Audio Amplifier Clipping and Speaker Power Mismatch - WHY?

Discussion in 'General Electronics Chat' started by Voda, May 23, 2016.

  1. Voda

    Thread Starter New Member

    Apr 2, 2011
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    I don't understand *why* clipping between an audio amplifier and a speaker happens due to a mismatch between an audio amplifier output and speaker power rating. Nothing I've read explains why - just that it does. Well, I need a better answer than "cuz the internet said so", ha ha.

    The output of the power amp "sees" the impedance, and with the signal voltage we get a value of current. If I turn up the volume, I get more voltage, and more current. I don't understand why this would be any different if the speaker's power rating was perfectly matched to the amplifier or if it was 10 times the power amp - what is the amplifier "seeing" that would make it clip with one, but not the other?

    How does the amplifier know this impedance source's power handling is rated 10 times higher than the amplifier could provide?

    In my case, I have an 18W RMS stereo driving a pair of 100W RMS speakers (gift from family member who meant well). Why would clipping happen on this circuit any differently than with an 18W stereo driving a pair of perfectly matched 18W speakers?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    It wouldn't.
    I understand your confusion.
    The speaker power rating has nothing to do with amplifier clipping (wherever you read that is a completely miss-informed source). It only determines how much power the speaker can take before blowing.

    Amplifier clipping is determined by the speaker impedance and the power output to that impedance.
    If you try to output more power than the amplifier rating, it will start to clip the output voltage at some point.
    The high frequency components of this clipping can damage any tweeters connected to the output, so you generally need to be careful to stay below this clipping point (besides the fact that the music generally sounds terrible when the amp is clipping).
     
  3. Voda

    Thread Starter New Member

    Apr 2, 2011
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    Thank you for your help. So, in summary, my audio amplifier is going to clip at the same volume point regardless of what speaker is connected, 10 Watts or 1000 Watts, so long as the impedance is the same? That makes sense to me, actually.

    So...why is the volume pot not sized to prevent outputting more power than the amplifier rating? The load impedance is known, and the amplifier limits are known, so why does my radio allow me to cause clipping in the first place?
     
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The output power is a function of your volume control setting AND the input power.
    If your audio source is delivering a 1 volt peak to peak input, clipping may happen before you reach max volume. If the source only inputs a .5 volt peak to peak signal then the amp will clip at a much higher volume setting, or it might not reach the output stage clipping point at all.

    Edit (the input peak to peak values are for examples only and represent nothing in regards to the actual amp)
     
  5. Voda

    Thread Starter New Member

    Apr 2, 2011
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    Totally understandable when the radio (input source) and amp are separate. I guess what I meant was when the radio and amp are in one unit - a typical 18W stock car stereo. Since the input source is known and designed together with the amp, I would have thought that could be eliminated.

    Or are we getting into media source material now, where the program data is calling for higher voltages thereby affecting the radio output to the amplifier section? I may be getting a bit too far in the weeds.
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Probably a design decision on the manufacturers part. It would insure that EVERY amp can reach the rated output power.
    You most likely are getting the full 18 watts the spec claims for the amp but design and components are operating above the minimum design parameters, allowing for a higher than minimum input power to the amp stage.
    Your amp is working fine, actually better than fine, and that increased signal input power just means you reach clipping before your volume control max is reached.
    They do this so every unit can be relied upon to output full rated power.
     
  7. Veracohr

    Well-Known Member

    Jan 3, 2011
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    I expect a classical music program would have a lower signal level than a bottom-heavy hip hop song coming from the same radio receiver.
     
  8. crutschow

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    There's no particular reason to include a way to limit the output below clipping, since it's very obvious from the distorted sound when the limit is reached and the listener with then turn down the volume below that point.
    A short excursion into the clipping point won't hurt the amplifier or the speakers.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Is clipping strictly an output problem? I think of clipping as being more commonly caused by overwhelming the input. For instance by using a headphone signal in a line-in port. You can certainly get awful sound – that I would call clipping – from this without being anywhere near to clipping the output. Maybe what I'm describing has a different term.
     
  10. dannyf

    Well-Known Member

    Sep 13, 2015
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    you actually never said what this mythical mismatch is so it is hard for others to precisely help you understand it.

    If it is what I think it is (underrated speakers + overrated amps), a few factors contributed to it.

    1. solid state amps clip hard: when they do, they generate lots of odd-order harmonics;
    2. treble drivers are actually lowly rated in their power handling capabilities (as our ears are quite sensitive to them), so a little bit of too much high frequency content can damage them.
    3. crest factors in most what we hear is quite high. 10x for human speeches for example.

    Put all that together, it means you want to avoid clipping as much as you can. So if you think you listen to music at 1w, you may want to get a (solid state) amp with output rated at 10w or 20w or 50w. Thus the perception of "under-rated" speakers, which is actually more accurately described as "over-rated" (solid state) amps.

    The issue is much less severe for tube amps as they clip gracefully so they don't have to be as "over-"rated as their solid state counterparts.
     
  11. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    Clipping will depend upon the input voltage level, and/or the output level into the speaker load impedance.

    If the input voltage is too hot (high of a level), then the preamplifier may clip (go beyond the power supply voltage) before the volume control circuit; but, with an integrated system (radio/player) this typically will not happen. However it can occur if you are using an external input from a source such as an MP3 /iPod player (in which case, turn the player volume down). For clipping after the volume knob stage; clipping can be prevented by adjusting the volume control, but the knob may seem at a rather low position depending upon the input level.

    With 18W RMS into 4 ohms, it is about 8.5V RMS into the speaker(s), which is 12V peak (using a standard sine wave). Similar to above, clipping will occur when the volume knob is set to a level where the output signal is trying to go beyond the power supply voltage, and this is load dependent (2, 4, 8, ohms speaker). This is because there is more gain (amplification) in the unit than is absolutely necessary, and is typical for electronic design as "headroom" to account for various tolerances. So if you turn the volume knob up to max, there may be too much amplification and the output distorts. So, turn it down, because the maximum output of the amplifier has been reached (or exceeded) and you are getting about 18 watts (in this example).

    Clipping does not depend on the speaker power rating, other than making sure the speaker(s) can handle the rated amp output. If the speaker is rated too low, then it may blow when the amp output exceeds those ratings. At high volume levels, if a speaker is being stressed, there can be mechanical rattle and the like which may sound like electronic clipping, when it is actually the mechanical noise from the speaker cone.

    If the volume knob is at, say, halfway and the output clips in an integrated unit when listening to the built-in radio, there may be an issue with the quality or performance of the unit (bass/treble settings can affect this as well).

    The above is a basic explanation, and there is a lot more detail to amplifier types and operation than just the above.
     
    Last edited: May 24, 2016
  12. crutschow

    Expert

    Mar 14, 2008
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    Yes, clipping can occur anywhere in the amplifier chain.
    But for audio power amps, the circuit is normally designed so that clipping first occurs at the output, to insure that the maximum power output can always be achieved.
     
  13. Voda

    Thread Starter New Member

    Apr 2, 2011
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    Actually, I did. The last bit in the OP: "In my case, I have an 18W RMS stereo driving a pair of 100W RMS speakers (gift from family member who meant well).".

    I appreciate your post and sorry for the confusion. This, apparently, is a quite popular and stubborn myth I've stumbled across. But I think the posters here have straightened me out quite well.
     
  14. nomurphy

    AAC Fanatic!

    Aug 8, 2005
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    How is it guaranteed that the "output" clips first? The output level is always dependent upon the input level. Which, with an external source, is not controlled by the amplifier. To do so, there must be an input volume control.
     
  15. crutschow

    Expert

    Mar 14, 2008
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    It's guaranteed simply by having the dynamic range of the preceding stages clip at a higher relative level than than the output stage. That way the output stage is always the first to clip.
    An input volume control has nothing to do with the internal amplifier clip level (which is always at the same output voltage), it just determines the value of the signal voltage at the input that is required to cause the output to clip.
     
  16. Lestraveled

    Well-Known Member

    May 19, 2014
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    I absolutely agree. It would be a very bad design if a preceding stage were to clip before the output stage.
     
  17. marcf

    Member

    Dec 29, 2014
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    I would think that the main cause for clipping would be trying to output a 30Vpk-Pk signal from an amplifier with a 20VDC supply.
     
  18. crutschow

    Expert

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    That's true for a single-ended output amp.
    But 30Vpp from a 20Vdc supply is readily done with a bridge output amplifier.
    Such an amp can output nearly 40Vpp with proper design.
     
  19. marcf

    Member

    Dec 29, 2014
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    That was not really my point.

    My point is that the clipping point of an amp is kinda dependent upon its power supply.
     
  20. crutschow

    Expert

    Mar 14, 2008
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    Of course it depends upon the power supply.
    Our discussion was about where the clipping occurs in the amplifier, not what ultimately causes the clipping.
     
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