Clipping (audio) - Sources of Clipping

Sources of Clipping

In analog audio equipment, there are several causes of clipping:

1. The peak-to-peak of a solid-state transformerless amplifier (most integrated circuit and discrete solid state circuits) is limited to the power supply voltage less a small amount that depends on the design of the circuit (especially the driver configuration) and the saturation voltage (V_ce(sat) for bipolar transistors, or R_ds(on) for Field Effect Transistors), and further reduced if the output stage does not have a quiescent DC output voltage set to half the supply voltage. For example, with a typical operational amplifier the Absolute Maximum Rating for the supply voltage is 36 volts, but a safe operating design supply voltage is 30 volts; if this was supplied as a perfectly balanced +15V and -15V then the theoretical peak output for an ideal rail-to-rail output opamp would be 15 Volts peak (10.6V RMS, 30V peak-to-peak), but a real-world opamp such as the 741 is likely to only be able to drive about 10 volts peak into loads above 2 kilohms, i.e. about 7.1V RMS).
2. An amplifier may have an asymmetrical output swing, perhaps because a transistor is biased so its collector voltage is not half the supply voltage (or the "balanced" power supply rails aren't perfectly balanced); clipping may begin earlier on one half of the output waveform. Bootstrapping or a redesign of the circuit may alleviate this when it is caused by difficulties in driving emitter follower output stages.
3. If the power supply capacitor is no longer able to keep the voltage "flat" due to a massive current draw, the positive and negative voltage supply of the amplifier will fluctuate resulting in sort of a clipped signal that contains some AC line frequency harmonics.
4. A vacuum tube can only move a limited number of electrons in an amount of time, dependent on its size, temperature, and metals. Usually fall-off in amplification with increasing output current results in "soft clipping".
5. Amplifying devices may also have limits on their inputs, for example the maximum base current a bipolar transistor can take, and a vacuum tube may have problems with grid current if the input signal becomes too positive. These factors can distort (clip) the input signal, if it comes from a high enough impedance source, or destroy the device (and so the designer will probably employ other limiting circuits; see below).
6. A transformer (most commonly used between stages in tube equipment) will clip when its ferromagnetic core becomes electromagnetically saturated.
7. An amplifier can limit the current output, or the input voltage, for a variety of reasons both intentional or not. Intentional limiting circuits would not be expected to come into effect in normal operation, but when the output load resistance is too low or the system is connected to an exceptionally high signal level, for example. The result of this form of clipping might not create a flat top to the Voltage waveform, but rather a flat top to the current waveform.
8. Certain signal processing elements can produce a unique form of phase-inverted clipping when the input signal exceeds the common-mode input range of an opamp. The result is that the voltage waveform clips, but in the wrong direction.

Some audiophiles believe that the clipping behavior of vacuum tubes (especially when used with little or no negative feedback) is superior to that of transistors, in that vacuum tubes clip more gradually than transistors, resulting in harmonic distortion that is generally less objectionable. This gradual onset of clipping is known as gain compression or "soft clipping". Circuits can be designed using either tubes or transistors to achieve this effect, and the behavior can be simulated with digital processing.