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Got myself a rig, but...

Discussion in 'Amps and Cabs [BG]' started by apparatus, Feb 20, 2004.


  1. I originally wanted to buy an Ashdown ABM 500 evo II with an Ashdown ABM 410 head (600w), but for reasons I won't go into here I had to go with the second best head they could offer me, which happened to be a Laney RBH800 (300w rms). It was a lot cheaper though, of course. I still went with the Ashdown cab, thinking that I might get that ABM head sometime in a not too distant future..

    Anyway, I've heard that underpowering cabs may be dangerous. Also, the RBH800 has an automatic limiter, with a little LED. It flashes almost all the time when I play, (with gain up about 3-4 and volume at about 4) so I guess something is clipping. Otherwise, it sounds pretty good (it's hard to find an amp that sounds bad with my Cirrus, I guess), apart from the fact that the cab's tweeter farts (so i just turned it off).

    My questions are:
    1. Is the RBH800 an OK head?
    2. Am I putting the cab at stake by underpowering it; is the noticeable clipping dangerous?

    Thanks..
     
  2. BigWave

    BigWave

    Jan 12, 2004
    Salinas CA
    My SWR has a limiter that's lit most of the time when I play with a band. Never had a problem. If you really have a limiter, it will prevent clipping.

    As for underpowering being a problem, pushing the amp beyond it's limits is the problem. If you're not clipping, you should be fine.
     
  3. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    But he is clipping - or would be if the limiter wasn't there.

    You need an amp with more watts.
     
  4. Yeah, I'll get the ABM 500 as soon as I can.. but until then, I'm alright with the limiter?

    It also has a button called "defeat limiter", when I push it, the LED doesn't flash anymore. English is not my native language, but I guess that means that the limiter is turned off. I didn't notice much of a difference in volume when the limiter was "defeated" so I'll leave it on.
     
  5. What exactly is clipping? (Sorry apparatus for the off subject question)
     
  6. No, the light means that the limiter is working so that something isn't clipping. It means that you are getting all you're gonna get out of that amp and nothing more. Under powering a bass cab (imho) won't usually hurt the speakers. If you drive a SS amp into hard clipping it will produce a lot of power in the high frequency range (and really sound like crap) that can blow an unprotected tweater if the cab has one, but most modern cabs have some type of protection like a dome light that works quite well. If you're loud enough in the mix, I wouldn't worry about the limiter light coming on as I don't think you are hurting anything.
     
  7. Finger Blister

    Finger Blister

    Jul 8, 2003
    [​IMG]
    Fig. A - Normal Sound Wave

    [​IMG]
    Fig. B - "Clipped" Sound Wave

    If you ask the amp to go beyond its limits you'll end up with "clipping."

    CLIPPING distortion--

    This is when an amplifier has reached its maximum output capacity yet tries to keep up with the input signal gain ratio between the signal source "HU" and Amp. The amp hits an imaginary wall whereby the output signal is no longer a symetrical replication of the input signal. The wave form in, does not match the wave form out in shape or amplitude. (you can see this easily if you had a A/B channel oscilloscope; channel A connected to the amp input, channel B to amp output) The only difference you should see between channel A & B are signal amplitude values. If the signal shape varies considerably in channel B, you have a problem with clipping.

    The amp tries to put out the appropriate power, but runs out of voltage from the supply rails and we get a flat spot at the upper and lower peaks of the wave form. In an extreme case, "severe clipping", there is so much additional energy buildup (heat) into the voice coil(s), but the cone does not move (motivate) enough to cool the voice coil and former adequately. Hense, the voice coil over heats and either seizes in the gap or burns the voice coil windings.
    RESULT: OPEN CIRCUIT and a blown speaker!

    What happens to the speakers when they are underpowered? Under normal listening conditions... NOTHING! There is adequate signal voltage from the amplifier to motivate the speaker. This moves the speaker cone and draws/expells air to cool the voice coil adequately. No problems here... just modest output from the speaker.

    LOW POWER ON SPEAKERS?

    When we use a small amp to drive a high powered speaker, the speaker can take all the "clean power" the amp can deliver and more. But it's when we push the amplifer into high distortion ("clipping") mode, the speaker cannot move (motivate) in and out adequately to cool the voice coil. Eventually, this will even fry a very expensive speaker in this manner.

    CAUSES of "CLIPPING"

    The amp will try to meet the power demand placed upon it, but it cannot exceed its design capabilities. This in turn, produces the deadly "square wave" output to the speaker. The speaker sees this severely clipped signal as something similar to DC current. Speakers cannot deal well with DC inputs. The cone goes in or out and stays there. No motivation to cool the voilce coil and sooner or later, the speaker will fail.

    PREVENTING CLIPPING

    1. Use amps that closely match or modestly exceed the power rating of the speaker. A 350 watt speaker speaker cabinet will love getting 400 watts of "clean power" vs a 350 watt speaker cab. getting 150 watts of badly clipped (distorted) power.

    2. Know what distortion sounds like and prevent it by proper amp setup procedures.
    (
    gain matching, limited bass boost usage)

    NOTE: Perhaps due to the influence of "Hip Hop Beats" and
    "Drop 'Da Bomb' Rap" todays younger Bass Players have a habit of
    Boosting subsonicbass frequencies and thus waste most of
    their amp's headroom while stressing mechanical speaker parts
    beyond their design limits.


    Know your speaker rating and the cabinets tuned frequency limits and set Low EQ. accordingly.

    3. If you are not sure your system is clipping, just listen...

    a. Are the highs and mids clear and natural sounding or harsh, shrill? Speaker farting on the Lows? You are clipping the amp if you hear the latter!

    b. Does the bass sound full, tight, have a definite thump and smooth transitions from one note to another? If not, good chance the amp is clipping or your enclosure is not designed for the low frequencies that you are demanding.
     
  8. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    There's a lot of info about clipping on TB so I'll keep my answer brief and let you search around for more.

    When an amp clips, it's working too hard to produce a normal shaped sine wave. It chops (clips) the top part of the wave off. The danger is that an amp that's clipping puts out more power than it is rated for. At full 100% clip, it puts out it's rated power x2. If you're not careful you can unwittingly blow speakers and fry voice coils.

    There's a whole lot more to it but that will get you started.

    EDIT:- Oops! Finger Blister must have beat me to it by a second or 2.
     
  9.  
  10. Thanks a lot. What's a dome light?
     
  11. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    I'd better address this before someone jumps on and spreads the myth the clipping is DC. It's in no way DC. But if the clipping is bad enough, the amp can lose the ability to control the movement of the speaker. It can move wildly, or not at all.

    A Dome light is basically just a light bulb that sits in front of the tweeter. If the teeeter is getting too much signal, the light bulb lights up. It does 2 things. Firstly it gives you a visual indication that your pushing the tweeter too hard. The first time you see it light up it scares the #$%^ out of you because you see an orangy yellow glow and you think your amp is on fire (or was that just me? :))

    Secondly is absorbs the additional energy, thus protecting your tweeter. The tweet continues to produce sound even though the bulb it lit up, but it is usually a compressed and distorted sound, yet another warning to turn down.
     
  12. Okay, thanks for the explanation. When I had the tweeter on, it sounded like something was wrong.. As if there was some loose metallic part vibrating somewhere as I played. But I figure I can't have killed the tweeter already, I had just turned it on when it sounded like that. I've only played with this rig once so I'll probably learn more tomorrow :)
     
  13. MrBonex

    MrBonex

    Jan 2, 2004
    New Hampshire
    It's funny how hard it is to describe a simple thing like clipping. Here's my take:

    Look at Finger Blister's "B" picture.

    Notice the flat tops and bottoms of the sine wave? When the speaker tries to reproduce that, it literally has to hold the cone (and voice coil) out (or in) at that position. That "holding" is the part that generates heat.

    Because the coil and cone are not moving, they are not cooling the coil with the natural "round" back and forth movement of a non-clipped wave.

    More: because most solid state amps are more capable of producing square waves (what a clipped signal resembles), clipping will be more damaging than with a tube amp. Voila! Toasted driver!
     
  14. tkarter

    tkarter

    Jan 1, 2003
    kansas
    I run a Laney RBH 800 head. It will always light when you get things cranked up a bit but it is not yet clipping. You get a steady light it will clip. Adjusting mids usually changes the amount of time the light will stay on. I love the Laney head and haven't hurt it in over a year of playing it. I use an ampeg svt 210 he and svt115 en on this head.

    tk
     
  15. tkarter

    tkarter

    Jan 1, 2003
    kansas
    Reread the intial post. If you are running one 8 ohm you need to play with the settings on this head. It likes the 4 ohm load much better.

    If it is clipping the tweeter I suggest turning the enhance down and that should stop that.

    The light usally means it is preventing the clip. However I have had mine actually clip even with the limiter on if I pushed it upward after the light started coming on. This only with the single cab on it. Which in my case was a single 2x10 at 8 ohms. I tend not to have to push the amp at all with a 4 ohm load on it and the cabs I have. It gets plenty loud enough to be heard over six loud guitars trying to beat it :)


    tk
     

  16. great, thanks!
     
  17. seanm

    seanm I'd kill for a Nobel Peace Prize! Supporting Member

    Feb 19, 2004
    Ottawa, Canada
    Do you have the bass and/or mids boosted? Boosting the lows takes a lot of power, try a flat eq and see if the led stops flashing.
     
  18. yes, I have. especially low mid.. I'll try the flat eq next time. thanks for the tip. I'm curious if I'll like the sound though :)