The Power Amplifier

The basic design of a power amplifier consists of an input stage, which performs the voltage gain and a current gain stage.

The amplifier input is coupled to the voltage gain stage through a DC blocking capacitor, which in combination with an input load resistor forms a high pass filter with low cut-off frequency. To limit the bandwidth of the amplifier, a low pass RC filter is also added.

The voltage stage must be able to deliver the required slew rate for the voltage gain. To amplify 20kHz with a sufficient voltage swing for driving a high power speaker the following calculation can be made; for example:

  • T =1/f = 50us     (20kHz),
  • Vin,rms = 1V       (full drive) (Vin,peak = 2 * 1V = 1.41V)
  • Vout = 50V         (full drive, voltage per side for a full symmetric design)

Then, required slew rate (SR) = 50/(50us/4) = 4 V/us.
Generally the voltage gain amplifier does not require slew rates larger than 10V/us.

foto of power amplifier with MCU, click for larger view
A power or current gain stage follows the input stage. This is the part that actually delivers the power to the speaker and consists of a pre-driver transistor pair and a high current transistor/FET pair. The transistor pairs must be chosen on their complimentary features. The current stage is set into class AB by making use of a transistor zener that enables a quiescent current through the power transistors. All power transistors including the quiescent current driver are mounted on one cooler to increase temperature stability.
A feedback network from current stage to voltage stage determines the overall gain of the amplifier. Preferably, the current stage does not provide voltage gain but sometimes a factor 2-3 is convenient. For full range amplifier often a Boucherot network is added to maintain stability in the high frequencies and an inductor to handle the capacitive load of the speaker.

power_amp_block.gif (8271 bytes)

How much power do you need?

Better buy a very good 30Watt amplifier than a lousy 120Watt amplifier

Power is often an important issue to take into account when deciding on audio amplifiers/speakers. From a designers point of view speaker power and amplifier power is only relevant for bass reproduction. For mid and high, only a few Watt is required to obtain high sound pressure levels (SPL) (generally tweeters are not able to handle more than 10W electrical power). Above this value thermal breakdown is not unthinkable. For bass speakers, mechanical overload and non-lineair distortion can take place.
Referring to bass speakers, an efficiency of 90dB@1W/1m is common. This value indicates that 1W electrical power applied to the speaker will give a SPL of 90 dB at a distance of 1m. The dB-scale indicates that this value is already very much for home audio and most people will not tolerate this SPL for more than a few minutes when in their living room.
Doubling the electrical power according to the formula:

P( in dB) = 10Log(Pelectric/P0), where P0 is a reference power

shows that each time the electrical power from the connected amplifier is doubled, the speakers SPL will increase with 3dB.
The influence of the speaker cone area is derived from speaker physics with the formula:

where p0 =20uPa.

In fact, doubling the cone area, increases the SPL with 6dB! On the other hand we also observe that the power decreases with 6 dB when the listening distance is doubled.

Another interesting aspect is that this formula indicates the typical 12dB/octave roll-off seen at speakers. This can be used as an accurate model for the speaker roll-off in filter designs.

The following table illustrates the influence of above formulas. It clearly shows that a 64W amplifier is able to produce a tremendous SPL in the living room. Buying a 128W amplifier does not add anything to music listening. 
Its worthwhile to take notice of the "Audio Perception" part of this website to get an impression of the loudness perception of 114 dB. You can compare this to a disco level and in many cases to the level of a pop concert. With 100dB in your living room you are still facing the loudness of a heavy truck passing you nearby in your room. Most of the times, even when your listening to very loud music for a short while, the SPL in the living room does not exceed 100dB. This gives a fair impression of how much power you actually need.
So: Better buy a very good 30Watt amplifier than a lousy 120Watt amplifier.
SPL@1m 1 amplifier;
1 speaker
1 amplifier;
2 speaker
  (Watt) (Watt)
90 dB 1 0.25
93 dB 2 0.5
96 dB 4 1
99 dB 8 2
102 dB 16 4
105 dB 32 8
108 dB 64 16
111 dB 128 32
114 dB 256 64

Note: The third column assumes that each speaker gets the rated amount of power, no matter what the connection is of the speakers to the amplifier.

 

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