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Amp load question #3

Discussion in 'Amps and Cabs [BG]' started by fourstringbliss, Apr 13, 2005.


  1. I know that two 8 ohm cabs together make a 4 ohm load, and two 4 ohm cabs together make a 2 ohm load. What would a 4 ohm and an 8 ohm cab make together? Would it be 6 ohms?
     
  2. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    1/(1/4+1/8)= 2.666 ohms

    ( or: (4x8)/(4+8)= 2.666 ohms )

    Alex
     
  3. Thanks Alex!
     
  4. A flaming pile of debris where your power amp used to be... :D

    There are amps that can handle that load per channel, but none that I know of that could handle that load in bridged mode. The formula above is absolutely correct, its well below 4 ohms, I trust his calculation.

    Randy
     
  5. Thanks! I don't want a flaming pile of debris!
     
  6. Funkengrooven

    Funkengrooven Turn it down? You gotta be nuts!!

    The formula is correct...
    That having been said, what blows up the output transistors in an amp is HEAT.
    Excessive current makes excessive heat and heat does the meltdown thing....SO>>>>>> get rid of the heat and it is possible to run an amp at 2 Ohms.

    DON'T DO THIS IF YOU CANNOT FIX THE AMP YOURSELF IF YOU TOAST IT!!!!!
    IT WILL VOID YOUR WARRANTY IN A MILLISECOND!!!
    MAKE YOU CRY, TOO :crying:

    The Cray Super-computer uses components that normally would not run when pushed that hard and cools them with liquid nitrogen...-250F or something REALLY cold like that. cool your amp and 2 ohms is yours and so is 1 ohm...
    Nitrogen cylinders are heavy.....
    I replaced the slow fan in my SWR 550x with a maximum air blaster and run the normally 4 ohm amp at 2.6 ohms all the time...SWR does not like me much. I also have intake and exhaust ports on the rack to move the air through. The trick is to know what the max junction temperature of your output transistors is and never exceed that. Having some current headroom in your Power Transformer is also a good idea.
    SWR sent the amp schematics to me and I made the determination on my amp with what I had available and what I knew I was going to be able to support and completely researched all the individual components.
    FOR TECHNOGEEKS ONLY.....ALL OTHERS BE AFRAID, BE VERY ARFAID!!!!
     
  7. I think I'll run two 8 ohm cabs and take it easy on my amp. I don't know what I am doing so I'll play it safe. I have heard that an 8 ohm cab is only a little quiter than the 4 0hm version of the same cab - is this true?
     
  8. Funkengrooven

    Funkengrooven Turn it down? You gotta be nuts!!

    I have never heard that in the 35 some odd years I have been playing with this stuff.....Oops!...Old Guy Alert!!!
    I have always matched cabinet and amp in the best possible manner that was do-able at the time.
    The 2.6ohm SWR thing that I do was necessitated by the fact that the 18" JBL (4 ohm) I have only cost me $50.00 and to replace it with an 8 ohm model would cost $500.00.
    And I am not willing to trade the JBL for a Peavey, Carvin, Madison ( :spit: ) or anything else.
    Bottom line, match the amp with good speakers at the correct impedance and you will be happy and so will your amp
     
  9. How would an 8 ohm 212 sound compared to the 4 ohm version?
     
  10. Funkengrooven

    Funkengrooven Turn it down? You gotta be nuts!!

    I can't imagine one sounding any better or worse than the other.
    It is going to depend on what speakers you have in the 2x12 and in what kind of cabinet it is and what other cabinets you are using and what they have in them.
    And while we are on the subject, lets talk about speakers and speaker efficiency and cabinet efficiency. All those things count way more than the speaker impedance.
    Example. JBLs, Altecs, EVs, Peavey Black Widows, and other quality (expensive) cast frame speakers are usually more efficient louder cleaner and more durable than the pressed/stamped frame speakers although there are exceptions.
    If you put a great speaker in a marginal cabinet or a cabinet that is not tuned to the speaker it might not sound as good as a crappy speaker in the same cabinet. If you put a crappy speaker in a cabinet that is tuned to perfection for the crappy speaker it will sound real good. If you put a great speaker in a great cabinet that is tuned to perfection you have some SERIOUS sound. Those are the parameters that should concern you more than the impedance. I am personally biased toward JBL, just because :hyper: I am a JBL fanatic. and All the cabinets I build (no pictures yet) are tuned. Even a sealed cabinet has tuning, the interior volume is chosen to enhance some sonic aspect of the speakers used in it.
    Now you are really confused?? No? Good!!
     
  11. konfishily

    konfishily

    Jan 24, 2004
    Brooklyn, NY
    The difference between the 4ohm and 8ohm of the same model is 3db. The 4 ohm is 3db louder. Its a noticable difference but not a significant one. If i were you i would get the 8ohm, so you don't limit your self later in cab selection.
     
  12. I think he means a 212 cab from company ABC that's available in 8 or 4 ohms, maybe like Eden.

    See if there's an efficiency difference in the specs for the cab vs the impedance. That'll tell you what the volume diff would be.

    For individual speakers used in a cab, I "think" the same speaker rated 8 ohms has more voice coil windings (twice as many) compared to the same speaker in 4 ohm version. The 4 ohm driver gets twice the power, but the 8 ohm has twice the windings, so is twice as efficient. They cancel each other out.

    So I would suspect that at the same settings, the 4 and 8 ohm version of the same cabinet would be identical in volume, although the one running at 4 ohms would be using more power to do so.

    Back to cabs, if the 8 ohm is rated at 103dB efficiency, and the 4 ohm is rated at 100dB, they'd be the same volume at the same setting, since the amp would pump twice the power at 4 ohms, and 103dB is twice as efficient as 100dB.

    Make sense?

    Now if you added a 2nd 8 ohm cab, for a total 4 ohm load, you do get your 3 dB volume gain.

    Randy
     
  13. Joe Beets

    Joe Beets Guest

    Nov 21, 2004
    If you forget the formula for figuring out the total ohms of several cabs, unless they are wired in series, the ohms will always be less than the lowest ohmage cabinet. For example, if you hook up a 16 an 8 and a 4 ohm, your total ohmage will always be less than 4. ;)
     
  14. Highpipes just clued me into this great item: Speakermate. This would allow me to get the 4 ohm 212 and add a 4 ohm 210 later while still only loading my amp down with 4 ohms!
     
  15. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    No, it won't. It is just a series/parallel option connection device. There was a detailed thread on this device about a year ago, search for it.
     
  16. My mistake! Thanks for the clarification!
     
  17. Regarding the 4ohm versus 8ohm differences in the same cabinet model. While 3db is not that noticable volume-wise, I've found that putting a lot more wattage into a cabinet (which is basically what you are doing going from 8ohm to 4ohm) can greatly open up the low end sound. I assume that's from all the additional headroom you get running, for example, an amp that puts out 200 watts into 8 ohms versus 400 or so into 4 ohms. I really do notice a difference in the character of the sound, with cleaner, wider, more forceful lows. This is especially noticable if you are a) playing at a relatively lound volume on a 5 string's low B and b) playing out of a very wide range, relatively inefficient cabinet like an Acme or EA. I REALLY noticed a difference when I went from an 8ohm Low B II to a 4ohm Low B II using the same amp. Of course, the best way to get more sound is to double up two 8ohm cabs... then you get more watts and a lot more db's at the same time.
     
  18. Thanks for your imput! I'll probably just go with the 4 ohm 212. I probably won't be playing anything bigger than a small or medium sized club or a church (where I'd probably plug right into the mains anyway). Do you think the 212 would be loud enough with good enough sound for small situations? What would be the advantage of adding a 210?
     
  19. konfishily

    konfishily

    Jan 24, 2004
    Brooklyn, NY
    A 212 will be great for small situations i guess. I know the 212's can give as much low end as a 15'' speaker, but I'm not sure it it has the same punch as a 210. But the 212 do sound really good in my opinion.
     
  20. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    I don't know of any amps that fully double their power when going from an 8 ohm to a 4 ohm load...

    Alex