Amplifier Power Ratings & Other Mysteries Of The Universe Explained - Part 2

Amplifier Power Ratings & Other Mysteries Of The Universe Explained - Part 2

In last HUBs article entitled “Amplifier Power Ratings & Other Mysteries of the Universe Explained - Part 1” we examined the testing methodology and standards used to obtain and publish power output and mains draw ratings for audio power amplifiers.

In this article we will delve further into amplifier power ratings with emphasis on the time duration used and attempt to explain the meaning of “burst” and “continuous” power ratings.

Unfortunately, audio amplifiers are viewed as fairly utilitarian devices by a significant portion of the professional audio community. Most audio people would and do spend hours researching, and auditioning loudspeaker systems in their quest for their ultimate system, and once chosen will happily spend hours passionately debating their choice with other equally passionate folk who happen to prefer a different brand.

However swing the topic of conversation to amplifiers and the discussion will start to run dry after a few minutes. This is because other than possibly size and weight, most people only think of amplifiers in terms of maximum rated output power. All other specifications are usually glossed over. This has forced manufacturers of audio amplifiers into a marketing arms race, with each striving to publish the biggest numbers on their specification sheets. As a result some (not all) manufactures have favoured alternate measurement techniques or standards that return more impressive looking numbers. 

In very general terms, audio amplifiers are tested using the following procedure:

  • Connect the amplifier output to a resistive (dummy) load representative of ‘nominal’ loudspeaker impedance.  Variable No1: What load impedance is used? (2, 4 or 8 ohm).
  • Apply a stimulus (test) signal to the amplifier input. Variable No2: What signal? (Tone, noise, music, speech).
  • Drive amplifier channel(s) to full. Variable No3: How many channels driven during testing?
  • Increase input drive level until output signal(s) begin to distort. Variable No4: what level of distortion? (0.1%, 0.35%, 0.5%, 1%, 10%)
  • Hold this level for a defined period of time. Variable No5: How long?
  • Measure voltage and calculate power using voltage squared, divided by load impedance.


Clearly there are several variables involved in this process, each of which will influence the final power rating. This means a true comparison is not possible unless all variables are known and declared. It also means that some manufactures may be tempted to select variables that return larger more marketable numbers.

The automotive audio industry is notorious for this trick. Do a quick web search for car audio amplifiers and you will be inundated with products offering astounding output power ratings, some claiming in the order of 12,000W. It’s difficult not to become sceptical of such claims given that they rarely declare measurement methods used to obtain such impressive figures.

Fortunately, most reputable manufacturers of professional audio products do not stoop to such tactics. They generally follow industry practise and declare measurement conditions on their data sheets, albeit possibly buried in the fine print. There are however a couple of common variables that should be qualified prior to comparing published data, the most common are:

Total Harmonic Distortion (How hard)

This is a measure of the level of distortion present, expressed as a percentage of total power.  This is an indicator of how ‘hard’ the amplifier was driven to achieve the stated power rating. For example, maximum power output; 8 ohm load, 20Hz – 20 kHz, 0.1% THD = 150 watts. Professional audio manufacturers generally choose target values < 1% with 0.1%, 0.35% or 0.5% in common usage. Non professional manufacturers may choose a higher target THD such as 1% or 10% as this will result in a higher calculated maximum power rating.

Time Duration (How long)

This is probably the most misunderstood aspect of amplifier power ratings. Traditionally amplifiers were rated using a ‘continuous’ sine stimulus signal. However this term is confusing in itself as continuous in this context means around 30 seconds. Not non-ending as per the dictionary definition of the word. However in the world of audio 30 seconds is a long time, and most amplifiers will have settled to a more or less continuous state after this period of time.

It has long been argued that real world audio signals are nothing like sine waves, but ‘bursty’ in nature and as such a more music like test signal should be used to obtain amplifier power ratings. This has resulted in several manufacturers using tone or noise bursts to measure their amplifiers and publishing what has become known as “burst power” ratings.

It is not the intention of this article to debate the relative merits of continuous power ratings versus burst power ratings, that’s a topic best tackled in the pub on a Friday afternoon. The intent of this article is to illustrate the difference and to stress that it is not possible to make an apples for apples comparison between the two.

Several standards (official or not) have emerged for burst power ratings, with burst durations such as 22mS, 33mS and 100mS in common usage. Generally, the shorter the duration of the test signal, the higher the resultant power rating. So it is critically important to qualify the time duration for all burst power ratings.

Putting things into perspective: Something about a hare and a tortoise….

The marathon is a long-distance foot race with an official distance of 42.195 kilometres (26 miles 385 yards, or exactly 267⁄32 miles) that is usually run as a road race. The event is named after the fabled run of the Greek soldier Pheidippides, a messenger from the Battle of Marathon to Athens.

The world record time for men over the distance is 2 hours, 3 minutes and 59 seconds, set in the Berlin Marathon by Haile Gebrselassie of Ethiopia on September 28, 2008. This represents an average pace of under 2:57 per kilometre, or an average speed of over 20.4 km/h.

I decided to challenge this record but didn’t have the time, resources or motivation to run the required 42Km. I instead set up a 28m track across flat ground in the reserve beside my home and  persuaded (read bribed) one of my kids to time me as I ran multiple laps. My best lap time worked out at around 4 seconds. This equates to around 7m per second or 25.2km/h, slashing the current world record by an impressive 5km/h!

Anyone who knows me will testify that I am definitely not Olympic athlete material. A fact confirmed (much to the amusement of onlookers - my dog) by my sudden need for a lie down to catch my breath after my “record breaking” run. Obviously, my run does not qualify for marathon world record status.  No official body would ever consider comparing times set over such vastly different distances.

I chose a course length of 28m for my record attempt as it is 1/1500th of the official marathon distance of 42km. The exact same ratio of the time duration commonly used for “burst” (20mS) and that used for “continuous” (30 sec) power ratings. The moral of the story?...Directly comparing burst and continuous power ratings makes about as much sense as comparing my run time with that of an elite Olympic athlete.

A slightly less tongue in cheek perspective

Let’s take a slightly more practical look at time duration of typical audio signals. Figure 1 shows the amplitude response of a single kick drum beat. The wave form in question was captured from the Jeff Buckley track: “Everybody Here Wants You”. The kick drum envelope consists of an initial short duration high amplitude section which gives the ‘power’ of the initial beat, flowed by a slow decay which gives the drum a warm, round sound. The highlighted area represents a 20mS time window. It can be seen that the 20mS window contains only a very small portion of the wave form, barely one and a half cycles. 

Figure1 
Figure 1 - Kick Drum, 20mS time window

 

Figure2 
Figure 2 - Kick Drum, 30 sec time window

By comparison Figure 2 shows the same audio wave form expanded to 30 seconds. This much longer time window encompasses a significantly greater portion of the signal envelope.  We are no longer talking about a time period equivalent to a percentage of a drum beat; we are now looking at a time period equivalent to around 20 drum beats plus bass, guitar and vocals. Far more representative of the full program material.

What does this all mean?

Burst power ratings are a good indication of an amplifier’s ability to deliver very short duration bursts of energy, which is fine, however it tells us very little about the amplifier’s ability to deliver power over a sustained period of time. For example, an amplifier that delivered an impressive burst of power for 20mS but then quickly collapsed would be a disappointing performer as it would power compress at the first kick drum beat of every track. 

An amplifier with an impressive ‘continuous’ (30 sec) power rating is far more likely to deliver the goods under program material consisting of more than one kick drum beat every couple of seconds.  Implied in this rating is the ability to deliver short duration (burst) output greater than the continuous power rating; however this is usually not qualified. Clearly the most useful solution would be to publish BOTH continuous and burst ratings which would give a clearer indication of the amplifier’s abilities, plus assist when comparing power ratings between brands.

A blast from the past!

While researching for this article I came across a specification sheet for the Jands SR3000 (Figure 3) apologies for the poor reproduction, it was scanned from paper.  No electronic copies on record.

Figure3 
Figure 3 - Spec sheet, Jands SR3000

The SR3000 is a classic amplifier from the year 1996 when Jands locally designed and manufactured audio amplifiers. The interesting thing about the SR3000 spec sheet is it lists both continuous (sinewave) and burst (IEC tone) ratings. It even includes a graph which clearly shows the reduction in power rating with longer tone burst duration. If only other manufacturers had followed this example...

Should you have any questions, comments about this article or suggestions for future articles please feel free to email support@jands.com.au.

Jeff MacKenzie
Manager, Technical Resource Group – Jands Pty Ltd

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