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The power amplifier in a Hifi & High End audio installation has only 1 task. A very simple task, but a very important one. The task is to amplify the carefully built audio signal supplied by the preamplifier. That signal must be amplified and buffered so that the power amplifier can drive the speakers. The preamplifier supplies a maximum of a few volts and only a small amount of current. The power amplifier can supply many tens of volts and plenty of current to drive the speakers properly.
Watt's up?
The big question was always: How many watts does it have? The more watts, the better, people thought. The reality is quite different. You undoubtedly need some watts to drive your speakers, but exactly how many watts you need to optimally drive your speakers depends on, among other things:
The efficiency of your speakers
The complexity of the speakers
The music played
The size of the listening room
How loud you play
Etc.
The efficiency of speakers is always measured in decibels per 2.83 volts at 1 meter distance. That is, dB/2.83V/1m. That 2.83V is actually the equivalent of 1 watt at an 8 ohm speaker.
Why volts instead of watts? That is because volts indicate the amount of voltage needed to make the speaker move. How many watts that is depends on the resistance of the connected speaker, and that is different for each speaker and each reproduced frequency, so you cannot measure with that.
The efficiency of the speaker is usually mentioned in the specifications. For example, it might say: Efficiency: 88dB. Or 85dB. Or 91dB. The higher the number, the higher the efficiency. If you provide a speaker with 88dB with 1 watt of power, it produces 88dB of acoustic output, or sound, at 1 meter distance. A speaker with 91dB produces 91dB of sound at 1 watt. More sound at the same power. The 91dB speaker is therefore more efficient. The more efficient the speaker, the less power your amplifier needs to produce a certain amount of sound.
2. Decibels and power
For every 3dB of additional sound you need to double the power. At first it doesn't increase that quickly, but at some point it really adds up. Take a look.
We take a speaker with low 86dB efficiency as an example.
To get 86dB of sound, we need to input 1 watt.
To get 89dB of sound, we need to input 2 watts. And so it continues.
92dB of sound, 4 watts
95dB of sound, 8 watts
98dB of sound, 16 watts
101dB of sound, 32 watts
104dB of sound, 64 watts
107dB of sound, 128 watts
and now it gets fast......
110dB of sound, 256 watts
113dB of sound, 512 watts
And so on.
Now not many people play 113 dB at home, perhaps the guys from Rammstein….
If we now take a dynamic speaker with a particularly favorable (high) efficiency of 99dB (usually a horn speaker), the picture looks completely different.
99dB of sound, 1 watt
102dB of sound, 2 watts
105dB of sound, 4 watts
108dB of sound, 8 watts
111dB of sound, 16 watts
114dB of sound, 32 watts
117dB of sound, 64 watts
As you can see. A speaker with higher efficiency is much easier to drive than a speaker with low efficiency.
Speaker efficiency alone is not enough. There is another factor at play, and that is the speaker's impedance. This is measured in Ohms. You often see 8 Ohm or 4 Ohm printed on the back of your speaker. But it is not that simple. A speaker's impedance changes when you play music, and it actually depends on the frequency. So it can happen that a speaker has an impedance of 60 Ohm at 50Hz, 5 Ohm at 800Hz, 35 Ohm at 4000Hz and 2 Ohm at 18000Hz. This is often excellently represented in the red line of the graph.
You can see an impedance of 7 ohm at 20Hz, 27 ohm at 50Hz, 3 ohm at 150-225Hz, 14 ohm at 3,500Hz and 4 ohm at 20,000Hz.
Now, the thing is that you need a certain amount of voltage to make the speaker sound equally loud at all frequencies. And, converted to the amount of watts, it works like this.
The impedance of the speakers represents the resistance of the speaker. The higher the speaker's resistance, the easier it becomes for the connected amplifier to achieve a certain volume. That sounds strange, but it is true. You can compare it to a water pump with a water hose with a diameter of 1cm that sprays water. The pump represents the amplifier and the water hose represents the speaker. So there is a water pump and it delivers an amount of pressure. Suppose the water hose now sprays 4 meters far. That 4 meters represents the volume of a speaker. If we could now press a button to make the water hose 2 times thicker, so 2cm in diameter, what happens to the water jet? Right, it does not spray more than 2 meters. In speaker terms, the volume becomes 2 times weaker. To get the jet to spray just as far, the pump now has to deliver 2 times as much water. In other words, the amplifier has to supply 2 times as much current to the speaker. A high impedance may seem difficult, but it is actually advantageous for easily delivering watts to the speakers.
Ideally, an amplifier doubles its power when the speaker's impedance is halved. If that is the case, the volume does not change when the speaker's impedance changes.
So 2 x 100 watts at 8 ohm delivers a certain maximum volume. If the speaker's impedance drops to 4 ohm at a certain frequency, the amplifier will have to deliver 2 x 200 watts for the same volume. If the impedance drops to 2 ohm, the amplifier will have to deliver 2 x 400 watts for the same volume. And so on. The lower the impedance of the connected speaker, the more current the amplifier will have to supply.
There are not many amplifiers on the market that double the output power down to 1 ohm, but the good news is that's not always necessary. There are simply not that many speakers with a resistance (impedance) of 1 Ohm at certain frequencies. 1 Ohm is really very low and approaches a short circuit. But if you have a speaker with a minimum impedance of, for example, 2.5 Ohm at 400Hz, then it is disadvantageous to use an amplifier that delivers little or no power at 2.5 Ohm. Most tube amplifiers cannot do that. So if you connect a tube amplifier to such an unfavorable speaker, it may sound decent at some points, and when reproducing voices in the midrange, it can completely collapse and distort.
Choose the right amplifier for the speakers you have. Ohm-Audio BV can help and advise you.
The right power amplifier for your speakers must therefore be technically suitable for your speakers, but that is not the only thing. The sound character must also match the speaker. That is something personal, but we can often give some advice on it. Preferably here at the shop with a carefully brewed cup of coffee.
