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SM400 bridged into 4 ohms?

Discussion in 'Amps and Cabs [BG]' started by scottlee, Nov 5, 2003.

  1. scottlee


    Feb 17, 2003
    Has anyone ever had any problems with this configuration? I currently have one 8ohm cab and am looking at getting another 8 ohm cab.

    I have an old swr SM400. It is stated that it can be bridged into a 4 ohm load (efectively 2 ohms for each power amp). Some people don't reconmend doing this warning about frying the amp!

    Is anyone or has anyone used their SM400 bridged into 4 ohm load? Anyone have any problems?

  2. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    South Shore MA
    I've bridged my 400 into 4 ohm loads pretty regularly for hours at a time (up to 6 on occasion). Never had any problems.

    However, the amp will be hot to the touch by the you're done. Really hot. I ended up installing a couple of fans in the case to keep things cool.
  3. Same here, no problems with it, It just runs very hot like every other SWR head.

    I just bought a small fan from walmart and put it behind the amp blowing across it, otherwise you can fry eggs on it.

  4. Finger Blister

    Finger Blister

    Jul 8, 2003
    No problems running the SM400 at 4 ohms
    if it has a fan.
  5. gnome01


    Oct 30, 2001
    Bronx NY, USA
    hmmm, I wonder if it's a bad idea to bridge mine and keep it in a 2-space rack with no fan. Not that I'm doing that or anything - eh em...
  6. maxbass


    May 22, 2002
    Milano Italy
    I regularly run my SM400S with two 8 ohm 4x10, and it's all right
  7. scottlee


    Feb 17, 2003
    Cool! Thanks for everyones experiences with this - Sounds like its worth a try.

    My SM400 is the old one without a fan. I have it in a 3 space rack case with nothing above it. For those of you who have added an external fan, is it somehting that you just plug into mains power??
  8. Petebass


    Dec 22, 2002
    QLD Australia
    I bought a couple of small 12 volt fans from Jaycar, something like THIS.

    To power them, I bought one of those 12 V adaptors that plugs into the wall, something like

    My amp couldn't get hot if it tried!
  9. scottlee


    Feb 17, 2003
    Thats probably because you have a quality Australian made amp! ;)

  10. Petebass


    Dec 22, 2002
    QLD Australia
    I have 2 of those amps. They don't have in-built fans. Instead they have heat sinks on each side.

    The one without the fans does get a fairly hot when pushed hard, but it's not in a rack case so the heat has somewhere to go.

    The other one is in a fully floating rack case that I built. The heat wouldn't have been able to escape unless I incorporated some sort of ventilation (ie, fans). That's the one that will never run hot.
  11. chadds


    Mar 18, 2000
    The manual says they are supposed to get hot. The case is aluminum and wicks heat away from internal components. Years ago SWR service said they don't really like to run at 2 ohms. Two 4 ohm loads.
  12. Finger Blister

    Finger Blister

    Jul 8, 2003
    'Tis a stereo head with 2 amps.
    When bridged at the min. 4 ohms imp.
    each individual amp is at 2 ohms.

    Should be able to run the amp bridged at 4 ohms;
    no problem.

    Running bridged mono at 2 ohms would represent a
    1 ohm load on the R & L amps.

    'Tis a fact.
  13. Aussie Mark

    Aussie Mark I come from a land down under

    Oct 26, 2002
    Sydney, Oz
    Endorsing Artist: Fender; O'Neill Amps; Cave Passive Pedals
    My SM400 is in a 3 RU case, with half a unit space above and below. I've added a Jaycar 240v fan to the rear of the case, and now it stays cool to the touch. One thing to watch, you will blow dust inside the case via the effects loop jacks and any unused speaker outputs, so tape any unused jacks up and open the case every few weeks to give it a good blow out.
  14. Finger Blister

    Finger Blister

    Jul 8, 2003
    I use those 'foam' ear plugs in open jacks
    to prevent dust build-up.

    Tape is so tacky! :D
  15. For those in the US wanting to rig up a fan, you can find stuff like that cheap at http://www.allcorp.com/ (surplus electronics dealer). I've bought a bunch of stuff (wire, connectors, fans, etc) from them while building computers and had good service.
  16. Aussie Mark

    Aussie Mark I come from a land down under

    Oct 26, 2002
    Sydney, Oz
    Endorsing Artist: Fender; O'Neill Amps; Cave Passive Pedals
    Great idea! Thanks
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