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Amp output: 200w @ 4 ohms same volume as 200w @ 8 ohms??

Discussion in 'Amps and Cabs [BG]' started by bigcardinal, May 1, 2006.


  1. Will the perceived volume output be the same? (using comparible cabs of course). Thanks
     
  2. KJung

    KJung Supporting Member

    Yes.... assuming your amp is designed to run safely and with good specs (distortion, etc.) into a 4ohm load.

    For most relatively high quality amps, the distortion ratings increase very slightly running at 4ohms versus 8ohms, but many EE posts here have convinced me the human ear would never hear it.

    Also, identical cabs with drivers of different impedence do, from what I understand, have a slightly different sound, but again the EE's on the site have convinced me that the human ear would not notice this either.

    So.... Yes... for all practicle purposes... identical IMO and IME.
     
  3. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    No. An amp rated for 200 watts into 4 ohms has an output voltage limit of 28 voilts. An amp rated at 200 watts into 8 ohms has a voltage limit of 40 volts. Played at full volume into identical cabs the 200w/8 ohm amp will be 3dB louder.
     
  4. KJung

    KJung Supporting Member

    Interesting.... the voltage thing is over my head.. don't exactly understand it... but I guess then 'a watt is not a watt':eek:
     
  5. seamonkey

    seamonkey

    Aug 6, 2004
    But wouldn't power remain the same? the 4ohm 200w circuit would have 7+ amps and the 8 ohm 5 amps.
     
  6. Mo' bass

    Mo' bass

    May 4, 2000
    Netherlands
    Sure, but he isnt talking about identical cabs...

    I think he means one cab with a 4 ohm impedance and one cab with a 8 ohm impedance. All other cabs specs (like sensitivity and voicing) are the same.

    So its like:
    Amp 200W@8Ohm into 8Ohm Cab = xxx dB
    Amp 200W@4Ohm into 4Ohm Cab = yyy dB

    xxx=yyy? (Yes/No/Maybe if only...)
     
  7. KJung

    KJung Supporting Member

    +1 that's the way I interpreted the question, and from what I've learned from the techies on this site, it should result in the same 'volume'... i.e., a watt is a watt.

    I know that otherwise identical cabs with different impedence (e.g., an 8ohm Epi410UL and a 4ohm Epi410UL) will sound slightly different due to (I think I have this correctly) the differences in the voice coil windings (I'm getting over my head here!), but I understand the differences are so subtle as to be meaningless in the 'real world'.
     
  8. MacGroove

    MacGroove Brother of the Groove with a 'Pocket Full of Funk'

    Oct 5, 2005
    Calif.
    +1 Ken, I understand what you are saying. Although the techical language is over my head, same amp comparing two cabs, one 4 ohms and one 8 ohms, at the same volume, the 8 ohm cab is not as prominent as the 4 ohm cab.
     
  9. jokerjkny

    jokerjkny

    Jan 19, 2002
    NY / NJ / PHL
    i'll take anything that runs cooler/easier...

    so 200 watts @ 8ohms would be the winner, cause lower impedences tend to push an amp harder & hotter, which could lead to shutdown, which could lead to angry lead singers / bandleaders / managers, which could lead to firings. :(
     
  10. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    A watt is a watt, but watts don't determine how loud a speaker is. Volts do, but if you're not an audio engineer you wouldn't be expected to know that.
    They would not be identical. When you change a driver's impedance, which is done by using a different voice coil, you also change a dozen or so T/S parameters. The two won't sound the same. I interpreted the question as to whether a 200w/4 amp would be as loud as a 200w/8 amp into the same speaker, and it would not.
     
  11. KJung

    KJung Supporting Member

    Actually... it gets more complicated than that. +1 on your above comment... that's why most of us who use a single cab use a 4ohm version... might as well get all the power out of an amp that's designed to run that way.

    The thread originator's question is a little different. He is asking a comparison of two totally independent rigs....

    Rig one.... amp brand A with a power rating of 200 watts into 8ohms running into cabinet brand B wired for an impedence of 8ohms.

    Rig two.... amp brand A (this is key... the same preamp, the same amp design, etc.) now with a smaller power section that puts out 200 watts at 4ohms into cabinet brand B (again, key... same design, same driver brand, etc)... now in a 4ohm version.

    So... assuming you aren't running either rig past '200 watts', the question is... will the sound, for all practicle purposes... be the same?

    My answer is 'yes'...a watt is a watt, and unless you are driving the amp to clip, the amps should sound the same. Of course, the amp rated at 200 watts into 8ohms will put out more watts into 4ohms.... so this can quickly get circular.

    However... it's an interesting 'thought experiment'.
     
  12. KJung

    KJung Supporting Member


    I did not realize that... totally new info for me. Of course, the whole question is a little meaningless... since the amp putting out 200 watts into 8ohms would probably put out 350 watts or so into 4ohms... and would be louder than the 200 watt into 4ohm amp... so it again can get circular pretty quickly.
     
  13. Thanks for the input, everyone. It seems that this issue is a bit more complex than I had thought...Although it also seems that there is a bit of an advantage with the 8 ohm setup: it can drive 4 ohm cabs at higher volumes than 8 ohm cabs, and less likely to heat up and die.
     
  14. vyse933

    vyse933

    Mar 31, 2006
    Grand Haven, MI
    i'm quite confused..from listiening to people talk in person, i was told that 300 watts at 4 ohms is the same thing as 150 watts at 8 ohms. the example that was given to me was that the GK BKL 600 handles 300 watts and 4 ohms, but at 8 ohms it handles 180 (acording to the manual..but it clips at 150). the other analogy given to me was that it's like running water through a bigger or smaller pipe..and that's where the conversation ended
     
  15. KJung

    KJung Supporting Member

    The thread originator's question was theoretical, and was trying to get at the 'quality of the watt' comparing two similar amps.. one that puts out 200 watts at 8ohms, and another that needs a 4ohm load to put out the 'same' 200 watts. This can get confusing and circular.

    If you go to the FAQ thread on amps, and find the 'ohms and impedance' thread, it will explain everything very nicely. Basically, running a head into a cab that has its lowest recommended impedance (recommended is key hear... even some amps that are 'safe' at 2 ohms are still recommended by the manufacturer to be run at 4ohm minimum) will give you the most power and the optimum performance of the head. This is typically 4ohm load, obtained by either a single 4ohm cab or two 8ohm cabs.
     
  16. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Good choice. Once upon a time all amps were only rated at 8 ohms, and you could make logical comparisons between different models. Then some marketing genius, realizing that the average consumer didn't know an ohm from a volt from a cumquat, but could tell the difference between 100 and 200 watts, decided to start using four ohm ratings, and the race was on. Two ohm and even one ohm ratings soon followed, not because they have any real worth, but because once one company decided to use over-inflated wattage ratings to push sales they all had to follow suit. It's all smoke and mirrors.
     
  17. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The whole thing is completely meaningless unless you assume for starters that by some means the two speakers get the same power.

    As someone noted, that means different voltages, which suggests for instance testing one on the 8 ohm tap, and using the other with the 4 ohm tap, to match nominal speaker impedance.

    Then also, the original poster was, I think, assuming the speakers WERE 'identical' other than impedance. it's a "thought experiment", and doesn't need to be physically possible.

    Then it depends on the amplifier. If they are also "identical", in the sense of behaving the same power-wise at their respective impedances, there should be no difference.

    But a 4 ohm amp may be designed with a little more dynamic headroom, just because that makes the transformer a little cheaper. In that case, you MIGHT hear a little more transient capability from the 4 ohm amp.

    I think I could argue almost as well that the 8 ohm amp might be slightly more "dynamic", so.....

    Generally the sonic differences between specific amplifiers will be way bigger than the theoretical differences between a "generic" 8 ohm vs a "generic" 4 ohm amp of same power.
     
  18. KJung

    KJung Supporting Member

    Isn't the above a 'tube amp' sort of thing.... with 8ohm vs. 4ohm taps. I assumed the thread originator was talking about SS amps, which put out more power into a lower nominal speaker impedance.
     
  19. David Vega

    David Vega

    Aug 28, 2002
    Puerto Rico
    hey! you blow me with this answer!:) Can you explain with formulas how you get the 28V and 40V?
     
  20. Dan1099

    Dan1099 Dumbing My Process Down

    Aug 7, 2004
    Michigan
    He's not saying that at all, Ken. Let me see if I can simplify the matter for you. Volume is not determined by watts, it is determined by volts. Watts are a "power consumed" unit, which is described as (V^2)/R. V being voltage, R being resistance, or, in this case, impedence. Therefore, an amp that puts out 200 watts at 4 ohms works out to roughly 28 volts, maximum. One that puts out 200 watts at 8 ohms comes to 40V. The one rated for 200 watts at 8 ohms will hence be louder than the other, no matter the impedence of the cabinet, as long as they both are sharing the same cabinet.

    You know this to be true, just by looking at simple specs... Most amps that are rated at 200 watts at 8 ohms are rated to 350 or 400 watts at 4 ohms. Put this up against another amp rated to 200 watts at 4 ohms, and you know what will happen.

    All Bill is saying is that listing wattages at 4 ohms is sort of a misleading advertising technique, designed to generate bigger numbers, which equals more sales. What we need is some sort of a standardization, IE listing all amps at the same impedence, or, better still, listing voltages instead of wattages.

    I realize this is an oversimplification of things, but I hope it helps. If I have misconstrued any of your points, Bill, please correct me.

    I believe that the confusion comes from this: Bill is considering strictly the amplifier, while Ken is considering a setup with one 200w-@4ohm amp, into a 4 ohm speaker, and a 200w-@8ohm amp, into an 8 ohm speaker, with all other T-S parameters of the speakers being equal. While I realize that 2 speakers with identical T-S parameters but different impedences are impossible, what would be the result of this, Bill? Thanks for clearing this up.
     

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