50W Amp for 15W Speaker

Question

I am building a portable speaker and I am looking for a decent amp for my two 15W Phillips speakers. It is tough to find an amp that is efficient and cheap so I was wondering if there is something wrong with buying an amp that is rated for 50W2 while my speakers are only 15W2.

Answer 1

That has been a subject of debate for decades. Should the speakers be overrated or the amplifier?

1. If the speakers are overrated the amplifier might be strained at high volume levels and produce distortion (clipping) that not only sounds bad, it is bad for the speakers as they faithfully try to reproduce the distortion.
2. If the amplifier is overrated (but not by an outrageous amount like 10:1), then you get clean sound until the speakers reach their limit. The woofers may pop against their frame and the sound becomes distorted. This is a warning to back off the sound level or risk burning up a speaker.
3. Because there is seldom anything like a perfect match between speaker wattage ratings and that of the amplifier, the better match is to have an amplifier that is a little over rated, and be careful with the volume control. This way the amp is never strained or distorted, giving you clean sound up to the maximum volume the speaker can handle.
4. It is easy to burn up 15 watt speakers, but they are often very efficient and will be very loud at a safe maximum volume. A 50 watt amplifier will drive these speakers with ease and not distort from working too hard.
   

EDIT: For better mechanical range of the volume control, look for a gain adjust control on the amp or buy 2 RCA attenuators from most any stereo installation shop(car or home). Ones with -3db rating will cut signal level to 70.7%. Those with a -6db rating will cut sound level by 50%. Any of these options will allow you to use more of the mechanical 'rotation degrees' of the volume control before you reach the limit of the speakers. (To avoid confusion, I am referring to signal level, not power levels.)

Answer 2

There are many audio power amplifier products based on the Tripath TA2020 which is a 20W + 20W Class-T chip. This would seem to be an ideal solution for your product. Many of the most respected powered monitors available today use Class-T power amplifiers internally.

Answer 3

Well, if you absolutely had to use the 50W amp with 15W speakers, and wanted to be able to crank the amp to "10" without blowing them, then there is another option: add a series resistance (pseudo-pad) to the speaker.

If the speaker and amp are 8Ω, through which 50W can be delivered, then the volts must be: $$V = \sqrt{W \cdot R}$$ $$\sqrt{50W \cdot 8\Omega}$$ $$\sqrt{400}$$ $$V=20$$

To calculate the current (just for reference), $$I=\frac{E}{R}$$ $$\frac{20v}{8\Omega}$$ $$I=2.5A$$ Note already it should be apparent that if a 16Ω speaker were used instead, the current would be half, and thus the power delivered would be half also.

To spec the exact resistance needed to reduce a 50W amp to 15W output, one could setup an equality (using the inverse of resistance) and cross-multiply: $$\frac{50W}{8\Omega^{-1}}=\frac{15W}{x^{-1}}$$ $$50x^{-1} = 8^{-1}\cdot 15$$ $$50x^{-1} = 0.125\cdot 15$$ $$50x^{-1} = 1.875$$ $$\frac{50x^{-1}}{50} = \frac{1.875}{50}$$ $$x^{-1} = 0.0375$$ $$x = 26.667\Omega$$

Now subtract the 8Ω and that gives \$R_{PAD}=18.667\Omega\$. Add that to the speaker and the risk of "blowing it" by over-driving from this amp should be mitigated. Since in a worst-case scenario up to 15W could be dissipated by Rpad, choose high-power resistor(s); carbon, metal film, or other non-inductive types should be used. Preferably several in parallel, to spread out any heat.

Note that this does change the impedance as seen by the amplifier. For most small audio amps, a higher resistance is usually not a problem. (A class-D amp which depends on a low-value load being there may be a whole other story worthy of an "L-Pad", but that depends on the amp.)