Limiter average audio

This limitation does not include any audio or MOSFET or JFET transconductance amplifier difficult to understand and use, I'm not the author of this configuration but, place here this circuit to make it usable for all. Its gain variation is based only on diodes , is used per avoid excessive final amplifier saturation next, it is useful to have an amplitude of output that varies with the input amplitude

as the following graphic:

This is not the instantaneous voltage, but the average value.

The limiter does not stop the input signal, but attenuates it in its entirety, as if the amplifier volume knob was turned to “turn down the volume”.

Principle of the audio limiter

The limiter is based on a divider bridge with 2 resistors. The controlled resistor acts attenuation output. The lower this resistance, more the output is attenuated. The objective is to ensure that the variable attenuation is suitable to the input voltage.
The controlled resistor is based on dynamic change in resistance of a diode in “curve” of its usual. The dynamic resistance Rd (in Ohm) a diode is given by the relation: Rd = 0,026 V / I (a 25 ° C) I is the current (in Ampere) flowing in the diode. Most limiters shifts the average value of the signal (via a connecting capability for biasing a transistor), that can create annoying unwanted noise signals to the speakers when they come into action.

A symmetrical setup based on 2 series of diodes avoids this problem. Each diode is connected to a variable voltage symmetrically.

Diagram and study of audio limiter

First of all, we note that there are 5 diodes in series. This ensures a low signal distortion up to peak amplitude of between 1,5 V e 2 V. We have a margin compared to the standard signals (peak 1 V max). The diagram of the audio limiter is divided into blocks. The threshold voltage is an adjustable DC voltage. This is the audio limiter threshold. If you set 1,5 V, the output signal will be limited to between 1,5 V e -1,5 V (from peak to peak amplitude) remaining unaltered below this threshold.

Operating the Audio Limiter

If the input audio signal never exceeds (at most) the voltage Vref, the output of the comparator (A) It is always low. The control voltage (in verde) from the peak detector is zero. The diodes are blocked and there is no attenuation. In reality there is a bit 'attenuation because the impedance of the amplifier input connected to the limiter form a divider bridge with 10k + 1K (11k). If the input audio signal exceeds the threshold voltage, the comparator switches high and charges the capacitor by 4,7uF via the resistor 1k and the diode 1N4148 (peak detector). The salt control voltage a few volts (typical from 2 a 5 V). Conversely, the inverting amplifier (B) creates a reverse control voltage (in yellow). The variable attenuation comes into action and reduces the amplitude of the output signal until the non-inverting input reaches the threshold voltage. When the input audio signal decreases, the capacitor discharges through the circuit components. The time constant must be chosen sufficiently low so that the attenuation is interrupted rather quickly (from about 0,2 a 0,5 s = “release time”) if the audio input level back to an appropriate value. But it must be large enough so as to be weakly affected by the low input signal frequencies. Otherwise, there would be a severe distortion of the audio signal.

The maximum peak voltage adjustment occurs at the midpoint between the two input resistors. This means that once the limiter came into action, the greater the level of input, the lower the output level.

The maximum attenuation is achieved with a high voltage control (circa 5 V), remain the two resistors 3K3 if the dynamic resistance of the diode tends to zero. The equivalent model is then 3k3 // 3K3 = 1K65 connected to pulp.

The gain is then: 1.65 / (1,65 + 11) = 0.13

Attenuation is -18dB (= 20log (0,13)). This value is more than sufficient. In practice, the value that is obtained with this circuit is -16 dB seen that in any case the resistance of the diodes never drops to zero.

If they continue to increase the audio level, the audio output level will be attenuated proportional to the maximum attenuation value. For operational you can choose a TL072. The choice is not critical. Note that the audio signal does not pass through the operational amplifiers!

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