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AVR200 speaker impedance settings

Discussion in 'Arcam Owners' Forum' started by NinjaKi11a, Aug 22, 2004.

  1. NinjaKi11a

    NinjaKi11a
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    I've just bought myself an AVR200 and I'm keen to get the badboy going, but I'm unsure about the speaker impedance setting on the back. According to the Quad website, my 11Ls have a nominal impedance of 6 ohms, but it says they are 'suitable for use with 8 ohm amplifiers'. Should I leave the amp at the default 8 ohm setting or change it to 4-6?

    EDIT: my rears are Mission 771e - not got a clue about their impedance.

    EDIT:Just found my 771e (rear) manual: it says they are 8 ohms. Because of this, and because the Quads are apparently 'suitable' for 8 ohm amps, I've left the amp at 8 - assuming that switching it down would be unsuitable for the rears...

    Do you think it's advisable to change?

    (Originally posted in amps forum).
     
  2. Crustyloafer

    Crustyloafer
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    I would reccomend using the 4 ohm setting as this will supply more current into low impredances, although at the expense of a slightly lower maximum volume level. The 4 ohm setting on the amp is much more tolererant of big impedance drops which a lot of speakers rated at 8 ohms can present on occasions.
     
  3. mrkeeling

    mrkeeling
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    If there is more current available in 4ohm setting, shouldn’t it be beneficial to have this stetting even with 8ohm speakers (with the cost of slightly lover volume level)?

    In other words, is 8ohm setting only allowing louder playback (sacrificing sound quality)?
    :confused:
     
  4. Crustyloafer

    Crustyloafer
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    The 8 ohm setting gives a higher (wattage) power output into 8 ohms but drops dramatically as soon as the impedance drops whereas on the 4 ohm setting the output when impedance drops remains more stable. What most people arent aware of is that although speaker manufacturers quote 8 ohms as the impedance of a speaker this is only a nominal or average rating and many 8 ohm speakers can drop as low as 2.5-3 ohms at certain frequencies.
     
  5. mrkeeling

    mrkeeling
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    So should I be getting better results (sound quality) in normal listening levels (we are probably talking about 1-10W/ch average) using 4ohm setting with speakers rated 8ohm than with using 8 ohm setting (theoretically of course)?
    :confused:
     
  6. Crustyloafer

    Crustyloafer
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    just switch it to the 4 ohm setting and forget about it.
     
  7. mrkeeling

    mrkeeling
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    I’m sorry for being such a pest, but I really want to understand this. :lease:

    I am by no means expert in this field, but I do have some ideas about elementary physics (maybe that’s the reason why I’m not getting this). ;)

    So we have three basic elements here: voltage, resistance and current. From the basic physics I=V/r (current is equal to the voltage divided by the resistance). So, with the constant voltage, cutting resistance in half will double the current. (8volts/8ohms=1amp; 8volts/4ohms=2amsp; 8volst/2ohms=4amps).

    In other words, every dip in speaker resistance creates bigger demand for current. If the demand for current is too big, it can overstress the output stages of an amp or overheat it (assuming power supply can handle the demand).

    From what I understood, 4ohm setting on speaker impedance switch alters the voltage supplying the amps so that current demands (caused by lower speaker impedance) do not overstress or overheat output stage (4volts/4ohms=1amp; 4volts/2ohms=2amps; 4volts/1ohm=4amps).

    If this is anywhere close to being correct, what I am not getting is how is more current available in 4ohm setting?

    And we all know that more current means better speaker control, hence better sound! :thumbsup:

    What I was told is try with 8ohm setting and if amp runs too hot, switch to 4ohms.
     
  8. mrkeeling

    mrkeeling
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    So, nobody want to shed some light on this and help me understand?

    Or am I so far off that I’m beyond help? :rolleyes:
     
  9. ANDY_DUTTON

    ANDY_DUTTON
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    Your description of what is happening is correct.

    The reason more current will be available is because the V/I protection in the amplifier protects the output stages by keeping the output devices inside their safe operating area. To do this it simultaneously senses the voltage and current across the output stage. When the load is trying to draw more current than the output stage can cope with at that voltage, it levels off the signal. If the supply voltage is lower the V/I protection can allow more current through before the output stage gets into trouble. This is especially true with reactive (that is, non-resistive) loads like loudspeakers.

    Another benefit with a 4 ohms setting is that as less heat is generated in the output stage the amplifier will run cooler causing less thermal drift and thermal distortion in the characteristics of the amplifier. Even at fairly low power outputs an amplifier dissipates quite a lot of power in the output stage. This is because if the voltage rail is for example +/- 55V (typical of the 8 ohms setting) and the output power into the 4R load is 12W rms, then only approximately +/- 10V peak will be across the load - the rest of the voltage (+/- 46V or more) is dropped across the output transistors causing them to dissipate substantial power. If the rail voltage is dropped to say +/- 40V then the dissipation in the output transistors is reduced by one third. (This is a gross simplification of what is actually going on but it gives you some idea). The output power at which an amplifier dissipates the most heat in its output stages is around one third of its rated power output, not at its maximum power, as many people assume.

    It is for these reasons that we provided settings for both 4 and 8 ohms speakers in our AV receivers. Incidentally the drop in maximum output signal voltage when driving an 8 ohms load on the 4 ohms setting on an Arcam AVR is only around 2.3 dB, which is quite difficult to discern.

    It is difficult to predict which setting will sound best for any set of speakers but the safest setting is always the 4 ohms one. If you are using any 6 ohms speakers I would recommend you use the 4 ohms setting.

    Regards,
    Andrew
     
  10. mrkeeling

    mrkeeling
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    Now that’s an explanation! :thumbsup:

    Now I am (finally) really getting it! :clap:

    Thank you Andy for taking the tame and clearly explaining this. :smashin:

    You are the best
    :beer:
     

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