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TJ Cornish

Understanding Power Consumption, Part 2

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This is the second half of the article, split due to post length limits. The first part is here: Understanding Power Consumption, Part 1

Case study: Allen & Heath GLD80 mixing console

A&H’s datasheet lists the maximum power consumption of the GLD80 surface at 95w, and the AR-2412 stage box at 70w. Though there will be slight differences in power consumption depending on if the motorized faders are moving or if the analog outputs are driving a large signal, the difference between minimum and maximum power consumption will be a few watts at most.

Case study: Crown ITech 12000HD
The datasheet for the Crown ITech 12000HD lists the following:
Circumstance Current Consumption (A) Calculated Power Consumption (W) Claimed output (W) both channels
Awake idle 2 240 N/A
1/8th power 8 ohms per channel 8.3 996 4200
1/8 power 4 ohms per channel 14.6 1752 8000
1/3 power 8 ohms per channel 18.1 2172 4200+*
1/3 power 4 ohms per channel 35.1 4212 8000+*
* These values are not given in the datasheet, but are assumed to be at least as great as the 1/8th power values.

From the table you can see that this amplifier has a modest idle current of 2A (which is actually higher than many other amps due to its topology), and a large intermittent current requirement, depending on the connected speaker load, and of course how hard the system is driven.

Many amplifier manufacturers use 1/8th power as a real-world metric for estimating power consumption. This is generally specified as the level where the clip lights occasionally flash.

Keeping in mind that we are powering this amplifier from a 20A 120V circuit capable of 2400W of sustained power delivery, it is interesting to see that the amplifier’s claimed power output values are significantly higher than the input power available to the amp – 8000 watts of power output with 2400 watts of input. This is possible because of the intermittent nature of audio signals. Two mechanisms enable this: circuit breakers typically allow more current – possibly several times more than the rating – for brief periods of time, and the amplifier has a capacitor bank that stores energy when demand is low, to meet peak demand.

Death by dB – Powering a Danley TH118 Subwoofer
A Decibel (dB) is a ratio of two quantities. For reference, an increase of 3dB represents a doubling of power consumption. It is generally accepted that people perceive an increase of 10dBSPL to be twice as loud as before.

The potential output of a loudspeaker is typically expressed by the combination of its sensitivity – how much sound it produces with a given input signal, and the maximum input signal level the device can handle without damage.

Important note: Measuring and reporting loudspeaker performance is complicated, and is filled with a lot of “it depends” factors. Understanding this at any depth is outside the scope of this article; and no attempt is being made to be particularly rigorous.

The Danley TH118 subwoofer has a sensitivity rating of 108dB with an input of 2.83 volts. This translates to 2 watts into the nominal 4 ohm impedance of this cabinet. This, in rough terms, means that with a signal input of 1 watt, the loudspeaker will produce 105dBSPL over the operating frequency range. Increasing the input power from 1 watt to 2 watts – a doubling - gives us 3dB more output, or 108dBSPL. This table shows the calculated output of the TH118 for a variety of input power levels.

Input power (Watts) Output dBSPL
1 105
2 108
4 111
8 114
16 117
32 120
64 123
125 126
250 129
500 132
1000 135
2000 138
4000 141
7000 143

It is easy to see that the TH118, as well as all loudspeakers, actually make quite a lot of noise with only a few watts of input power. A 500 watt input signal produces 132dBSPL of output – something that even a very modest amplifier can supply. However, to double the apparent output of the speaker – an increase of 10dBSPL – requires ten times the input power – 5000 watts – something that is difficult for all but the largest amplifiers on the market.

This example is a simplification, but the main point is that it takes 10X the amplifier power to produce 10dB more acoustic output. The corollary is that if you are short on available power, turning your system down 3dB will cut your current requirement roughly in half.

Case Study – Saturated blue comparison, 500W PAR64 vs. LED wash fixture
One of the greatest advancements in efficiency in the last decade is the rise of LED lighting. This improvement is particularly striking when you are trying to produce saturated colors.

A conventional incandescent bulb radiates over a broad range of the spectrum – both visible light, and infrared light, which we feel as heat. When you place a gel filter on an incandescent fixture, most of the output of the bulb is absorbed by the gel, turning all output except the gel’s pass range into heat. The more saturated the gel is, the greater the loss of efficiency. In contrast, LED emitters have a very narrow output band at a particular color, which means that the LED is very efficient at producing that color.

For this exercise we will consider the LEE Filters 071 Tokyo Blue, a deep saturated blue color: http://www.leefilters.com/lighting/c...#071&filter=cf The data sheet lists this particular gel as 0.3% efficient from a Tungsten source. A GE 500PAR64MFL lamp produces 6500 lumens, or 13 lumens per watt. If we put the Tokyo Blue filter on the 500w GE PAR bulb, 99.7% of the light is converted to heat, and wasted, leaving the equivalent of only 20 lumens of saturated blue light output.

In contrast, LED elements have narrow-band output, which translates to much more visible output per unit of power. Consider a stage fixture with 36 1w LED elements, of which 12 are blue. The typical efficacy of a blue LED is 37 lumens per watt, so our 12 watt LED fixture should produce around 440 lumens of saturated blue light on perhaps 30 watts of power consumption, allowing for the control circuitry of the fixture.

So, we can have 20 lumens of output with 500 watts of input, or we can have 440 lumens of output with 30 watts of input. Not exactly a hard decision – LED fixtures are as much as 300 times more efficient – at least for saturated colors. This math is validated in real life. In the photo below, the fixture on the left is a 575w leko with a deep blue gel, and the fixture on the right is a Blizzard Q12A LED fixture with measured power consumption of about 30 watts.
Click image for larger version. 

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Summary and Application
The advantages of minimizing power consumption are significant. You may get a competitive advantage if you can deliver a larger show on available power than your competitors. You will likely reduce your out of pocket costs if you can use available wall power, rather than hiring an electrician to do a tie-in, or renting a generator. You may also make your patrons happier if the venue isn’t sweltering due to the heat of inefficient lighting and amplifiers.

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