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sorry but another OHM question

Discussion in 'Amps and Cabs [BG]' started by Whil57, Aug 12, 2017.

  1. Whil57


    Aug 7, 2013
    Long Island
    OK, read the old sticky on OHMs. Great post. Read some users questions. Still left wondering though. This question is in regards to a single speaker. Lets take one amp head- rated at 8ohms for a single cabinet, but 4 ohms for two cabinets. So you get and 8 ohm cab. But if you use the second speaker output, you now have four Ohms operating (based on the formulas ive seen posted). But can the amp head, SAFELY use a single cabinet by a single of its outputs on a 4 ohm cabinet. It says the head operates at 4 ohms sometimes, so how does it know. I've seen in the articles that if the single cabinet was 2 ohms the lack of resistance would push back to the head. So i am confused i guess how the head would work with a single speaker or cabinet.
  2. /\/\3phist0

    /\/\3phist0 Mesa Engineering! Warwick-G&L-Source Audio Supporting Member

    May 7, 2007
    The amp sees the load, not the outputs in use, not the number of drivers in said load.
  3. jazzbass_5


    Sep 1, 2007
    NY, Albion
  4. Whil57


    Aug 7, 2013
    Long Island
    so the amp therefore has a tolerance, then. 4-8 ohms is good and it doesnt matter where it falls in between.
    Rip Van Dan likes this.
  5. Yes, the amp doesn't mind, as long as the load doesn't get below 4 Ohms
    but, for tube amps you shouldn't overload them with too many Ohms
    and for SS amps, you definitely don't want a short

    It doesn't, which is why us humans have to be careful what we connect to them.
  6. Whil57


    Aug 7, 2013
    Long Island
    Thanks, that helps.
  7. /\/\3phist0

    /\/\3phist0 Mesa Engineering! Warwick-G&L-Source Audio Supporting Member

    May 7, 2007
    Right as a rule of thumb (some tube amps excluded) higher #ohm rating will always be safe. The marked low limit (in your example) is 4 ohms.
    One 4 OHM cab
    2 x8 OHM cabs
    4×16 ohm.
    Equals a 4 OHM load.
    2x4 ohm cabs would be 2 ohms and unsafe.
    Unless the head is rated for 2 OHM operation.
  8. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Are you talking about a tube or a solid state output section?
    Eric Bates likes this.
  9. Dwight


    Apr 9, 2012
    It doesn't matter. neither amp will like being mismatched to the speaker load. If the impedance is too high you will not get maximum power transfer. If it is too low, you risk damage to the amp from excessive current flow.
  10. BogeyBass


    Sep 14, 2010
    yes a single output can run a single speaker, or even multiple speakers. doesnt matter as long as the load is not lower than 4ohms. how many plugs/jacks/ or cables it takes doesnt matter. as long as the load and whatever combinations of speakers in series or parallel is not lower than the minimum allowed impedance. Which is usually 4ohms for most amps 2ohms if they are designed to do so. as far as tube heads its little different cause they use transformers. so usually require the speaker to be plugged into or switched to a specific tap for whatever impedance is needed.

    nothing gets pushed back to the amp with a low resistance. a 2 ohm load is just fine for a amplifier if its designed to operate a 2ohm load. there is 2 ohm load stable amps. basically a low impedance load needs less voltage and more current. the extra current will put more of a demand on the output devices, with solid state basically transistors. extra current causes more heat so it has to be able to deal with the extra heat, likewise have a power supply that can provide enough current. also enough output devices to provide the needed current.

    on the other side of the scale amps are just fine with 8 ohm 16 ohm 32 ohm loads as well, it takes more voltage to get higher power with high impedance loads, so depending on how much voltage swing the amp can provide is how much power it can provide. likewise whatever current is required at that voltage. same with 2ohm loads still needs voltage swing just less, and a buttload of more current. nothing magically changes, a amp is what it is, it has so much voltage swing and so much current available. depending on impedance. it can provide said amount of power with whatever voltage or current it has. Basically how ever much voltage and current the power supply of the amp can provide. and how much current the output devices can provide without overheating or becoming unstable
    Last edited: Aug 13, 2017
  11. Whil57


    Aug 7, 2013
    Long Island
    Ok. Bogey, you just hit upon part of my not specific enough question. (and to answer beans its a solid state Peavey Tour 700). Since i see no ohm load switch, then from what you said, if i plug in one cabinet rated 4 ohm, the amp output would be at 8 ohms regardless of the cabinet being 4 ohms, if thats the way it operates and i wouldnt get a 4 ohm load unless i activate the other output jack? (I believe the specs said parallel outs).
  12. shastaband

    shastaband Supporting Member

    Dec 12, 2006
    NO, if you plug in one 4 ohm cabinet your Peavey will see a 4 ohm load. Doesn't matter if the other speaker jack is open. The jacks are paralled. If you plugged one 8 ohm cab into one speaker jack, and plugged another 8 ohm cabinet into the first cabinet, then your amp would also see a 4 ohm load. Even if the other output jack on your amp has nothing plugged into it. (Jacks on cabinets are almost always wired in parallel also.) It's about the total load presented to the amplifier, and it doesn't matter if you use the first jack, second jack, or both jacks. This is true for most solid state amplifiers like your Peavey. (There are a very few exceptions, in some tube amps and combos, but not with your Peavey.)
    Last edited: Aug 13, 2017
    bassrich likes this.
  13. MDBass

    MDBass Supporting Member

    Nov 7, 2012
    Los Angeles, CA
    Endorsing Artist: Dingwall-Fender-Jule-Dunlop-Tech 21-Darkglass-Nordstrand

    If you plug in a single 4ohm cab, the amp gets a 4ohm load.
  14. Munjibunga

    Munjibunga Total Hyper-Elite Member Gold Supporting Member

    May 6, 2000
    San Diego (when not at Groom Lake)
    Independent Contractor to Bass San Diego
    Yes. You will have access to the full power of the amp, so be careful not to exceed the speaker's power handling capacity.
    Downunderwonder likes this.
  15. There is only one amp output. It goes to two output jacks connected in parallel.

    You can connect whatever you like to either or both so long as the combination doesn't overload the amp with too little impedance.

    One 4 ohm cab should be a robust one or the amp will blow it up.
  16. Nev375


    Nov 2, 2010
    No. Think of it like a wall outlet. They usually come in pairs. You can use either one or neither or both of them at once. They are wired to the same breaker, but if you plug too much stuff in you are going to draw a lot of current and have a fire on your hands if that breaker fails to trip.

    Many amps, especially newer ones, have protection circuits built into the output. But it's not a good idea to test them.
    Rip Van Dan likes this.
  17. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The Peavet Tour 700 has two output connectors wired in parallel. Offering two connectors is a convenience for you, it allows you to daisy chain two cabinets (connect in parallel) from the back of the amp. Many cabinets have two connectors for the same purpose, you can connect two cabinets in parallel from the back of the cabinet. Whether you connect them both at the back of the amp or connect one cabinet to the other and then into the amp amounts to the same thing.

    The specs of you amp are 500W into 8 ohms impedance or 700W into 4 ohms impedance. The nominal impedance of a speaker cabinet is specified in ohms. You should not connect a cab or cabs that total less than 4 ohms. That is the minimum that your amp can handle. Less than 4 ohms and you risk damaging your amp. Keeping these limits in mind is important, you have to adhere to them.

    So to get the most power output from you amp, you would need one 4 ohm cabinet or two 8 ohm cabinets daisy chained together. Two 8 ohm cabs daisy chained together will yield a total of 4 ohms.

    A musical instrument amp with a solid state output section does not have an output transformer. The power output will depend on the speaker load that is connected to the amp. In your case, a 4 ohm load will deliver maximal output power, 700W. If you connect a higher impedance speaker cabinet, the power output will be reduced. In your case, 500W with 8 ohms.

    This will not harm the amp as long as your total speaker impedance is not below 4 ohms. For example, two 4 ohm cabs connected in parallel would yield 2 ohms total, this would damage your amp because it is below the minimum allowable impedance of 4 ohms. So you do have to be careful what the total impedance is that you connect to the amp.

    A musical instrument tube amp is different as most have an output transformer. Often transformers have different output taps that allows the amp to safely operate with a variety of cab impedances. There could be multiple output connectors or one connector and an impedance selector switch, it amounts to the same thing. For example, it might have separate 8 ohm and a 4 ohm outputs. For maximum power output, you have to be careful to connect either an 8 ohm cab to the 8 ohm output OR a 4 ohm cab connected to the 4 ohm output. This ensures that the impedance of the cab is matched to the amp and that the rated power will be delivered. A different combination such as a 4 ohm cab connected to the 8 ohm output, would result in an impedance mismatch. People do run tube amps with impedance mismatches but it is not recommended. The amp design dictates what you can get away with. Too high or too low a mismatch will damage the amp.
    Last edited: Aug 13, 2017
  18. john m

    john m Supporting Member

    Jan 15, 2006
    The amp only " knows" how much current to supply to the load.
    It does not " know" what that limit is an will try to supply the amount of current asked for by the load ( as much as infinity for a dead short on the output terminals).
    More current = more heat. Heat is the enemy.
    Our brain is in control of the heat via how much current we ask the amp to supply to the load--- don't hook up a load below the rated impedance AND don't run the amp at loud volumes that are heavy in the low end--- this saves speakers too.
    Note: rules are slightly different for most tube amps.
  19. The amp sees and is concerned with the *total load*. If the total impedence is 8 ohms the amp doesn't care if it is one 8 ohm cabinet or a combination of cabinets totaling 8 ohms. Same goes for 4 ohms or any impedence.
  20. n1as


    Mar 29, 2013
    This is a misleading way to state the amp spec and if it is your understanding than no wonder you're confused. Amps are not rated for "number of cabinets" but the amp is rated to work over an impedance range down to a minimum of (usually) 4 ohms. The number of speakers and the way they are wired determines the speaker load (ohms) and it has nothing to do with the amp.

    Plugging an 8-ohm cabinet into either output jack will give the amp an 8-ohm load.

    Plugging a 4-ohm cabinet into either output jack will give the amp a 4-ohm load.

    Plugging two 8-ohm cabinets, one into each jack gives the amp a 4-ohm load since the output jacks are wired in parallel.

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