OHM question??

Discussion in 'Amps and Cabs [BG]' started by PATTY1, Mar 22, 2011.


  1. PATTY1

    PATTY1 Guest

    Dec 28, 2009
    my SM400 SWR says it has two amps, each puts out 150watts into 8ohms or 250 watts into 4ohms.....and then it goes on to say that in "bridge" mode ( a combination of the two amps), that it puts out 500watts into 4ohms or 400 watts into 8ohms.......should the 8 ohm bridge mode really be 300 watts ??
     
  2. somegeezer

    somegeezer

    Oct 1, 2009
    England
    Could be a typo, yeah.
     
  3. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    No, because series bridging ideally doubles the voltage swing, which quadruples power. But that's an ideal figure, which few amps are actually capable of. And be aware that bridging into the minimum impedance load is seldom of any benefit; it's only when driving a higher impedance load that bridging makes sense.
     
  4. Zooberwerx

    Zooberwerx Gold Supporting Member

    Dec 21, 2002
    Virginia Beach, VA
    ...and this is what confuses me when dealing with higher powered amps (i.e QSC RMX 1450, Crest CA-6, etc.). Then again, I'm befuddled by a box of Corn Pops.

    For example, let's take two 8 ohm 2 X 10 enclosures. In stereo mode, each cab will see ~300 watts per side using the aforementioned amps. If we switch to bridged mono and daisy-chain the enclosures, the resulting 4 ohm load will "see" ~1400 watts or 700 watts per enclosure. This is the minimum impedance load so is there indeed limited benefit? OTOH, running the same amp in bridged mono feeding a single 8 ohm 4 X 10 makes perfect sense as per your post. This is actually an accurate description of my gear on hand, including 8 ohm Acme B-2 and B-4 enclosures.

    Riis
     
  5. I'm sure Bill will post the real answer to this question, but I'm going to take a stab at it and say that the benefit is nil due to the fact that an increase in wattage is rarely ever enough to make a noticeable change in volume. I believe it takes something like ten times as much power (ie 200 watts to 2000 watts) to double volume (3dB) while we don't perceive a change in volume until there's a 10dB change. Something like that...
     
  6. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The problem with bridging is that all they talk about is watts, which is moot, when what should be discussed is voltage swing and current delivery, which is pertinent. But manufacturers assume, rightfully, that all that would be achieved by discussing voltage swing and current delivery is rolling of the eyeballs by the reader.
    IMO if you don't really really know what you're doing don't bridge, run one cab per channel. If 300w per channel into two 2x10 isn't loud enough for you you're probably already going deaf from playing too loud.
     
  7. +1

    IMO, way too much enthusiasm is placed on bridging by uninformed players.
    (IMO again) If you feel you must bridge, you don't have the right amp anyway.
     
  8. BassmanPaul

    BassmanPaul Inactive

    This is a post I whole heartedly agree with! I have never had the need to bridge ANY of my amplifiers. Running them in parallel with one cab each has always provided enough volume for my needs. Remember you need TEN times the power to get TWICE as loud.
     
  9. Zooberwerx

    Zooberwerx Gold Supporting Member

    Dec 21, 2002
    Virginia Beach, VA
    What would be the correct amp for an 8 ohm Acme B-4 or, more specifically, what would be the most efficient configuration? I'm on board with stereo mode when powering the 2 X 10's.

    Riis
     
  10. IvanMike

    IvanMike TTRPG enthusiast, Happy, Joyous, & Free. Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    the few times that i've bridged, it's been with an 8 ohm cab that could take a lot more power than one side could provide. Even so, as posters have mentioned, the increase in volume wasn't staggering, just enough to give me some more headroom.
     
  11. SanDiegoHarry

    SanDiegoHarry Inactive Supporting Member

    Aug 11, 2008
    San Diego, CA
    Add to that, those old SWR amps are fairly infamous for being underpowered - that is to say, it may SAY 150 watts, but good luck getting it.

    That doesn't mean they aren't useful amps - just don't expect big power and match your speakers accordingly. I've known more dudes who expected those old SM400's to *PUMP* and were sorely let down.

    I had and SM900 and let me tell you, it was underpowered too. Some companies rate their amps conservatively, some not so much.
     
  12. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    +1. The point of bridging is to get adequate voltage swing to drive an 8 ohm or higher load to full output, not to 'get all the watts out' into a 4 ohm or lower load. There are cases where a particularly anemic amp might be better off bridged into a 4 ohm load, but they're rare.
     
  13. father of fires

    father of fires Commercial User

    Nov 29, 2006
    BALTIMORE CITY
    Chief of Medicine at Damnation Audio
    I have an SWR SM400s and the amp is rated at 300 watts/2 ohm per side.

    It seems to me like this amp was made to be bridged into 4 ohm.
     
  14. IvanMike

    IvanMike TTRPG enthusiast, Happy, Joyous, & Free. Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    it can be, but in terms of what it was made for, It was sort of made to be bridged into the 8 ohm load of a goliath 410. If not, then that certainly was the most popular use for the bridge feature on that amp. SWR really didn't have any 4 ohm cabs that could handle the sm-400 bridged back when the 400 and the goliath were there flagship products. Wow, 1987 seems like yesterday, hard to believe it was 24 years ago!.
     
  15. father of fires

    father of fires Commercial User

    Nov 29, 2006
    BALTIMORE CITY
    Chief of Medicine at Damnation Audio
    That makes sense. I use to run it with a Goliath II.

    Now, I just run one side into an 8 ohm Ampeg 15".
     
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