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does a pre amp clip?

Discussion in 'Amps and Cabs [BG]' started by a e i o u, Dec 22, 2003.


  1. In an amp head, does the preamp cause for clipping, or is it the poweramp? What if you were too add a new preamp to a head, would that change clipping in anyway, and would you be able to use your amp head's pre gain or EQ's or is that all by passed?:cool:
     
  2. inazone

    inazone

    Apr 20, 2003
    Colorado
    You can clip your preamp, and/or your power amp. You can even clip your basses pu and onboard pre.
     
  3. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Technically you're not "clipping" until the signal is leaving the amp with the top of the sine waves chopped off (ie the output stage of the power amp). However you can cause that to happen a lot of different ways, including at the pre-amp stage of the signal chain. Try to think of it as a balancing act between the preamp input and poweramp output.

    If you're asking wether changing the pre-amp will change the amount you can get before clipping starts, the answer is no. At the onset of clipping, a power amp starts to put out more power than what it is rated for. If if starts clipping at 400w with pre-amp 1, it will start clipping at 400w with preamp 2 as well. So it stands to reason the best way to get more noise before clipping is the get a power amp with more watts.

    If you've by-passed the head's existing preamp, I'd say that you can't use it for anything. Even if it still works, you can get yourself into trouble by using multiple pre-amps and overcook the signal going into the power amp. Ideally you want it going in there at between 1 and 2 volts, depending on the amp. Multilpe outboard preamps run the risk of feeding the amp much more than that, making it easier to clip and harder to control your levels. It can be done but you'll have to be hella-careful.
     
  4. If the preamp is sending a clipped signal to the power amp, then the output of the power amp will be a clipped signal, just the cause would not be the power amp.

    I am not sure that I totally follow what you are trying to do, but as Petebass said, putting a hotter input into your poweramp will not give you a louder signal. The poweramp will still clip at the same output level (though the poweramp volume will actually be lower).

    Geoff
     
  5. I was hoping to plug an RBI into the power amp in on my b2r head, which is supposed to bypass the b2r's internal pre amp. I would be happy with this as long as I could get the same volumes I've had with the b2r head alone, but was hoping that the RBI would give me more pre gain before it clipping started so I could be louder.

    but, by putting an RBI into the power amp in on the b2r, would I be able to get at least the same volume I had when the head was alone?
     
  6. yeah you cant get any louder with a new preamp but you can still achieve the same volume as before.
     
  7. If the preamp is sending a clipped signal to the power amp, then the output of the power amp will be a clipped signal, just the cause would not be the power amp.

    Like Pete said, Technically you're not "clipping" until the signal is leaving the amp with the top of the sine waves chopped off.
    And he is talking about the point where the maximum output voltage is reached.
    When this maximum is reached the output voltage will stay for the duration of the clip constant. And this can cause serious damage to your speakers.
    For a 500W amp this voltage is about 34V RMS or 110V pp (!!)
    You don't want to put this 110 V for a very long period on a 2ohm resistor.....

    When a preamp clips it will not have to result in a clip of the power amp. (For instance when the master volume is low)

    Anyway...technically spoken....
     
  8. I'm sorry, but I disagree.

    A 10 millisecond period of constant voltage does not damage a speaker. The waveform of a signal has no relation to speaker damage at all. In another thread you stated that a clipped amp output is a DC voltage. This is also not true. A 50 Hz square wave (an infinitely overdriven sine) is hardly DC. It's a very harmonics-rich signal with absolutely no DC content.

    - That what burns a speaker's voice coil is sheer power, which turns to heat, which melts the coil wire or fixing.
    - That what rips a speaker cone is overexcursion (past Xdamage) or paper fatigue.
     
  9. Richard Lindsey

    Richard Lindsey

    Mar 25, 2000
    Metro NYC
    Thank you.:cool:
     
  10. notanaggie

    notanaggie Guest

    Sep 30, 2003
    Joris and Richard Lindsey:

    There is SOME justification for the "clipping = DC" myth. I agree it is not so, as a rule.

    However, some less well-behaved SS amplifiers will drift off into a net DC output when clipped hard. Old ones often did that, especially ones without a full differential front end.

    They can also "block" where a capacitor internal to the amplifier gets a charge on it and takes a while to discharge, not passing signal during that time. This can have the effect of "stretching" the lows and giving an out-of-audio-range low frequency output pulse that can cause a speaker to exceed its excursion.

    Also, when a lot of the coil is out of the gap, (easy on a long throw woofer) that part of the coil is not cooled by the magnet structure, and can heat rapidly. It depends on moving back into the gap to cool it, but with a long DC pulse (OK, contradiction in terms) it cannot be cooled.

    So there is a way that clipping can give DC and burn speakers.

    But most modern amplifiers are relatively safe and free from that problem.

    All tube amplifiers are of course DC blocked by the transformer.


    I will join the chorus: Clipping kills speakers usually by simple overpowering. The square wave has an RMS value equal to its peak, instead of only 70% of peak. So power goes up in clipping.
     
  11. OK, good arguments. I agree that ofcourse only power will turn into heat in a speaker and that's the only thing what a speaker will burn.
    So let us simply state that overpowering will damage speakers. But I think we know that all. The question is why the danger is bigger when the power amp clips.

    And notannagie made it very clear: The square wave has an RMS value equal to its peak
    A square wave itself will not damage the speaker. So Joris you are right.
     
  12. Maarten:

    A clipping amp is dangerous mainly because it puts out more power than it's rated for, as was said by notanaggie. Also, it may put out low frequencies due to instability, which drive speaker cones into overexcursion.

    You are correct about the drifting. I prefer to keep it out of clipping debates, because it would get very tiresome. Some amps suffer from unstability when clipped. My small amp head has this. When you severely clip it, the speaker cones start to shimmy around. I increased the amp's stabillity by increasing the value of the decoupling capacitors. This "shimmying" might be regarded as DC. However, my measurements, back then, showed a very-low-frequency amplitude of a few volts, against 40 volts peak of the normal signal. So near-DC content is still very minimal. And you would have to constantly clip the amp, in such a way that it won't have a shot at correcting its offset.

    I'm not sure about the second part of your post, the coil is not in contact with the magnet structure and is solely cooled by convection, IMO. And besides, once the magnet assembly heats up, it would no longer have that cooling capacity anyway.

    Ahhh, nerd talk! Great!

    All in my humble opinion.
     
  13. notanaggie

    notanaggie Guest

    Sep 30, 2003
    The coil is mostly cooled by radiation across the small gap in the magnet structure. There isn't enough airflow to cool it by convection, the same air is moving back and forth.

    Think about it, even 100 watts dissipation is a lot, a simple resistor would be large and get very hot. The coil is small, thin, and one side is often covered by plastic (kapton hi temp stuff).

    That is why the magnet gets hot. But even when hot, it may be 100 deg cooler than the coil (deg F at least). So to the coil it still "looks" cool and can accept heat flow.

    Sounds odd, but its true.