How to best understand head to multiple cab impedance issues.

Discussion in 'Amps and Cabs [BG]' started by Nick Donn II, Dec 1, 2019.

  1. Nick Donn II

    Nick Donn II

    May 20, 2019
    If I have a head that is 500w at 4ohm and 250w at 8ohm and 2 cabs that are rated 300w at 8ohm. Is this ok? Optimal? How should I hook them up? Series correct? Any other pointers or places to go to better understand the whole thing would be greatly appreciated.

    For reference I have a hartke LH500 head. A hartke hydrive 210 cab and a Phil Jones Bass Cab 47 on the way. The Phil Jones has 2 connectors a xlr input and a xlr link.
  2. Take a look at the stickie thread on this.
    Nick Donn II likes this.
  3. DeepFriedBass


    Nov 21, 2019
    Yep, just wire each cab seperately from each speaker out on the head.
    Nick Donn II likes this.
  4. Stumbo

    Stumbo Guest

    Feb 11, 2008
    You'll get 4 ohms by plugging in two 8 ohm cabs into your amp, one cable per cab and get the full 500 watts out of your amp. Done.
    thetragichero and Nick Donn II like this.
  5. There is often confusion in newbies about ''series''. Electrical series wiring nearly always requires the user to take convoluted steps with special extra cables or boxes. What the newbie usually thinks series means is actually parallel aka daisy chaining.
    john m, kentiki, Relsom and 5 others like this.
  6. The PJB47 has Speakon style speaker connector sockets not XLR which are a completely different design (nowadays only used for low level mic and line signals). As the LH500 has standard 1/4" jack sockets you will need to connect to the PJB47 with a 1/4" jack to speakon speaker cable. The Hartke cab requires a standard 1/4" jack to 1/4" jack speaker cable. Make absolutely sure that you only use speaker cables (most times there is writing on the cable wire sheath that states it) not instrument cables to connect from your amp to the cabs. Using instrument cable could overheat/melt /damage your amp. This is a relatively common mistake that I have even seen in rehearsal studios!
    Pulverizor, bbh and Nick Donn II like this.
  7. Nick Donn II

    Nick Donn II

    May 20, 2019
    Meant to write nl4 but thank you for all this information. This was so helpful and to be able to get it all in one place so quickly is priceless. I’ll be sure to post about the results I have a double Four and really can’t get over how pure of a tone pjb delivers. While pure isn’t always the goal it is nice to have it as an option. Thanks again you guys all explained it well enough for a welder to think he understands electrical so that says something.
  8. Nick Donn II

    Nick Donn II

    May 20, 2019
    my only other question would be which is preferable to have two CABINETS that together are rated to 600 watts at 4 ohms (300 watts at 8 ohms each) and a HEAD with rating of 500 watts at 4 ohms OR 2 CABINETS that together are rated for 400 watts at 4 ohms (200 watts at 8 ohms each) and a HEAD with a 500 watts at 4 ohms rating.
    Last edited: Dec 1, 2019
  9. jeff62

    jeff62 Supporting Member

    Oct 24, 2005
    Central FL
    Can’t really tell what you’re trying to convey here but I’ll say this: use your ears. Your ears will tell you what is preferable to you. Use the volume knob appropriately — if you hear the speakers complaining (farting), then adjust your volume and/or your eq.

    As a general rule, you typically won’t hook up more than two cabs to your amp. Each cab will likely be 8 ohms in a multi-cab set up. When you have multiple cabs it’s generally better to have the same number of speakers in each cab to ensure equal power distribution. These are just general rules (and not set in stone) and I’m explaining it this way because it’s clear that you are new to this, thus general rules may help you from making an expensive mistake. Happy hunting!
    JeezyMcNuggles and Nick Donn II like this.
  10. ctmullins

    ctmullins Dominated Gold Supporting Member

    Apr 18, 2008
    MS Gulf Coast
    I'm highly opinionated and extremely self-assured
    Those ratings
    • Are very approximate,
    • Indicate thermal limits, not mechanical,
    • Don’t account for variations in EQ, compression, overdrive, etc.
    They’re very nearly pointless.

    As @jeff62 said, use your ears. If you hear your drivers making ugly noises, you’re pushing them past their limits.
    Nick Donn II likes this.
  11. DeepFriedBass


    Nov 21, 2019
    With a 500 watt RMS @ 4ohms head, I would prefer to have two 8 ohm 300 watt RMS cabinets than two 8 ohm 200 watt RMS cabs, so the cabs have a bit of thermal headroom
  12. Tim Skaggs

    Tim Skaggs

    Sep 28, 2002
    I'm thinking your question has an error that I suggested a correction of in parentheses. If not, my answer may still be of value.

    Two 8-ohm, 300 watt cabs (600 watt total) driven by a 500 watt amp would be a good match from purely a power to load perspective. It doesn't mean it would sound good or be worth the effort to carry both speakers every time you play, but having an impedance load capable of handling a bit more power than your amp output at that impedance is generally a good thing.

    Two 8-ohm, 200 watt rated cabs (400 watt total) load driven by a 500 watt amp wouldn't be as good from a power to load perspective, but could be ok anyway if you aren't attempting to achieve max volume from the rig. Still no guarantee it would sound better with both cabs over one cab. Having a speaker load that is not rated to handle the max output of the amp puts you in a situation where you could damage a speaker or two, but you would probably notice you were pushing things too hard.

    I don't know which scenario would be preferable for best tone / sound. That would depend on the speakers. Using a speaker configuration with a power handling capacity less than the output of of the amp can be done if you aren't trying to make it rain, but still comes with some risk if you don't use some restraint.
    Nick Donn II likes this.
  13. Nick Donn II

    Nick Donn II

    May 20, 2019
    alright you guys cleared it up for me. I saw my typo and corrected it just In case someone else stumbles upon this. Either way you guys answered it and more. I also read the other threads which went into detail with clipping, tweaters, heat disipation etc. I’m much more relaxed about testing this out worst case I’ll have to buy another head. Oh darn! LOL wish you guys happy holidays and thanks for such detailed responses and patience with the new guy
    ctmullins likes this.
  14. ctmullins

    ctmullins Dominated Gold Supporting Member

    Apr 18, 2008
    MS Gulf Coast
    I'm highly opinionated and extremely self-assured
    One more comment from me: Remember it’s the drivers (individual speakers) inside the cabinets that make the noise. The cabinet shapes the lower couple of octaves, to fill out the low end, but when you’re concerned about making noise, it’s mostly the drivers that matter. Sound is measured in deciBels, not Watts.

    100 Watts into an 8 ohm 4x10 will sound fuller than 200 Watts into a 4 ohm 1x10 (generally speaking, and assuming roughly comparable drivers and reasonable cabinet tuning).
  15. Ric5

    Ric5 Supporting Member

    Jan 29, 2008
    I like 5, 8, 10, and 12 string basses
    Don't mix 4 ohm and 8 ohm cabs together … the results can be bad or catastrophic
  16. SactoBass

    SactoBass A retired civil engineer who likes all-tube amps! Supporting Member

    Jul 8, 2009
    Lake Havasu City, AZ
    Nick: if you are thinking that having 600 watts of cab wattage capacity (with your two cabs rated at 300 watts each) and only 500 watts of amp power means you will need to buy a more powerful amp, please do not think that! Here's why:

    The wattage rating of cabs is its "thermal" rating, not its "mechanical" rating. Nearly all manufacturers of mass-produced speaker cabs publish the "thermal" rating of the cab (that's the wattage that would melt the voice coil). A cab's *mechanical* rating (which most manufacturers wanna keep hush hush), is usually only about half its thermal rating. Manufacturers wanna sell speaker cabs, so they don't publish the less impressive sounding mechanical rating. So, for example, if you are looking at a cab rated at 300 watts continuous (or RMS), you should assume that it can actually safely handle about 150 watts before causing mechanical damage to the speaker (commonly referred to as "blowing the speaker"). As always, there are exceptions. Boutique cabs such as Barefaced, Audiokinesis, Bergantino, and Greenboy cabs are much more capable of handling their published wattage ratings.

    So, with your 500 watt amp and your pair of cabs rated at 300 watts each (600 total), you still have more amp power than you need, which is perfectly fine as long as you listen for speaker stress as mentioned by other posters in this thread. Unless you will be cranking your amp all the way up and adding in lots of low end EQ, it is unlikely that you will have any problems. Even so, it's always good to keep an ear out for signs of speaker stress. Since each of your cabs is 8 ohms, each cab will be receiving exactly the same amount of wattage from the amp. Let's say, for example, that you have your amp volume set such that your amp is producing 400 watts. That means each of your cabs will be receiving 200 watts. If your cabs have the same number of speakers in both cabs, then each speaker will be receiving the same wattage. But if, for example, one cab has two speakers in it, and the other cab has four speakers in it, then that means each speaker in the cab with four speakers will be receiving 50 watts (200 divided by four), whereas each speaker in the cab with only two speakers in it will be receiving 100 watts (200 divided by 2). So in that scenario, with cabs that are rated for the same wattage, I recommend placing the 2-speaker cab on top of the 4-speaker cab so that the 2-speaker cab is closer to your ears so that you can listen for signs of stress, because the 2-speaker cab will be the one to stress before the 4-speaker cab will (in the case where both cabs are rated the same).

    Hope that info helps! :thumbsup:
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