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Monitoring voice-coil temperature

Discussion in 'Amps and Cabs [BG]' started by alexclaber, Jul 2, 2003.


  1. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    I currently use a pair of Acme Low-B2 cabs which I used to power with a Mackie M1400 amp (425W RMSinto each cab) and I have played outdoor gigs with this with heavy compression and the amp riding the limiter without blowing any speakers. I play 4-string bass in standard tuning (though I occasionally use the octaver on my Deep Impact) and have 12dB/octave highpass filters set at 30Hz on both my preamp and poweramp.

    I'm planning on getting a QSC PLX 3002 to replace the Mackie because I'm sick of the weight and think a little (a lot!!!) more headroom would be nice, as would having enough power off one channel to drive a single cab for practices or very small gigs. This would put 900W RMS into each cab which, though a lot, is less than quite a few people use that bridge their amp into a lone B2, and if one B2 can go pretty loud with 1000W+, imagine what a pair, each with about 1000W, can do!

    Because I play 4-string and use those high-pass filters I think the speakers are unlikely to be blown by exceeding Xmax (anyone think otherwise?) so the only thing likely to damage the Acmes would be the voice-coils melting. Having seen that the Acmes can handle the Mackie at beyond its limits for a whole gig, I think the 3002 will not be too much as long as it doesn't get pushed *that* hard.

    However I'm paranoid so would it be feasible to attach some sort of electronic thermometer to the voice-coil or close enough that it will pick up the approximate temperature, so I can monitor how hot the voice-coil is getting and then turn down if the speakers are at risk?

    Alex
     
  2. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    You're paranoid.
     
  3. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    You'd be paranoid if you were intending to power cabs rated at 350W RMS with an amp rated at 900W RMS per channel, had a tendency to end up in bands with very loud drummers and shared Larry and Bootsy's view on how loud bass should be...

    Alex
     
  4. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    But the temperature rise would occur so fast that you couldn't do anything anyway.

    And, although your setup exceeds the "2 x power" rating, you'll be safe IME, since you never use the full power over a longer period of time.
    You'll hear clipping before anything happens anyway IME.
     
  5. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    OK. That makes sense - I guess it wouldn't take long for 900W to heat up something that small.

    Would I? The amp won't be clipping, so that'll be speaker clipping - why would this occur, is this over-excursion or something completely different?

    Alex
     
  6. redneck2wild

    redneck2wild

    Nov 27, 2002
    Memphis, TN
    It is possible that the Acme cabinet is not very efficient. It may require a large amount of power to produce the volume that you are trying to produce. That may be why your amp is clipping.
    According the Acme's website the sensitivity of the Low B-2 is 93 dB 1W/1M. It takes takes twice the power to increase volume by 3dBs. Many bass cabinets have a sensitivy around 99dBs. It would take 4 times the power going the Acme to produce the same volume as a cabinet with a 99dB sensitivity.
    On the other hand, Acme cabinets tend to go lower than many other Bass cabs on the market. For example the freq response is 41hz-22kz for the Acme Low B-2 while an Ampeg SVT-210HE has a freq response of 58-18KHz (but a sensitivity of 97dB).
     
  7. Mcrelly

    Mcrelly

    Jun 16, 2003
    Minnesota, USA
    it might be easier to monitor average power into the cab if you really want to "see" it. maybe ask around on some electronics forums or maybe www.harmony-central.com forums?? I'd be curious too! but possibly the easiest way to know if you're pushing too many watts is to know when distortion is happening in your speakers and turn them down at that point.

    let me know about the watts meter.
     
  8. fast slapper

    fast slapper

    Dec 11, 2001
    Fresno, CA
    How long do you usually play?
     
  9. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    That's exactly why the amp is clipping, hence my desire for a more powerful amp. The peak power handling of the Acmes should be plenty high enough to get seriously loud, given enough clean power. I could replace the Acmes with more efficient cabs but I like their hifi sound too much - the only similar sounding cabs AFAIK are Accugroove and (to a lesser extent) Euphonic Audio, and they're way out of my price-range.

    So will this be speaker distortion, and if so, what does it sound like? I presume it'll be noticeable when I'm using a clean bass sound, but would I notice it over overdriven or fuzz bass sounds?

    Alex
     
  10. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Nexo make PA systems that revolve around a processor that monitors the voice coil. Well not really, it's actually a simulaion based on a computer algorithm. The result is really small PA's that can blow away really big PA's.

    The technology is very new and very expensive. And it will only work on Nexo speaker cabs (I believe they're loaded with B&C speakers.

    Have you thought about Polyswitches? Though with that amount of watts, you'd probably need too many to make it feasable.......
     
  11. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Just having a look at the Nexo site - very cunning technology indeed. Overkill for my needs though, all I want is something that stops me melting my voice coils, who'd have though it would be so difficult!

    I'm looking at Polyswitch's site and it looks like they'd just limit the total power going into the speakers, i.e. the peak power not the average power. I wonder if you could use something like this to replace the bulbs in HF protection systems?

    Alex
     
  12. My daughter's band played a car show in Bakersfield last weekend. The sound folks did the PA system the old fashioned way: brute force. There were (12) 18" subs, 6 on each side of the stage, and each sub was powered with its own Crown Macro 2400 amp. The mains were flying high from masts as an angled array. There were three Mains in each array on each side of the stage.

    Ambient was 107 degrees, so I'm sure the voice coils were hot. You'd never know it from the chest hits those subs put out.

    IMO, it is much easier to add more speakers, so each gets half its rated power capacity. This combination will be louder overall by avoiding power compression due to over heating.
     
  13. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    They're made of a polymer that partially melts when overloaded, restricting the current if overheated. As soon as the current is reduced or removed, is solidifies again and restores the original signal. All this happens in a split second.

    I've never used them and to be honest they look like they're designed more for low powered, Hi-Fi applications. And they won't protct against mechanical overload which to be honest, is probably more of a concern in your case.
     
  14. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Easier from a technical point of view, definitely, but not from a space point of view. One big difference I've noticed between UK and US bassist's rigs (apart from NYers) is that we use much smaller/fewer cabs. Any more than my two B2 cabs wouldn't fit in my house, let alone my car!

    With the Mackie at full blast my Acmes were loud enough for pretty much every situation, but the dynamics and huge bottom was somewhat diminished. I'm thinking that judicious use of a more powerful amp could ensure that when they're running at that volume their tone remains just as good as when quieter. Is that likely?

    Alex
     
  15. Full power means full heat on the voice coils. You will get power compression for sure, at least -3dB and probably more. I'm not at all surprised they sound better at a lesser input power.

    My rig lives in the back of my Toyota pickup truck and never enters the house. Too big. Too much of a pain in the butt to load/unload between gigs.
     
  16. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    so there aren't any thieves in Sacramento. Interesting.

    If I tried that, my gear would be stolen quicker than you can say "pawn shop".
     
  17. First, keep it ugly.
    Next, keep it hidden under something ugly.
    Keep it in an ugly truck, preferably unwashed.

    :D

    Last secure the tail gate inside with a big chain and lock so it won't open. The gear is too big to get out the lid, so they'd have to remove the entire shell to get the gear out. The shell removal bolts are blocked by all the gear.
     
  18. If you're worried, get a reasonably-powered amp with built-in clip-monitoring limiters and leave them turned on.
     
  19. Trevorus

    Trevorus

    Oct 18, 2002
    Urbana, IL
    you might want to check out those polyswitches. They seem like a pretty good idea. They'll protect against temerature, voltage and current. By the way, does anbody know the equation for figuring wattage by current and voltage?
     
  20. Jeff Moote

    Jeff Moote Supporting Member

    Oct 11, 2001
    Beamsville, ON, Canada
    Power (in Watts) = Potential Difference (in Volts) x Current (in Amperes)

    So P=V*I

    Is that what you needed?