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Audio signal clippers.

Contents


Introduction.

Audio signal clippers are used for a number of applications:


Soft-clippers I.

2 simple soft-clipper schematics.
Fig.1: 2 Simple soft-clipper schematics.

Fig.1a is the simplest possible soft-clip circuit. It relies on the logarithmic relationship between diode voltage and current.
The output voltage of the circuit is shown in fig.2 below. It can be seen that the output voltage depends on the current through the diodes.
The output voltage can be increased by adding more diodes in series or by using other types of diodes.
I have seen suggested that germanium diodes should give a "softer" clipping, but I do not have any to test with, so I can not comment on that.
A "softer" clipping can also be obtained by adding a small resistor ( 5 to 10 Ω ) in series with the diodes.
The clip level is temperature dependent ( approximately 1.5 dB over 50 °C for silicon diodes ), but this is irrelevant for many applications.

AC relay with power factor correction.
Fig.2: Soft-clipper output voltage.

Lime: Output signal with silicon diodes, 2 Vp input voltage and series resistor of 470 Ω ( top ), 1 kΩ, 2.2 kΩ, 4.7 kΩ and 10 kΩ.
Blue: Output signal with schottky diodes, 2 Vp input voltage and series resistor of 470 Ω ( top ), 1 kΩ, 2.2 kΩ, 4.7 kΩ and 10 kΩ.

Fig.1b is a circuit I have seen in some amplifiers powered from an unregulated supply.
The clip level track the supply voltages with reasonable accuracy provided there is a V or more on the transistor bases.
The capacitors to GND reduce the mains-ripple on the bases.


Soft-clippers II.

Soft-clipper / limiter schematic.
Fig.3: Soft-clipper / limiter.

Very simplified schematic for soft-clipper / limiter.
The circuit detects the amplifier clipping point by measuring that the amplifiers loop-gain begins to fall.
This happen when maximum output voltage or current is reached and the signal level inside the feedback loop will increase.
When this happens, the LED will light up and the resistance in the LDR fall, decreasing the input signal.
In this way, the feedback loop is held in regulation, hard internal saturation is avoided and the output impedance remains low.

This circuit is not really a clipper ( although the manufacturer labeled it "soft-clip" ), but a limiter as the release time constant for the LDR is around 50 ms.


Clipper.

A clipper is normally used if you have an absolute maximum signal level.

AC relay with power factor correction.
Fig.4: Clipper.

The 2 amplifiers A2 and A3 detects when the output voltage is outside the reference voltages Vref+ and Vref-.
When that happens, their outputs will clamp the output signal to the same level as the reference voltages.
A1 is required to drive the rectifier circuits.
The plot below is a simulation of the circuit with TL072 OP-AMPs.

Clipper waveforms.
Fig.5: Clipper waveforms.

Lime: Input signal, 10 Vp, 20 kHz.
Blue: Output signal.
Red: Signal on A2 output.
Cyan: A1 output current.

As can be seen from the plot, there is around 10% overshoot on the output waveform.
This overshoot is reduced at lower frequencies.
It is caused by the slew-rate ( simulated 13 V/µs ) of U2 and U3 and the 22 pF capacitors.
The capacitors were included in the simulation as a real circuit may need them for stability.


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