Many of you have questions about pairing amplifiers and speakers based on their power. We are often asked if the power delivered by the amplifier and the power handling capacity of the speakers have to be identical, or if an amplifier that is too powerful can damage a pair of speakers. Here are some answers and solutions to prevent any problems, notably for home theater installations.
Understanding the power of an amplifier
The power of a hi-fi amplifier or A/V receiver is given in watts. Contrary to what is often assumed, the indicated output isn’t the power delivered constantly – as is the case with a lightbulb, for example – but the maximum output that the amp or receiver can deliver. Depending on the intensity of the incoming audio signal and the gain applied to it (position of the volume potentiometer), the power delivered is variable. A 2×50W stereo hi-fi amplifier can therefore deliver anything from a few milliwatts to 2×50W.
What is speaker power handling capacity?
In practice, a speaker’s power handling capacity is the amount of electrical current that the coil of the speaker’s woofer can handle, as it is the driver that is most called upon. It is assumed that the woofer receives about seven times more power than a tweeter, for example. This is due to the lack of sensitivity of the human ear to low frequencies. The mechanical properties of the coil determine its power handling: material used (copper, aluminum or silver), diameter and length of the wire. If this power handling capacity is exceeded, the coil may melt or burn.
Electrical and acoustic power
Some manufacturers specify a power handling capacity that is not the electrical power handling capacity of the voice coil. For example, if the woofer’s coil can handle 100W, such a power will cause an excessively high excursion of the woofer’s cone, which will potentially expose it to mechanical damage (tearing) and generate high distortion or even expose the amplifier to dangerously abnormal resistance (impedance).
Is a 50W amplifier dangerous for 100W speakers?
Potentially, yes, if it is used to its full potential and the speakers are difficult to power (large drivers, large enclosure, low nominal impedance). The enemy of speakers is not so much the power as the distortion generated by an insufficient power supply. A poorly powered 50W amplifier can, under extreme conditions, generate square signals or supply continuous current to 100W speakers, whose coils can then deteriorate and melt. The speaker is then “destroyed” even though its power handling capacity is technically higher.
Is a 100W amplifier dangerous for 50W speakers?
Yes, if it is used to its full potential. However, if used at about half its power, it will power the speakers with ease and provide a coherent sound, in short a better musical experience.
Do manufacturers mislead consumers with the power of their amplifiers?
No, but for home theater, the output is often rated at 1kHz and not from the lows to the highs (20Hz to 20kHz), and with a high distortion (1% to 10% sometimes). Why is this? The answer is somewhat complicated. Ideally, one should know the power delivered simultaneously to all the channels, from 20Hz to 20kHz, with a low distortion (0,08 % for example), as is the case for hi-fi amplifiers. The problem is that although the 2 channels are used constantly in hi-fi, it is rarely the case in home theater, especially since the surround channels are not intended to reproduce very low frequencies. So manufacturers do not indicate the simultaneous multichannel output, which would (wrongly) worry their customers, given the low values. Let’s take for example the Yamaha RX-V4A A/V receiver, a midrange model that excels in its category. Yamaha announces 115W for 1 powered channel, at 1kHz and with 0.9% distortion. But 80W for 2 channels powered from 20Hz to 20kHz with 0.06% distortion. In other words, the receiver can deliver 160W from 20Hz to 20kHz with 0.09% distortion. This, divided by 5 channels, is 32W per channel, when all are used. Admittedly, 5×32W is not a very appealing figure. However, it is more than enough in absolute terms and as we explained before, no movie will require the same amount of power from all channels at the same time.
Is 5×32W really enough?
In most cases, yes. This statement will be all the more true if an active subwoofer is used to reproduce the low frequencies instead of the A/V receiver. In this case, the 32W will be used to reproduce the midrange and highs. With speakers of average sensitivity (89 dB for 1 W of power), this is equivalent to a sound level of 104 dB at 1m from each speaker: this is colossal. If no subwoofer is used, then these 32W will be used for the low frequencies and about 7 times less power will be devoted to the midrange and highs, that is to say about 4W, in other words a sound level in these frequencies of 95 dB at 1m from the loudspeakers: this is also very high and it is necessary to have very understanding neighbors…
Home theater is more exposed to risks than hi-fi
A movie soundtrack is often much more challenging for amplifiers and speakers. The dynamic shifts are much more significant than with most music. Moreover, the low frequency level can be massive, whereas this register is often “flattened” on audio CDs. Furthermore, we often like to watch movies at very high volume.
Properly configuring an A/V receiver
Although most A/V receivers come with a microphone and a good auto-calibration system, the measurements and the corrections made by the on-board software must be checked, especially regarding the crossover frequency and any eventual equalization. This is essential in the absence of a subwoofer, as the low frequencies of the surround, center and LFE (.1) channels will be mixed to the main speakers.
It is therefore necessary to check that the low-pass frequency of the speakers is not lower than that communicated by the manufacturer. For example, a speaker that reproduces frequencies from 80Hz to 20kHz should not under any circumstances receive frequencies below 80Hz. This is because the coil of the woofer can heat up and melt. This is especially true for compact or satellite speakers with small drivers (3” or 4”) and very small coils.
If the amplifier, after auto-calibration, has set the crossover frequency to 60Hz, you will have to correct it manually. Also, it is important to pay attention to the possible equalization of low frequencies by the A/V receiver. An increase of 2 dB at 50Hz is equivalent to doubling the power output.
The many advantages of subwoofers
In addition to having a matching driver and amplifier designed for low-frequency reproduction (large voice coil, high amp output), the subwoofer – in combination with an A/V receiver – protects the other speakers from a possible massive influx of current from the receiver. The requirement is to declare the speakers as SMALL in the A/V receiver’s menu. The risk of “blowing” the speakers is therefore minimal.