Are low-power amplifiers acceptable in hi-fi systems?

It depends on the system and what you expect from it. Sometimes yes, usually no.

Sound pressure level (SPL) and dynamics are two aspects of high-fidelity sound reproduction. The fidelity of these two parameters in a hi-fi system depends on amplifier power output and speaker sensitivity, and these two specifications are inextricably linked.

Low-power amplifiers can create realistic sound pressures and dynamics, but only when partnered with high-sensitivity speakers. Conversely, the more power available, the lower the speaker sensitivity can be whilst still being able to achieve realistic SPLs and dynamics.

This all comes down to physics. A 20W or 30 W per-channel amplifier can’t offer high-fidelity dynamics and realistic sound pressures with ‘normal’ sensitivity speakers, ie speakers with around 89 – 91dB/Watt sensitivity. It’s just not physically possible, no matter how good the amplifier.

In these cases, hi-fi listening at low levels may be possible, but realistic musical scale and SPLs will be missing. Cranking up a system with marginal power and dynamic headroom will result in distortion and clipping that produce square waves with excessive high-frequency content that may damage or even destroy an amplifier and/or speakers.

Think of this as like power-to-weight ratio with vehicles. My 1250cc (~140 HP) motorcycle engine gets my bike moving very rapidly, with a mass of around 250kg. That same engine would prove unacceptably puny if it were installed in my 1700kg Subaru Outback that needs its 255 HP engine to get it moving acceptably. It simply wouldn’t work, except maybe at a crawl.

When you partner a high-quality, low-power amplifier with higher-sensitivity speakers, say 93 – 95dB/Watt, then you can achieve real heavyweight dynamics and punch. Systems like this can generate crushing sound pressure levels, so it’s a sliding scale. Decrease power, and you must increase sensitivity. Increase power, and you can sacrifice a little sensitivity.

You might wonder why all speakers are not made really sensitive, then? Again, it’s all about trade-offs. With high sensitivity often comes colouration and poor bass performance. That’s why really high-fidelity speakers are normally around 90dB/Watt and the best and most accurate speakers are often lower sensitivity, between 83 and 88dB/Watt. Such speakers demand a more powerful amplifier to drive them properly. Speakers with a low 85dB/Watt sensitivity, for example, will need a seriously beefy solid-state amplifier to drive them. This is a statement of technical fact, not opinion, BTW.

This FAQ covers the concept in more detail.