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  1. #1
    New Member
    Join Date
    Sep 2009
    Brownsville Tx

    Power amp and setup doubt

    Hello, I have two doubts about power amps. First of all i have two 4 ohm Dual 15 600 watt speakers which i power with an amp which is Stereo 500 watts on 8 ohms, 700 watts on 4 ohms and Mono 600 watts on 8 ohms, 850 watts on 4 ohms.

    First of all, I am planning on getting two 8 ohm 550 watt subwoofers and am not sure what power amp to get in order to power up both subwoofers. Since the subwoofers are 50 watts over the amp i mentioned earlier, would there be any problem if i hooked up the subwoofers to another of the same amp? I was also thinking of using one of the dual 15's with the subwoofer on the same amp since it has a crossover setting but i am not sure of it since one channel will be running on 4 ohms while the other on 8. Can someone please clarify if it is ok or if i will have to get another amp with more wattage.

    My second doubt is on monitors. I have two 8 ohm 550 watt monitors which right now are powered by my powered mixer and I am thinking of buying two more and another power amp. I am planing on daisy chaining two monitors on each channel of the amp. My question iso the watts from the monitors add up? Do the two 8 ohm monitors add up to 4 ohms on one channel? If so, how many watts of power is needed for each channel? What is a good solution?

    I would really appreciate an answer since i have no other reference on this. Thank you.

  2. #2
    A couple of questions, to get you a better answer.

    What is the make/model of said amp?
    What are the makes/models of said speakers?

    Hypothetical situations cannot lead to correct answers.

    Most consumer-level amps will be safe to run one top off each side (at 4 Ohms each), but you will not be able to run the tops and any other speakers from that amp.

    If you want to run the subs off that amp, it should be fine, the 50 watts is not going to reduce your available output by that much. (less than 1dB).

    You could run your tops off that amp, just be sure there is no chance of overload/distortion, since the amp is rated higher than the speakers, and your High Freq horns may be subject to being blown by any harsh signal.

    For the monitors, you are right, two 8 Ohm boxes = a 4 Ohm load on the amp, the power could go as high as 1100 watts (if the speakers really are rated at 550 each). How much power you will NEED is going to vary based on how much overall volume you want/need. I can tell you that feeding 350 (give or take) watts into a box rated as 500 watts RMS, will give plenty of power for monitors.. It also depends on the kind of music and the stage volume of the instruments... I've done sound at gigs with 500 watts on nice monitors and people couldn't hear themselves over guitar rigs that were cranked and a drummer who was beating the living daylights out of his set...

  3. #3


    I am of the school of thought that you want twice as much power than you speakers are rated. This gives you whats know as head room and prevents the amps from clipping out as easy. Clipping causes power spikes that can damage the voice coil of speakers. under powering speakers has similar effect it can destroy your speakes. wattage isnt really a good indicator of output of speakers you need to know what there SPL rating is that gives you acurate acustic output such as if you crowd is small say 100 people and your an acustic act you would want to have at least one set of monitors at 100db or better 500 watt speakers would prob be at least 100db and be fine in this instance. However if your are a in a rock band your gonna need twice in power that at least, and be at 130db for vocals to cut through stage volume. I would get amps with more power. also if impedance is a concern of yours if your amp is on the lower end of the spectrum in quality you can run your speakers in Serrial/Parallel in your case your speakers both hooked to amp would maintain there 8 ohm load, and the SPL rating of your speakers. alittle on the advanced side but its not that tough. Good luck

  4. #4
    New Member
    Join Date
    Sep 2009
    Brownsville Tx
    the speakers i have are B-52 dual 15's and the power amps are qsc gx5s.

    Thanx thanx madmax and bassred for your help. It is starting to make sense to me. And i have heard of the method of having double wattage at the amp for the cabinet but i also heard that having a couple hundred watts is enough. But its better to be safe than sorry.

    and i didnt know impedance affected that much so thanx madmax.

  5. #5

    Bridging amplifiers means using more than one power source to drive a load, which is typically a loudspeaker in audio applications. Bridged amplifiers are a setup for producing greater output voltage than one amplifier can generate. This results in a cleaner sound at lower volume levels and greater power when you want to crank it up. To do this, the output of the second amplifier must be inverted and the loudspeaker is hooked up to the two outputs of the amplifiers. It is strongly recommended that only "bridgeable" amplifiers be used together. If in doubt, consult the owner's manual and verify the amps are capable of being bridged. If not, do not attempt to bridge them together. This article explains how to connect a pair of bridgeable power amps."Power amp and setup doubt audio engineering"

    audio engineering

  6. #6
    MadMax, Sir, please reconsider your answer. Headroom is having powered all your equipment to the spl's you want/need. and each componant having just a little more to give to get you higher, and is the product of proper gain structure throughout a system. Your school of thought is incorrect.

    Just having double the power available in an amp than needed to drive a speaker is not proper gain structure, and is rather inefficient. That is what leads to clipping in the voice coils when driven too hard by an oversized amp. That leads to frying the coils. Underdriving a speaker cannot damage it, since you are turning the volume down, not up.

    Please remember that current * voltage = watts, and that watts is a measurement of heat radiated from the load, whether that load is an amp or a speaker. Impeadence is similarly linked with the math. You can't change one without changing the other three.

    You are very correct, however, about spl's being a better indicator of the needs of a system, and basing a design on this rather than putting everything together by using power ratings, which is guesswork at best, is a prefered method. I always figure out spl's and acoustics first and work backwards from there.
    'I think my intimate relationship with electronics started as a child when I was playing with a screwdriver and a wall plug, Doc, and...'

  7. #7


    ok I see Pennies,
    Good looking out. I musta worded my comment wrong I didnt mean double power equals head room. I am under the understanding that for speaker you want to get bigger amps to prevent the amp over heating as a result of haveing to turn it up to give the speakers the power they require. I guess I am wrong its what I have read. I understand the deal with head room I thought it tied together with the additional power the amp had when driven less than full. when you got large spikes in input. the amp would not clip maybe I missunderstood. No biggy thanks for the assist.

  8. #8
    The main reason you want bigger amps than you need is to avoid clipping. Most of the power in speech and music is in the lower frequencies, but when an amp clips, it generates a lot of high-power, high-frequency garbage.

    That's a tweeter blower. With few exceptions, speaker systems are designed with tweeters that can only handle a tiny fraction of the overall rated power. In the home stereo world, a 100-watt speaker system may have a 5-watt tweeter. (Don't know what the ratio is in the live-band biz.) That actually works, since music has very little high-frequency energy, but once an amp starts clipping, all bets are off.

    A comparatively small amp, overdriven, can produce enough high-frequency energy to blow a tweeter. Thus the argument for big amps: NOT to make more noise, but to make the same amount of noise with no clipping.

    With a big amp, you have to be disciplined. If you use it to stay out of clipping, good. If you use it to push harder and make more noise, you better have a healthy bank account, cuz you're going to be buying new speakers, and not just the tweeters either.

  9. #9
    I wish you guys would get clipping straight. Clipping an input to an amp will only cause distortion in the output (each succesive stage in the amp, or any signal chain for that matter, gets clipped too, tho they don't cause it , they just amplify it or pass it through), and if the speakers can't handle it, they are not strong enough. True, high end is more fragile, and more care to be taken there.

    Clipping at any stage causes all the next stages to amplify/feed through that clipping; you can't just turn down the next stage and hope it will go away, and because speakers are a stage, the last, having smaller speakers makes no sense. This is why it is so important to have a well thought out gain structure throughout any system. Bigger amps is not an answer, nor is bigger speakers.

    Knowing the equipment, the limits, studying the specs IN DEPTH, understanding how every piece of gear interacts with each other, are key to getting a good system together and working right.

    Understanding just what clipping is, is a prerequisite to sound system design. It happens when the peaks of a waveform are cut off by a circuit when it is overdriven; ie biased against the input so to speak (I'm not refering to transister biasing, though the model would work). Clipping an amplifier at its input or at any stage produces that clipped waveform at the speaker. That produces, first, distortion you can hear. It will usually sound less loud than if it is working right. Many will try to compensate by turning stuff up louder, but that only makes it worse, and heats up the voice coils to saturation; the next thing that happens is speaker damage. Getting bigger speakers will not help here, because the problem is in the way the amps are being used. Getting bigger amps is not a proper solution either, because it will not solve gain structure problems.

    If you are clipping an amp, find out why. 9 out of 10 times it is because the inputs ore overdriven, not because of a lack of power in the amp itself.

    I'm not going into all the Fourier stuff about distortion here, But generally Karl is right about high end distortion, and the damage it can cause.

    As far as spikes are concerned, most amps have overcurrent protection for it.

    One more thing; speakers don't "require" power, MadMax, they just dissapate it in the form of heat, given off when the voice coils are forced to move due to the current*voltage. The byproduct is sound.

    Later, man.
    Last edited by penniesfromheaven; 21st Sep 2009 at 03:40.
    'I think my intimate relationship with electronics started as a child when I was playing with a screwdriver and a wall plug, Doc, and...'

  10. #10
    Good point, clipping can occur anywhere in the chain, and it always has the same effect - it wrecks the sound and potentially the speakers.

    Been a while since I've put together a system, but my standard practice has been to hook up a signal generator and a scope and find the limits of every piece of equipment in the chain, not just at one level control setting but over a range of settings. A major pain in the asterisk, but it's also a pain to spend a lot of money and not get the performance you're paying for.

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