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Thread: Ampflier Power

  1. #1

    Ampflier Power

    Hi all,

    I've got a question about ampflier power because I don't think I'm quite understanding the meanings between wattage, gain and voltage.

    Just say I have an amp that provides 1200 Watts per channel at 4 Ohms, and I have a 4 Ohm load on each side. If I have 0dB levels from my desk into the amp, and the amps channel attenuators fully clockwise, does this mean 1200 Watts is going to the speakers?

    Alternatively, if the attenuator was at, say, half way, does this mean 600 Watts is going to the speakers?

    Any guidance would be helpful, I'm quite confused by it.

  2. #2
    I think maybe if I put that in a scenario:

    I have four speakers at 350 Watt RMS, 800 Watt Peak each. I will be running two off each side of the amplifier, making a load of 4 Ohms, 700 Watt RMS each side.

    The amplifier is 1200 Watt RMS per channel at 4 Ohms. So technically the combined Peak of the speakers is higher than the amps output, but that's still enough to drive the speakers at more than RMS power, hence potential damage.

    Basically the question is, if I had the amp attenuators fully open, would this mean the speakers are being given 600 Watts each (bad)? In which case should I only turn the amp up to about half that to give approx 300 Watts to each speaker (good)?

  3. #3
    OK. to start, here are some layman definitions for the terms you listed.

    Watts-this is power, or work done. it is the amount of energy used, and it is measured by the amount of heat given off while the work is being done. (watts can be converted to btu's and to degrees Farenhiet). Watts = heat.

    Gain-this is the amount of amplification of a signal as measured from the input to the output of a stage in the signal chain. It is measured in decibels (most common), and also in volts (power potential) or current (amperes, or rate of work done). Measured in db's or volts.

    voltage-you don't really need to worry about this as far as power amplifiers go, but this is the potential to do the work. Think of a dam with alot of water behind it. It can't do anything untill you turn something on to let the water through (current, or amperes).

    "If I have 0dB levels from my desk into the amp, and the amps channel attenuators fully clockwise, does this mean 1200 Watts is going to the speakers"?
    No. 0db from your desk should mean zero watts of work done in the speaker (except for noise generated throughout the signal chain). The watts will be directly proportional to the amount of input signal times the total gain of the amplifier. (note: I'm over-simplifying this because I don't know what reference you used to get the 0db number).

    Many companies list RMS wattage and this is generally close to what is called "continuous" power. It is like the average of the stronger parts of the program material. Peak power, on the other hand, is more like the loud transients in a song, like cymbal crashes, snare drum, you get the idea?

    It is my experience that speaker damage is mainly caused by this continuous power overdriving the speaker to the point where the speaker voice coil glue melts. As a rule of thumb use speakers that are rated at least 20% higher wattage-wise than the amplifier, for both continuous and peak power.

    "Basically the question is, if I had the amp attenuators fully open, would this mean the speakers are being given 600 Watts each (bad)? In which case should I only turn the amp up to about half that to give approx 300 Watts to each speaker (good)?" Amplifier attenuators are logorithmic, meaning turning them down halfway does't mean the power is cut in half. They are usually graduated in db's. A 3db drop in gain means the power is cut in half. Another 3db drop means the power is cut in half again. (If you are measuring voltage instead of watts 6db shows a halving or doubling of the voltage).

    "I have four speakers at 350 Watt RMS, 800 Watt Peak each. I will be running two off each side of the amplifier, making a load of 4 Ohms, 700 Watt RMS each side." I assume you are connecting 2 speakers at 8 ohms each in parallel per channel, in which case you are correct. However, given that the amplifier can generate 1200 watts rms, or roughly continuous power I would start with cutting the amps down to about 1/4, then bring them up and listen for any kind of distortion, especially at the low and midranges, and let your ears be your guide.

    You definately have more power available from the amps than the speakers can handle, so be careful, huh?
    'I think my intimate relationship with electronics started as a child when I was playing with a screwdriver and a wall plug, Doc, and...'

  4. #4
    Lots of helpful advice, thank you. But that raises some other issues that I'd appreciate any help on:

    As a rule of thumb use speakers that are rated at least 20% higher wattage-wise than the amplifier, for both continuous and peak power I've been advised in the past that its good idea to use an amp one and a half times the speakers so that you need never turn the amp up full, therefore having headroom on the amp and prolonging its life (advice from a sound tech btw, not a salesman trying to sell me a costlier amp). Would anyone agree with that?

    Also, my reference for 0dB is +4dBu (according to the desk). Input sensitivity on the amp is 0.775 volts, and it also has "Voltage Gain: 70x" if that allows you to elaborate more on that.

    Thanks

  5. #5
    Well, having headroom in the amp of 50% IS a good idea, but you should be basing that on the room requirements, and then get speakers that can handle 20% more than that. Our amps at Church are at about 75%, and I find they perform very well with speaker arrays rated at 26% over the amp's max. So, use amps that are rated 50% above the rooms requirements, and then get speakers rated 20% or more above that and you should never need to worry about speaker damage, assuming you always will have control of the program content. The worst case for this is distortion caused by overdriving the amp inputs, in which case turn down your LR mix. There is alot of acoustic mechanics involved here in order to properly fill a room with a desired volume level

    The 70x number tells me you have 100mm faders with a standard op-amp circuit driving the the channels output to the LR mix. It means that with the fader all the way up the output is 70 times greater than the input. The formula for the db gain is 20log output voltage/input voltage. In watts it is 10log output power (watts)/input power.

    dbu is a reference to telephone stuff. It goes back to T Edison and the telegraph, and means 0dbu = 1 volt peak to peak across a 600 ohm load. Notice that low impedance audio equipment is 600 ohms; not a coincidence. Balanced circuits were invented by Bell Labs.

    The sensitivity is the lowest input level your desk will pass without going below the rated noise floor (a number picked by the manufacturer, since there is no standard).
    'I think my intimate relationship with electronics started as a child when I was playing with a screwdriver and a wall plug, Doc, and...'

  6. #6
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    You Are Correct! - Don't Listen!

    Don't listen to the advice about an amp with less power than your speakers. That is how you end up with distortion when you need to push them. One and a half times the power is fine. That gives you plenty of overhead for your system.

    But don't use the amp to control your power/volume. Go ahead and turn up the amp to full blast and use your board to adjust the volume. You have to be able to tell if you are pushing your speakers and amps too hard but your sound is much better this way.

    Check your system and see how hot it is running. You'll be able to tell if you are pushing the speakers too much. My suggestion is to buy high wattage speakers that are more than you will ever need and the amps to push them. Also remember that it takes more power to push the low end than it does the high end. So if you are outdoors you are going to work your speakers and amps harder in order to get enough lows. Indoor is a lot easier on your speakers and amps.

  7. #7
    Pam, hold on a sec.

    Think about it. Power is voltage times current. But it is current that drives the speaker, and the amp produces the current. With the amp up full blast you risk pushing too much current through the speaker if the amp is capable of producing more current than the speaker can handle. (it is the voltage that does the pushing).

    Please remember that POWER, in electronics, is a measurement of HEAT. Too much heat in a speaker causes distortion and damage.

    I know my answers are long winded and technical, but the rules of nature are absolute. My answers will stand up to severe scrutiny.
    'I think my intimate relationship with electronics started as a child when I was playing with a screwdriver and a wall plug, Doc, and...'

  8. #8
    Quote
    Quote: penniesfromheaven
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    Please remember that POWER, in electronics, is a measurement of HEAT. Too much heat in a speaker causes distortion and damage.
    this is right. you should not use an amp that is rated for more power then the recommended power of the speaker!

    Quote
    Quote: penniesfromheaven
    View Post
    I know my answers are long winded and technical, but the rules of nature are absolute. My answers will stand up to severe scrutiny.
    Long winded for sure! But Right!!!!!
    Manoni Productions
    Pass me another beer...You are still ugly!

  9. #9
    Thank you, kawsakimx6, very much, and I hope you had a pleasent and blessed Easter
    '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
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    Hellllp I`m On Fire

    Posted - 06/04/2007 : 2:59:05 PM Show Profile Email Poster Edit Topic Reply with Quote
    Hello,
    my name is Chris from NJ, I was wondering If any Bass players/Bass Techs out there could help me figure out a technical problem I had yesterday. I performed yesterday at an outdoor function. A super long power line running from a building was extended to the location where we performed. I have a Sunn 1200 S Bass Amp connected to a David Eden 4X10 Bass Cabinet. I have used this Rig for many years at many functions, and never with any Issue. Yesterday after we had played about an hour long set, we took a break. As I came back to begin our second set,before plugging in,I heard some kind of pop noise and then some kind of a hum out of my speaker, and then my bass cabinet started to smoke. I turned off the Bass Amp and disconnected the wire from the Bass Amp to the Cabinet.When I tried connecting it again,I heard the hum again and the Cabinet smoked again. After a few minutes I tried powering on the Bass Amp alone and the cooling fan turned for a second then stopped, so I powered off the Amp. The rest of the band finished up the final hour of the gig without me.The next day, I tried powering up the Sunn Amp again and within 2 seconds It began to smoke this time without being connected to the 4x10 Cabinet... Does anyone out there recognize these electronic symptoms?? Can anyone please help me to figure out this mess?.... my sincere appreciation in advance, Chris "Martine" Marashlian/New Jersey/USA
    http://www.catsrevenge.com

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