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Background

When selecting an amplifier and speaker, you are usually provided ratings or output in watts. For a speaker you get maximum and nominal watts rating, while for an amplifier you get output in watts.

I am confused, because I have read that you want your amplifier to have a higher output in watts than you speakers watts rating, as this means it does not need to work so hard to power the speaker, so will avoid clipping.

I have also read, that you only want to supply the speaker on average its nominal rating, while only supplying its maximum rating in bursts. If you exceed these numbers you will damage the speaker.

Question

So, which is it? Should my amplifier have a higher output in watts than my speakers maximum or nominal power rating? Or should my speakers have a high maximum or nominal rating...and if so, which one, nominal or maximum?

Which of these configurations is recommended?

1. Speaker (Max Rating 2W, Nominal Rating 1W) and Amplifier (3W Output)
2. Speaker (Max Rating 2W, Nominal Rating 1W) and Amplifier (2W Output)
3. Speaker (Max Rating 2W, Nominal Rating 1W) and Amplifier (1.5W Output)
4. Speaker (Max Rating 2W, Nominal Rating 1W) and Amplifier (1W Output)
5. Speaker (Max Rating 2W, Nominal Rating 1W) and Amplifier (0.5W Output)

Context

To provide some context, I am designing a solution for a low cost consumer product that does not require high fidelity audio or high volumes. The amplifier will need to run off 3.3v or 5v, which will be provided by an adapter connected to mains power. Audio will be provided by a line-level input, most likely the audio device will be a phone, mp3 player or computer.

I am currently looking at these ICs, as I just need a low cost mono amplifier (class AB) to drive a small speaker (most likely 8 Ohms, unless I can be convinced otherwise) - http://www.ti.com/product/tpa6211a1#parametric

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    \$\begingroup\$ 4, but it depends on how your amplifier is rated. Rarely is there truth in consumer audio electronics. \$\endgroup\$
    – HL-SDK
    Nov 8, 2013 at 0:48
  • \$\begingroup\$ But if it is a consumer product, I can't imagine you not having the budget to evaluate and protoype all 5 designs. \$\endgroup\$
    – HL-SDK
    Nov 8, 2013 at 0:48
  • \$\begingroup\$ Time factor for the first prototype. I will be evaluate more options including exciter for a speaker once I have got together 1 or 2 prototypes. Also, no point wasting time, if there is an accepted choice. \$\endgroup\$
    – Remixed123
    Nov 8, 2013 at 0:51
  • \$\begingroup\$ The IC you linked looks great -- I'd just add input attenuation so that a max input level doesn't introduce clipping. It will probably be empiracally determined either way. The TPA6211A1 is neat, but remember not all audio outputs have the same signal levels (different cell phones have different volume levels!) \$\endgroup\$
    – HL-SDK
    Nov 8, 2013 at 0:53
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    \$\begingroup\$ With expensive speakers, you don't want to accidentally blow them by overpowering them (of course you want to test how loud the are at full amp output power and if you don't, probably your friend will some day). I'd say go for an amplifier with a lower output power than the speaker's rated power. \$\endgroup\$
    – jippie
    Nov 8, 2013 at 8:36

2 Answers 2

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None of the options is good. The speaker produces a sound pressure level for a certain wattage inputted and this is the criteria you need to look at. Some speakers are more effective than others and this effectiveness is largely determined by their enclosure. Luckily, for any given size and shape of speaker the enclosure is roughly constant.

Decide what SPL you need and look at speaker specs to shortlist a handful then design your enclosure to suit the shortlisted items then do comparative studies. You are obviously looking for bandwidth, loudness and distortion as the key ingredients of the selected speaker. Choose also, the speaker that is capable of withstanding the power output that can be thrown at it. Choose an amplifier that won't melt down ie has protection built in.

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  • \$\begingroup\$ Thanks! None of the speakers I have looked at gave a SPL rating, so not sure how I select based on SPL? The product I am designing for is an inexpensive consumer product that has an inbuilt speaker as a tick off feature, most people will likely use the audio out to their stereos. So it is very unlikely any time and money will be spent on designing an enclosure that has been tested for acoustics, and any comparative studies will be minimal and involve selecting a few different amplifiers, speakers and messing around with the circuit to try out a few filters to reduce noise and distortion. \$\endgroup\$
    – Remixed123
    Nov 8, 2013 at 13:19
  • \$\begingroup\$ The first cheap 1W speaker I searched (alibaba) had a decent spec - alibaba.com/product-gs/567592356/… - where are you looking for this speaker? When I say decent I mean it had enough details for you to make comparisons with other cheap speakers. \$\endgroup\$
    – Andy aka
    Nov 8, 2013 at 13:26
  • \$\begingroup\$ Here is an example - australia.rs-online.com/web/p/speaker-drivers/7191964 - I did notice it does have SPL in the datasheet though. However, not all the datasheets on this site do. Can you recommend an online store that has a large selection of speakers, and sells in small orders for evaluation purposes? \$\endgroup\$
    – Remixed123
    Nov 8, 2013 at 13:40
  • \$\begingroup\$ I can't recommend an online store but it's interesting to see the speaker you linked - comparing mine and yours, yours is a better speaker for a couple of reasons that I can see but this may be of no consequence for your product and what I'm trying to say is this... If you don't know what you are looking for, how will you know good versus bad - this is why comparitive tests can help you. \$\endgroup\$
    – Andy aka
    Nov 8, 2013 at 13:47
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    \$\begingroup\$ It's not really attenuation you need - what you really need (I would have thought) is soft clipping to prevent overloading the speaker - this is best done at the input either digitally or in analogue - maybe raise another question on this? \$\endgroup\$
    – Andy aka
    Nov 8, 2013 at 14:10
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Amplifiers have more of a hard maximum limit whereas speakers are softer.

The power limit of an amplifier is basically frequency independent, more or less. It's determined by the voltage at which it clips. When given sine waves, it clips at the same voltage at 100 Hz as at 10,000 Hz (if it is any good, anyway). It also clips any other material at the same voltage. The calculations from that to RMS values are straightforward.

With speakers, what the maximum rating means is not quite so clearly defined.

If you naively match a 50W amplifier to a 50W speaker, you may not be able to explore what that speaker can "really do". Perhaps the speaker can handle being over-powered for short periods of time. Maybe the speaker is okay with 60W for five minutes, and the distortion is not even bad. Or maybe it can handle large transients that occur in material whose average power is low.

In a nutshell, a power amplifier's headroom is very sharply and precisely limited by its power rating (with regard to a given load impedance), whereas that of a speaker not necessarily so.

So it makes sense for an amplifier to be overrated with respect to a speaker rather than vice versa, not just from the perspective of avoiding clipped signals going to speakers, but from the point of view of headroom.

Another regard in which it is good for an amplifier to be over-specified is load handling. An amplifier that can drive 2 ohm loads is more robust than one which can drive only 4 or 8 ohm loads. It's not necessary to have such an amplifier for 8 ohm loads, but the beef doesn't hurt. You never know: a speaker voice coil could melt in such a way that it suddenly has less impedance. Someone could connect too many speakers in parallel. Etc.

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  • \$\begingroup\$ But which of the 5 options above? \$\endgroup\$
    – Remixed123
    Nov 8, 2013 at 3:21
  • \$\begingroup\$ @JMoney True; actually even for amplifiers, the wattage figures can be deceptive. Even if the amplifier can put 50W into a certain load and even if there is no clipping, that doesn't mean it's in its safe operating area. A typical push-pull emitter follower output stage suffers the worst-case power dissipation not at the voltage extrema, and not a the zero crossing, but at an intermediate voltage, when there is both voltage drop and current across the output devices. \$\endgroup\$
    – Kaz
    Nov 28, 2016 at 21:32
  • \$\begingroup\$ The power ratings of speakers and amplifiers can have different meanings so 50W does't mean much unless it specifies 50W for which method. Then the method needs to be understood to know what it indicates about the device. The most useful is probably RMS or something that roughly correlates. Once they are in the same form the two can be compared. If you don't want to blow your speakers their RMS power rating should be at least that of the amplifier. And if the amplifier is going to be turned up enough to clip the speakers should be even twice the amp's RMS power. \$\endgroup\$
    – J.Money
    Nov 28, 2016 at 21:34

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