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4ohm amp output into 8ohm speakers?

Discussion in 'Miscellaneous [BG]' started by johnbegone, Dec 25, 2004.


  1. johnbegone

    johnbegone

    Sep 16, 2004
    Chicago, IL
    This problem is related to my PA powered mixer and my main PA speakers but I was hoping the geniuses of talkbass could help me out. I've got a Samson 500watt powered mixer, with two separate 250w 4ohm channels. When I first got it I didn't have big speakers so I was running both speakers with one of the channels (splitting the 250w). When I finally did get the bigger speakers the other channel of my amp was not working at all....... blah. So for awhile I used the single channel for the bigger speakers and all was working fine, just quiet. Eventually I had the money to get it repaired, and I was then able to run both the speakers on their own channel with 250w going into each.

    The speakers are SoundTech ST15R which are 15" woofers with a horn. They are 8ohm 300w RMS, 600 program. Now here's the problem. At one show I was running sound at one of the horns went out. Ok, I though, must be cheaply made speakers (even though they're about $900 for the pair... I got a great deal on ebay). I went to plug them in the next day to figure out which one needed to be fixed, and now BOTH horns are gone. I couldn't understand what happened, but they are under warrantee for the next 3 years, so they'll be taken care of. Is this all happening because I'm running them with a 4ohm mixer? I thought it could work that way, just not the other way (like putting a 2ohm load on a 4ohm amp would be bad, obviously). Do I need to just get a new power amp with 8ohm out?

    Thanks for any help anyone can provide. It is certainly appreciated.
     
  2. No you do NOT need to get an 8 ohm amp. The ohms rating is the minimum load the amp is rated at, 4 is better than 8, read the Ohms FAQ.

    You can happily plug an 8 ohm speaker into an amp rated to go down to 4 ohms. Here's an analogy: suppose you have a lamp and the receptacle is too far away from the lamp to plug it in. Suppose the receptacle is ten feet too far away. The store only has a six foot extension cord and a twelve foot extension cord. You could buy the twelve foot cord and use it, you'd just have two feet of extra cord. But if you bought the six foot cord it wouldn't reach.

    The minimum ohms thing is sorta like that (only the numbers run in reverse). Having an amp that could run 4 ohms but using an 8 ohms speaker is good. Having a 4 ohm speaker but an amp that's only rated to 8 ohms is bad.

    Are you sure the horns have gone, or have the tweeter protection bulbs gone kapput? Many pro speaker crossovers have a bulb in them to protect the speaker. This usually looks like a regular 1156(?) automotive bulb.

    With an ohmeter, you can check the bulb and the horn.

    It may be worth having the powered mixer checked again to make sure there isn't anything weird going on with the signal coming from the amp. But I suspect you clipped the tweeter signal and the bulbs burned out. Even though your amp is rated 250 watts into 4 ohms, it'll only deliver about 180 into 8 ohms, so you may have cranked it a little too loud and distorted the highs enough for the tweeter protection to kick in.

    Tweeter protection bulbs are not covered under warranty probably.
     
  3. IvanMike

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

    Nov 10, 2002
    Middletown CT, USA
    nashville bill is correct.
    read the following link for some clarification.
    btw, ignore the program rating of your pa speakers. RMS wattage is the only rating that you should look at.

    read this link
     
  4. johnbegone

    johnbegone

    Sep 16, 2004
    Chicago, IL
    Darn, this just makes me more confused as to what I should do. So should I send the speakers in to the company? And what are some things that could be wrong with my powered mixer's output that would cause it to blow horns? Like some technical examples I could tell my tech.

    Thanks guys/gals
     
  5. IvanMike

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

    Nov 10, 2002
    Middletown CT, USA
    i dont think its a problem with your powered mixer, i think its a problem with those cabinets.
     
  6. I'd open up your cabinet and look at the crossover. See anything that looks like a light bulb? If so, take it out and see if it's burnt out.

    I wouldn't worry about the amp yet, check the bulbs first.
     
  7. IvanMike

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

    Nov 10, 2002
    Middletown CT, USA
    +1, i was thinking that i should have mentioned that
     
  8. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Yep, 8 ohm load to a 4 ohm amp is fine - HOWEVER.......... it does reduce the amount of watts produced by the amp, typically by about 30%. So your powered mixer wasn't putting out the full 250w per side. It would have been more like 175W. In practice this doesn't amount to much of a volume drop, but it's something you should know.

    Did you encounter any feedback squeals at the gig in question? Or any other sudden high frequency spikes? Horns hate this. You also might want to check that it isn't a case of the wire becoming seperated from the horn.
     
  9. Sorry to revive a dead thread. Hope it's not offensive. Using the search engine this was the closest on point. And I did go to the FAQ thread that IvanMike linked first. I just want to make sure I understand what I'm doing before I make a mistake.

    I recently picked up an old Crate B350 that is according to the label by the speaker jacks:

    350 watts @ 2 ohms.
    200 watts @ 4 ohms.

    I also picked a Peavy 4x10 cabinet that says it has a net of 8ohms. That also presumes it is what it says it (which I'll believe after I take it a part and verify).

    They way it is now. I understand that I shouldn't run the head directly into this cabinet as the head has lower ohms than the speakers.

    However, running two cabinets with 8 ohms would give me a net of 4 ohms and being okay?

    After opening the cabinet, if I find I have 4 speakers with 8 ohms each and they are wired in parallel/series to get the net 8 ohms, I could rewire the 4 speakers in parallel for 2 ohms net on the cabinet and should be good to go --presuming of course the I have speakers that add up to 350 watts RMS.

    Thereafter, if for some crazy reason, I wanted to run a second cabinet with this amp, as long as together the net is 4 ohms or lower, it should be okay. Even though I'm really not too interested in running different cabinets at different ohms.

    Is my math correct?

    Ultimately I'd prefer to get the cabinet at either 2 or 4 ohms. It's not important which to me as I really don't have any need to be louder than the amp provides to keep up with the drummer, if I need more volume to keep up with the drummer, I'll get a new drummer.

    I'm not married to this equipment. I bought the head for $150 and it seems in pretty good shape for it's age. The Peavy 4x10 cabinet was free. I knew a friend of a friend who knew somebody getting rid of it. I merely needed to go pick it up. I've never spoken with the previous owner of the cabinet, but it looks like he poked four small pinholes in one of the speakers. I can only guess that he was trying to go for some kind of overdrive sound and heard that Dave Davies did that back in the 1960s for his guitar. Replacing the speakers is going to happen. I just want to make sure I don't make a mistake dropping in 4 speakers at 8 ohm and wiring them to a net of 2.

    Thanks for hearing me out.