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4ohm vs 8 ohm?

Discussion in 'Pro Audio Equipment' started by lastounce, Aug 20, 2005.

  1. lastounce

    lastounce Guest

    Ok - this may be a really stupid question....

    but can you connect an amp that says "100 Watts per channel 8 ohm" into a pair of speakers that accept 4ohm? is it going to be the same thing or am I going to damage something?

    would the speakers have to have an 8ohm jack or the amp specifically be able to switch to 4ohm?

    just wondering...
  2. jonyoung

    jonyoung Well-Known Member

    Dec 31, 2003
    You could fry your amp. Best thing is to add some 4ohm resistors in series with the speaker inputs, effectivley making them 8 ohm speakers. Make sure they're rated to handle the wattage from your amp.
  3. Kurt Foster

    Kurt Foster Distinguished Member

    Jul 2, 2002
    77 Sunset Lane.
    It all depends. Is this a solid state or a tube amp? Tube amps usually need to see whatever load they are rated at ... solid state amps are more forgiving.

    When you see a S/S amp rated at 100 watts @ 8 ohms that usually means that how much power the amp will deliver at that load. However you can run a 4 ohm load on that amp and it will deliver almost twice the watts although with less headroom. You should be cautious when running loads below 4 ohms. There are amps like some Peaveys, that can run at 2 ohms but that's really almost a dead short ... use caution and refer to the owners manual if you are not sure.

    Professional sound companies usually run 16 ohm systems because they operate with more headroom and sound cleaner ... although they require more watts to reach a specific sound pressure level.
  4. lastounce

    lastounce Guest

    that makes sense...to a certain extent.

    How come Dynaudio's BM15 (passive) only accept a 4 ohm load then I wonder? Wouldnt it be better for them to have made them 8 or 16 ohm? Also, the amp I'm referring to is a bryston 3B (the old one, not ST)

    actaully while we're at it...

    how would you control the volume on a bryston amp? since it has no front volume controls will I need to run my signal into my mixer first? (I dont have a good mixer) Would I be able to control the volume from within ProTools (I'm sure this is a bad idea somehow...) how does everyone do it?

    (My setup: 002R into RME ADI 8 Pro DS AD/DA into an amp like bryston 3B or something cheaper that I can buy at my local shop (ie alesis RA 500) and Dynaudio BM15 passives....)
  5. sushifish

    sushifish Guest

    If you're running protools I would run your monitors through your mbox or 192 interface. I believe you use a "master fader" and assign it to your outs. (monitors) I've done this before in a studio but you must keep an eye on your levels. (I had a close call on someone elses big genelecs. :shock: almost a very expensive mistake)

    Hope this is accurate and helps :cool:
  6. iznogood

    iznogood Guest

    this is soooooo wrong!

    never ever do that!!

    you would need HUGE resistors to do that.... they would cost an arm and a leg!

    furthermore they would probably spoil the damping factor of the amp!

    any amp designed after 1990 should be able to drive a 4? load.... unless it's a highly unlinear impedance curve (electrostatics) or a capacitive load!

    and anyway... unless it's designed really poorly,,,, you would only blow the fuses!

    and certainly a bryston falls into the well-designed category!
  7. lastounce

    lastounce Guest

    lol thanks guys - great info from both
  8. jonyoung

    jonyoung Well-Known Member

    Dec 31, 2003
    iznogood wrote:
    Aw, c'mon! Power dissipating resistors don't cost that much. I wired up a pair of NS-10's to a solid state amp with fixed gain that had different output impedance AND 2x the gain of the monitors for $20. I used this setup for a long time and nothing blew up.
  9. dpd

    dpd Active Member

    Sep 29, 2004
    Putting series resistors in line is stupid. You are doing two things: throwing 1/2 of your power away, and raising the effective damping factor of the amp.

    The amp will drive 4 ohm speakers. Depending on how well it is designed (output transistor current and power supply stiffness), it should double its power into 4 ohms. But, it probably won't go to double. As long as you don't drive the amp into clipping driving the lower loads (rotten sound) or overheat (not enough heat sinking).

    Solid state amps are NOT impedance-matched devices - they are basically designed to be (somewhat ideal) voltage sources that will continue to provide greater output current as the load impedance continues to drop. Up to a point - for the reasons above.
  10. Fozzy

    Fozzy Guest

    OK, so you have an amp that will give 100W into 8 ohms and put a 4 ohm resistor in series with the speaker. That way the speaker gets 50W and the resistor gets 50W. That's quite a resistor if it can hack that!

    As dpd said solid-state amps are designed to have near zero output impedance so matching impedance is not a problem.

    By halfing the load impedance you will have doubled the power gain of the amp (the voltage gain remains the same), i.e. if it previously gave the 100W output with 500mv input it will now give the 100W at 250mv in.

    As you go above the 100W power level though there will probably be some headroom before anything notable happens but after that one of several things could happen:

    1. Distortion because the PSU is unable to supply enough current for the power you're trying to get.

    2. An internal fuse put there to protect the output transistoors blows. In the absense of such a fuse the output transistors themselves can fail.

    3. Too much heat. A quality amp should have thermal protection circuitry in which case it will just shut down. If they've been too cheap to include it then again it can lead to component failure.

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