Thomas Cornish Consulting

By TJ Cornish, May 2014

Note: This article was originally written for the Soundforums.net live audio forum, which classifies people as “Varsity” - their full-time employment is at a regional or larger sound production company, or “Junior Varsity” - part-time or smaller-scale operators.  Some of the language below reflects the original audience of the article.

One of the foundational necessities of a successful gig is having adequate safe power.  This can be a particular challenge at the Junior Varsity level.  This article will be a high-level flyover, providing some basic insight into how the gear we use works.  Future articles will go deeper.

 

What’s a Watt?

Power is measured in watts, and is the energy available to do work.  The formula to calculate power in watts is P_watts = Voltage * Current.  A 20A 120V circuit theoretically has 2400 watts of power available. 

 

How much power do I have to work with?

All of the loads on a circuit count against the available power.  For example, if a 20A 120V circuit – potentially 2400 watts of available power - powers a 300w light bulb, a 75w cash register, and a 1000w refrigerator, the consumption is 1375 watts, theoretically leaving 1025 watts available before the circuit is at maximum capacity.  Exceeding this capacity will cause the circuit breaker to trip.

This is not an exact scenario.  Most circuit breakers are “slow-blow”, meaning that a circuit breaker will allow more than its rated current to pass for a certain amount of time before enough heat builds up to trip the breaker. 

A 20A breaker will pass 40A for up to 60 seconds before tripping, and 30A for up to 225 seconds before tripping. Manufacturing tolerances of the circuit breakers and other factors like ambient temperature in the breaker panel may alter the point at which the breaker trips, with higher ambient temperatures causing breakers to trip sooner and/or at a lower current than when temperatures are lower.

 

Continuous and Intermittent Power Consumption

The power consumption of a device can be broken down into the continuous component – the amount of power a device always draws whenever it is turned on, sometimes called idle current, and the intermittent component – additional energy required when the device is doing work, such as lighting a lamp, or amplifying a signal to be sent to a loudspeaker.

We saw above that the duration of the power consumption matters.  If left long enough, most 20A circuit breakers will trip in the close vicinity of 20A, however we can actually draw significantly more than 20A on a circuit as long as we don’t draw it for very long.  This behavior allows devices with high intermittent current requirements – greater than the circuit’s nameplate capacity – to still operate, as long as the amount and duration of that intermittent current does not exceed the breaker’s trip curve.

 

Power usage by device classification

Generally speaking, mixing consoles, signal processing equipment, backline instruments, and video projectors have almost no intermittent current requirement, so calculating the power required is as easy as reading the sticker on the back of the device.

Audio amplifiers and self-powered loudspeakers have a small continuous power requirement and a large intermittent power requirement when the amplifier is driven hard.

Lighting systems, including both tungsten and LED, have a small continuous power requirement to operate the control circuitry, and a comparatively large intermittent power requirement when the lamp is on.

Moving light fixtures with a discharge-type light source are somewhat similar to video projectors in that they have a high continuous power requirement, and a small intermittent power requirement when the fixture is moving.  Tungsten-based moving light fixtures are similar to a tungsten dimmer system, as the lamp is the vast majority of a fixture’s power consumption.

 

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:

* These values are not given in the datasheet, but are assumed to be at least as great as the 1/8^th 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 1/8^th 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.

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/colour-details.html#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.

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.