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Impedance yet again

Discussion in 'Amps and Cabs [BG]' started by ToddZilla, May 6, 2004.


  1. ToddZilla

    ToddZilla Guest

    Apr 12, 2004
    Out of the 500 threads a search for "impedance" yielded, I couldn't find my answer. So I pose this question to my fellow TBers.

    My Trace head (AH250 GP12 SMX) has 2 8 ohm speaker outs. Do these need to see an 8 ohm load? Or is it safe to show them 4. I have to replace subs anyway, but I wanted to know if I need to get 8 ohm for a total imp. of 4; or 16 ohm for a total imp of 8.

    Also, as they're listed as 8 ohm outs, will running each to a seperate 8 ohm cab result in a 4 ohm load? Which would lead me to believe that showing a single out 4 ohms is safe. Am I on the right path with that thinking?

    Thanks,
    ~Steven

    For clarities sake...I think

    8 ohm -------> 4 ohm cab
    8 ohm -------> empty

    yields same results as

    8 ohm -------> 8 ohm cab
    8 ohm -------> 8 ohm cab
    does that total to a 4 ohm load?

    Also Trace/Gibson was helpful enough to send a manual, as I bought the amp used. But it was unable to answer my question.
     
  2. Yep, you are right. I checked the manual for the GP-12X, which is probably the same. It says use (2) 8 ohm cabs (which combine to be a 4 ohm load), or just one 4 ohm cab.

    Chris
     
  3. ToddZilla

    ToddZilla Guest

    Apr 12, 2004
    hrm, I dug through my manual and didn't find that....lack of coffee perhaps. :)

    So say I wanted to change the impedance on my cab. It's a 210, both 10s are 8 ohm, and the tweeter is 8 ohm. The resulting load is 4 ohms. If I wanted to increase it to 8 ohms, I would swap for 2 16 ohm 10s. But what about the tweeter, do I match it as well, or is it okay to leave at 8? Or what effect on load does the tweeter give?

    right now its 2x 8ohm subs + 8 ohm tweet = 4 ohms
    does 2x 16ohm subs + 16 ohm tweet = 8 ohms?
    and what load would 2x 16ohm subs + 8ohm tweet give?


    ~Steven
     
  4. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    The tweeter will not affect your overall impedance, since impedance is somewhat frequency variant, and the high frequencies really don't affect the equation.
     
  5. ToddZilla

    ToddZilla Guest

    Apr 12, 2004
    So if I decide to turn it into an 8ohm cab, I'll be able to leave the original tweeter in there with no ill effects? Just making sure I understand, would hate for the magic smoke to leave my amp :p

    Steven
     
  6. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Wait. Usually cabs with tweeters have a built-in internal crossover. The crossover outputs are impedance specific, so if you change the aggregate impedance of the woofers for instance, you'll have to change the crossover too, otherwise the frequencies won't be correct. It's worse than that even, if you change the woofer impedance then the tweeter's response might change too. I'd double check with the manufacturer, to see whether they offer a 4-ohm version of the crossover (or whatever, maybe they have a mod you can do to the original crossover to make it work with different load impedances). I wouldn't just alter the woofers without looking into the crossover too, that could have dire consequences for a solid state amp that's operating right on the hairy edge of maximum power. Just a heads-up...
     
  7. I'd say remove the crossover and the tweeter all together. That should simplify everything. I mean, we are playing bass, right? :D You'll sound a lot better when you don't have a tweeter messing up your sound.

    Chris
     
  8. Call 1-800-4-GIBSON.

    They actually answer their phones and have real people at their customer service department.
     
  9. ToddZilla

    ToddZilla Guest

    Apr 12, 2004
    Well I found a local place that had a couple of eminence delta 10s. All they had were 8 ohms, so I still in my original configuration. Plus I usually keep that silly tweet as low as the control will let me :) So I still have a 4 ohm 210 to deal with...but it sounds amazing with the eminence's in.

    But say I decided at some point to wire it series so I could add a 115 or something. Beyond they power loss I'd see, would I still need to change the crossover out to avoid bad things? Was thinking I could do an external crossover with the 210 and a 115, have it kick to the 15 at the delta's 60 hz dropoff and be gravy. But that tweeter xover might present a problem in my thought process.
     
  10. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Yes, you still need to worry about the crossover. Here's a couple of thoughts.

    One is, you could go bi-amp. That would alleviate most of the issues. Then you could leave your 4 ohm cab alone, and run it on the top end, and run your 15 on the bottom end.

    Second thought is, if you use cabs with different impedances in parallel, you'll have unequal power distribution. With a single 15" 8-ohm speaker and a dual 2x10 at 16 ohms, your 15 will get about 2/3 of the power from your amp. This may or may not be what you want. Sometimes it works fine, especially if your 10's are mid-heavy or the tweeter is too bright.

    And the last thought is, you could buy an external crossover that allows you to integrate all of your speakers into a single functional unit. This would be a three-way crossover, using the 15 on the bottom, the two 10's as mids, and the tweeter for the highs. EV and SWR and Peavey make some 3-way cabs with this configuration, it seems to work very well. Perhaps one of those companies would be willing to sell you a crossover. If so, you can probably specify whether you want the mids to work at 4 ohms or 16 ohms (or, they'll tell you to do it one way or the other).
     
  11. ToddZilla

    ToddZilla Guest

    Apr 12, 2004
    Thanks, definately some things to think about now.

    Would I able to get a rack mount crossover, or do they just alter the frequency path and not the load?

    For the external crossover, you said that most companies mount them in a cab, would this be something I could build a small enclosure for? For instance, could I come out of my amp at 4 ohms (presented by xover) and from there run to a 4ohm cab and an 8 ohm cab?


    oh...and how does biamping work?

    pardon tha noob

    ~Steven
     
  12. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Hi Steven, crossovers are generally of two types: either active or passive. The active ones are usually the rack mount types, and they go "before" the power amp. So you'd take your preamp output and feed it into the crossover, then take the two (or three) crossover outputs and feed each one into a separate power amp. That's the way bi-amps work too, except usually the crossover is integrated into the amp (for instance, this is the way the early SWR heads and the GK 800RB work). These units actually have two power amps built into the head.

    The passive crossovers are the ones that are usually mounted directly into the speaker cabs. You can definitely build a separate box for the crossover though, there's no need to have it reside in the cab, it's mainly done for convenience (fewer pieces of gear to lug around). I have an external crossover like that, it's in a little metal box about the size of a shoebox, and it has three 1/4" jacks (one input and two outputs). With a passive crossover there's only one power amp, the output from the power amp goes to the input of the crossover, and the each output from the crossover goes to a separate speaker cab. So in this case the crossover goes "after" the power amp.

    The main advantage of the passive crossover is that you only need one power amp. Its disadvantage is that it has to handle as much power as your amp puts out, so the electronic components tend to be physically pretty big if you're running a kilowatt. And if you have a level control on your tweeter, it will usually be pretty big too, a big pot or rheostat (an L-pad or T-pad), and that probably should go into the external box as well, if you're taking that direction.

    The main advantages of the active crossover is that it works at much lower signal levels, so the active units can be physically very small, and they're easy to "configure" on the fly, so if you had to use subs for a particular outdoor gig or something, you could very easily set up the crossover on a one-time basis for that purpose. With a passive crossover it's not so easy to change things on the fly, 'cause the coils and capacitors are tuned for the specific speaker impedances and the particular crossover frequencies you need.

    I like passives, 'cause I don't like to haul heavy power amps around, and I'm willing to sacrifice the flexibility 'cause I always use the same rig. I used to use an active, I was bi-amping for several years, but eventually went back to a single big power amp.

    There are other options, but they're relatively rare. Mostly they're for specific purposes. For instance, the passive crossover I'm currently using is a hybrid, it's more like a low pass filter, and its only purpose is to drive the subs when I'm playing outdoors. I run a separate power amp for that purpose, and I take the line out from my WW amp and run it through the crossover into a Stewart 2.1 that drives a pair of 18" EVX-180A's. That way, when I'm not using the subs, I just disconnect the cable from the line-out and I'm good to go.
     
  13. ToddZilla

    ToddZilla Guest

    Apr 12, 2004
    Okay, biamping is out then, lack of funds and all.

    My amp is rated to 4ohms, so based off that....

    I can go out the amp / into a xover that presents a 4ohm load / from xover I can hit my 4ohm 210, AND a ?ohm115

    Am I understanding how that works correctly? Or will it just not work like that at all?

    I'll be purchasing a new tweeter for the 210 soon, as it's rated to handle about half of what my new Eminence's do.

    So with that, if I were to just wire the 10s in series to get an 8ohm load, get a new crossover for that cab, I could buy a tweeter that will work with the new setup.

    Another point of confusion for me. It was stated earlier that tweeters have minimal effect on total load. Right now the cab has 2 8ohm 10s and an 8ohm tweeter. Is the tweeters desired ohm rating dependent on the crossover, or does it need to match the subs?

    ~Steven

    oh and thanks for lettin me pick your brain non :D
     
  14. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    Most crossovers I've seen in bass cabinets are high pass only...they protect the tweeter but allow the woofers to run full range. Having typed that, I believe the Peavey TX cabinets actually do have a low pass section to filter the highs out of the woofers. I've also read that owners of those cabs say they sound a lot better if the choke in series with the woofers is shorted out running the woofers full range.

    If the cross over in your cabinet is high pass only, it wouldn't really matter what impedance woofers you put in as the crossover will continue to protect the tweeter as the cabinet designers had planned.

    If the crossover has a low pass section, it would have a large coil in it wound with large diameter copper wire. It may also have an iron core (done to save money). If the crossover just has one capacitor or maybe a capacitor and a small coil or like Eden and SWR two capacitors and a coil (wound on a plastic bobbin) then the crossover is high pass only and you don't have to worry about it.

    It sounds like you bought new speakers already, but if you hadn't, you can get amazing prices on Eminence Delta 10s from Dave at Avatarspeakers.com...way cheaper than Parts Express or anyone else I've encountered. He would have 16 ohm units you wanted.
     
  15. ToddZilla

    ToddZilla Guest

    Apr 12, 2004
    Ya I got some already, though I did talk to Dave at Avatar. Was just a case of me needing them faster than he could get them here at that price....overnight on 2 10s would've been nuts we decided. Unfortunately all I could find local were 2 8ohms. Which is direct replacement for whats in there, so I'm happy enough...plus they sound so much better than the creased cone *expletive* that was in there. So I'm happy enough.

    Now I'm just trying to figure out how I could add a 115 or something along those lines. I'm stuck with an amp that'll do 4 ohms, and a 4 ohm cab, I could swing a 115 in the near future, but a new amp may be far off. Plus I dig the way the Trace sounds.

    Overall I'm happy with my gear now...need to swap the tweeter cause it distorts...but "off" fixes that for now :D
    Just trying to learn/figure out how I could expand from here....or if I'm stuck with what I've got for a bit.

    ~Steven
     
  16. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Crossover design is pretty straightforward. You can design a crossover to work with any impedance speakers, and present any impedance load the amp needs. Search Google and I'll bet you'll find some design equations, it's basically just passive filter stuff. Also check the Audio Cyclopedia, there's a great chapter on crossovers in there. Thing is, you need to know the frequency at which you want to cross over, so you can either get that by looking at the speaker specs, or you can reverse engineer the existing crossover to figure out the frequencies.

    Your previous post sounds good, sounds like you got the idea. The tweeter doesn't have to be any particular impedance, it's just whatever the crossover wants to see. If the "crossover" is just a series capacitor like BillyB_from_LZ mentioned, then it's just a matter of replacing the cap if you change to a different impedance tweeter. If there are coils in there, those are a little harder to get off-the-shelf, but they can be found with a little effort. Plus you can wind your own, it's not too hard.

    What model cab is it? I have some Trace info, maybe I have the schematic in my files.
     
  17. ToddZilla

    ToddZilla Guest

    Apr 12, 2004
    The cab is a Genz Benz, the older XB model according to them. The thing is a beast, but sounds good, and I got a good deal on it at the time.

    Thanks for the education fellow TalkBass'rs

    ~Steven
     
  18. Mindabout

    Mindabout

    Apr 11, 2004
    Australia
    Whilst on the topic of impedances I have a question for you guys. I'm looking at an amp that is rated 300 W at 4 ohms but all the speaker cabinets I've been shown in stores seem to be rated 300W at 8 ohms. I only want to have one speaker cabinet and I'm hoping to use the amp at the full 300W.

    Just wondering:

    1) How is the impedance rating for an cabinet established? Is it determined by the size or amount of speakers?

    2) Are there speakers commonly available rated at 300W at 4 ohms? Would they tend to be in a higher price range?

    3) Is there any way other than trial and error to find amps with the correct impedance rating?

    4) How would it go matching a speaker cabinet with a rating of 400W at 8 ohms with the afforementioned amp?
     
  19. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    Chicago
    A loudspeaker cabinet's impedance is a product of the impedance of the speakers within.

    A loudspeaker's impedance is determined by the design of the voice coil, literally a coil of wire that makes the cone move. The speaker's designer decides which impedance the speaker will have (typically 4, 8 or 16 ohms) along with other performance characteristics and then designs the coil accordingly. Often loudspeaker manufacturers will offer the same speaker in different impedances. Eminence is great about this.

    The impedance is really the key parameter to worry about when matching cabinets to your amp. While some solid state amps will drive a 2 ohm load (two 4 ohm cabinets connected in parallel), not all can. This means that if you ever want to use two cabinets with an amp that can't go below 4 ohms, you'll need to buy 8 ohm cabinets.

    Price range is always the factor... Avatar Speakers sells 4 and 8 ohm cabinets of the same type for very reasonable prices. Cabinets produced by larger companies are often limited to one impedance because they find that in a particular design 4 ohm or 8 ohm cabinets sell better. Both Ampeg and SWR offer some models in either 4 or 8 ohm impedance. There are others, but you get the idea.

    Again, the key parameter to match is the impedance. If you are really going to use one cabinet and will never, ever want another, look or 4 ohm cabinets with power ratings in the area of your amp's rated output. If you don't drive the amp into clipping regularly, you really don't need to worry about blowing up your speakers.