All these ohm questions: Can we stop the bad advice?

Discussion in 'Amps and Cabs [BG]' started by bwildt, Mar 19, 2019.

  1. bwildt

    bwildt

    Mar 21, 2017
    Wichita, Kansas
    You know, this question about ohms and multiple cabs shows up about every day. Along with this question comes some responses that are either bad advice, just plain wrong, or even dangerous for the equipment.

    1) You cannot rewire a cab from 4 to 8 ohms or vice versa. At least I can't think of a cab configuration where you can do this

    2) Don't tell people they can run a 4 ohm and an 8 ohm cab together unless a) the 4 ohm cab can handle twice the power of the 8 ohm cab or b) the amp can in question can handle sub-4 ohm impedances.

    3) Don't suggest going to series connections or some combination of parallel/series via cables outside of the cabinet. Besides the power handling mismatch of the cabinets (see #2), going to a higher impedance results in less power from the amp, even if it is feeding an increased speaker area. The net gain is usually zero or not worth it. Also, building up some kind of series or series/parallel speaker cable is just asking for trouble. You now have a non-standard cable that is probably unmarked and may cause problems if used somewhere else. Besides that, unless you also build a spare cable, you are in trouble if your special cable fails. Plus, inadvertently plugging in standard cables risks damaging your equipment. Don't tell me that you will be the only one plugging your rig together. Band mates or sound guys helping out may not be aware of your special configuration.

    Discuss.
     
  2. bholder

    bholder Affable Sociopath Gold Supporting Member Supporting Member

    Sep 2, 2001
    Vestal, NY
    Received a gift from Sire* (see sig)
    I've been guilty of a few "well, you could..." type responses, but yeah, you're right, it's just not worth it in the long run...
     
  3. I don’t know. I don’t often see very much bad advice on the amps forum here. When it is given, it’s often quickly called out with a ton of (sometimes overly technical) facts to the contrary. If anything, the TB Amps forum is heavily biased toward the factual, thanks in no small part to the number of educated posters taking time out between designing and building stuff to lend expert advice free of charge. We have it pretty damn good here on TB.
     
  4. Wasnex

    Wasnex

    Dec 25, 2011
    1. Requires dual voice coil drivers, so in most cases this is true. Here's an example where it actually works: D.I.S.T. Series Dual-Impedance Bass Speaker Cabinets — Epifani Bass Amps Reviews of these cabs have been favorable. I think the idea is a bit gimmicky.

    2. The uneven distribution of the power is definitely relevant, but the power rating the speakers require is determined by how much power they will be fed.

    If you feed 300W to an 8 ohm and 4 ohm speaker connected in parallel, the 8 ohm speaker will get 100W and the 4 ohm speaker will get 200W. So in this case the 4 ohm speaker needs to be able to handle at least 200W. The power rating of the 8 ohm speaker is irrelevant to the power rating of the 4 ohm speaker, but in this case the 8 ohm speaker needs to be able to handle at least 100W.

    It would be entirely okay to run an 8 ohm speaker with a 500W rating and a 4 ohm speaker with a 300W rating.

    3. You can buy an off the shelf series adapter that is clearly labeled. The series connection doubles the impedance which cuts the power in half and drops the SPL by 3dB. This applies to solid state amps. The power remains constant with a tube amp as long as the amp sees the expected load.

    The series connection also causes the voltage to be split between the drivers, which drops the SPL 3dB. As you mentioned, this works out to essentially a zero net gain because you get up to +6dB from mutual coupling in the low end.

    So a series connection is not beneficial with a solid state amp if your looking for more volume, but if your looking for a fuller tone, it might be the ticket. For example if you have an 800W amp that is rated for a 4 ohm minimum load and want to run a pair of Ampeg 810Es for massive stage presence, you need to hook the cabs up in series so you don't damage your amp.
     
    Last edited: Mar 19, 2019
  5. JimChjones

    JimChjones

    Aug 6, 2017
    SE England
    I'm with bwildt.

    Non standard (ie series) speaker connections are a dead amplifier waiting to happen. Sooner or later it will be misconnected. AH has also highlighted other potential issues.

    I'm also of the opinion that mixing cab impedances requires rather more planning and understanding than a substantial percentage of users are actually going to provide and is best dealt with on a case by case basis.
     
  6. Wasnex

    Wasnex

    Dec 25, 2011

    I agree, there are lot's of way to kill an amp through ignorance or lack of proper attention.

    You may be interested in this post: My apologies for being obtuse.. I know this is questioned ad nauseam, but I am still confused...
     
  7. Goatrope

    Goatrope

    Nov 18, 2011
    Sarasota Florida
    I don’t think the issue is so much bad advice going unchecked, but rather the threads devolve into a battle of wits between some very smart folks, but unfortunately there’s always one more scenario or clarification. Like trying to reach infinity, we never get there, but we certainly try.

    So the OP gets a ton of information, and often increasingly complex answers, because it’s a much more complicated question than they anticipated, and like I wrote, we got some really dialed-in people here. :)

    So can we just jump in with the short “yes” or “no” answer, rattle off the math, and be gone before it goes to 99 pages? Is it possible to throttle-back if the OP’s question has been answered, even if we feel the answer is incomplete?
     
    FenderB, gepettus, cazclocker and 3 others like this.
  8. Happy Face

    Happy Face Supporting Member

    May 3, 2007
    Portland, Maine
    Quick thread veer - what ohm meter do you folks recommend?
     
  9. Too much of this post makes sense and is what kept me away from Amps altogether for a long time. It wasn’t too long ago when things would get heated pretty quickly, and not in a fun way...it was more like hard math (and I say that as layman), but it’s come a long way and not without causualties. It’s definitely one of the more confounding corners of our nook, where art meets science. Inconvenient truths abound.
     
  10. 1bassplayinfool

    1bassplayinfool -Nowhere Man- Gold Supporting Member

  11. Jim C

    Jim C I believe in the trilogy; Fender, Stingray, + G&L Supporting Member

    Nov 29, 2008
    Bethesda, MD
    Fluke; I've got 2 from the mid 80's that work perfectly
    Not a good idea to drop them (which I haven't)

    I think another thing that has not been mentioned is the sheer number of people and the lack of use of the search button. I am on a motorcycle board that if you don't use the search for obvious questions, you won't get an answer.

    The other thing I hate is the geniuses that posts on a long thread without reading, since his brilliant idea has already been posted at least once before.
    or
    Posting of incorrect information and not having the balls to say I stand corrected.

    At least with bikes we are reasonably sure that the poster is at least 16, and if the bike is expensive or maintenance intensive, much older.
     
    Happy Face likes this.
  12. Killed_by_Death

    Killed_by_Death Snaggletooth Inactive

    Most recently there was a case of a 2 Ohm cabinet that could be rewired to 8 Ohms.
    However, the 16 Ohm cabinet was a lost cause w/o changing the drivers.
     
  13. JimChjones

    JimChjones

    Aug 6, 2017
    SE England
    The trouble with the talkbass "FAQs", for what little my opinion is worth, is that they aren't conventional FAQs, but links to long threads, often containing a variety of opinions, some even contentious.

    They don't in my opinion really provide the simple easy to assimilate information that I would like to present to someone without a reasonable understanding of the technology.
     
  14. bwildt

    bwildt

    Mar 21, 2017
    Wichita, Kansas
    I agree that some post responses do something offer alternatives, although they are often kind of esoteric in nature, i.e. dual voice coil switchable 4 or 8 ohm operation. However who is going to run out and buy something like that?

    Most often the OPs of these threads are noobs or someone of limited ohmic (??) knowledge that are just asking if a couple of cabs they have laying around will play together. They don't have the finances to go out and buy anything to replace what they have. In a lot of these cases the answer is simply "No, you don't want to do that. That might let the magic smoke out."

    I determined quite a while ago that I wanted to be modular and would only buy 8 ohm cabs that I could generally safely double up if needed or could use as an extension cab for a combo as long as the combo was not already running at 4 ohms by itself (like my Fender Bassman 150).

    I generally don't need big honking amps any more. If I need more volume than about 350 watts into a 2-12 8 ohm cab can do, there will be a PA doing the heavy lifting. I don't have any real need for a 2 ohm-capable amp. However, I also admit that I am getting to be a cranky old coot that doesn't play really loud genres and doesn't like high stage volumes.
     
    NKBassman likes this.
  15. bwildt

    bwildt

    Mar 21, 2017
    Wichita, Kansas
    Happy Face, my understanding is that metering the ohm reading of a cabinet is not easy with a simple meter, as it is a capacitive load and a resistive load that most meters read. I just use any old meter that can meter ohms. However, the reading on the meter will be about 20% lower than the ohm rating of the cab. An 8 ohm cab will meter at about 6 ohms. A 4 ohm cab will meter at about 3 ohms.
     
    clockworkcorpse and Happy Face like this.
  16. darwin-bass

    darwin-bass Supporting Member

    Mar 29, 2013
    Salem OR
    IMHO the roor cause is a lack of understanding of the fundamentals of resistance and impedance.

    Folks don’t understand and ask simplistic questions looking for simplistic answers. Ofhers with limited understanding provide partially correct answers when the complete answers are often more complex.
     
    ThisBass and agedhorse like this.
  17. Killed_by_Death

    Killed_by_Death Snaggletooth Inactive

    Impedance is the term.
     
    petch, n1as and fuzzer like this.
  18. Wasnex

    Wasnex

    Dec 25, 2011
    Your approach is fine, but other people have different needs and interests. Depending upon the circumstances, mixing dissimilar cabs or running cabs in series may be useful, or the sound quality may suck. You find out by experimenting.

    My focus is to help people understand why combining random speaker designs may not work well, and also help them avoid connecting their gear in a way that may cause damage. I believe the helpful advice of the Talkbass community helps a lot of people avoid destroying their amps.
     
    berman3313 likes this.
  19. JeezyMcNuggles

    JeezyMcNuggles Supporting Member

    Feb 23, 2018
    Santa Maria, CA
    I suck, but nobody really notices
    On the subject of #1, you could most definately rewire any cab to more or less ohms. As long as it isnt a single speaker cab. You just run the leads from speaker to speaker to different posts. Therefore changing the impedance of the cabinet. Exactly like I just did with my sub. I ran the positive and negative from one voice coil's posi/neg posts to the other, and then to the box. All red/black to red/black. Parallel. My speaker went from four ohms to 2 ohms. You could do that to any combination of speakers. You just have to do some math.
     
  20. I've yet to have a serious issue, mixing impedances. I haven't done it that often, but I've done it enough times to realize gear can handle it under nominal conditions and expectations. Its not optimal, but stuff isn't going to blow up.