DE3337072C2 - Low frequency amplifier - Google Patents

Abstract

The invention relates to a low-frequency amplifier, consisting of a processing circuit (1) and a power amplifier (2) with loudspeaker (3), in which a switch (4) in the area between the processing circuit (1) and to suppress crackling noises when the supply voltage is switched on and off the loudspeaker (3) and a control circuit (5) are provided which controls the switch (4) in such a way that both within a first period of time from the time the supply voltage is switched on until at least the time the steady state of the individual DC voltages located at the switching points, and no signal transmission from the processing circuit (1st ) to m loudspeaker (3).

Description

The invention relates to a low-frequency amplifier according to the preamble of claim 1.

When switching on and off the supply voltage of such an amplifier occur in the loudspeaker Clicking noises that are very much to the human ear are uncomfortable and can damage the loudspeaker as the power amplifier is full is controlled through .. These clicks can occur in all stages, especially if they contain numerous DC-coupled individual amplifiers, as is always the case in integrated circuits

There are already circuits known (DE-PS 23 22 317, DE-AS 27 08 055), with which the emergence

half of a second period of time from a point in time fts), which is shortly after the point in time (U) when the supply voltage (U ₀ ) is switched off, until at least the point in time (te) when the decay

the individual circuit points are equal to 20 such clicks within a power voltage, the signal transmission line is avoided, for example by being short-circuited accordingly to ground, characterized in that the control circuit (5) contains a low-pass filter and a high-pass filter, both of which are a resistor (6, 8) with a capacitor (7, 9) lying in series 25, the inputs of both passes being connected to the supply voltage (Uo) , while the output of the low-pass filter (6, 7) is via a series resistor (10) and the output of the high-pass filter (8, 9) via one of the polarity 30 approximate circuits, etc., is due to the large number of DC-coupled stages corresponding to the supply voltage (U ₀ ) on the voltage wave blocking diode (11) with the input of a switching element (12 ) are connected to a threshold characteristic, the output of which controls the switch (4) influencing the signal transmission. 2 . Low-frequency amplifier according to claim 1, ge

Corresponding dimensioning of a negative feedback from the output to the inverting input or by delayed build-up of bias voltages at certain Differential amplifier stages within such a power amplifier.

The generation of crackling noises in the preamplifier, in which z. B. also demodulators, source switches, volume controls, sound controls, matrix

characterized by a diode (13) which is arranged parallel to the resistor (6) of the low-pass filter (6, 7) and is polarized so that it short-circuits this resistor (6) when the supply voltage (U ₀ ) falls.

3. Low frequency amplifier according to one of claims 1 and 2, characterized by a

40 The above-mentioned manner is not possible without significantly impairing the transmission quality of the entire low-frequency amplifier.

Another circuit is known (DE-OS 22 61 139), in which the cracking noises by short-circuiting the signal line to the power amplifier in the respective periods of time can be suppressed with a damping transistor. Because with the control circuit for the damping transistor, the short-circuit times are not optimally attainable, both when switching on and when switching off the operating voltage of the Beginning or end of the working state of the preamplifier via delay circuits accordingly

resistor 45 arranged in series with the diode (11) can be delayed until the attenuation transistor respectively

4. Low-frequency amplifier according to one of claims 1 to 3, characterized by a resistor (24) arranged between the input of the switching element (12) and reference potential.

5. Low frequency amplifier according to one of claims 1 to 4, characterized in that the Control voltage for the switch (4) can be taken from the switching element (12) via a voltage divider (14, 15) is.

6. Low frequency amplifier according to one of claims 1 to 5, characterized in that the Switch (4) consists of a transistor (16) with its main electrode path parallel to a is fully switched through.

The invention aims to provide a remedy here. The invention is based on the object of the click disturbances to keep away from the loudspeaker without the effort required in the known circuits is necessary. This task is done with the characteristics solved according to the characterizing part of claim 1

With. The advantages achieved by the invention are in particular that during or shortly after the start or switching off the supply voltage of the low-frequency amplifier, no clicking noises can be heard and that the transmission quality of the amplifier is still maintained.

An implementation example! the invention is in the Resistor (17) is arranged, which is shown between the 60 drawing and will be explained in more detail below

The input of the power amplifier (2) and a bias voltage source bridged by a capacitor (18) are connected, while the input of the power amplifier (2) is connected to the output of the preamplifier (1) via a coupling capacitor (21). connected is.

7. Low frequency amplifier according to one of claims 1 to 6, characterized in that in Se- purifies.

It shows

F i g. 1 is a block diagram of the low-frequency amplifier according to the invention,

F i g. 2 a more detailed circuit diagram of this amplifier and

F i g. 3 voltage diagrams for the voltage curve in the control circuit.

According to FIG. 1 contains the low frequency amplifier with a preamplifier and a power amplifier 2 a loudspeaker 3. A switch 4 at the input of the " , Power amplifier 2 is controlled by a control circuit 5 such that within the time periods in which cracking disturbances can arise in the processing circuit 1, namely after the Eir.-i and switching off the supply voltage i / o, no audio frequency signals are transmitted from the conditioning circuit 1 to the power amplifier 2, namely by short-circuiting its input terminal to ground.

According to FIG. 2, the control circuit 5 contains a low-pass filter consisting of a resistor 6 with a parallel-connected diode 13 and a capacitor 7, and a high-pass filter consisting of a resistor 8 and a capacitor 9, the inputs of both passes being connected to the supply voltage U ₀ . The output of the low-pass filter 6.7 is connected via a series resistor 10, the output of the high-pass filter 8.9 via a diode 11 and a series resistor 23 to the base of a transistor 12, which serves as a switching element with threshold characteristics and connected to ground via a resistor 24 . At the collector of the transistor 12 there is a voltage divider 14, 15, to whose tap the control electrode of a switch 4 (FIG. 1) influencing the signal transmission is connected

This switch 4 consists of a transistor 16 which is arranged with its emitter-collector path parallel to a resistor 17, which is between the input of the power amplifier 2 and a bias voltage source consists of a voltage divider 19, E) The signal voltage supplied by the processing circuit 1 is running Via a coupling capacitor 21 and a series resistor 22 to the input of the amplifier 2, at which Output of the loudspeakers 3 and a negative feedback circuit 25, 26, 27 connected to the inverting input are.

In Fig. 3 shows the voltages Uo to Us as a function of time, as they occur when the supply voltage U _{0 is switched} on and off at the corresponding points in the circuit according to FIG. 2 occur.

It mean

ii = time of switching on the supply voltage U ₀ ,

t ₂ = time at which the steady state of all voltages within the conditioning circuit I _{1 is reached}

f3 = time at which the switch-on threshold is reached of transistor 12,

U = time of switching off the supply voltage LO,

f ₅ = time of reaching the turn-off threshold of transistor 12 and

/ β = time at which all stresses disappear within the processing circuit 1.

The supply voltage LO shows a relatively steep rising or falling edge when it is switched on and off (ti and / 4). The steepness of these edges depends on the time constant of the voltage supply circuit.

The voltage U \ at the output of the low-pass filter 6, 7 has a very flat rising edge when switched on (u) , which depends on the time constant of the low-pass filter 6, 7, with the diode 13 being blocked. When switching off (u), on the other hand, the diode 13 becomes conductive because the voltage on the capacitor 7 is initially maintained while the voltage U ₀ drops 13 and the capacitance of the capacitor 7 is formed. This results in a steeper falling edge when switching off (U).

The voltage LA at the output of the high-pass filter 8, 9 has a steep rise when switched on (ti) (like the voltage Co) and then a flat drop corresponding to the time constant of the high-pass filter 8, 9. When switching off (u) , the same lead results , but in the negative direction. Of these positive and negative voltage profiles, the diode 11 only allows the negative ones to pass, so that the voltage profile I / 3 appears behind this diode 11.

The voltage Ua results from the addition of the voltages U \ and L / 3. When it is switched on (ti) it has a flat rise (corresponding to the voltage Ui), when it is switched off (u) it has a steep fall, which is even faster than the fall in the voltage U3.

This course of the voltage Ua ensures that the switching thresholds U _em and U _{out of} the transistor 12 Cf ₃ ) are exceeded for a sufficiently long time after switching on (t 1) or very quickly (ts) after switching off (u), whereby at the collector of the transistor 12 results in the voltage Us with which the transistor 16 is activated, which thus removes the short circuit of the connection line between the preamplifier 1 and the power amplifier 2 long enough (h) after switching on (ti) and immediately (t 5 ) after switching off (u)

This switching process itself does not cause any clicks because the DC voltage conditions at the input of the power amplifier 2 do not change. B: i non-conductive transistor 16 is this input over the resistor 17 at a fixed bias. Since the transistor 16 in the conductive state only the resistance 17 bridged, the input of the power amplifier 2 is then also connected to this fixed bias voltage, via the emitter-collector path of the transistor. A jump in DC voltage occurs accordingly not on.

60 2 sheets of drawings

65

Claims (1)

Patent claims: 1. Low frequency amplifier, consisting of a preamplifier (1), a power amplifier (2) with Loudspeaker (3), a switch (4) for short-circuiting the signal transmission line in the area between the preamplifier (1) and the loudspeaker (3) and a control circuit (5) that controls the switch (4) controls in such a way that both within a first period of time, which extends from the point in time ft \) of switching on the supply voltage (U ₀ ) to at least the point in time ft ₂ ) when the steady state of the DC voltages at the individual circuit points is reached, as well as inner A series resistor (22) is inserted into the coupling capacitor (21)