Psst... Ready to join TalkBass and start posting, make new friends, sell your gear, and more?  Register your free account in 30 seconds.

Speaker impedance trouble

Discussion in 'Amps and Cabs [BG]' started by mpfoust, Nov 9, 2001.


  1. mpfoust

    mpfoust

    Nov 9, 2001
    I am using an Ampeg B2R with a 4-10 Cab paralleled with a 1-15 Cab. After playing a few gigs my ohms dropped to almost 3. This made the amp quit working. Is there anything I can do short of replacing the speakers. Right now I can only use one of the Cabs.
     
  2. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    How do you know this? Did you measure the impedance?
    (Impedance is not constant, because it depends on the frequency - the rating is only a nominal value. But impedance won't change from one gig to another. Please excuse my bad English...)
    So:
    If both cabs are rated at 8Ohms the total load will be 4Ohms and you are fine.
    If one cab is rated at 4Ohms and the other at 8, total load will be 2.67Ohms. This is bad for the B-2R. In this case there is indeed no other way than replacing the 4Ohm cab (or using it alone)

    Hope that helps

    Matthias
     
  3. mpfoust

    mpfoust

    Nov 9, 2001
    I put a meter on the cabs together. They are both rated at 8 ohms. the 1-15 went to 7ohms. the 4-10 went to 6.8ohms. Together they are close to 3 ohms. Please help!
     
  4. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    Well, as I said, 8Ohms is only a nominal value related to a certain frequency. It's not unusual that the meter does not read exactly 8Ohms.

    If you put your values in the equation for parallel impedances you get:
    1/(1/7+1/6.8)=3.45
    Voil√°.
    Still no problem.

    But you may have driven your amp too hard although the cabs are matched properly.
    Maybe it overheated.
    Did you crank it?

    Matthias
     
  5. mikemulcahy

    mikemulcahy

    Jun 13, 2000
    The Abyss
    Its probably more a function of underpowered speakers and your amp clipping. Not eneoug power is far worse than too much. whats the rating of your cabs/amp?


    Mike
     
  6. Gabu

    Gabu

    Jan 2, 2001
    Lake Elsinore, CA
    If underpowered speakers are the problem... you could get a power amp.
     
  7. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
  8. mpfoust

    mpfoust

    Nov 9, 2001
    My amp is rated at 200 at 8ohms 350 at 4ohms. The cabs are 400 nominal and 800 peak. I do crank it up at gigs (probably more than I should).
     
  9. Sounds like you are just overheated the amp. I've read where some people with this problem just put a small fan blowing on the back of the amp to cool it.

    200 watts will only go so far with a bass. If you need more, you'll need a new (or additional) amp.
     
  10. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    The cheapest solution is tweaking the EQ:

    Try not to boost low frequencies too much (or even attenuate them) and boost mids instead.
    Lows eat up a lot of amp-power, but do not make you sound significantly louder.
    Set the EQ while the whole band is playing to hear how the bass sounds in the mix.

    Matthias
     

  11. This is normal. The DC resistance of a speaker is ALWAYS lower than the rated impedence. An "8 Ohm" box will show anywhere from 5.5 - 7 Ohms, depending on the driver. My guess is that the impedence of the cabinets has nothing to do with your problem. I could probably diagnose the problem if I coud see the amp, but I can't, so take it to an authorized service tech. If there's nothing wrong with the head, you're probably driving it too hard and it's going into thermal protection. In which case you need more power. :D
     
  12. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    Or make the band play at lower volume. :D
     
  13. Thought I would share my problem that is very close to yours.

    I also own two Ampeg classic cabinets (410 rated 200w rms and 115 rated 400w rms, the 15 being much newer, both 8 ohms cabinets) and I was using a Hartke 4000 amp (400w into 4 ohms).

    Now, I used to always clip my Hartke with my Rickenbacker 4003 bass (which has very hot pickups for a passive bass) and in the process blew my 15 and the Hartke (after 2 years of giging with it). Still not sure exaclty what happened with the 15 , but I know I blew the Hartke. I am getting the cabinet serviced as we speak using the 5 year warrenty that comes with the ampeg it (very usefull), but trashed the Hartke.

    Since then I bought a Gallien-Krueger 1001RB (540w at 4 ohms) and barely clip at all. So we will see if i blow up anymore cabinets!

    I used to not care about cliping, but I now know how important that is, so all your bass players be aware! Buy an amp with proper headroom, may seem expensive at first, but instead of buying two amps like me you just by one!

    alright just thought i'd share