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Impedance and Ohm question - newbie!

Discussion in 'Home Cinema Speakers' started by jpb2001, May 1, 2003.

  1. jpb2001

    jpb2001
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    My speakers are Mordaunt-Short 904 Floorstanders at the front, 304C centre and 302 rears. According to the MS website the impedance for all speakers is 4-8 ohms. My amp is a Sony STR-DB780 which has a selector for 4 ohms or 8 ohms.

    My question is - which selection should I choose - 8 ohms, or 4 ohms? Which is likely to give the best sound from my speakers? And how will the setting affect the loudness of the output? I am not a sound technician so please reply in laymans terms if at all possible. BTW, the fronts are set to Large, everything else to Small (no sub as yet).

    Thanks in advance for any help.

    Cheers,
    JPB
     
  2. Reiner

    Reiner
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    According to practical experience there is no LS with 4 to 8 Ohms, it's either 4 or 8 or anywhere in between. :)

    There is more to it though, if interested run a search on "impedance". :boring:

    However, to keep it simple as you requested, 4-8 usually turns out to be 4 Ohm, thus the switch belongs into that position, too.

    This does not affect the sound quality nor the volume/loudness, just limits the max. power the amp will deliver to protect itself from becoming overloaded.
    Ok, so it does limit the max. volume but it's not the volume you listen to constantly. If you are not comfortable with that you can leave the switch in the 8 Ohm position as long as you don't run a disco in your garage, i.e. keep volume at normal levels. The manufacturer might however refuse warranty in such a case.
    While unlikley that the receiver will actually get damaged (most have a protection circuit) I suggest better to be safe than sorry and use the 4 Ohm position.
     
  3. wookie

    wookie
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    This so called feature, used by some manufacturers, is designed to prevent overheating of the receiver or damage to its output transistors because of excessive current flow. The manufacturer accomplishes this in one of 2 ways: 1) Stepping down rail voltage supplied to the power amp or 2) feeding half the signal strength to a voltage divider of power resistors. Both of these methods severely limit dynamics and current capability of the power amp. This results in an audible decrease in bass capability and dynamics transient sound because the 4 ohm setting effectively increases the receiver's output impedance. Unfortunately many manufacturers put these features on their products to ease customer concerns with driving low impedance loads and for safety reasons when getting UL approvals. Note: In order to meet UL requirements, a receiver cannot be rated down to 4 ohms without having this switch onboard. Receivers without this switch are usually rated down to 6 ohms. In most cases, well designed receivers can easily handle 4 ohm loads safely and efficiently. It is highly recommend to keep the impedance switch set to 8 ohms regardless of your speakers impedance and make sure your receiver has plenty of ventilation.

    :lesson:
     

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