Transcript
LIVE SOUND MIXING 9: AMPLIFIERS
especially if there is no crossover or equalizer to bump up the level. So this switch will let the amplifier run up to full power on say 0.775 volts. Switching this in will not give you more power out of the amplifier! It just means the amplifier will get to its maximum power with less input. Input Link switch. If you want the same signal to go to both channels of the amplifier, normally you’d have to make up a Y lead. This switch saves you the trouble. Switch it in and whatever goes into Channel A goes into Channel B too. Bridged Mono mode switch. Many amplifiers can be bridged to run as a mono unit delivering twice the power, into a single load. Usually one of the inputs is designated as the one to use in bridged mode, and the + speaker cable is connected to the Red binding post of one channel, and the - cable is connected to the Red binding post of the other channel. With Speakons there is often a separate bridged mono output Speakon connector. Always power down the amplifier before changing this switch and connecting the cables. Check the wiring very carefully before powering up the amplifier. Limiter Switch (sometimes with a Threshold control as well). This switch stops the amplifier from clipping and trying to put out more power than it can. However it won’t stop the amplifier putting out a distorted signal if you feed a distorted signal into it.
What is Clipping?
Always power down the system before switching to bridged mono mode
Once an amplifier reaches its maximum power output, you can’t get any more out of it. If you keep pushing more Input into it, the amplifier will give up trying to accurately reproduce the signal, and start chopping off, or squaring off, the top and bottom of the signal. This is called clipping. You can see it on an oscilloscope, and you can hear it as distortion. You want to avoid this as much as possible.
CLEAN SIGNAL SHOWN ON AN OSCILLOSCOPE
A SIGNAL THAT IS CLIPPING
Professional speakers will usually handle momentary peaks far in excess of the their rated power, provided that the peak is a clean signal and not distorted. Once the signal to the speaker becomes distorted, though, failure is not far away.
Last On, First Off This is how you power up the system. 1. Make sure the amplifiers are turned down and switched off 2. Switch on the AC power to the Mixing consoles and the effects racks 3. Wait 10 seconds then switch on the AC power to the amplifier racks 4. Turn on the amplifiers one after the other, with 5 seconds in between each one 5. Check that all the fans are turning, then turn the amplifiers up one after the other; just a little at first, then if everything sounds OK, turn them up all the way In live sound systems it is usual for the amplifiers to be turned up to maximum, as the level going to each one is controlled firstly by the mixing console, and secondly by the crossover. By powering up this way you avoid putting any clicks and thumps through the system, and you give each amplifier a chance to charge up its power supply before you turn on the next one. This will help to avoid blowing any power fuses or circuit breakers in the venue, which can happen when you turn them all on at once. Page 88
Live Sound Mixing 4th Edition ©2005 D.R.Fry
