1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  
    TalkBass.com has been uniting the low end since 1998.  Join us! :)

another 4ohm or 8ohm cab question (for use w/ separate pre and power amps)

Discussion in 'Amps and Cabs [BG]' started by hgregs, Aug 10, 2012.

  1. hgregs


    Sep 25, 2008
    ct/ny border
    i'm having a 2x15 built right now, and i'm torn as to having it made as a 4ohm or 8ohm. i've read the prior posts on this issue, but they don't address my situation. i currently use a kern preamp, and don't intend to change that. my power amp (overkill lab gruppen fp3400) has enough power to run it out of one side (at any impedance).

    i'm concerned that if i get a 4ohm, i'll be unable to use smaller power amps in 8ohm bridge mode (a 1u amp would be nice). and as far as i know, most power amps are 2 channels. so to get the most out of them with one cab, they would be run bridged at 8 ohms.

    the other amps i have include an epifani ps1000 (also 500x2 @4ohm, or 1000 bridged @8ohm).

    it'd be great to buy a micro integrated head, but i'm doubtful i am going to like one enough to replace the kern+power amp.

    so after all that rambling... if you wanted to use separate power/pre amps, would you go w/ an 8ohm for the same reason, or am i missing something?
  2. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    most power amps that I've come across are quite happy running into 2Ω per channel. That means that bridging into 4Ω is no problem. For myself I have never seen the need to run any of my amps in bridge. That said, I always use two cabinets on a gig. When I last had a single cabinet, a 2x15, I wired it so each driver had its own power amp. Amps were run in parallel mono.
  3. hgregs


    Sep 25, 2008
    ct/ny border
    thanks man... i didn't even realize my amp would do 4ohm bridged (it does), i assumed 8ohm.
  4. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    I don't think it matters. If you think you'll ever need four 15"s, then it would matter. Otherwise, not so much, and going 4 ohm for the cab gives you more choice in speakers by a longshot.
  5. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    yeah, a 4Ω 2x15 gives you the option to rock with most any "normal" bass amp, while the 8Ω 2x15 (aside from needing weirder speakers) would demand either bridged amps or really big amps to deliver any real juice into it.
  6. If your 2x15 is 8 ohm, is probably a pair of 4-ohm drivers wired in series.
    The various Eminence "C" suffix models are 4-ohm drivers.
    The chances of your cab being two 16-ohm drivers in parallel is almost nothing, unless custom drivers are used.

    There is indeed a difference in the results of the wiring.
    Series gives you more power handling, Parallel gives you more noise for the same voltage.

    Using the 3015LF as an example, this driver will accept 60 volts (450 watts) input.
    If you wire in series, your amp has to produce 120 volts for each series-wired driver to see 60 volts.
    The same drivers wired in parallel will both see 60v with a 60v input. No bridging required.

    I would be asking the builder what drivers are used, and how they are wired.
    My choice is the 2x15 in parallel.
    To add a second cab, rewire each 4-ohm cab to Series (16 ohms) and parallel the two cabs.
  7. walterw

    walterw Supportive Fender Gold Supporting Member Commercial User

    Feb 20, 2009
    isn't this just a confusing way to say that the higher impedance of series means the amp produces less power?

    the power handling of the cab would be the same either way, however many watts per speaker x 2.
  8. Mschiff


    Jul 22, 2012
    Memphis, TN
    i run mine bridged mono with two 115 cabs (for portability). two 8 ohms cabs making a 4 ohm load on the amp. remember, the amp "sees" the load of your cabs whether 8, 4 or 2. they are quite versatile themselves. i bridge mine just to get the full 500 watts out of it at 4 ohms. if it was 500 per channel, i'd probably only need one side. i'm just stuck with what i have for now. good luck
  9. Nope.
    It is the accurate description of how it works.

    Parallel wiring makes more noise per volt (knob setting).
    Amps are voltage devices, the power rating is the limitation of how much current an amp can tolerate at a given voltage.

    As noted above, bridging is required to drive a pair of series-wired 3015LF at 60 volts each.
    There are increased safety risks when bridging at 120 volts.
    Bridging is the most strenuous mode for an amp.

Share This Page