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    Oct 2, 2009
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    You can use an amp with 4Ω output to drive an 8Ω speaker.
     
  7. veenife

    Thread Starter Member

    Jan 16, 2014
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    my first question was going more on the direction of impedance measurement....

    is there a way to measure with a multi meter the output impedance from a power amplifier?
     
  8. MrChips

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    Yes, put a variable resistive load on the output and adjust it until you get halve the output voltage.
     
  9. veenife

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    Jan 16, 2014
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    sorry i cant really grasp the method... what would be a resistive load?

    and how do i know what the half of the output voltage is?

    ...

    if it helps on the front of the power amp is written...

    A 400 -4 .... which i guess is the name of the model.... maybe it means something in electronics language???
     
  10. Jony130

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    Feb 17, 2009
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    The output impedance of a solid state power amplifier is usually lower then 0.5ohm.
    And you need a scope and signal gen to measure it.
    But what I don't understand is why you need to know the amplifier output impedance?
     
  11. MrChips

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    Here is an example:

    Suppose the output impedance of the amp is 4Ω.

    Input a 1kHz sine wave signal to the input of the amp and adjust the level to give a 1V signal at the output. You can choose what ever amplitude measurement, RMS, peak, or peak-peak. Let's set it to 1V peak.

    Now put a 4Ω resistor across the output and measure the output voltage again. If you measure half the original voltage, that is, if the new measurement is 0.5V peak, then the output impedance is indeed 4Ω.
     
  12. veenife

    Thread Starter Member

    Jan 16, 2014
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    i dont really NEED to know... i just dont what blow it up if i connect with the worng impedance... but yeah... if you say the solid state doenst matter than okay... but i dont know if the thing is a solid state... but must be somehow
     
  13. Jony130

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    Do you ever seen a SS power amplifier with output impedance equal to 4Ω?
     
  14. MrChips

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    As pointed out, a standard speaker with 8Ω impedance will work fine. If you don't want to blow the amp or speaker, then don't drive it too hard.

    It is a known trick to put a small incandescent light bulb in series with the speaker to prevent blowing the speaker or the amp.
     
  15. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    If you speaker have 4Ω impedance you can connect it but you must be careful with the volume knob. Don't go full one the volume.
    Is you have a 8Ω speaker you don't need to worry about any think. Except maybe the volume knob to not overdrive the speakers. Because we don't know the amplifier output power.
    But what about the brand ? Any info?
     
    Last edited: Aug 20, 2014
  16. Lestraveled

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    May 19, 2014
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    Post a picture of your amp so we can identify it and stop all this guessing.

    Mr.Chips, the output impedance measurement method you are describing works well with transformer output type amplifiers. It does not work for direct coupled solid state amplifiers where the dynamic (synthetic) output impedance is very low (tenths of an ohm or less). Another way to look at it is as a big audio op-amp. You would not be able to load an op-amp at its real output impedance because it could not supply the current.
    Solid state amps base their advertized output impedance on distortion and heat dissipation.
     
  17. MrChips

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    OP is only concerned about blowing the amp or speaker.

    I would just put a 12V 20W lamp in series with the speaker.
     
  18. Papabravo

    Expert

    Feb 24, 2006
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    A 400 -4 is meaningless unless it can refer you back to a manufacturers data sheet or product manual.


    A variable resistor that can handle at least 1.5 times the output power of the amplifier. What you do is adjust the variable resistance so as to maximize the power output. This means voltage and current. At this point the variable resistor is measured and that will approximate the output impedance of the amplifier.

    With a true-RMS voltmeter you measure the output of the amplifier with no load. Then you insert the variable resistor and adjust it until you measure one half the voltage with no load. At this point one-half the available power is dissipated in the output stage of the amplifier, and the other half is dissipated in the load.
     
  19. Lestraveled

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    Does it look like either of these?
     
  20. The Electrician

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    Oct 9, 2007
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    Are you trying to suggest that dynamic (synthetic) output impedances are different from other, more ordinary, impedances?

    The method Mr. Chips suggested works just fine. You just have to use an appropriate output level.

    If the signal level at the output of the opamp is such that it is supplying an output current within its ratings, Mr. Chips method works quite well.

    If the output impedance were as low as a typical solid state power amp, then using Mr. Chips method directly would require connecting various low value resistors to the output until the no load signal level was reduced by half. Rather than doing that, just connect a suitable low value resistor and make note of the signal level when so loaded. Then the output Z is given by Z = R(Vo/Vl-1), where Vo is the unloaded output voltage, Vl is the loaded output voltage, and R is the load applied.

    For example, I have an old Crown DC-300. I applied a 1 volt, 1 kHz sine to an input, adjusted the volume control until the unloaded output voltage was .1 volt. I then connected a .1 ohm resistor across the output and noted that the output voltage dropped to .062 volts. The output impedance is then given by .1*(.1/.062 - 1) = .061 ohms.

    I also used a DE-5000 LCR meter to measure the series mode resistance (Rs) at 1 kHz and got a value of .066 ohms.

    It's not that hard to measure the output impedance of an audio power amp, whether it's dynamic, synthetic, or whatever it is.
     
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