GB2214372A - Gain control circuit - Google Patents

Abstract

A gain control circuit PL has positive feedback means A1, R1 coupled to the output of the circuit PL so as to increase the gain of the circuit PL. The positive feedback provided by A1, R1 compensates for the wiper resistance of the potentiometer PL. Described is a sound control circuit having substantially identical circuits for both the left and right channels. The signal is fed in each channel through a respective potentiometer (PL, PR) for varying the gain of each channel. The output of each potentiometer (PL, PR) is fed back by way of a positive feedback circuit (A1, R1 : A2, R2) incorporating a buffer amplifier (A1; A2) configured to have a small gain. The positive feedback increases the off-ratio of the circuit and optimises the control law. <IMAGE>

Description

ELECTRICAL CIRCUIT APPARATUS The present invention relates to electrical circuit apparatus.

According to the present invention there is provided apparatus comprising an electrical circuit including control means for varying the gain of said electrical circuit, and positive feedback means coupled to the output of said electrical circuit, wherein said feedback means include gain increasing means and are coupled to the electrical circuit to augment the gain.

Generally, positive feedback is avoided in electronic circuitry, other than in oscillators, because it can cause instability. However, in the apparatus of the invention the control means control the augmented gain of the electrical circuit.

The gain increasing means of the feedback means is arranged to have a gain which is small compared to the maximum gain of the electrical circuit. Of course, when the gain of the electrical circuit is substantially completely attenuated, the signal fed back by the feedback means is zero.

The electrical circuit may be any circuit or combination of circuits including said control means.

For example, the electrical circuit could be a series combination of a summing amplifier and a fader circuit.

Said control means is generally a potentiometer.

In a particular embodiment said positive feedback means is connected to the wiper of said potentiometer.

In an embodiment said feedback means comprises an amplifier configured to have a small gain, the output of the amplifier being connected to the potentiometer wiper by way of a resistance.

In a preferred embodiment the electrical apparatus comprises a sound control circuit having substantially identical circuits for both the left and right channels, each channel circuit comprising a linear potentiometer arranged to receive the signal, the potentiometer wiper providing the output, and feedback means as defined above connected to the wiper of the potentiometer.

Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a conventional sound control circuit with a panoramic potentiometer and an associated routing matrix, Figure 2 is a graph showing the levels of the left and right channels of the circuit of Figure 1 with respect to the position of the potentiometer, Figure 3 shows a sound control circuit of the invention, and Figure 4 is a graph showing the levels of the left and right channels of the circuit of Figure 3.

All mixing consoles have a stereo sound control circuit to move the sound left or right between a stereo pair of loudspeakers. This is done by varying the proportions of the sound sent to the left and right channels, for example, by way of a potentiometer circuit. Ideally, the amount sent to the left or right channels should vary as the sine or cosine of the movement of the wiper of the potentiometer such that the sum of the squares of the two channel amplitudes is kept constant. This prevents the apparent loudness of the sound changing as it is moved between the speakers. However, the cost of using precision potentiometers to produce such a characteristic is prohibitive and generally a standard linear potentiometer is used and its normally linear law is "bent" by the use of additional fixed resistors.

67hen the audio system is in stereo the two channels reaching the listeners ears are not strictly in phase because of the different distances between the ears and the loudspeakers. The two channels therefore only partially add giving a 3dB increase in level. Hence in this situation in its mid-position, the potentiometer circuit is required to attenuate each channel by 3dB to keep central images at the same loudness. However, when the signal received is in mono, for example from PPl radio, the left and right channels have already been electronically summed exactly in phase and so there is a 6dB increase in level. In this situation, in its mid-position the potentiometer circuit should attenuate the signals of each channel by 6dB.Furthermore, instead of a sine/cosine law, the potentiometer should provide a sine-squared/cosine-squared law.

In practice, to satisfactorily deal with both stereo and mono input signals, potentiometer circuits are given compromise characteristics providing an attenuation of 4.5dB at their mid-position and having a characteristic which attempts to approximate between a sine law and a sine-squared law.

This compromise is illustrated in Figure 2 which shows the level in dB's of the left and right channels of the conventional sound control circuit of Figure 1 with respect to the position of the circuit potentiometer. In Figure 2, the trace L is that of the left channel and the trace R is of the right channel. In addition, the curve SIN shown in dotted lines is an ideal sine characteristic, and the curve SIN2, again shown in dotted lines, is a sine-squared curve.

Figure 1 shows a conventional sound control circuit panoramic potentiometer and a routing matrix circuit. It will be seen that the circuits for each of the left and right channels are substantially identical. The audio signal is input at terminal S and each channel is fed to a respective potentiometer PL,PR. These potentiometers PL,PR are ganged together with their wipers moving in opposite directions. In Figure 1, each of the potentiometers PL,PR is illustrated by an equivalent circuit showing not only the potentiometer P itself, but also a representation of its unwanted internal resistances comprising a wiper resistance RNZ and terminal resistances RT. In each channel a respective "law-bending" resistor R1, R2 is connected between the potentiometer wiper and its input terminal.The resistor R1, R2 is arranged to give an attenuation of 4.5dB to the input signal when the potentiometer wiper is in its central position rather than the 6dB level which would be obtained without these resistors.

Mixing consoles typically have a bank of switches on each input channel to determine the destination of the signal by selecting a group to which it is to be fed, for example, as is illustrated in Figure 1.

Since consoles may have up to 32 of these groups, the size and cost of the bank of switches, referred to as the routing matrix, is significant. It is therefore a practice to cause the switching to be done in pairs to halve the number of switches required. Selection between the two groups of the pair is then done by moving the signal fully to the left channel or fully to the right channel. Of course, to achieve a good performance it is necessary that when the potentiometer in one of the channels is moved to its off position, its attenuation is substantial, for example at least 80dB.

In a circuit such as that of Figure 1, the resistors R1 and R2 produce a poor off-ratio in each channel of about 65dB. This is because, when the wiper of the respective potentiometer PL,PR is at its fully off position, there is a voltage drop through its wiper resistance RW and a terminal resistance RT which reduces the attenuation.

It will be seen that Figure 1 includes respective buffer amplifiers Al and A2 between the potentiometers PL,PR and the routing matrices. These are required to prevent the routing matrix producing an adverse loading effect on the potentiometer circuit.

A sound control circuit of the invention is shown in Figure 3. Again, the signal is fed in each channel through a respective potentiometer PL,PR and the output is taken off via the wiper of the potentiometer. Of course, these potentiometers PL,PR will have wiper and terminal resistances but these are not illustrated in Figure 3.

The output of each potentiometer PL,PR is fed back by way of a positive feedback circuit which incorporates a respective law-bending resistor R1,R2.

Each positive feedback circuit ensures that the respective resistor R1, R2 is not effective when the potentiometer wiper is positioned to give maximum attenuation such that there is no reduction of the maximum attenuation by way of the internal resistance RNl.

It will be seen that each positive feedback circuit comprises a buffer amplifier Al, A2 which is configured to have a small gain, typically 2dB. The output of the buffer amplifier A1,A2 is fed back to the wiper of the appropriate potentiometer PL,PR by way of the "law-bending" resistor R1, R2. The small increase in gain attributable to the buffer amplifiers can be allowed for later in the mixing console circuit.

(;hen the potentiometer is at its fully on position there is no significant effect on the signal output from the potentiometer. 67hen the potentiometer wiper is at the other end of its track, there is virtually no signal at the output of the appropriate amplifier A1,A2 and therefore nothing is fed back through resistor Rl or R2 so that there can be no unwanted voltage drop across the wiper resistance R.

However, when the wiper of the potentiometer is substantially in its midway position the signal fed back to the wiper through R1 or R2 augments the gain as in this position the signal from R1 or R2 is not greatly shunted by the output impedance of the potentiometer.

Figure 4 shows the signal levels achievable by the circuit of Figure 3 with respect to the position of the potentiometer wiper. It will be seen that the left and right traces are a much better approximation to a sine/sine-squared compromise. In addition, the off-ratio of the circuit of Figure 3 is much better than with the conventional circuits. The circuit of Figure 3 has been measured at a consistent - 90dB.

This is an improvement of 25dB, or in other words, by a factor of about 20, as compared with a circuit as the one shown in Figure 1.

In the embodiment particularly described above, the sound control circuit includes means, specifically a potentiometer, for varying the gain of the circuit.

The positive feedback circuit is used to increase the off-ratio and to optimise the control law of the circuit. However, the use of positive feedback with an electrical circuit incorporating controllable gain varying means has other applications, not only in optimising control laws, but also in improving parameters such as noise and headroom. This technique, for example, can be used to improve the control law of active gain controls which use linear potentiometers to control the gain of two stages at once. Alternatively, the positive feedback technique can be used to improve the off-ratios of fader circuits employing linear potentiometers. It can also be used to improve the performance of a summing-amplifier connected in series to a fader. In this respect, the positive feedback loop would couple the output of the fader to the summing input of the summing-amplifier.

It will be appreciated that other modifications and variations in the circuits as described above may be made within the scope of the present invention.

Claims (9)

1. Apparatus comprising an electrical circuit including control means for varying the gain of said electrical circuit, and positive feedback means coupled to the output of said electrical circuit, wherein said feedback means include gain increasing means and are coupled to the electrical circuit to augment the gain.

2. Apparatus as claimed in Claim 1, wherein the gain increasing means of the feedback means is arranged to have a gain which is small compared to the maximum gain of the electrical circuit.

3. Apparatus as claimed in Claim 1 or 2, wherein the electrical circuit is a series combination of a summing amplifier and a fader circuit.

4. Apparatus as claimed in any preceding claim, wherein said control means is a potentiometer.

5. Apparatus as claimed in Claim 4, wherein said positive feedback means is connected to the wiper of said potentiometer.

6. Apparatus as claimed in Claim 5, wherein said feedback means comprises an amplifier configured to have a small gain, the output of the amplifier being connected to the potentiometer wiper by way of a resistance.

7. Apparatus as claimed in Claim 1 or 2, wherein the electrical circuit comprises a sound control circuit having substantially identical circuits for both the left and right channels, each channel circuit comprising a linear potentiometer arranged to receive the signal, the potentiometer wiper providing the output, and positive feedback means connected to the wiper of the potentiometer.

8. Apparatus as claimed in Claim 7, wherein said feedback means for each channel circuit comprises an amplifier configured to have a small gain, the output of the amplifier being connected to the respective potentiometer wiper by way of a resistance.

9. Apparatus comprising an electrical circuit substantially as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.