Anti-pop circuit for AC amplifiers

Details Anti-pop circuit for AC amplifiers Anti-pop circuit for AC amplifiers

1999-12-14

2001-07-03

Shingleton, Michael B (Department: 2817)

Amplifiers

Combined with automatic amplifier disabling switch means

C330S149000, C381S094100, C381S120000, C381S094500, C381S094800

Reexamination Certificate

active

06255904

ABSTRACT:

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 T
1
(which may coincide with the turn-on instant To), the signal output is equal to Vout=A×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.


REFERENCES:
patent: 5317280 (1994-05-01), Zimmer et al.
patent: 5642074 (1997-06-01), Ghaffaripour et al.
patent: 5642076 (1997-06-01), Naokawa
patent: 5796303 (1998-08-01), Vinn et al.
patent: 5982234 (1999-11-01), Compagne

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