Speaker – Amp – Match

The articles that I have found on power amplifier sizing keep speaking of this “Rule-of-Thumb” and I find differences of opinions on this “Rule-of-Thumb”. By conducting my own study and research, I think I can elaborate on this some to make more since and explain why each of these rules were selected.

I found the three “Rule-of-Thumb” to recommend:

• 80 to 125% RMS
  • First, there is no RMS in speakers. This value is referred to as continuous for speaker per AES organization.
• 100 to 150% of RMS
  • If the specification show a difference between the Continuous (RMS) and Peak in only 3dB (2x RMS) then a maximum of 150% of the RMS should be used.
• 150 to 250% continuous.
  • In case of the specification show 250 Continuous (RMS), 500 Program and 1000 Peak then we are looking at using 200% of the Continuous.

The goal here is to get as much as you can of your speaker with protection of the speaker as a priority. These above rules-of- thumb are worthless.

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Power Amplifier Notes

Speaker to Amplifier Matching

The best comment that I have run across has been from Martin Sound, that stated that ideally the amplifier should deliver full peak power without the risk of clipping. This is meaning that a speaker that is rated at 1000 watts peak should be provided with an amplifier with 1000 watts peak power. Now the question is, how do I interpret these values from the ratings within the specifications.

First, the speaker specifications states a rating of 250 watts that is tested under EIA or AES specification. This results in a continuous rating using pink noise. Since this value is a continuous signal of all frequencies define for this driver, the real world rating is the maximum power handling rating, that is also referred to as program or music power, will be equal to two times this continuous rating. Another two times this value will be the peak rating of the speaker. The result will be:

250 watts continuous;
Continuous x 2 = 500 watts program;
Continuous x 4 = 1000 watts peak

For the amplifier, you will need to look closely at the footnotes of the specifications. Typically the rating is defined as the maximum average or continuous output in watts at a given THD level at 1 kHz. Now one instance I found the rating to be maximum output power. In this case, this is peak power. One customer was complaining because the specifications stated 1600 watts but he was only measuring 1131 watts. This person did not interpret the specifications correctly because 1600 times 0.707 will equal 1131 watts. In this case 1600 watts peak (maximum) will be 1131 watts continuous or RMS. The result will be:

707 watts continuous or RMS;
Continuous x 1.414 = 1000 watts peak;
Maximum (peak) x 0.707 = continuous watts

In this case above, a 1000 watts peak speaker should require a 707 watt continuous amplifier.

Maybe to simplify this, you could say that:

250 watt continuous speaker times 2.8
would require a 700 watt amplifier.

500 watt program power speaker times
1.4 would require a 700 watt amplifier.

This is the optimum requirement but this will usually play havoc on your budget. To compromise and fit your budget better, the minimum amplifier requirement for the matching of a speaker would be:

500 watt program power speaker
would require a 500 watt amplifier.

Summary

The initial requirement provides for the amplifier peak power to be equal to the speaker peak power with no Peak limiter would be required. This results in the amplifier RMS power above the speaker thermal limit and an RMS limiter would be required with a -1.5 dB threshold to provide thermal protection.

The optimum solution in turn provides for the RMS amplifier to be equal to the thermal limit which is the program power of the speaker. This allows for the peak power of the amplifier to be 1.5 dB below the speaker peak power and might require a Peak limiter. If a Peak limiter is not used, it would be recommended to closely monitor the Clip indicator on the amplifier.

While conducting a test and set all pieces of equipment thresholds in my chain to 0UV using pink noise. I used this pink noise and I adjusted the amplifier gain to the point where I was not getting the CLIP LED. This result was about 75% of the gain adjustment on the amplifier. Now I only get the CLIP LED occasionally during the loudest passages. This has covered for 95% of my peak issues.

The optimum solution would be the best choice which would reduce the cost of the amplifier but still no limiter would be require eliminating any extra cost for your system.