I Need Help with My Rig (several questions)

Discussion in 'Live Sound [BG]' started by saxdragon, Mar 21, 2012.

  1. Hey everyone. I just picked up a couple of empty speaker cabinets. They carry two 10-inch speakers each and have a port that runs the length of the cab. I took two 16-ohm speakers out of a Peavey guitar amp and installed them in one cabinet, and plan to do the same with the other cabinet. I used it last night for the first time at rehearsal, combining it with my Acoustic 150b (an old one from the seventies) and it's two 15-inch speakers, plugging the two tens into the "speaker 2" jack on the back of the amp. I liked the sound better right away, enjoying more definition from the guitar and the expanded tonal spectrum. But several questions occurred to that i hope you guys here could help me with.

    1.) How likely is it that I'll blow those ten-inch speakers? The amp I took them from was rated at 100 watts. The Acoustic 150b is rated at 120 watts RMS, i think.

    2.) Should I stuff up the cracks? Will it sound better and protect the speakers from tearing themselves apart if I jam something in the port?

    3.) I'm fuzzy on impedence. I have the two 15's coming out of the speaker 1 output and the two 10's coming out of the speaker 2 output. Assuming the two 15's are rated at 8 ohms each that means speaker 1 has a load of 4 ohms, right? And speaker 2 has a load of 8 ohms. Am i right so far? Will that work? Is it safe?

    4.) I'd like to be able to control the volume of each cabinet separately, as I noticed i wasn't getting as much of the low frequencies as I'd like. What are the options for controlling the volume of the two 10's?

    Thanks for reading my post. I'm looking forward to reading your thoughts on the matter.

  2. Pep


    Oct 5, 2010
    Louisville, KY
    I see a lot of potential problems here.

    (1) Guitar speakers for bass. My guess is that these will blow.
    (2) Acoustic Amp rated at 120 w/RMS but at what impedance?
    If the cabinet was designed to run with that amp then adding speakers will only decrease the power available to push the two 15s.

    Too many new players believe that too much power is what causes bass speakers to blow, but it's the exact opposite in most cases. When it comes to bass it's all about having more power available than your speakers can handle. This is known as headroom. It provides a clean signal and little distortion.

    If you have too little power you run the risk of clipping the amp (driving it to distortion) and frying the voice coils of your speakers.

    You're not the first to venture into this territory and you won't be the last. I learned the hard way many years ago after blowing at least 6 JBL 15s in a matter of weeks. Yes, I simply added a 15" cabinet to my existing rig, which not only changed the impedance of the load but seriously compromised the power output of the amp.

    I'm now at the point of "overkill" on this. My last rig was a Peavey Maxxbass preamp, a 2-10" cabinet rated at 600 w/RMS @4 ohms and a QSC RMX2450 bridged / 2400 watts@4 ohms.

    Yes that is overkill but I didn't blow the speakers and local bass players wanted to know every detail of the rig and if I'd sell it to them.

    So my suggestion is this. If you like the clarity of the cabinet with 2-10s, there are bass systems with a 2-10 cabinet out there that will provide you with enough low-end and clarity without having to haul a 2-15 cabinet around. There are a lot of affordable bass rigs from Peavey, Carvin, Gallien-Krueger and Fender. If you want to go a bit more towards the higher-end units, look at MarkBass and Genz Benz. You may want to stay away from Behringer due to quality issues.
  3. Thanks for the info, Pep. I think the amp is 120 watts at 8 ohms and 200 watts at 4 ohms.

    What are the differences between guitar speakers and bass speakers if they are rated the same for power handling and impedance? Are guitar speakers not built to handle the low frequencies and liable to fail in the cone?

    Is there any kind of interface you can think of (besides using two separate amps) that I could use to control the power or the volume going to the two 10's? Players mix speakers of different sizes right? I like the deep sound of the 15's but also want to hear the higher frequencies, too.

    Thanks again
  4. This is utter nonsense, and has been discussed/debunked here over and over again. See 'Underpowering/overpowering' in the Amps FAQ area.

    Guitar speakers are inappropriate because, among other reasons, they lack the excursion required to get meaningful volume at low frequencies. Bad idea and they will not last long used this way.
  5. Dave W

    Dave W Supporting Member

    Mar 1, 2007
    White Plains
    Do yourself a huge favor and go into the Amps Forum and read the FAQ sticky at the top of the page. You are going to blow the speakers, your amp, or both if you continue using it the way your are.

    Probably pretty likely.

    No. The port was a design of the cab. Just plopping any old speaker into the cab was a bad idea in the first place. As far as bass cabs are concerned, the box is an integral design...and the speakers are carefully chosen to work in that design.

    You said the amp can do 200w at 4 ohms. Without knowing the ohm ratings of the cabs it's impossible to tell you. Lets assume you're right, and the 2X15 cab is in fact 4 ohms...that is all you can hook up to your amp. Connecting anything more is bad.

    You can't. You'd need a separate amp. You're probably not getting the lows you'd like because you are using guitar speakers and cabinets that are very poorly designed. Get some real bass cabs and be happy.
  6. Pep


    Oct 5, 2010
    Louisville, KY
    Discussed: Yes.
    Debunked: Not hardly.
  7. Floyd Eye

    Floyd Eye Banned

    Feb 21, 2010
    St. Louis
    Oh it has been debunked. You apparently just don't believe the engineers that debunked it.

    Back to the OP: There are so many things wrong with what you are doing I don't even know where to begin.

    For starters. Guitar speakers, in a bass cab? Really? I would address the mismatched cabs thing, but several people will chime in and tell me how I don't know what I am talking about and why would a company sell a full rig with mismatched cabs, blah, blah, blah. Use the search feature.

    Then we can address the fact that you are just sticking random speakers into a box and assuming it will sound good.

    My advice is to sell the Acoustic, put the 10" speakers back in the guitar cab and sell it, then save some more money and get a decent 4x10 and a decent amp to drive it.
  8. Pep


    Oct 5, 2010
    Louisville, KY
    After spending the last hour rereading every point and counterpoint in that section, I fail to see where anything was proven, or disproven for that matter. If a moderator hadn't closed the thread they would still be volleying.

    I'm sticking with my statement because it has yet to fail me in the past 15 years while the other side of the coin has repeatedly cost me big bucks, even in situations where I used the what the manufacturer's design engineers recommended.
  9. Floyd Eye

    Floyd Eye Banned

    Feb 21, 2010
    St. Louis
    Well, manufacturers design engineers are going to tell you that what they are selling is what's best. That is why Carvin sold a lot of full bass stacks that had one 8 ohm 4x10 cab and one 4 ohm 1x18 cab along with a head that was called RL1000, yet would only put out about 625 watts with those cabs.

    Can you give me an example of someone blowing a cab by under powering it?
  10. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA
    Don't let the argument regarding under/over powered stray us from the real point here. There is a LOT More that goes into speaker cab design than the OP thinks. He has some reading to do.

    Now, as far as ohm ratings go, if the minimum ohm rating of your amp is 4 ohms, the 2-15s are already there. If you are running in parallel (which is very likely) adding an 8 ohm cab will actually DROP you to 2.67 ohms, well BELOW your amp's minimum. (I know. The math is funny. We don't have time to get into reciprocal formulas and stuff here. Just trust me. It's not good for your amp.) Plus, some speakers will get more power than others, and it's just a big hot mess.

    As far as frequency response and blowing the guitar speakers, you probably will (and without jumping on the argument bandwagon too much, it will have nothing whatsoever to do with watts). Just about the only way I can think of to avoid that from a frequency standpoint would be some kind of crazy crossover rig job that would be a bunch of trouble and probably not produce awesome results. And that still wouldn't get us around good old Ohm's Law.

    Here's the thing. The underlying problem seems pretty "clear" to me (ironically). You lack the clarity in your bass rig that you desire. Somehow, those guitar speakers gave you a tiny glimpse of that clarity and you liked it. You just need a good bass speaker cab (or two) that will give you that. If it were me, I would pop those guitar speakers back into whence they came, sell of everything speaker that you own, and go buy a really nice used 4-10 rated at 4 ohms. It will move almost as much air as the 2-15s, and probably give you some clarity you lack (unless it is tuned very boomy..... again, do some reading and searching here for advice on a nice clear 4-10, because 10s alone don't automatically make for a clearer sound than 15s. I told you it was complicated, didn't I?)

    Short version: (Well, it's about dang time you got around to a SHORT version, TF.) Get a speaker cab that is 4 ohms and sounds like what you want it to. It's a LOT simpler than carrying around a speaker yard sale with you in hopes of gaining clarity. Or just consult a monk. They seem to have the whole clarity thing down. Good luck!
  11. Floyd Eye

    Floyd Eye Banned

    Feb 21, 2010
    St. Louis
    I recommended 8 ohms in anticipation of adding another cab at a later date, but yeah, I agree.
  12. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA
    Not a bad point either. If you (OP) think you will be adding another cab later, get an 8 ohm first time out. That way, when you add another later, again, you will be running at 4 ohms total. The difference in volume with just the one 8 ohm versus one 4 ohm is not very much. (Confused yet? I know I was when I jumped into all of this.)
  13. Pep


    Oct 5, 2010
    Louisville, KY
    Here's one that's a combo. GK MB212. The speakers still work but not at a volume that's acceptable even for a small jam session.
  14. :eek::confused::banghead:

    OP, PLEASE read the faq stickies at top of amps forum before you damage yr amp-by running it below its recommended impedance (ohm-age):p
    You won't damage speakers by under powering, but you can surely damage ur amp.
    Take time & read em & ask questions in amp's forum.
  15. will33


    May 22, 2006
    Long and short of it.......

    1. If your amp is 4ohms minimum, you will blow it adding any more cabs than your existing 215.

    2. If the amp is stable to 2 ohms, you can add the guitar cab. It would be a good idea to add a capacitor such as this http://www.daytonaudio.com/index.php/dmpc-50-50uf-250v-polypropylene-capacitor.html in line with the positive speaker wire in the guitar cab. That will begin to roll off the low frequencies in that cab at a rate of 6db/oct. somewhere around 400hz and help protect the guitar speaker from LF over excursion or thermalling (not as much power is present once you get above the fundamental and first couple of overtones).

    This would send roughly 2/3 of available amp power to the 4ohm bass cab and 1/3 to the guitar cab, which is a good thing as it takes more power to make lows than it does mids and highs.

    That would be a fix and allow you to run both cabs safely.

    The proper way to do this is through biamping but the above description will work.

    Again, if your amp is not stable to 2 ohms, you can stop reading at #1.
  16. will33


    May 22, 2006
    Actually, even with a parts tolerance of -20%, a 100V cap should be enough, I figure the extra is just for peace of mind. Setting it up at 400hz should allow you to dial in a bit of bass/lowmid to fatten up the tone and keep the guitar speakers protected, at least with a smallish powered amp like that. A stronger amp shoukd use a 12db/oct. filter.

    Again, only if your amp is good for 2ohm operation.

    As far as the whole underpowered/clipping thing, yes, it's been debunked many times over by, among others, Mr. Bob Lee and Mr. Bill Fitzmaurice. Both very learned, well respected engineers with decades of experience under their belts. Also members of AES. That's more than good enough for me.

    Basically comes down to being 2 ways to damage a speaker, mechanically (overexcursion), or electrically (overpowering). You send it more juice than it can handle, it burns. The coil doesn't care what form that comes in (sine wave, dynamic instrument signal, clipped signal, DC, etc.)