How To Pick Out An Amplifier For Your Speakers

So you’ve got a speaker picked out and its time to get an amplifier. You see the speakers have a wattage number and the amplifiers all have one prominently listed too. How do you know how many watts you need? How do you know if an amplifier will work well with your speakers?

Forget Everything You Think You Know
The first thing you need to do is forget everything you’ve been told by marketing reps, sales people and your own intuition. They want to sell you more expensive amplifiers you don’t need. They aren’t selling you snake oil, they are just convincing you you need more amplifier than you really do. When it comes to your own intuition, don’t trust it; marketing divisions know how to advertise components so that you think you are doing your due diligence and choosing based on specifications, but in reality, you are falling into their trap.

Now that you’ve forgotten everything, lets begin learning. The first thing you need to know is the wattage rating on your speaker is like the height limit on a tunnel. If you exceed this wattage regularly, you’ll be doing irreparable damage to your speaker. Just know that you can damage your speaker without exceeding these levels and that you can sometimes get away with exceeding the number on occasion.

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Amplifiers advertise power ratings as well. In this case, it is how much power it can deliver under certain conditions. These conditions can make a huge difference. For instance, the NAD 3020D costs $400 and advertises a 35W power rating. On the other hand, a Lepai LP-2020+ will cost you $28 and gives you a 20W power rating. Here’s the big difference: the NAD has 2000 times less distortion at 35W than the Lepai does at 20W. The criteria for giving a power rating simply is not consistent between manufacturers, or even models. It makes things difficult when comparing amplifiers.

Figure Out How Much Power You Need To Make Things Loud

So the first step in choosing an amplifier is figuring out how much power it will need to deliver to the speaker to be as loud as you want it to be. You’ll need three numbers for this: the speaker’s sensitivity (dB/Watt), the distance you will be from the speaker (in meters) and a target volume for the listener. You’ll need to choose the target volume. Use this chart and figure out how loud you want the peak sound to be. My normal movie watching peaks at 85dB. A movie theater would use 105dB as their figure.  Once you’ve got all three numbers, follow the following steps.

  1. Take the difference of the speaker’s sensitivity and the volume level you chose. Then look up the result in the table below
  2. Multiply the wattage you got from the table and multiply it times the distance (in meters) you are from the speaker.
  3. Take that result and multiply it by the distance again. That’s how much power you need.
Volume – Sensitivity Watts Needed
-20dB 0.1 W
-10dB 0.316 W
-6dB 0.5 W
-3dB 0.707 W
0dB 1 W
6dB 2 W
10dB 3 W
14dB 5 W
18dB 8 W
20dB 10 W
26dB 20 W

Here’s an example. I sit 2.5 meters from a speaker with 87dB/Watt @ 1m sensitivity, and I’d like to listen to it at a peak of 96dB. 96dB – 87dB = 9dB. The nearest value on the chart is 10dB, so I’ll take 3W off the chart. 3 * 2.5 *2.5 is 18.75 Watts of power I need to listen to music as loud as I’d like to.

Picking an Amp Based on Specifications

Now that you know the most power you’ll be sending to your speakers, you can start judging some amplifiers. You don’t want to get an amplifier with a rating less than the number you calculated, but there are a few other important considerations to take into account.

The first is you want to worry about distortion. That Lepai Amp I mentioned has 10% distortion at 20W, but only 0.1% distortion at 2W and 1% at 10W. More available power is going to generally lead to less distortion. My rule of thumb is that distortion and power are directly proportional, up to a factor of 100 reduction. I’d aim for a 0.1% distortion figure at your peak listening volume. That means, using the case above, I’d aim for an amplifier that has 0.1% distortion at 18.75W, or 1% distortion at 187.5W. The graph below shows how the distortion characteristic an amplifier has as you increase the power output.


The last thing to consider is the impedance rating of the amplifier. If your speaker lists 6 ohms or 8 ohms, any speaker amp will work. If it lists 2 ohms or 4 ohms, make sure your amplifier is rated for it. A low impedance speaker creates more heat than a high impedance speaker at the same power level, so you need to know you aren’t going to roast your amplifier trying to power your speakers. If there isn’t a level listed for lower impedance, assume the amp can produce half the power for a 4 ohm impedance than an 8 ohm impedance, and likewise, assume the amp can produce one quarter the power into a 2 ohm load that it can into an 8 ohm load.

For good amps that can handle large currents, this effect is actually reversed (the NAD 3020D I listed puts 35W into 8 ohms, and 105W into 2 Ohms), but my goal here is to keep your equipment from smoking.

If should also be noted that the impedance ratings on most speakers are averages and not minimums. Most “8 ohm” speakers have a minimum impedance of 4 ohms or 6 ohms. This is something to consider if you are stretching the limits of any defined specifications.

Optional: Consider Amplifier Type

There are many ways to build an amplifier. There are 4 major topologies you’ll see advertised.

  1. Class AB amplifiers is the standard amplifier type today. They are a compromise between power hog Class A amplifiers and distortion heavy Class B amplifiers.
  2. Class A amplifiers minimize distortion, but run hot and therefore are large and need venting.
  3. Class D amplifiers are digital amplifiers which create a very fast stream of pulses and are filtered before being sent to the speaker. These are very low power but tend to be high distortion.
  4. Class T amplifiers are a special kind of Class D amplifiers which have theoretically better performance than regular Class D’s.

Amplifier Brands

Now that you know how to pick out an amp, here are the brand’s I’d consider, without jumping into tube amps or very expensive models. If there are any I’m missing, let me know in the comments.

Cheap Stereo Amp Brands

Lepai – The cheapest amplifier’s you’ll find. Small, Class T amplifiers.

Topping – More expensive, and better class T amplifiers.

SMSL – Similar to Topping

Dayton Audio – Some of the best bang-for-your buck amplifiers, T-amps and AB-amps.

Integrated Stereo Amplifier Brands

Yamaha – A nice range of Class AB amplifiers

Marantz – Very nice amplifiers, some are Class AB, some use special topologies.

Cambridge Audio – Well known for their stereo integrated amps, Class AB

NAD – They make excellent amplifiers in general, their small amplifiers are Class D, larger ones are Class ABs

Simple, High Quality Amps

Emotiva – Great bang-for-your-buck amplifiers at the high end

Home Theater Receivers

Onkyo – Well known receiver brand, watch out for weak amplifiers on the center/surround channels

Denon – Same as Onkyo, but a better reputation for reliability.

Marantz – High quality amplifiers on a Denon receiver is the best description

Yamaha – Has their own version of many features, but in general is comparable to Onkyo/Denon.

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