GB2315173A - Audio amplification systems - Google Patents
Audio amplification systems Download PDFInfo
- Publication number
- GB2315173A GB2315173A GB9614470A GB9614470A GB2315173A GB 2315173 A GB2315173 A GB 2315173A GB 9614470 A GB9614470 A GB 9614470A GB 9614470 A GB9614470 A GB 9614470A GB 2315173 A GB2315173 A GB 2315173A
- Authority
- GB
- United Kingdom
- Prior art keywords
- amplifier
- output
- gain
- amplification system
- control circuit
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
- 230000003321 amplification Effects 0.000 title claims abstract description 25
- 238000003199 nucleic acid amplification method Methods 0.000 title claims abstract description 25
- 230000005236 sound signal Effects 0.000 claims abstract description 17
- 230000003278 mimic effect Effects 0.000 description 9
- 239000003990 capacitor Substances 0.000 description 4
- 238000010586 diagram Methods 0.000 description 4
- 230000006835 compression Effects 0.000 description 2
- 238000007906 compression Methods 0.000 description 2
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000000034 method Methods 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03G—CONTROL OF AMPLIFICATION
- H03G3/00—Gain control in amplifiers or frequency changers
- H03G3/20—Automatic control
- H03G3/30—Automatic control in amplifiers having semiconductor devices
- H03G3/3005—Automatic control in amplifiers having semiconductor devices in amplifiers suitable for low-frequencies, e.g. audio amplifiers
Landscapes
- Engineering & Computer Science (AREA)
- Multimedia (AREA)
- Amplifiers (AREA)
Abstract
An audio amplification system comprises an audio amplifier of controllable gain 2, a power amplifier 3 for connection to a loudspeaker drive unit 4 and a gain control circuit 5. The power amplifier 3 is in general connected to an unregulated power supply to obtain a short term power significantly greater than the sustainable output power (advantageous when the input audio signal is a music signal containing a series of impulses). According to the invention, the amplifier (8) of the gain control circuit 5 receives some information regarding the instantaneous voltage of the power amplifier's power supply by deriving its power supply therefrom and applies a signal to control the gain of amplifier 3 when clipping occurs in the amplifier (8) output thereby preventing clipping in the power amplifier 3. The gain control circuit 5 may include an error amplifier, a full wave rectifier and a low pass filter between the amplifier (8) and the control input of controlled amplifier 2.
Description
Audio Amplification Systems
This invention relates to audio amplification systems in which a power amplifier is used to drive a loudspeaker drive unit.
Audio signals occurring, for example, in hi fi systems have a wide dynamic range so that if the average power level is set high, the loudest signals can be beyond the available capacity of the system. If the loudest signals are within the capacity of the amplification system but beyond the capacity of the loudspeaker drive unit, the speaker drive unit will be driven against its end stops and distorted sound will be the result.
Driving an amplifier beyond its capacity gives rise to the phenomenon known as "clipping" in which the extreme excursions of a waveform are cut off because the system is unable to follow them. The peaks of a sine wave, for example, become clipped off into plateaux.
Again, the sound heard by the listener will be distorted.
It is known to use "compression" on audio signals in order to increase the average level of the audio signal.
In an implementation of such a technique, the audio signal passes through a stage of variable gain of which the gain is decreased as the level of the audio signal increases. By that means, the dynamic range is reduced at the expense of the introduction of some distortion which is, however, not so objectionable as the "hard clipping" when the peaks of a sine wave are changed into plateaux. Nevertheless, it is still possible to overload such a system and cause hard clipping to occur by making the average power level too high.
It is an object of the invention to provide an amplification system in which the risk of the occurrence of hard clipping can be reduced.
The present invention provides an audio amplification system comprising:
an audio amplifier of controllable gain having an input to receive an audio signal to be amplified and an output for the amplified audio signal,
a power amplifier having an input connected to the output of the audio amplifier and an output for connection to a loudspeaker drive unit, and
a gain control circuit having an input connected to receive the audio signal and an output connected to control the gain of the amplifier of controllable gain, characterized in that:
the gain control circuit includes an amplifier arranged to receive its power supply from the power supply of the power amplifier by way of voltage reduction means,
the said amplifier of the gain control circuit is arranged to amplify the audio signal and to produce a signal to reduce the gain of the amplifier of variable gain when clipping occurs at the output of the amplifier of the gain control circuit.
Most power amplifiers in current use operate from an unregulated power supply. This is for reasons of economy of circuitry, and to gain some short-term headroom in respect of the output power. With an unregulated power supply, the short term output power can be significantly greater than the sustainable output power. This can help increase the perceived loudness from the amplifier since music generally consists of a series of impulses. The problem encountered in operating a compressor into such a power amplifier is that a conventional compressor does not take into account the power output capability of the amplifier at any particular instant. Thus its compression has to be set to limit at the sustainable output power from the amplifier. Additional loudness can be gained when, according to the invention, the compressor is fed some information regarding the instantaneous capacity of the power amplifier's power supply.
By suitable choice of the voltage reduction means and the design of the amplifier of the gain control circuit, it can be arranged that clipping occurs in the amplifier of the gain control circuit in advance of clipping in the power amplifier. By this means, it can be arranged that the gain of the amplifier of controllable gain is reduced to prevent clipping in the power amplifier just before the power amplifier would otherwise have begun to exhibit clipping.
Advantageously, the voltage reduction means comprises resistance means. That is a simple and inexpensive way of reducing the voltage of the power supply of the power amplifier.
Advantageously, the output of the amplifier of the gain control circuit is connected in a negative feedback arrangement to an input of that amplifier. In such an arrangement, the negative feedback will normally hold the input signal to that amplifier at a low level, but when the amplifier commences clipping a large error signal appears at its input and can be used to reduce the gain of the amplifier of controllable gain.
Preferably, the amplifier of the gain control circuit is a differential operational amplifier and has its output connected to the inverting input through a resistor. That provides a simple and inexpensive way of implementing the negative feedback circuit.
Advantageously, the differential amplifier is a CMOS type operational amplifier. Such an amplifier has the advantage that its output can swing to within a few millivolts of its available power supply.
Advantageously, the said resistance means comprises a resistive potential divider chain connected across the power supply to the power amplifier. That provides a simple and inexpensive way of providing the amplifier of the gain control circuit with a power supply that can follow variations in the power supply to the power amplifier and at the same time it enables a cheap, low power operational amplifier to be used which would otherwise be damaged by the relatively high voltages normally used for the power supply to power amplifiers.
Preferably, the potential divider chain comprises first, second, third and fourth resistors connected in series, the power supply to the said amplifier of the gain control circuit being derived from the junction of the first and second resistors and the junction of the third and fourth resistors, and the junction of the second and third resistors being connected to common potential. Such an arrangement is simple and inexpensive.
By way of example only, an audio amplification system in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
Fig. 1 is a schematic block diagram of the audio amplification system;
Fig. 2 is a schematic block diagram of a gain computation circuit of the audio amplification system;
Fig. 3 is a circuit diagram of a first amplifier of the gain computation circuit; and
Fig. 4 is a circuit diagram of a second amplifier of the gain computation circuit.
Referring to the accompanying drawings, an audio amplification system 1 comprises an amplifier with voltage controlled gain 2 having an input terminal I for an audio signal to be amplified. The output of the amplifier with voltage controlled gain 2 is connected to the input of a power amplifier 3, to the output of which a loudspeaker drive unit 4 is connected. The output of the amplifier 2 is also connected to an input of a gain control computation circuit 5 to which the state of the power supply of the power amplifier 3 is also supplied and shown as input 6. The output 7 of the circuit 5 is connected as a gain control input to the amplifier 2.
As shown in Figure 2, the gain control computation circuit comprises an amplifier 8 which receives the output of the amplifier 2 as an input signal and has positive and negative power supply inputs 9 and 10 derived from the power supply to the power amplifier 3.
The power supply inputs 9 and 10 mimic variations in the power supply to the power supply amplifier 3 and the associated amplifier 8 is here referred to as a mimic amplifier.
The output of the mimic amplifier 8 is connected to the input of an error amplifier 11. The output of the error amplifier 11 is connected to the input of a full wave rectification circuit 12. The output of the full wave rectification circuit 12 is connected to the input of a low pass filter circuit 13 and the output of the low pass filter circuit provides the input 7 to the amplifier 2.
The circuit of the mimic amplifier is shown in
Figure 3. The power supply to the power amplifier 3 consists of a +40 V rail 14 and a -40 V rail 15. Four resistors R1, R2, R3 and R4 are connected in series from the rail 14 to the rail 15. The junction of resistors R1 and R2 provides the positive power supply line 9 to the mimic amplifier 8, the junction of resistors R2 and R3 is connected to 0 V common potential, and the junction of resistors R3 and R4 provides the positive power supply line 10 to the mimic amplifier.
The active element of the mimic amplifier 8 is a
CMOS type operational amplifier A1. The output of the voltage controlled gain amplifier 2 is connected to the inverting input of amplifier A1 by way of a resistor R5.
The non-inverting input of amplifier A1 is connected to common potential. The output of amplifier A1 is connected back to the inverting input by way of a resistor R6. A high frequency decoupling capacitor C1 is connected across the power supply to the amplifier A1.
The output to the error amplifier 11 is taken from the inverting input of amplifier A1.
The circuit of the error amplifier 11 is shown in
Figure 4. The active element of error amplifier 11 is an operational amplifier A2. The output taken from the mimic amplifier 8 is connected to one end of an input capacitor C2. The other end of input capacitor is connected to one end of a resistor R7, and the other end of resistor R7 is connected to the inverting input of amplifier A2. The non-inverting input of amplifier A2 is connected to common potential. The output of the amplifier A2 is fed back to the inverting input by way of a resistor R8. The power supply to the amplifier A2 consists of +12 V and -12 V and a high frequency decoupling capacitor C3 is connected across the power supply to the amplifier. The output to the full wave rectification circuit 12 is taken from the output of amplifier A2.
In a practical example, the components were of the following values and types:
R1 6.8 kn
R2 1.5 kfl R3 1.5 kfl R4 6.8 kfl R5 100 kn
R6 220 kfl R7 100 kn R8 820 kn
A1 LPC661
A2 TL071 C1 100 nF
C2 100 pF non polar
C3 100 nF
The operation of the amplification system will now be described. The audio signal applied to terminal I is amplified by the amplifier 2 and is applied to the end of resistor R5 remote from the amplifier A1. The potential divider chain of resistors R1 to R4 provides, if one ignores the current drawn by amplifier A1, a potential of 40 x R2/(R1+R2) V on line 9 and -40 x R3/(R3+R4) V on line 10, that is, approximately +7.22 V and -7.22 V for the values given above. The amplification provided by amplifier A1 for the signal from the amplifier 2 is theoretically -R6/R5 (assuming the amplifier to have a very high gain and infinite impedance), that is, -2.2 for the values given above.
For small signals, the operation of amplifier A1 will approximate to the theoretical operation just described and the signal at its inverting terminal will be very small and at "virtual earth" as it is usually described. The amplifier A1 cannot, however, provide an output voltage greater than the value of its power supply and for large signals the amplifier start to exhibit clipping. Moreover, for these signals the load imposed by the power amplifier 3 on its power supply will be relatively great and the power supply voltage to the power amplifier will drop. That in turn will be reflected by a corresponding drop in the power supply voltage to the amplifier A1 increasing the extent of the clipping by that amplifier. In these circumstances, the inverting input of amplifier A1 will no longer be a virtual earth but will exhibit a significant signal.
That signal may be considered as representing the error difference between what the output of amplifier A1 ought to be and what it is in reality. This error signal is amplified by the error amplifier A2 which will have a theoretical gain of -R8/R7 in respect of that signal, that is, -8.2 for the values given above.
The amplified error signal is rectified and low pass filtered and used to decrease the gain of the amplifier 2. The gain control constitutes a feedback loop tending to reduce the error signal to zero. Thus, ignoring any variations in the input audio signal, the output of the amplifier 2 will now drop and so in consequence will the error signal from the mimic amplifier A2.
In this way, the amplifier A2 can be caused to clip at, say, 95% of the maximum voltage excursion of the power amplifier 3, so preventing the power amplifier from entering the last 5% of its maximum voltage excursion.
Although a system in which an error signal is fed back to an amplifier of controllable gain, the invention can also be applied to a system in which a controlling signal is instead fed forward to an amplifier of controllable gain.
Instead of using resistance means to reduce the voltage of the power supply to the power amplifier, a zener diode circuit or a voltage regulator circuit could instead be used.
Claims (12)
1. An audio amplification system comprising:
an audio amplifier of controllable gain having an input to receive an audio signal to be amplified and an output for the amplified audio signal,
a power amplifier having an input connected to the output of the audio amplifier and an output for connection to a loudspeaker drive unit, and
a gain control circuit having an input connected to receive the audio signal and an output connected to control the gain of the amplifier of controllable gain characterized in that:
the gain control circuit includes an amplifier arranged to receive its power supply from the power supply of the power amplifier by way of voltage reduction means,
the said amplifier of the gain control circuit is arranged to amplify the audio signal and to produce a signal to reduce the gain of the amplifier of variable gain when clipping occurs at the output of the amplifier of the gain control circuit.
2. An amplification system as claimed in claim 1, wherein the voltage reduction means comprises resistance means.
3. An amplification system as claimed in claim 1 or claim 2, wherein the output of the amplifier of the gain control circuit is connected in a negative feedback arrangement to an input of that amplifier.
4. An amplification system as claimed in claim 3, wherein the amplifier of the gain control circuit is a differential amplifier and has its output connected to the inverting input through a resistor.
5. An amplification system as claimed in claim 4, wherein the differential amplifier is a CMOS type operational amplifier.
6. An amplification system as claimed in any preceding claim, wherein the said resistance means comprises a resistive potential divider chain connected across the power supply to the power amplifier.
7. An amplification system as claimed in claim 6, wherein the potential divider chain comprises first, second, third and fourth resistors connected in series, the power supply to the said amplifier of the gain control circuit being derived from the junction of the first and second resistors and the junction of the third and fourth resistors, and the junction of the second and third resistors being connected to common potential.
8. An amplification system as claimed in any preceding claim, wherein the output of the said amplifier of the gain control circuit is connected to the input of a second amplifier of the gain control circuit.
9. An amplification system as claimed in claim 8, wherein the output of the second amplifier is connected to the input of a rectifier circuit, the output of the rectifier circuit is connected to the input of a low pass filter, and the output of the low pass filter is connected to the control input of the amplifier of variable gain.
10. An amplification system as claimed in any preceding claim, in which an error signal is fed back to the amplifier of controllable gain.
11. An amplification system as claimed in any one of claims 1 to 9, in which an error signal is fed forward to the amplifier of controllable gain.
12. An audio amplification system substantially as herein described with reference to, and as illustrated by, the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9614470A GB2315173A (en) | 1996-07-10 | 1996-07-10 | Audio amplification systems |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9614470A GB2315173A (en) | 1996-07-10 | 1996-07-10 | Audio amplification systems |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB9614470D0 GB9614470D0 (en) | 1996-09-04 |
| GB2315173A true GB2315173A (en) | 1998-01-21 |
Family
ID=10796661
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB9614470A Withdrawn GB2315173A (en) | 1996-07-10 | 1996-07-10 | Audio amplification systems |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2315173A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN101271726B (en) * | 2007-03-20 | 2010-05-26 | 普诚科技股份有限公司 | Audio processing system |
| GB2561902A (en) * | 2017-04-28 | 2018-10-31 | Soundtrap Ab | Digital signal processing |
Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4581589A (en) * | 1983-04-28 | 1986-04-08 | Toa Electric Co., Ltd. | Apparatus for avoiding clipping of amplifier |
| US5001440A (en) * | 1989-12-21 | 1991-03-19 | Ford Motor Company | Audio amplifier limiting for a remote power amplifier |
-
1996
- 1996-07-10 GB GB9614470A patent/GB2315173A/en not_active Withdrawn
Patent Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4581589A (en) * | 1983-04-28 | 1986-04-08 | Toa Electric Co., Ltd. | Apparatus for avoiding clipping of amplifier |
| US5001440A (en) * | 1989-12-21 | 1991-03-19 | Ford Motor Company | Audio amplifier limiting for a remote power amplifier |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN101271726B (en) * | 2007-03-20 | 2010-05-26 | 普诚科技股份有限公司 | Audio processing system |
| GB2561902A (en) * | 2017-04-28 | 2018-10-31 | Soundtrap Ab | Digital signal processing |
Also Published As
| Publication number | Publication date |
|---|---|
| GB9614470D0 (en) | 1996-09-04 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| 732E | Amendments to the register in respect of changes of name or changes affecting rights (sect. 32/1977) | ||
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |