Low Distortion Audio Power Amplifier Design

Audio power amplifiers are, in principle, very simple. If the input voltage is 1 volt at a certain instant of time and the output voltage is then 20 volts, then the amplifier has a gain of 20. In an ideal amplifier this number, 20, would be a complete description of the performance, and the output voltage would always be simply 20 times the input voltage. In a real amplifier with an input voltage varying with time the output voltage has a maximum level, and a maximum rate of change, and the gain falls at high frequencies, but within these limits it is still possible to approach the ideal behaviour.

There is a very simple way to find out how close to the ideal a given amplifier comes. If the gain is supposed to be 20, then all we need to do is divide the output voltage by 20 using an accurate resistive divider, and for an ideal amplifier the result will be identical to the input voltage. The most informative way to observe any differences is to subtract the input voltage from the attenuated output signal and display the result on an oscilloscope. For useful results careful balancing of amplitudes and phase shifts is needed so that the nonlinear distortion alone can be extracted and investigated. The earliest published example of the method I have found appeared in Wireless World in 1953 (E.R.Wigan, "Diagnosis of Distortion" June 1953 pp261-266), and another article (F.Jones, "Dynamic testing of audio amplifiers", Hi-Fi News & Record Review, November 1970 pp 1655, 1657.) mentions that the Acoustical Manufacturing Company Ltd (Quad) had been using this method for the previous 25 years. It was also the subject of my own M.Sc. dissertation in 1978.

Some forms of distortion are of greater importance than others. For example, crossover distortion in class-B amplifiers can increase in percentage as signal level falls. Music often has a low average level and only short duration peaks, and then much of our listening is done at low levels, so reducing crossover distortion needs to be a high priority. I published an output stage design using feedforward to dramatically reduce crossover effects, and this appeared in Electronics World April 1998. This worked well, but I found practical circuits difficult to stabilise. I later tried a simple mosfet amplifier idea just using conventional negative feedback, this was the MJR6, and to my surprise I could get distortion figures as low as the feedforward designs but without the stability problems. This eventually evolved into the MJR7, with even lower distortion, while keeping a simple circuit and a single pair of lateral mosfets at 100mA quiescent current. Adding more parallel mosfets and running them at higher current would reduce distortion further, but unless far higher power output is needed this is an inefficient way to reduce distortion. A more effective method is feedforward, and adding this to make the MJR9 reduced all measurable audio frequency distortion to under -120dB. With distortion already very low that seemed a step too far, with no serious point. It also needed accurate adjustment using test equipment capable of detecting distortion down to around -130dB, so it was not an ideal DIY project. The MJR7 needs no selection or matching of components, and the adjustments need only a standard multi-meter.

Harmonic distortion figures are believed by some to be an inadequate measure of amplifier performance, so to check that there are no overlooked effects only apparent when listening to music with a speaker load I also carried out tests using the direct comparison method mentioned above. Using a Mordaunt-Short MS20 speaker, with music output a little above my normal listening level, recording the extracted and amplified 'error' signal of the original MJR6 and later listening to this alone the amplifier noise became clearly audible, together with some low level uncancelled music, but distortion is less easily identified. An example of the recorded error signal is included as a ten second wav file (1.69MB). Although amplified considerably this is still at a very low level.
If you think you can identify a distortion component in this sample with the volume turned up high, then try reducing the level to the point where the noise component becomes only just audible with an ear close to the speaker, then listen at a typical listening distance, say 3 metres, then imagine how audible the distortion component would now be with loud music being played at the same time. This gives some idea how far below audibility any non-linear distortion from this amplifier really is in normal operation.

This approach is only useful if we wish to design amplifiers to be accurate. Conventional listening tests may lead to entirely different conclusions, and amplifiers which change the signal in some audible way may be preferred by some listeners.

Many published designs are incomplete, requiring the addition of further circuitry to provide speaker protection in the event of an amplifier fault. Without such protection a direct coupled design could apply the full supply voltage to the speaker, causing serious damage. I previously used speaker protection relays, but found these to be too unreliable. My recent mosfet designs include speaker protection in the form of capacitor coupling. This at first looks like a step backwards, output coupling capacitors normally add some distortion, and reduce speaker damping at low frequencies, but by including the capacitors inside the feedback loop these problems are solved, and the damping factor, measured as 1500 at 20Hz, is better than for many direct coupled designs. All my distortion testing, both sinewave and music, includes any effect of the output capacitor, which must therefore be very low.