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Slightly Complicated Impedance Question

Discussion in 'Amps and Cabs [BG]' started by therealting, Jun 26, 2007.


  1. Firstly let me preface this by mentioning that the reason I'm posting this thread is because I'm looking at getting an amp that does 500w into 2 ohms (and of course lower wattages for higher impedances, presumably about 300w into 4 ohms and so on).

    I have a Flite Sound cab (the 110MHD), which has three drivers:

    • 10" Eminence Beta (8 ohms, 250w RMS)
    • 5" Pioneer cone midrange (8 ohms, 50w RMS)
    • 4" Pyle Pro horn tweeter horn (4-8 ohms, 200w)

    The cab has attenuators for the tweeter and horn (which I pretty much always run full out anyway).

    I measured the resistance at the jack terminals, and my multimeter stabilises at 6.3 ohms, it doesn't seem to matter how the attenuators are set. Interesting as the cab is rated at 8 ohms... is it common for ratings to be "approximated" this way?

    I'm also wondering how this figure is arrived at... parallel wiring of three speakers (at 8 ohms each) would give 2.67 ohms (or 2 ohms if the mid-range is 4 ohms, the opposite end of the rating range).

    Any input would be appreciated, as I would love to rewire the cab to a lower overall impedance to take advantage of the amp's power output. I am aware that I'd probably need to be careful with the 50w midrange driver... but even rewiring the tweeter and woofer in parallel might be good enough, eliminating the midrange driver altogether, etc.
     
  2. 12bass

    12bass

    Jan 2, 2003
    Victoria, Canada
    AFAIK, there is no reasonable way to reduce the impedance of your cabinet, save replacing the drivers. As I understand it, the midrange and tweeter do not significantly affect the impedance. A reading of 6.3 ohms DC resistance is about right for a nominal 8 ohm impedance speaker (AC impedance usually around 4/5 of DC). The only realistic option I can suggest is to add another cabinet.
     
  3. seamonkey

    seamonkey

    Aug 6, 2004
    Impedance is AC resistance. Your three speakers are on a cross over, each gets power at difference frequencies. They're not really wired in parallel.

    The woofer is what matters the most. So you could change it, but it'll shift the crossover. And it'll be tough to find a good match for the cabinet.

    Or you can add an autoformer something like this: http://www.peavey.com/products/brow...umber/00720310/Automatch(TM)IITransformer.cfm
    or get Plitron to wind you a custom one.
     
  4. Bryan316

    Bryan316 Inactive

    Dec 20, 2006
    Detroit
    Sounds like this cabinet isn't designed for bass.

    TRAITOR! CRUCIFY THE HERETIC!!!


    Heh heh. Yeah, when you measure a speaker/cabinet for it's impedance, you'd set your multimeter to read ohms worth of impedance. That multimeter is designed to read ohms in DC voltage and current. Not AC voltage and current like speakers are designed to function with.

    The innacuracy you're gonna get is 75% of what it actually is. So a 4-ohm speaker would measure 3 ohms on a multimeter. An 8-ohm cab would measure 6 ohms.

    So your measurement of 6.3 ohms seems on the money. Don't forget... that crossover board also adds some impedance, so your actual cabinet's impedance is probably like 8.5 - 9 ohms. But that's really insignificant.

    Trying to get mad amounts of sound out of a single 8-ohm cab is a difficult thing. Especially since that cab ain't designed for putting out tons of low end. This doesn't seem like a bass guitar cabinet at all, but hey, it's yours, do what you will. But getting another 8-ohm cab, like a 15" cab, would do you justice.

    As for getting an amp that does 500 watts into 2 ohms... seems overkill when you've only got one 8-ohm cabinet. Why not get an amp that does 500 watts into 4 ohms and just get one additional cabinet? Cuz as far as I know, I don't think many amp manufacturers build amps that weak to go down that low. You see amps designed to go down to 2 ohms, that are designed for HUGE wattage, not minimal to average like 500 watts. It'd just be inefficient.
     
  5. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The Beta 10 will only take about 40 watts before farting out anyway. No ten will take even 150 watts before farting out, let alone 300.
     
  6. Wow, lots of responses in a really short time! This is helpful information.

    The fact is that I only really play bass on one gig nowadays, and that is in pubs around London. I don't care too much about tone in this context... I bring out my cheapest bass (an OLP 5-string), and if it's loud enough, then it's good enough. Also these places are terrible gig environments... last Saturday, drunken revellers fell into my microphone stand four times (which is painful on the teeth), and spilled pints of snakebite all over my stage monitors and effects pedals. I actually have been getting by with using a Crate Powerblock to power my Flite cab, and it's done an passable job. I like it mainly because it weighs 5lbs, and it's one less heavy thing to carry up the stairs when I get home at 1am.

    The two main problems I have with it are:
    • it has annoying RF problems... I can hear clicking if I point my pickups at the amp from less than three feet away (i.e. whenever I want to adjust the amp while playing).
    • it has a horrible momentary shutdown when hit with a transient like a hard slap or pop. The only good thing about this feature is that my dynamics control has improved significantly... I can ride the limit for most gigs without the limiter kicking in. Which makes me think that...
    • the amount of power is just a little bit too low. It's rated 150w RMS bridged into 8 ohms, but I know it's not a bass amp and headroom isn't great.
    The amp I am looking at replacing it with is the Yamaha BBT500H (500@2), which is also pretty lightweight at 11lb, and I have a line on a reasonably priced used one. The other options I am looking at are a used Ampeg B2R (350@4, 200@8) which is 15lb, or an inexpensive power amp (a cheaper solution but with a weight sacrifice). I string my bass EADGC so I'm not fussed about stunning B-string response.

    Thanks again for your thoughts, this is helpful.
     
  7. Bryan316

    Bryan316 Inactive

    Dec 20, 2006
    Detroit
    Eden WTX260 Slingshot - 3.8 lbs, 300 watts @ 4 ohms, far better tone than that Crate Powerblock which is designed as a guitar amp, not for bass.

    Eden Nemesis NC210 - 47 lbs, 300 watts @ 4ohms, perfect match for the WTX Slingshot.
     
  8. Would love an Eden rig, unfortunately WAY too nice for me to bring out on the gigs I'm doing on bass! I'm trying not to spend too much really. If possible I'd like to stick with my current cab and get a better amp, with the option of another light cab later on maybe.
     
  9. Dan1099

    Dan1099 Dumbing My Process Down

    Aug 7, 2004
    Michigan
    The first problem is with your bass, not your amp. It needs shielded.

    The second problem is almost assuredly a thermal shutdown issue, which means you need more power. An extra 50 watts, which is what you'll get from the Yamaha, isn't likely to make a significant difference.
     
  10. My bass is fully shielded and star-grounded (I did it myself), the problem only happens when the front of the bass is in the direction of the amp (which is the only unshielded bit). I've had this problem with every bass I've tried in this amp.

    It definitely sounds like I need more power (hence my question about maximizing power utilisation). Perhaps I should go with a poweramp instead, seems like the most cost-effective way to get lots of clean headroom without thermal shutdown issues.
     
  11. Dan1099

    Dan1099 Dumbing My Process Down

    Aug 7, 2004
    Michigan
    Yeah, but there shouldn't be RF coming FROM your bass. From your amp into your bass, yes. My guess is you need to take a look at your shielding with a multimeter, and make sure it reads 0 ohms between all the points of the shield.
     
  12. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    That's because it's normal. The cause is electromagnetism radiating from the amp power transformer, and nothing less than a half-inch thick mu-metal shield will stop it. Move away from the amp.
    Don't expect miracles from a 1x10.
     
  13. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Eh....of course it's true, but...... a speaker guy saying it!

    With so many driver manufacturers claiming huge power ratings AND claiming LF capability, without mentioning the small detail that they can't necessarily go together simultaneously....

    It's refreshing to see someone just come out and SAY it!
     
  14. seamonkey

    seamonkey

    Aug 6, 2004
    I've heard you can squeeze out more out of driver by horn loading it. Seems it will go to burnout with minimal distortion (farting) being heard. Maybe this is where you can push the driver at rated power.
     
  15. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    You will get more output with horn loading, but that doesn't necessarily mean it will take any more or less power. Folded horn subs in particular have extremely low distortion and generally don't give any warning when they're being overpowered, but still 9 times out of 10 driver failure is the result of operator error.
     
  16. What is horn loading?
     
  17. Primary

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