Impedance? What's it mean?

Discussion in 'AV Receivers & Amplifiers' started by doogz, Jan 11, 2012.

  1. doogz

    doogz
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    Afternoon all,

    Looking for a little bit of advice.

    I had an LG surround system, 5.1 system, non-powered woofer, with a built i dvd player. The amp itself is fooked, so i've acquired a Pioneer VSX421:

    Pioneer VSX-421

    The rated impedance at output level is rated at 6 Ohms.

    The speakers i'm using, from my LG setup are quoted as being 4 Ohms.

    Is this likely to cause me any issues? Am i likely to cook my amp?

    Apologies for what is probably a simple/stupid question, this is not really my "thing"

    And thanks for any replies

    Doug.
     
  2. Raitzi

    Raitzi
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    Check that 4 is not below the lowest allowed rating. Also if receiver has setting for impedance, adjust it to 4. (read manual)

    In reality, you can use those speakers even if the 6 ohms is lowest allowed but you have to limit max allowed volume to 66% of normal maximum.
     
  3. doogz

    doogz
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    Thanks for the quick reply.

    I plan on dealing with the mess of cables and get everything up and running tonight, so i'll check the manual when i get home.

    I'll probably get around to buying some better speakers soon, hopefully the stuff i have will suffice in the mean time.

    Thanks again.
     
  4. bumpymark

    bumpymark
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    hi there to be honest you would be better gettin a decent set of speakers and active sub in order to do justice to your nice new reciever depending on your budget theres a good deal to be had look at richer sounds for example and see what they have go to offer hope this helps and good luck!
     
  5. doogz

    doogz
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    Sorry, i meant to mention, i've swiped my big active polk audio woofer from my music room to use with this new amp just now, because the LG one was no use.

    Been having a look on richer sounds actually, bought most of my stereo and my new TV from there. Problem is, i want floorstanding fronts and rears, but nothing too chunky. I think little dinky satellites on stands look a bit daft tbh.

    I'll keep looking, as it seems the speakers i was planning on using aren't very suitable after all. Found a .pdf of the receiver manual on the Pioneer website, quotes 6-16 Ohms as "Guaranteed Speaker Impedance".

    Cheers
     
  6. gangzoom

    gangzoom
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    I found these website very helpful:

    Speaker Impedance Explained - Ohms

    Connecting 4-ohm Speakers to an 8-ohm Receiver

    From what I understand the lower the impedance of the speaker the more power is required to drive them (hence a chance of blowing a low powered amp). My M&S speakers are rated at 4 ohms, when connect up to my Denon 1909 and you turned the volume up you could hear the strain the amp was been put under. Where as when connected to the 4311 theres absolutely no drama.

    I'm acutally surprised a LG one box does it all amp could drive 4 ohmn speakers in the first place, it took me a lot of looking around to find a AVR that could drive 4 ohmn speakers without a power-amp!!
     
    Last edited: Jan 11, 2012
  7. Raitzi

    Raitzi
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    I gave the simple answer :) But here comes the longer one. Lower impedance does not mean that it requires more power from amplifier. Thing is that lower impedance means higher current at same voltage level. You adjust output voltage with volume knob of your amplifier. Actual volume depends on the amount of current (because current moves the coil in magnetic field and produces movement to the element).

    I use just resistance and skip the variable impedance part which depends on frequency...
    P=power, U=voltage, I=current, R=resistance (impedance)
    P=U*I
    R=U/I
    Amplifier is limited by power and current limits.(but as voltage is controlled by amplifier, we do not have to worry about power limit)

    Now if the lowest rated impedance is 6 ohms. It means that electrical current limit at maximum power happens at max volume(voltage) setting from the knob.
    Imax=U1/6ohm
    this is our limit.

    Now for 4 ohm speakers:
    New maximum voltage U2 (how far you can turn the knob)
    Imax=U2/4ohm ->
    U2=Imax*4ohm=(U1/6ohm)*4=U1*(2/3)= U1*0.6666...

    So you get same output when volume setting is turned just 66% of what it was with 6ohm speakers. If you go over this you eventually can burn the over current fuse from the amplifier or of course even damage your speakers.
     
  8. dante01

    dante01
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    This does nothing more than reduce rail voltage to the amplification stages and in turn reduces both dynamics and fidelity. It could even theoretically cause clipping! The manufacturers only include impedance switching in order to comply with US regulations and curb consumer fears about overloading their amps. In real terms, it makes no real difference and simply shifts the risk of damage away from the amp and onto the speakers. It is better practice to leave amps set to 8ohm regardless of the actual rated impedance of the speakers being used. You simply need to keep a check on the amp for signs of overheating and you need to take more care with the volume dial. Most AV amps have built in cut out and protection circuitry anyway, so you are unlikely to cause damage to an amp even if you do push things that little bit too far.
     
    Last edited: Jan 12, 2012
  9. dante01

    dante01
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    I find the following explanation of impedance easy enough to follow and it doesn't require a physics degree to understand:
     
    Last edited: Jan 12, 2012
  10. Raitzi

    Raitzi
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    This does not make any sense. If it lowers the output voltage of the amp, how on earth it would stress speakers more?
     
  11. dante01

    dante01
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    I never mentioned causing stress to the speakers, I said that the rail voltage is lowered to the amplifier. This can effectively under power speakers and potentially cause clipping.


     
    Last edited: Jan 12, 2012
  12. doogz

    doogz
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    Cheers guys.

    I guess the only part that's actually confusing is why it's called "Impedance" instead of plain old resistance.

    Ohm's law is pretty simple to understand.

    Think i'll treat myself to some new speakers anyway :)
     
  13. noiseboy72

    noiseboy72
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    In very simple terms, its called impedance becuase it is a dynamic value, not a fixed resistance. The nominal value might be 6 ohms, but over the whole frequency range it could change from less than 2 ohms to more than 600 ohms! It is derived from other important characteristics of the speaker including inductance.
     
  14. BlueWizard

    BlueWizard
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    For general conversation, we use resistance and impedance interchangable. Also note that the NOMINAL impedance of the speaker is the average more or less impedance. At the resonance frequency of the speaker, the impedance cna be 10 time higher than the nominal, and on either side of that peak, it can drop down to about 2/3 to 3/4 of the nominal.

    In truth we have a complex combination of ever changing Resistive impedance, Capacitive Impedance, and Inductive impedance. The difference between them is reflected in the relationship between Voltage and Current.

    In a pure resistive load, voltage and current are in sync, when voltage peaks, current also peaks. This is not true of pure capacitive or inductive load, in one case, voltage leads the current by 90°, in the other voltage trails current by 90°. That makes the current and voltage a quarter cycle out of phase with each other. This is also true of inductive motor, which is why you see motors rated in VoltAmps (VA) rather than Watts.

    Now back to 4 ohms vs 6 ohms. This is really about the amount of current that is drawn for a fixed input voltage. If you apply 10v to 6 ohms, you consume 2.5 amp of current. If you apply the same 10v to 6 ohms, you consume 1.7 amps of current, 8 ohms consumes 1.25 amps.

    A power supply has a limit to how much current it can supply and more current means more heat in both the power supply and in the power stages of the amp.

    Most amp have a capacity range, many decent amps, especially stereo amps, have no problem with a 4 ohm load, as long as it is reasonably stable. Some AV amps are limited to 6 ohms, other better amps can handle 4 ohms.

    ALSO, the impedance rating for the power of the amp means nothing. Typically, amps use an impedance lower than the standard 8 ohms simply to raise the apparent power. A lower impedance load will consume more power, but doesn't necessarily give you anything for that power. The real limits of an amp are Voltage and Current. Voltage is limited by the voltage available from the power supply. If the Power supply is ±20 volts, then the power will be 50 watts to 8 ohms. If you put that same 20v into a 4 ohm speaker the seeming power goes up, but you are still limited to the same 20v.

    A 20v amp run at 4 ohms seems to have 100w. But a true 100w amp will have 28.5 volts available to 8 ohms.

    The point is, the impedance that the power is rated at is merely a reference point that you can use to compare the power of this amp to other amps. It has nothing to do with the speakers that can be used on any given amp.

    Now some amps give multiple power ratings, for example, NAD will rate power a 8 ohms, 6 ohms, 4 ohms, and 3 ohms. The 3 ohm rating simply implies that the amp would be marginally stable at that impedance. However, if it is rated at 3 ohms, that means it is very likely to be stable at 4 ohms.

    The standard working range for a vast majority of amps is 4 ohms to 16 ohms. Below 4 ohms the current demand are too high, which in turn mean to much internal heat. If the impedance goes above 16 ohms, the amps become unstable and tend to self-oscillate (like feedback).

    Now some PA amps can handle loads down to 1 or 2 ohms and remain stable and functional, but you are not ever in your lifetime likely to come across amps like this.

    Most amps will imply or come right out and say they are 4 ohm stable. Amps like Sony, mention nothing about 4 ohms, and like would not like speakers below 6 ohms.

    Keep in mind, another factor in heat is volume or loudness. The louder you play, the more current you use, and the more heat that results. I ran my Yamaha and Onkyo amps (stereo) with a combined 8 ohms plus 6 ohm speaker load. That makes the total load about 3.4 ohms, which is beyond the stated capacity of either amp. At normal volumes both amps were fine, but when I started to run the volume up near 50% (for a movie), the amp shut down.

    So, yes, 4 ohms will likely work. But how well and for how long and for how loud is anyone's guess.

    The spec sheet/owner's manual for this Pioneer amp does not rate the power at any impedance other than 6 ohms and 8 ohms, which does not bode well for 4 ohms speakers.

    Steve/bluewizard
     
    Last edited: Jan 14, 2012
  15. doogz

    doogz
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    Excellent, thanks for the advice guys, really appreciate it.

    TBH, i've only ever posted on here before when i've had a problem, but i think i might stick about, seems i could learn a fair bit.
     
  16. scottthehat

    scottthehat
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    my onkyo 606 only has the option for 4 or 6ohm(god knows why 6 and not 8) but the manual says it will run 4 to 16ohm speakers. my new speakers state nominal impendece is 6ohm but will run on 4 to 8 ohm amps.
     

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