GB2303991A - Microphone amplifier with phantom power - Google Patents

Abstract

The microphone amplifier comprises two complementary signal inputs each being connectable to a microphone via a respective phantom power blocking capacitor 30. One or more amplifying stages 210, 220 for amplifying differential signals at the signal inputs provide two complementary output signals, the amplifying stages providing a common mode gain substantially equal to unity for common mode signals at the signal inputs. The output signals are combined by resistances 150 to generate a common mode mix signal that is coupled by capacitor 180 and resistances 102, 104 to each of the signal inputs. By providing bootstrapping of a common mode signal in this way, the common mode impedance of the circuit can be increased and so the effect of mismatch in the dc phantom power blocking capacitors can be reduced.

Description

MICROPHONE AMPLIFIER WITH PHANTOM POWER This invention relates to microphone amplifiers with phantom power.

Phantom power involves using the signal connection lead from a microphone to carry a dc voltage to power the microphone remotely. Commonly the dc voltage is supplied by the device receiving the audio signal from the microphone (such as a mixing console). This avoids the need for the microphone to use local battery power.

However, in a microphone amplifier input stage, the dc phantom power supply must be isolated from the microphone amplifier to prevent saturation of the operational amplifiers within the microphone amplifier.

In order to achieve this isolation, previous designs have included a pair of dc blocking capacitors, one on each input arm of the microphone amplifier. Although satisfactory in some aspects of its operation, this type of circuit suffered from a number of disadvantages.

A main disadvantage is that the two capacitors have to be chosen so that their respective values were in almost perfect agreement. This is because any disagreement between the two capacitor values reduces the common mode rejection of the amplifier by generating a differential noise signal from a common mode input.

In one attempt to overcome this requirement for the values of the capacitors to be identical (which is very difficult in practice), the values of the two capacitors were greatly increased. This does not reduce the amount of differential noise, but can move the effect to such a low frequency that it is not a noticeable problem in an audio system. However, this measure also introduces the problem that the amplifier circuit then takes an unacceptably long time to reset when the phantom power is switched on or off. During this resetting period, slight imbalances in the two capacitors can cause large differential noise signals to be transiently generated.

This invention provides a microphone amplifier comprising: two complementary signal inputs, each being connectable to a microphone via a respective phantom power blocking capacitor; one or more amplifying stages for amplifying differential signals at the signal inputs to provide two complementary output signals, the amplifying stages providing a common mode gain substantially equal to unity for common mode signals at the signal inputs; means for combining the output signals to generate a common mode mix signal; and means for coupling the common mode mix signal to each of the signal inputs.

In the invention a common mode signal is returned substantially equally to the inputs as a bootstrap signal. As with conventional bootstrapping techniques, this common mode bootstrapping arrangement can increase the input impedance of the amplifier to common mode inputs, with respect to the input impedance for differential mode inputs. By increasing the common mode input impedance in this way, the effects of any mismatch in the dc blocking capacitors is reduced. This is because the differential component generated from a common mode input by the mismatch in the dc blocking capacitors is dependent upon the common mode input impedance, so that a higher common mode input impedance leads to a lower differential noise component. This in turn reduces the need for very large capacitors (with correspondingly large settling times) to avoid differential noise.

Preferably the coupling means comprises two substantially identical resistances connected between the two signal inputs, the common mode mix signal being connected to a junction of the two resistances.

In order to avoid any effects on the dc bias settings of the amplifier, it is preferred that the common mode mix signal is connected to the junction of the two resistances via a capacitor.

Preferably the dc blocking capacitors have nominally matched values and are mounted adjacent to one another on a circuit board.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 is a schematic diagram of a microphone amplifier.

Figure 1 is a schematic diagram of a microphone amplifier which receives balanced positive and negative input signals 10, 20 via dc blocking capacitors 30.

The purpose of the blocking capacitors 30 is to isolate the input of the microphone amplifier from a phantom power supply Vp (which may be supplied by a part of the amplifier itself) or other dc signal present on the microphone leads. The capacitors are matched to one another to within, say, 1% of their nominal value of about 2 microfarads. They are also mounted adjacent to one another on a circuit board so that any thermal effects or drift are reasonably well matched between the two capacitors.

The microphone input signals which are passed by the dc blocking capacitors 30 are amplified by two respective symmetrical amplifiers, each based around a transistor input stage (a transistor 210) and an operational amplifier stage (an operational amplifier 220). The microphone amplifier generates a balanced output comprising positive and negative output signals. The amplifier has a common mode gain which is very much lower than its differential mode gain. In fact, with the components shown, the common mode gain of the circuit of Figure 1 is substantially unity.

Two resistors 150 are connected between the outputs of the two operational amplifiers 220, so that the signal at the junction of the two resistors 150 represents the common mode output of the amplifier. This signal is passed back as a common mode bootstrap signal via a capacitor 180 to the junction of two input resistors 102, 104.

As with conventional bootstrapping techniques, the common mode bootstrapping arrangement of Figure 1 increases the input impedance of the amplifier to common mode inputs, with respect to the input impedance for differential mode inputs. Because the common mode bootstrap is via the capacitor 180, this is also achieved without affecting the dc bias of the input transistors 210.

By increasing the common mode input impedance in this way, the effects of any mismatch in the dc blocking capacitors is reduced. This is because the differential component generated from a common mode input by the mismatch in the dc blocking capacitors is dependent upon the common mode input impedance, so that a higher common mode input impedance leads to a lower differential noise component.

It will be understood, of course, that the present invention has been described above by way of example only and that modifications may be made within the scope of the appended claims.

TABLE 1: MISCELLANEOUS COMPONENTS IN FIGURE 1 ReSiStOTS 100, 102, 104 100000 ohms 110 3000 ohms 120 100 ohms 130 22000 ohms 140 680 ohms 150 10000 ohms 160 4700 ohms 170 2400 ohms CaDacitors 30 2 microfarads 180 47 microfarads (electrolytic) 190 22 nanofarads 200 150 picofarads Inductors 90 40 microHenries Semiconductors 210 LM394 transistor 220 NE5532 operational amplifier

Claims (6)

1. CLAIMS 1. A microphone amplifier comprising: two complementary signal inputs, each being connectable to a microphone via a respective phantom power blocking capacitor; one or more amplifying stages for amplifying differential signals at the signal inputs to provide two complementary output signals, the amplifying stages providing a common mode gain substantially equal to unity for common mode signals at the signal inputs; means for combining the output signals to generate a common mode mix signal; and means for coupling the common mode mix signal to each of the signal inputs.
2. 2. An amplifier according to claim 1, in which the coupling means comprises two substantially identical resistances connected between the two signal inputs, the common mode mix signal being connected to a junction of the two resistances.
3. 3. An amplifier according to claim 2, in which the common mode mix signal is connected to the junction of the two resistances via a capacitor.
4. 4. An amplifier according to any one of claims 1 to 3, in which the dc blocking capacitors have nominally matched values and are mounted adjacent to one another on a circuit board.
5. 5. An amplifier according to any one of claims 1 to 4, comprising means for supplying a direct current phantom power signal to the microphone.
6. 6. A microphone amplifier substantially as hereinbefore described with reference to the accompanying drawings.