This is topic Amplifiers AC needs in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.
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Posted by Antonio Marcheselli (Member # 373) on 06-02-2002, 05:53 AM:
Just a curiosity: is there any relationship between the power that an amplifier can handle and the power absorbed from the socket? Is this varying or is it always the same?How much power should absorb a, say, 200W amplifier?
Bye
Antonio
Posted by John Anastasio (Member # 607) on 06-02-2002, 08:38 AM:
Absolutely. You can't get something for nothing. If you need to produce 200 watts continuously, you need to draw more than 200 watts of power from the mains, since no electrical device is 100% efficient. Don't confuse continuous power with Peak power, however. It is possible to draw less than 200 watts if the amplifier is only rated at 200w PEAK, since the peaks are only drawn from the power supply capacitors over short periods of time.
Posted by Antonio Marcheselli (Member # 373) on 06-02-2002, 02:08 PM:
John,But Speakers don't work at 220V... 
Once I monitored my sound rack in my theater (7 Crown MT) and power absorbed didn't go above 500W. And was always 500W, despite the sound in the theater.
So what?
Antonio
Posted by John Anastasio (Member # 607) on 06-02-2002, 03:23 PM:
Think of your amplifier's power supply as a big bucket full of power. It draws a constant amount of power from the mains to keep itself full. The amplifier draws from that power supply, not from the mains. Normally, the power drawn away from that "reservoir" is not enough to deplete it. Sometimes, however, it does run out...that's what happens when the amplifier is clipping. The power supply simply can't supply enough power to comlete the waveform. The power supply draws a constant amount, and the amplifier uses what it needs.
Posted by Dave Macaulay (Member # 813) on 06-02-2002, 04:04 PM:
The laws of physics are hard to break.
Power in = power out + overhead. The overhead is simply the idle load of the amp plus inefficiency.
If you had 500W whether loud or not, something is wrong. You are either running very low audio power and pretty much reading the idle power, or something is oscillating and you're continuously feeding inaudible power to the speakers. I've seen places where the booth lights flicker when a loud film section goes through, obviously the amps are drawing some serious power.
Our rule on rack power distribution is to allow 80% of the circuit rating at amplifier full rated current. This means no more than 12A on a 15A circuit, using the max amps as shown on the amp labels. This will guarantee that the circuit will not trip with any audio load possible, and shouldn't trip even with inrush current when switching on. The resulting stable voltage will probably increase the life of the amps as well.
In Ontario at least, new installations are subject to government supervised electrical inspection. A rack that isn't wired this way will fail the inspection and that will result in the building not getting an occupancy permit. It's easier to specify correct wiring than to try and fix it a week before opening, and explain it to the customer. Retrofits are rarely inspected but if we can get the circuits we will.
I've seen installations that ignore common sense with amp rack AC power circuits, let alone use a conservative system like we do. 6 X 900W amps and the processor on one 15A circuit? Nuts.
Posted by Antonio Marcheselli (Member # 373) on 06-02-2002, 07:06 PM:
John and Dave.I'll double check my amperometer...
So, a 2000W amplifier will absorb 2000W from the outlet (and more).
Bye
Antonio
Posted by Gordon McLeod (Member # 33) on 06-02-2002, 07:27 PM:
Since Energy can not be created nor destroyed that is basically correct 2000 watt amp will draw 2000 watts plus the efficiency of itself when running full out
Posted by Manny Knowles (Member # 1171) on 06-02-2002, 08:26 PM:
So - for somebody who's just reading this and wondering - why is it important to know this? In terms of the actual cinema equipment scenario, what problem does this help us solve? I'm just being curious.
Posted by Antonio Marcheselli (Member # 373) on 06-03-2002, 03:17 AM:
Manny,It is for my Home Theater... 
Bye
Antonio
Posted by Gordon McLeod (Member # 33) on 06-03-2002, 05:47 AM:
Actually as Dave mentioned inadequaete power to a rack is a bifold problem
Legaly in most of the world the nameplate ratesings must not exceed 80% of the capacity of what feeds it
Also with heavy inrush on powerup or a loud bass passage it is possible to create a brown out type condition on the circuit feeding the rack and that can damage equipment