Anti-pop circuit for AC amplifiers

The anti-pop circuit includes a unity gain buffer with an input coupled to the source of the reference voltage and an output coupled to the input of the amplifier to accelerate the charging of the input coupling capacitor of the amplifier at every turn-on. The capacitor-charging buffer is automatically disabled before the turning-on of the amplifier. The charging buffer may be enabled at start up by generating an impulse of a pre-established duration at the turn-on instant by a monostable circuit or by disabling it upon verifying the decaying to zero of the charging of current of the input coupling capacitor. The circuit eliminates the popping noise at the turn-on without an excessive delay of the turning-on of the amplifier.

FIELD OF THE INVENTION
 The present invention relates to low frequency AC amplifiers, and more
 particularly, to single-ended audio amplifiers biased at a fixed reference
 voltage.
 BACKGROUND OF THE INVENTION
 When single-ended audio amplifiers biased at a fixed reference voltage are
 turned on, the coupling capacitor of the AC signal at the input of the
 amplifier is uncharged and at the turn-on instant the charge transient of
 the input capacitance may cause an undesired typical noise (pop). This
 disturbance occurs when the final stage of the amplifier is turned on
 while the charging of the input capacitance is still insufficient.
 The phenomenon is highlighted in FIGS. 1A and 1B, in which the functional
 scheme of a single-ended amplifier and the relative diagrams of the
 turn-on transient are depicted. If at the instant To the source of the
 reference voltage Vref is turned on, because of the still incomplete state
 of charge of the input capacitance Cin, the input voltage Vin (that is,
 the voltage on the noninverting input (+) of the xA amplifier) will not be
 equal to the reference voltage Vref applied on the inverting input (-) of
 the amplifier. If the output power stage of the amplifier is turned on,
 for example, at the instant T1 (which may coincide with the turn-on
 instant To), the signal output is equal to Vout=A.times.Vref
 exp(-t/(Rin*Cin)) and causes a typical popping noise.
 The level of this popping noise is commonly reduced by delaying the turn-on
 of the final stage of the amplifier in respect to the turn-on instant of
 the source of the reference voltage Vref. However, this approach cannot
 completely overcome the problem unless the delay is very long. Another
 known technique includes doubling the input stages, in order to "mute" the
 amplifier during the turn-on transient, however more complex circuits are
 needed and an appropriate time constant is programmable through a
 dedicated pin of the device to retard the switching from a mute to a play
 condition.
 SUMMARY OF THE INVENTION
 An object of the present invention is to provide an anti-pop circuit that
 overcomes the above noted drawbacks of the known techniques. The circuit
 of the invention uses a simple approach and entails markedly shorter time
 constants than those that are commonly required according to the known
 techniques.
 The anti-pop circuit of the invention includes a unity gain buffer with an
 input coupled to the source of the reference voltage Vref and an output
 coupled to the input of the amplifier to accelerate the charging of the
 input coupling capacitor of the amplifier at every turn-on. The
 capacitor-charging buffer is automatically disabled before the turning-on
 of the amplifier, i.e. the turning on of the output power stage of the
 amplifier. The charging buffer may be enabled at start up by generating an
 impulse of a pre-established duration at the turn-on instant by a
 monostable circuit or by disabling it upon verifying the decaying to zero
 of the charging of current of the input coupling capacitor.
 The device of the invention eliminates the popping noise at the turn-on
 without an excessive delay of the turning-on of the amplifier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 With reference to FIGS. 2A and 2B, starting from the turn-on instant To,
 the input coupling capacitor Cin is charged, up to the reference voltage
 Vref by the unity gain buffer x1 for a time interval established by the
 disabling command CONTROL. The buffer x1 is disabled to avoid any effect
 on the normal functioning of the amplifier xA. The effect of the buffer x1
 is highlighted in the relative diagrams (FIG. 2B) of the turn-on transient
 where it may be observed how the voltage on the input node Vin of the
 amplifier xA rapidly reaches the reference value Vref by virtue of the
 quick charging of the input coupling capacitance Cin by the buffer x1.
 The time of enablement of the charging buffer x1 may be easily established
 to ensure a complete charging of the input capacitance Cin to Vref. The
 delayed turn-on, at the instant T1, of the output power stage of. the
 amplifier xA occurs simultaneously or after the disabling of the charging
 buffer x1 and, as highlighted in the diagram of the output voltage, any
 pop phenomenon is eliminated.
 The CONTROL command, enabling/disabling the charging buffer x1 of the input
 capacitance may be generated by a monostable circuit, referred to as
 one-shot timer, or by a dedicated circuit monitoring the decay to zero of
 the charging current of the input capacitor Cin when it reaches the Vref
 voltage. That is:
EQU Vin=VreforIC=0
 Preferably, the buffer x1 used to speed-up the charging of the input
 coupling capacitor Cin at turn-on should have a low offset because its
 offset will be replicated at the output of the amplifier xA multiplied by
 the gain of the amplifier.