What happens running 4 ohm amp connects to 8 ohm speakers?

Discussion in 'General Audio Discussion' started by 70WR, Jun 13, 2011.

  1. 70WR

    70WR Super Member

    What happens - short/long term?

    I just found out that on one of my amps, the 4 ohm speaker connects (banana plugs) I had feeding 8 ohm speakers - been this way less than 2 weeks.

    I found how to do it, both 4 into 8 and 8 into 4, but I don't want to know how to do it - just what happens when its done.

    How I did it - I was running my tube amp into 4 ohm Maggie's, then over a week ago I pulled the banana plugs from the Maggie's (MG-I) and just plugged them into 8 ohm speakers, not thinking the other end of the wires were fed from the tube amp 4 ohm speaker connects instead of the 8 ohm speaker connects.

    So to add to to my wondering, what if it was the other way around, amp 8 ohm fed to 4 ohm speakers.
  2. icenine

    icenine Super Member

    No problem running 8 ohms on a 4 ohm tap....you just do not want to do the reverse i.e. run a 4 ohm speaker on an 8 ohm tap....
    Last edited: Jun 14, 2011
  3. 70WR

    70WR Super Member

    ^ Whew! I didn't know of harm to the speakers and/or amp. For over a week, I ran it like this daily from 2 to 8+ hours. I didn't push the amp hard so I figured maybe I'll be alright.

    So if amp at 4 ohms to 8 ohm speakers is okay - why not leave the speakers wires connected to the 4 ohm connects on the amp all the time - then when I swap speakers between 4 ohm and 8 ohm - nah, scratch that - I learned a lesson and hope to always make it a point on checking this whenever switching speakers/amps etc.

    I figured running amp (8 ohm) to speakers (4 ohm) is worse than what I had done. Impedance mismatch would be worse.

  4. icenine

    icenine Super Member

    Best bet is too know what load your amp can run.....read the manual. And be careful running four speakers (A+B outputs) at same time.....playing four 8 ohm speakers creates a 4 ohm load to the amp. Playing four 16 ohm speakers at the same time would make an 8 ohm load, which would be safer for most amps but there are not a lot of 16 ohm speakers out there. I know it gets confusing.
  5. shelly_d

    shelly_d Not An Audiophool

    4 ohm output into 8 ohm speakers = less power output and perhaps less distortion. This can be a good thing if you end up not being shy on power, that is not pushing the amp all the way into clipping.

    Reversing the mismatch and you could get excessive heat build up in the amp's output and may increase distortion, neither is a good thing.

  6. donberry

    donberry Super Member

    actually, many will recommend trying it out on both and see if there is a difference in sound and use the one you prefer. For tube amps anyway.
  7. MelodyMaster

    MelodyMaster 78s galore.

    I run 8 ohm speakers on the 4 ohm tap anyway because I never approach full output and it does sound a bit sweeter that way, smoother bass and highs.
  8. 70WR

    70WR Super Member

    I didn't know there were recommendations for this - and here I nearly had a stroke the moment I saw it, a panic shut down - for no legit reason I guess other then I thought at the moment "what am I doing to my amp!" - all for nothing

    Thanks again all - maybe I'll do a comparison, I actually didn't notice it but wasn't doing it looking for a change.
  9. gushtone

    gushtone Member

    On a related note, I run my old Marshall head at 8 ohms into a 16 ohm 4x12 cab and I always preferred the sound to the 16=>16. Let's me push it a bit harder and sounds warmer.

  10. mhedges

    mhedges Super Member

    You will lose some potential power by running 8 ohm speakers on a 4 ohm tap but other than that nothing will be hurt.
  11. BADBOB

    BADBOB New Member

    Hello . Newbie here so please forgive me for not knowing the rules yet.
    This is a new thread about bi-amping .
    I have a pair of infinity kappa 8.1 . I have a pair of mcintosh mc2500 amps. How do i hook these up and what do i need to hook these up ?
    Any info would be appreciated .
  12. positron

    positron Super Member

    Nice points Shelly. To add a little more, running 8 ohm speakers on 4 ohm amp output will reduce peak plate currents. This is helpful when running class AB and B as plate dissipation is reduced. However, as Shelly mentions, output power is usually reduced a little as well. Damping factor is increased with such a setup compared to 8 ohm speakers on 8 ohm tap.

    As Shelly also points out, reverse mismatch will cause excessive peak plate current, increasing plate dissipation, which is not good for the output tubes.
    Damping factor is also reduced.

  13. rucyrius

    rucyrius New Member


    If the two 8 ohm speakers are wired in parallel and tapped into 4 ohm on the amp isn't that cutting the speakers to 4 ohm and it's all running at 4 ohms?

    Also what happens if you have two sets of 8 ohm speakers running from a 4 ohm tap running at the same time ... I have a fisher 500c set on 4 ohms.. To a speaker switcher .. Most of the speakers are 8 ohm. ( one set in living room are 4 ohm super sensitive(95)) there are 4 sets of speakers in all ... I never run them all at the same time .. But am I asking for trouble if I have two sets on at a time... And what if I run one set of the 8 ohm speakers at the same time as the 4 ohm set ... They sound ok when I do it but I'm worried I'll blow up the vintage amp... Thoughts?
  14. shelly_d

    shelly_d Not An Audiophool

    2 sets of 8 ohm speakers in parallel are 4 ohms to the amp. Period. Treat them the same as a 4 ohm set of speakers.

    4 Ohm speakers in parallel with 8 ohm speakers makes the amp see 2.6 ohms. Place such a pair on any 2 ohm tap you may have. If you don't have a tap that goes down to 2 ohms, you should not run them in parallel. In my opinion it is just too low for the amp and could cause heat related issues in the long run, even if not causing issues in the short run.

  15. rucyrius

    rucyrius New Member

    Then if I run the 4 Ohm speakers along with the other two sets of 8 ohm speakers .. What does that do to the amp?

    Otherwise I should make sure the 4 ohm speakers are only run by themselves. Ie: without other 8 ohm speakers switched on?
  16. rucyrius

    rucyrius New Member

    One other thing .. If I'm using the 4 ohm tap... Is it better to run the two sets of 8 ohm speakers ? Is that better than running one set of 8 ohm speakers?
  17. shelly_d

    shelly_d Not An Audiophool

    You have all these questions that you are asking. Instead of laboriously answering each possible permutation of speaker hookup, here is a lesson in how to calculate the resulting impedance of any combination:

    First lest us assign the variables to the speakers.

    Let Z1 be the impedance of the first set of speakers,
    Z2 the impedance of the second set and
    Z3 the impedance of the 3rd set.

    Lets call the Impedance that the amp will see Ze (for effective impedance).

    The formula is an addition of fractions. That is:

    1/Ze = 1/Z1 + 1/Z2 + 1/Z3 (this continues for however many speakers you connect in parallel.

    Just set up these fractions, find the lowest common denominator like in grade school and add them together.

    Note that the left side of that equation is one OVER the effective impedance so once you have added all the fractions, flip it over (i.e. take the reciprocal) and then do the division (top or numerator divided by the bottom or denominator). The results of that calculation is what the amp thinks it is driving.

    So: For 4 ohms in parallel with 8 ohms:

    1/Ze = 1/4 + 1/8​

    The lowest common denominator is 8 so use that:

    1/Ze = 2/8 + 1/8​

    Combine like terms

    1/Ze = 3/8​

    Now take the reciprocals of both sides:

    Ze = 8/3​

    Perform the division of 8 ÷ 3

    Ze = 8/3 = 2.6 ohms​

    I hope this helps.

  18. sb47

    sb47 Well-Known Member

    All speakers are rated at an "average" wattage and ohms load, its not set in stone. Depending on the frequency the ohms level will vary somewhat anyway.
    Not sure if any harm will be done using ether tap but the distortion levels will go up as the ohms load goes down. So running an 8 ohm speaker on a 4 ohm tap may not do harm “unless” you drive it hard, should be no problem, but you may be robbing yourself of a cleaner signal.
  19. rucyrius

    rucyrius New Member

    Thanks Shelly
    I think I have it now...
  20. nickxp

    nickxp Member

    What about if I have 1 meter 94db 8 Ohm speakers. I find them to sound much better on the 4 ohms tap. With 94db being easier to drive? Would this make since

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