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Cab and Amp querry

Discussion in 'Bass Amps & Cabinets' started by Avram, May 10, 2008.

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  1. Avram

    Avram Thread Killer

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    Ok...so I think I understand most of the 8 ohms vs 4 ohms thing.

    When hooking up a 500w-4ohm amp to one of the following....
    1. One 500w-4ohm cab - will use full power of the amp, and be loud.
    2. One 500w-8ohm cab - will use about 2/3 the power of the amp, and not be as loud.
    3. Two 500w-8ohm cabs - will use all the power of the amp, and be loud.

    My questions are:
    1. Will the two 500w 8ohm cabs be under-powered in setup 3?
    2. Would it be better to use a 4ohm 1000w amp with two 4ohm 500w cabs, or is this simply louder?
    3. Does achieving the full wattage capabilities of a cab produce a better range of tonalities than only using partial wattage, such as in setups 2 and 3?
    4. Does none of this affect the tonal characteristics and its all a question of decibels (loudness)?
    5. Am I insane, or the only purpose for using 8ohm cabs is to hook them up together with smaller amps for the purpose of achieving full amp power (and maybe hooking two different cabs to the same amp)?

    :shock:
     
  2. Rieno

    Rieno

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    1. Underpowering doesn't exist, to be blunt.

    2. If an amp can run at a minimum of 4 ohm, don't hook up two 4 ohm cabs! You will get a total impedence of 2 ohms and your amp will run to hot and eventually die.

    3. Speakers will usually sound beter with volume (soundwise, not size), as does your amp. This has nothing to do with wattage of the cab, as all they do is let you hear what is given to them.

    4 & 5. Mainly a question of loudness and versatility. The possibility of hooking up two 8 ohm cabs means you can vary the speaker sizes.

    Of course, most of this goes out the window when using tube/valve amps. They will usually only run well at a given impedence (although going up in ohms does less damage than going down). Some older ones even take nothing under 16 ohms!
     
  3. Avram

    Avram Thread Killer

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    so...wattage ratings for cabs indicate how much they can take, maximum wattage, not necessarily at what wattage they will sound best. Got it...

    Does that mean then...that if I hook up two (2) 500w-8ohm cabs to a 1000w-4ohm amp I will split the wattage between the two cabs and get their full 500w potential?

    Thanks a lot for the reply Rieno!! :D
     
  4. Rieno

    Rieno

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    Jup..when you hook up two cabs in parrallel (which you normally do), the amp will send half of its power to one cab and the other half to the other cab.

    Using a 750w cab and a 250w cab in this setup would put the lower rated cab in danger :)

    And no prob man, I've had to learn all of this and I remember how happy I was when people helped me ;)
     
  5. Avram

    Avram Thread Killer

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    Yeah thanks...it seems so simple now that I understand it...like algebra
    :wink:

    I see now why people get an 8ohm 500w 4-10 cab with a 4ohm 500w amp. Though they can't use all the amps power right away, they can later add another cab like an 8ohm 500w 1-15 cab to get more low-end rumble and complete the full wattage-usage from the amp. And then later...then can trade up the 500w amp for a 1000w amp to get double the volume out of the same cabs...neat. 8)
     
  6. Benjamin

    Benjamin

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    It's no problem if you have a 600 watt cab with a 300 watt head. Or two 600 watt cabs with a 500 watt head. The head will give out 500 watts and split it between the two 600 watt cabs. Having a maximum production of 1200 watt, will give you plenty of headroom, and if you screw gain to the max and rape your strings you're head still shouldn't be clipping :)
    The extra watt in cabs over the head is headroom, and it's nice to have ;)
     
  7. Rieno

    Rieno

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    Now you're wrong. Headroom is having more power in the head than you need, so you have power left for spikes in the in- and output stages. Has nothing to do with cabs :)

    Cabs don't produce anything in terms of wattage.

    That's why I always advise having a head with more watts than the speakers can handle (RMS). When you put too much power through the speakers, they will distort very audibly and they can take a beating, believe me. When you have cabs with more capacity in watts than the head can deliver, you can (if you're not paying attention to sound) push the head into clipping. If you keep that up, you can wreck both your amp and the cabs pretty easily :)
     
  8. Avram

    Avram Thread Killer

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    so...

    500w-4ohm-amp + 2(500w-8ohm-cabs) = full 500w usage from amp

    but

    the combined available watt usage from both cabs is 1000w, so there is a 500w available space left unused by the cabs. And this isn't headroom?

    and

    1200w-4ohm-amp + 2(500w-8ohm-cabs) = 1000w usage from amp with 200w left unusable by the cabs. This 200w is headroom?

    I wondered what headroom was but never got around to asking.... :?
     
  9. Florin

    Florin Warwick Forum Administrator Staff Member

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    Actually this is the problem... In general, the power amps are not meant to be used to the max. Hellborg actually is, but, the rest of them not... So what happens is that after let's say 3 o clock, the amp will start distorting, and it will produce square waves that will destroy your speakers.
    Using big cabs powered by little amp cranked to max is the way to blow your speakers.

    Using a 250 W amp witha 500 W cab is NOT headroom. The amp will clip before the cabs.
    Using 1000 W amp with 500 W cab IS headroom, you will use the amp with lower settings, the amp will actually sound better, nice and clean.
     
  10. Rieno

    Rieno

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    Mwa, headroom is just having more power than you need. Nothing to do with the cabs :)

    If you have a 1000 watt amp, that's probably way more power than you're ever going to need. Simply put:

    When using an amp at half the volume and don't need to go over this, the power you leave alone is headroom. When using an amp at full power, you have no power left for spikes and therefore no headroom. Spikes can happen when slapping and popping for example (or when there's a faster piece in the music and you accidentaly start playing harder ;)).

    Edit: Florin kinda says the same..having your cabs at a lower rating than the amp, just makes sure you're not even able to push your amp to the max :)
     
  11. Avram

    Avram Thread Killer

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    So would two 500w 8ohm cabs be too much drain on a single 500w 4ohm head?

    Since the head is splitting its power between the two, and each cab is only getting 250w, is this bad for the amp then?

    or is this different from under-powering a single 500w cab with a 300w head?

    damn, now I'm confused again. :lol:
     
  12. Rieno

    Rieno

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    Underpowering doens't exist :D

    At it's max, a 500 watt @ 4 ohm head will send 250 watt to each of the two 8 ohm cabs. If you don't use it at it's max, it will be less.

    The head sends whatever you want it to send. A cabinet doesn't drain your head of power..it only handles what it gets.
     
  13. Avram

    Avram Thread Killer

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    right, sorry, I guess Florin's comment about square waves threw me off. :lol:
    I think I got it now...basically, don't crank up the volume all the way, watch out for clipping, underpowering doesn't exist. :D

    Thanks again for help guys!
     
  14. Rieno

    Rieno

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    Yup, that's the general idea :)

    Florin's comment on square waves is true when you clip your amp hard and keep that up for a short time. With those you can wreck head and cabs.
     
  15. Florin

    Florin Warwick Forum Administrator Staff Member

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    Underpowering exists!
    a 1000 W cab cannot be used with a 100W solid state amp, for two reasons.
    1- it will not sound right. The cab has an optimal power, where it sounds good, with all the harmonics and stuff. It is not made to play at 100W. SOme says that the drivers are cooling by the movement, so if you play them at 100W they can overheat and fry :)

    2- If you use 100 W in 1000 W cab you will want p-ower, you will crank the gain and output, ... square waves.... burn!

    SO as you see, underpowering does exist, and it will make you sound bad, and will burn the drivers too.
     
  16. Rieno

    Rieno

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    For your second point, that's just plain common sense not to crank the amp up to those levels ;) Square waves are bitches, but easy to prevent.

    First point is valid though, should've thought of that. My bad! It's just that people sometimes seem to think that a 300 watt head can 'underpower' a 400 watt cab and wreck it and when that comes up for the umpteenth time, I get all squigly :p
     
  17. Avram

    Avram Thread Killer

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    Ok...thanks Florin :)

    so would the following setup be considered 'underpowered'?

    4ohm 500w head
    8ohm 400w 115 cab
    8ohm 600w 410 cab

    or would this be healthier?

    4ohm 1000w head
    8ohm 400w 115 cab
    8ohm 600w 410 cab

    or just louder? :?
    I don't mean to drag this on, just trying to wrap my head around it. :D
     
  18. Florin

    Florin Warwick Forum Administrator Staff Member

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    Actually if you read the previous posts, there are still many of us that think it is safe to crank the amp, as far as the cab can take it. So is our duty to remind that from time to time :)

    Avram, both setups are fine. Of course the second one is the best, but the first will work ok, just remember to use the master about 12- 2 o clock.
    Cheers/ Flo
     
  19. Avram

    Avram Thread Killer

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    Thanks Florin! :D

    And thank you too, Rieno, for all the input! :D

    I'm pretty sure I get this now...I have been reading many of the older threads here in the Amp section and it all helps.

    And thanks to you as well Benjamin for your post! :D

    I think we can consider this thread closed.

    Cheers!
     
  20. Eberbachl

    Eberbachl

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    I'm very sorry Florin, but this is simply NOT true.

    It is completely safe to use a 100 watt head (or less!) with a 1000 watt cab, assuming you don't send the head into clipping.

    It's just like running a 500 watt head at low volume. Speakers don't require large amounts of cone movement for cooling.

    Of course a cabinet will be at it's most efficient if you are using a head that produces clean power at something close to the cabinet's power rating, and of course Florin is right that ANY head that is sent into clipping will damage ANY cabinet.

    But - it is not true that it is unsafe to use a lower power head with a higher rated cab as long as you refrain from clipping the head.

    ;)
     
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