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watts vs ohms

Discussion in 'Amps and Cabs [BG]' started by sandman357, Mar 23, 2005.


  1. Hey,
    This is probably a stupid question, but what is the difference between watts and ohms? I am looking at two Ashdown 410 cabs. One is 300 watts at 4 ohms the other is 450 watts at 8 ohms. Both are the same money, $299 no tax or shipping from Zsounds, is there a big difference?
    (for the time being I am going to use an old 200 watt Carven head to drive it.) I am thinking of the 450 watt option. Any advice?
    Thanks.
     
  2. the simplest way to determine that is if the speaker output on your amp says 4 ohms, get the 4 ohm one, if it says 8 ohms, get the 8 ohm one
     
  3. Ohms = The resistance

    Watts = The power

    even tho the 8ohm one is 450 watts, it is probably going to have a lower output than your amp, unless it has a switch

    I doubt your amp will be 200 watts at 8 ohms and 4 ohms, its possibly 200 watts at 4ohms, and 125(ish) at 8ohms, or, as i said, could have an ohmage switch . . .
     
  4. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Step 1:- Look at the back of your amp. It will say something like xxWatts at xxOhms. Most amps are 4 ohms, meaning you can run 4 ohms minimum. If this is the case, 8 ohms cabs are are fine, as are 16 ohms cabs or anything above that. The kicker here is that the more Ohms, the less watts your amp produces.

    Step 2:- Determine if you are ever going to want a second cab. If you get the 4 ohm cab now, and your amp is rated for 4 ohms minimum, you can not drive a second cab with that amp. Two 4 ohms cabs creates a total load of 2 ohms - an amp with a 4 ohms minimum rating will overheat and fry. However, two 8 ohms cabs create a total load of 4 Ohms, perfect for a 2 cab arrangement on a 4 ohm amp.
     
  5. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    In any case neither watts nor ohms will tell you much about how loud you can expect either to be with your amp, as watts measure power, ohms measure impedance, and sound level is measured in decibels (dB). With accurate senstivity charts you could make an informed decision, but getting those is not likely to happen. The majority of manufacturers don't even have them available for their own use. Absent said charts the only way to be sure which is best is to A/B compare. Buy both, send the one you don't want back.
     
  6. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Yep. And specs can never tell you what a cab will "sound" like in terms of tone. All this stuff led to the creation of my sig. Once the technical side is taken care of, you really do have to audition things with your ears and base your decisions on that.
     
  7. stropsrats

    stropsrats Owner: www.kennedyaudio.com

    Feb 14, 2005
    Valders, Wisconsin
    I go to my books on microphones for reference when it comes to impedence match up. They tell it like it is, pros and cons. Higher impedences sound better but are noisier. Then again, you can always add impedence to a low impedence match, but you can't subtract it. That is why some microphone companys only supply their microphones as a low impedence microphone. You have to add the impedence using a little unit you get at radio shack. If your amp has an XLR outlet jack to the speakers, then this is what the manufacturer intends. Otherwise, it's best to ask a lot of questions to the technical support people of your amp untill they come up with some answers. You can also add impedence by putting a capacitor across the line, but be careful and get some help if you decide to do that. Then like the man said, your ears will be your guides.
     
  8. Selta

    Selta

    Feb 6, 2002
    Pacific Northwet
    Total fanboi of: Fractal Audio, AudiKinesis Cabs, Dingwall basses
    I just have to play devil's advocate and say that he can run 2 4ohm cabs if he wants to. :) It's all in the wiring...

    Ray
     
  9. stropsrats

    stropsrats Owner: www.kennedyaudio.com

    Feb 14, 2005
    Valders, Wisconsin
    Here is a watt puppy to look at while you are deciding:
    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=14993&item=5761690934&rd=1&ssPageName=WDVW

    An expert reviewed a pre-production model and found it worked better when a small resistor was added in series.
    Link:http://www.stereophile.com/features/127/index2.html

    As this article points out, with tube amplifiers, you just can't tell except by listening to the match of the amplifier and the particular speaker, and, it seems, the L-pad too! With solid state amplifiers, the choice is not as critical.
     
  10. If you're amp is rated to drive 4 ohm loads, you can get either one. If you're amp won't handle 4 ohms, get the 450w 8 ohm.

    If you ever want to run 2 cabs, get the 8 ohm cab. Running 2 8 ohm cabs together is a 4 ohm load, most amps can handle that. 2 4 ohm cabs run together is 2 ohm load, most amps cannot handle that load.

    Your amp will be rated different wattages at different ohm loads. It may put out almost twice as much at 4 ohm loads (2 8 ohm speakers, or 1 4 ohm speaker) compared to 4 ohm loads. If you're amp won't put out more than 450 watts @ 8 ohms, it probably won't drive the 8 ohm speaker properly, you'd have to get the 4 ohm speaker to get max volume.

    For maximum flexibility, get the 8 ohm. That way you can always upgrade the power amp later to drive it, and if it goes to 4 ohms you can add another cabinet also.


    NOTE: If you're planning on bridging your stereo amp, and each channel only goes down to a 4 ohm load, when its bridged you must run an 8 ohm load. To bridge an amp and connect a 4 ohm load, each channel of the amp must be able to handle a 2 ohm load when in stereo mode. That's not real common, not terribly rare either. Something to consider.

    Randy
     
  11. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Technically, two 4 ohms cabs wired in series gives a 8 ohms load. I didn't mention it because wiring cabs in series is not something I recommend for someone who is still learning about the basics of impedance.
     
  12. embellisher

    embellisher Holy Ghost filled Bass Player Staff Member Supporting Member

    Dude, God love ya, and no offense intended, but your post makes no sense at all.

    Do you honestly think that this post is useful or helpful in answering the original posters question, or is this just a joke?
     
  13. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    I'll be charitable and assume that he was trying to be helpful but lacks adequate knowledge on the subject to do so. His references would apply to connecting pre-amps, power amps and source devices, but have nothing whatsoever to do with speakers.
     
  14. stropsrats

    stropsrats Owner: www.kennedyaudio.com

    Feb 14, 2005
    Valders, Wisconsin
    There is no fundamental difference between a microphone and a guitar pickup. They are essentially the same or similar things. As far as a joke goes, some people on this board seem to think everything they say is interpreted in good humor, when in fact some of us are being quite serious. The posts ask the basic question about impedance matching to what is apparently a tube amp. You are the ones who want to get into other discussions. Discuss it with Paul, I notified him of your off color remarks.
     
  15. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    and pickups were mentioned exactly where in the thread??????
     
  16. stropsrats

    stropsrats Owner: www.kennedyaudio.com

    Feb 14, 2005
    Valders, Wisconsin
    My old friend ivan. OK, that's the kind of comeback I like to see, or can deal with. Now we are at least on the same page. I don't mind some criticism, maybe I'm just sensitive about sarcasm. You're right, pickups aren't exactly the topic. I was just waiting for that one. Buy hey, I'm thinking electronics here, not so much hardware selection per sae. To me a pickup is a speaker in reverse, just like a microphone. Well, in my mind's eye it's the same structure, and similar theory in operation. I'm into technology. Call me a geek or whatever, but it's just that that's what I'm trying to communicate with. But to be fair, somebody should have said that and I can relate. But the main point is a good enough one that I thought I should just skip with all the explanatory phorensics or be accused of jargon again, you know?
    Which do you want, 'jargon' or just the crude thinking?
    Any by the way, all this is very helpful in that I added more explanation about just this sort of thing on my web page;(consolid). I have to just resign to the fact that not everyone will understand it right away, but give it a chance and maybe you will gather it up and go with it. If not, no harm intended.
    It is a subject that has a lot written about on the internet.
    It's called bridging. Bridging applies to both input and outputs.