GB2062373A

GB2062373A - D.C. power supplies

Info

Publication number: GB2062373A
Application number: GB8033263A
Authority: GB
Original assignee: Morecroft D N
Current assignee: Morecroft D N
Priority date: 1979-10-16
Filing date: 1980-10-15
Publication date: 1981-05-20

Abstract

A D.C. power supply for electrical equipment, more especially audio- electronic equipment, in which after the conventional full wave rectifier bridge (4) and smoothing circuits (6), a plurality of voltage regulators (VR1 to VR4) are arranged in cascade between the smoothing circuits and electrical equipment to be supplied. Associated with each voltage regulator is a parallel circuit, comprising a resistor and capacitor (R5, C5 to R8, C8), connected between an intermediate point in the regulator and the earth or negative line (7), and a feedback resistor (R9 to R12) is connected between said intermediate point and the other line on the output side of the regulator. By selectively dropping the voltage across each voltage regulator, a substantial reduction in noise, both originally present in the A.C. mains and also generated in the rectifying and smoothing process, can be achieved. <IMAGE>

Description

SPECIFICATION D.C. power supplies The present invention relates to improvements in power supplies to electrical equipment. In particular it relates to power supplies to electronic-audio equipment, such as radio receivers and hi-fi record players or tape recorders.

More particularly it relates to the effect which power supplies have on amplifiers used in electronic-audio equipment.

As reproduction has improved very substantially over the last twenty-five years, it is desirable to reduce the "noise" in such systems to as low a level as possible.

The broad definition of "noise" is "any sound unwanted by the listener".

In the output of a radio receiver/record player/tape recorder, four types of noise can be distinguished. These are as follows:- (a) Interference, such as by an unwanted station, a lightning discharge, a commutator machine, car ignition or extra-terrestrial noise from space; (b) Noise of a mechanical origin due to bad or faulty electrical contact; (c) Hum, from the alternating current supplying power to the audio amplifying equipment of the radio receiver/record player/tape recorder; (d) Thermal, random or fluctuation noise which sounds like soft breathing.

In a radio receiver, there is very little that can be done to completely remove unwanted interference particularly that which is not man made. V.H.F.

frequency modulated receivers show a large improvement in eliminating unwanted noise, particularly interference by other stations. Noise of a mechanical origin can be avoided by ensuring that there are no bad contacts.

Hum is not fundamental and can be eliminated by using batteries for the power supply. However, it is not economical to use batteries for most audio-equipment other than small transistor radios or casette recorders.

Even when the high-tension supply to a receiver or audio equipment is adequately smoothed, hum at mains frequency and its harmonics may appear in the audio output.

The majority of power supplies for electronic audio equipment utilize a transformer to step down the voltage and some form of half-wave or full-wave bridge rectifier, smoothing capacitors and a choke to rectify and smooth the output of the transformer.

A typical power supply which is used comprises a transformer, a full wave rectifier and a smoothing capacitor. The ripple output frequency will be twice mains frequency i.e. 100 Hz and the amount of ripple will depend upon the size of the capacitor and the current drawn by the load. The low level circuits are usually fed by a zener diode stabilized output after such a circuit, perhaps even from a separate small supply. In the most expensive amplifiers a simple stabilizer is often used to keep the voltage constant and/or control the current level. More often than not the main purpose of this circuit is to assist in protecting the amplifier in the event of misuse rather than to improve its performance. Some amplifier manufactures even state that it is preferable to use unstabilized supplies so that more voltage.is available for the short peaks typical in music.

The disadvantage of these power supplies and their variations is that they overlook the absolute necessity of providing the various circuits with as smooth and noise free supply as is possible. The mains supply itself is full of electrical noise covering a wide range of frequencies apart from its fundamental 50 Hz generated frequency, this noise mainly falling within types (a) and (b) listed above. If any of these noises find their way into the signal path they will add and cancel the small items of signal, and sometimes even the larger ones, with the result that the total subjective performance of the amplifier is often quite drastically altered from that which the specification would have us expect.

It thus seems necessary to accept that more expenditure needs to be allocated to the power supply section of any given design. The power supply is as important if not more important than the type of amplifier circuit. Having accepted the necessity for this we can now divide the problem into three sections as follows:- (a) The problem of residual ripple resulting from the remains of the generating frequency and any sub-harmonics of this.

(b) Various harmonics of the fundamental frequency, high frequency noise originating from the mains supply and additional noise resulting from high frequency noise interaction with circuits and cables generating thereby further noise.

(c) Circuit noise generated by sections of the amplifier demanding current in line with signal level i.e. transient changes in current and voltage.

The best mains commercial regulated supplies for low level circuits achieved typically noise and ripple levels of about 100 mv peak to peak. Power supplies in power amplifiers operate at ripple levels of volts rather than fractions of a volt. Even in class A amplifiers, having stabilized supplies, ripple and noise runs up to a significant fraction of a volt.

Tests carried out on low level circuits, i.e.

moving coil pickup and pre-amplifier circuits, indicate that noise levels (ripple and noise) as low as 20,uv peak to peak in the supply voltage are still distinctly inferior to a battery reference on auditioning. Therefore it is obvious that there is much room for improvement in order to achieve noise levels as low as 1 5yv peak to peak from a mains source.

It is therefore an object of the present invention to overcome partially or wholly the above referred to disadvantages by providing an improved power supply for electrical equipment which is substantially free from noise both originally present in the A.C. mains and also generated as a result of the rectification and smoothing of the A.C. mains to provide a D.C. power supply for the electrical equipment.

According to the present invention there is provided a D.C. power supply for electrical equipment including: a transformer and rectifier means for obtaining rectified D.C. from an A.C.

mains supply; means for smoothing the rectified output; and multi-stage voltage regulator means connected between the smoothing means and the electrical equipment to be supplied, each said voltage regulator means effecting a dropping of the rectified voltage to thus provide a D.C. power supply which is substantially free from noise.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawings, wherein: Figure 1 is a circuit diagram of one preferred form of power supply suitable for supplying D.C. to electronic-audio equipment; Figure 2 is a circuit diagram illustrating a typical monolithic voltage regulator which can be used in the power supply shown in Figure 1; Figure 3 is a circuit diagram showing a second embodiment of a power supply suitable for supplying a power amplifier used in the electronicaudio equipment; and Figure 4 is a circuit diagram illustrating a typical voltage regulator which can be used in the power supply shown in Figure 3.

Referring first to Figure 1, the power supply comprises a conventional step-down transformer T1 having primary and secondary windings 1 and 2 respectively, a filter F and a full-wave rectifier bridge 4 having diodes arranged in conventional manner. The primary winding 1 is connected to the 240 volt A.C. mains supply and the filter F is connected in parallel with the primary winding 1.

The output from the full-wave rectifier bridge 4 is connected to a filter and smoothing circuit 6 comprising three resistors R1 to R3 and four capacitors C1 to C4, which components are connected in conventional n configuration to form three cascaded 7e networks with the capacitor C2 being common to the first and second 7r networks and the capacitor C3 being common to the second and third 7r networks. The capacitor C4 is provided both to shunt the capacitor C1 (C4 having good high frequency characteristics) and to ensure stability in the first voltage regulator of the following circuit.

The output from the filter and the smoothing circuit 6 is connected to a voltage regulator circuit 8. The voltage regulator circuit comprises four monolithic voltage regulators VR1 to VR4 arranged in cascade. Associated with the first voltage regulator VR1 is a parallel network comprising a resistor R5 and a capacitor C5 connected between an intermediate point of the voltage regulator and the earth line 7 of the power supply. A feedback resistor R9 is connected between the output of the voltage regulator VR1 and the said intermediate point. Likewise the voltage regulators VR2, VR3 and VR4 have parallel networks C6-R6, C7-R7 and C8-R8 associated therewith and feedback resistors R1 0, R1 1 and R12 respectively.Across the output from the final voltage regulator VR4 is a parallel network consisting of two capacitors C9 and ClO.

The capacitors C5 to C8 preferably have good high frequency characteristics, and their values are only critical in that they must absorb the line frequency effectively and they must not affect the switch on time of the voltage regulators too drastically.

The capacitors C9 and C10 are selected for their high and low frequency characteristics to absorb as wide a noise bandwidth as possible.

A typical circuit for each voltage regulator VR1 to VR4 is shown in Figure 2. The circuit comprises two NPN transistors Q1 and Q2, resistors R13 and R14, and zener diodes Z1 and Z2. The collectoremitter path of the transistor Q1 is in series with the main current flow between the positive terminals of the circuit. The resistor R13 is connected across the collector and base electrodes of the transistor 01. A series circuit comprising the zener diode Z1 and the resistor R14 is connected across the output side of the circuit, i.e. across the terminals Vout of the voltage regulator VR.

The junction between the zener diode Z1 and the resistor R14 is connected to the base electrode of the transistor Q2. The zener diode Z2 which acts as a reference voltage, is connected between the emitter electrode of the transistor Q2 and the negative or earth line of the power supply.

The collector electrode of the transistor Q2 is connected to the base electrode of the transistor 01.

It will be appreciated that the negative or earth rail shown in Figure 2 may be the earth line 7 shown in Figure 1 and that the parallel arrangement of the capacitor and resistor can be connected directly to the base electrode of the transistor Q2.

It will be further appreciated that instead of using a plurality of zener diodes Z2 as reference voltages, it may be more convenient to provide an external reference by means of a circuit designed for this purpose.

The operation of the circuits shown in Figures 1 and 2, will now be described in greater detail.

The filter F is designed to attenuate the mains noise. The full-wave rectified voltage obtained across the rectifier bridge 4 is smoothed in the resistor-capacitor filter network comprising the resistors R1 to R3 and the capacitors C1 to C4.

The output from the filter and smoothing circuit 6 is D.C. with a small ripple voltage at 1 OOHz superimposed thereon, plus harmonics of 50Hz mains supply and some other mains induced noise. The magnitude of the D.C. voltage at the output of the circuit 6 is greater than the magnitude of the stabilized voltage supply to be supplied to the electronic-audio equipment.

Accordingly further voltage drops are produced across each of the voltage regulators VR to ensure that the correct magnitude of stabilized voltage is available to the electronic-audio equipment.

In the circuit shown in Figure 2, each transistor Q1 regulates this voltage drop according to the bias applied to its base electrode. This bias voltage is determined by the conduction of the transistor Q2 which causes the voltage drop across the resistor R1 3 to vary. The conduction of the transistor Q2 is controlled by the zener diodes Z1 and Z2. The voltage at the emitter electrode of the transistor Q2 is fixed by the breakdown voltage of the zener diode Z2, and thus acts as a reference voltage.

When the voltage Vout drops, the voltage at the base electrode of the transistor Q2 drops by the same amount due to the zener voltage of the zener diode Z1. As the voltage at the emitter electrode of the transistor Q2 is fixed at the reference voltage, the conduction of the transistor Q2 is reduced and the voltage at the base electrode of the transistor Q1 rises which increases the conduction of the transistor Q1 and effectively raises the output Vout since there is less of a voltage drop across the transistor Q1 . The capacitor C5 has the effect of reducing any ripple voltage which may be induced across the zener diode Z2 which acts as a reference.

The cascade chain of the four voltage regulators VR1 to VR4 thus ensures that the output of the last voltage regulator VR4 is always stabilized at a constant value irrespective of fluctuations in the A.C. mains voltage. Moreover the presence of the parallel circuit consisting of a resistor and capacitor connected to the base electrode of the transistor Q2 and to the emitter electrode of the transistor Q1 through a further resistor ensures that mains ripple is further reduced and that the harmonics of the mains voltage are virtually eliminated.Each voltage regulator VR1 to VR4 is preferably designed to have a ripple rejection which is better than 80 dB, so that when four are used as in the preferred embodiment, this rejection for 100 Hz ripple can be much higher, although this rejection will not be as high for some of the high frequency harmonics of the rectified A.C. mains ripple.

It is generally accepted in power amplifier design that class A operation provides the clearer output, and this is believed to be largely due to class A mode of operation. Class A designs invariabiy use a fully regulated power supply whereas the majority of class B or class AB designs do not use a fully regulated supply. It will be appreciated that modern equipment is so accurately matched that the minute switching residuals in class B amplifiers are virtually inaudible. However, at the very instant that class B amplifiers are drawing current from the power supply, the supply itself is generating most noise because the very act of drawing current causes the capacitors of the power supply to be topped up by the rectified supply. Also, at this very instant, the rejection of noise by the output stages of the amplifier is less because they are partially switched on.Hence, it is necessary to further improve the power supplies to a power amplifier or pre-amplifier stage in order to eliminate subjective noise induced by such stages operating in either class B or class AB.

Referring now to Figure 3, the second embodiment of a power supply is specifically designed for use with a power amplifier. As in the first embodiment, the power supply circuit includes a transformer T1 having primary and secondary windings, a filter F1 across the primary winding, and a diode rectifier bridge 4. The smoothing circuit comprises a single capacitor C1 1. The voltage regulator circuit comprises three monolithic voltage regulators VR5 to VR7 arranged in cascade. Associated with the first voltage regulatorVR5 is a parallel network comprising a resistor R18 and a capacitor C12, connected between an intermediate point of the voltage regulator and the earth line 7.A feedback resistor R19 is connected between the output of the voltage regulator VR5 and the said intermediate point. Likewise, the voltage regulator VR7 has an associated parallel network comprising a a capacitor C13 and a resistor R20, and a feedback resistor R2 1. The voltage regulator VR6 is in the form of a pre-regulator and has a first feedback resistor R22 from its output to its associated intermediate point, and a second feedback resistor R23 connected between the output of the voltage regulator VR7 to a second associated intermediate point. A capacitor C14 is provided across the output of the final voltage regulator VR7 and performs substantially the same function as the parallel capacitors C9 and C10 of the circuit of the first embodiment shown in Figure 1.

A typical circuit for each voltage regulator VR5 to VR7 is shown in Figure 4. The circuit includes a PNP transistor Q3 whose emitter-collector path is connected in the negative supply rail. The base electrode of this transistor is connected to the output of an amplifier 10. The input to the amplifier 10 is connected to firstly a reference voltage Vr supplied by a battery 1 2 through a resistor R15, and secondly to a feedback resistor R16. The battery 12 provides a stabilized reference and variations in the output voltage Vout are fed back to the amplifier 10 through the feedback resistor R16 so as to control the conduction of the transistor Q3 and thus offsetting any fluctuations in the load due to class B or class AB operation of the power amplifier.

The two parallel networks comprising the resistors R18 and R20 and the capacitors C12 and C13 which networks are associated with the voltage regulators VR5 and VR7 are connected between an input X to the amplifier A and the negative rail of the power supply.

In this arrangement the reference voltage for each voltage regulator VR5 to VR7 is provided by a battery 12. It may be more convenient to provide an external reference by means of a circuit designed for this purpose.

It should be noted that the resistor R23 associated with the pre-regulator VR6 is connected between the output of the voltage regulator VR7 and the input to the amplifier 10 within the pre-regulator VR6.

The provision of the tracking pre-regulator VR6 improves load regulation when driving transient loads, which makes this embodiment of the power supply circuit suitable driving a class B amplifier. It is possible to achieve a load regulation better than 0.06% and a very good line regulation.

It will be appreciated that the detailed circuits of the voltage regulators shown in Figures 2 and 4 can be modified to supply power to a pre-amplifier or a power amplifier on a split rail basis. For example, in connection with the circuit shown in Figure 4, a mirror image circuit can be integrated into the circuit already shown in this figure, such that the positive lines of the two circuits form a common zero line. The negative line of the mirror image will then form the positive line of the combined circuit and will contain an NPN equivalent of the transistor Q3 in the negative line.

Claims

1. A D.C. power supply for electrical equipment including: a transformer and rectifier means for obtaining rectified D.C. from an A.C. mains supply; means for smoothing the rectified output; and multi-stage voltage regulator means connected between the smoothing means and the electrical equipment to be supplied, each said voltage regulator means effecting a dropping of the rectified voltage to thus provide a D.C. power supply which is substantially free from noise.

2. A D.C. power supply according to Claim 1, wherein said smoothing means comprises a plurality of rr networks containing series resistors and parallel capacitors.

3. A D.C. power supply according to Claim 1 or 2, wherein there are four identical voltage regulators arranged in cascade between the smoothing means and the electrical equipment.

4. A D.C. power supply according to Claim 3, wherein each voltage regulator comprises a first transistor whose emitter-collector path is in one line of the power supply, a second transistor whose emitter-collector path is connected between the base electrode of the first transistor and the other line through first voltage reference means, the base electrode of the second transistor being connected to the junction between a series circuit cotmprising second voltage reference means and a resistor connected across the power supply on the output side of the regulator.

5. A D.C. power supply according to Claim 4, wherein said first and second voltage reference means are zener diodes.

6. A D.C. power supply according to Claim 4 or 5, wherein between the base electrode of the second transistor and the other line of the power supply, there is connected a parallel circuit comprising a resistor and a capacitor.

7. A D.C. power supply according to Claim 6, wherein a feedback resistor is connected between the output of the voltage regulator and the base electrode of the second transistor.

8. A D.C. power supply according to Claim 1, wherein there are three identical voltage regulators arranged in cascade between the smoothing means and the electrical equipment.

9. A D.C. power supply according to Claim 8, wherein each voltage regulator comprises a transistor whose emitter-collector path is in one line of the power supply, a reference voltage means and amplifier connected between the other line of the power supply and the base electrode of the transistor.

10. A D.C. power supply according to Claim 9, wherein said reference voltage means is a battery, which is connected to the input of the amplifier through a resistor.

ii. A D.C. power supply according to Claim 9 or 10, wherein between the input to the amplifier and the line containing the transistor on the output side thereof, there is connected a parallel circuit comprising a resistor and a capacitor.

12. A D.C. power supply according to Claim 9 or 10, wherein the second of the three voltage regulators acts as a pre-regulator and a part of the output of the third voltage regulator is fed back through a resistor to the input of the amplifier, of the second voltage regulator.

13. A D.C. power supply according to any one of the preceding claims, including at least one capacitor in parallel with the output from the final stage of the multi-stage voltage regulator means.

14. Audio-electronic equipment in combination with a D.C. power supply according to any one of the preceding claims.

1 5. A D.C. power supply for electrical equipment constructed and arranged to operate substantially as herein described with reference to and as illustrated in Figures 1 and 2, or Figures 3 and 4 of the accompanying drawings.