Amplifying an audio signal

1. A method comprising amplifying a differential signal produced by a microphone or an audio transducer so as to introduce a controllable degree of distortion, said method including steps of:

receiving a differential signal from a microphone or an audio transducer, said differential signal comprising two complementary parts;

applying the two complementary parts of said differential signal to an inverting input terminal and a non-inverting input terminal of a differential amplifier, said differential amplifier being configured to produce an output signal at an output terminal, and wherein said differential amplifier is provided with a negative feedback path between said output terminal and both said inverting input terminal and said non-inverting input terminal, thereby introducing into said differential signal a feedback signal related to said output signal, thus controlling the gain of said differential amplifier; and

providing non-linear circuitry in said negative feedback path that introduce distortion in said feedback signal that is related to the level of said output signal.

2. A method according to claim 1 , wherein the non-linearity of said non-linear circuitry is adjustable so as to control the extent to which distortion is introduced.

3. A method according to claim 1 or claim 2 , further including a step of providing a compensation circuit for selectively compensating the non-linearity of said non-linear circuitry so as to substantially eliminate said distortion.

4. Amplifying apparatus, comprising:

an input configured to receive a differential signal produced by a microphone or an audio transducer, said differential signal comprising two complementary parts;

a differential amplifier having an inverting input terminal and a non-inverting input terminal to which the two complementary parts of said differential signal are applied, said differential amplifier being configured to amplify said differential signal to produce an output signal at an output terminal, and wherein said differential amplifier is provided with a negative feedback path between said output terminal and both said inverting input terminal and said non-inverting input terminal, whereby said negative feedback path is configured to introduce into said differential signal a feedback signal related to said output signal, thus controlling the gain of said differential amplifier; and

non-linear circuitry in said negative feedback path configured to introduce controllable distortion in said feedback signal that is related to the level of said output signal.

5. Apparatus according to claim 4 , further including a manually operable switch for selectively switching from substantially no distortion to a distorted operation.

6. Apparatus according to claim 5 , wherein said switch is a relay, and said apparatus further comprises a relay-control switch configured to operate said relay after a short delay, and said relay control switch is also configured to operate a muting procedure so as to mute audible clicks as the relay is operated.

7. Apparatus according to claim 4 , wherein the non-linear circuitry is configured to introduce even harmonic distortion.

8. Apparatus according to claim 7 , wherein said non-linear circuitry comprises a pair of field effect transistors configured to operate under variable current conditions.

9. Apparatus according to claim 8 , wherein an increase in the current supplied to said field effect transistors improves their linearity and the degree of distortion introduced to said feedback signal is increased by reducing current supplied to said field effect transistors.

10. Apparatus according to claim 9 , wherein each input path is connected to a respective constant current feed back path, wherein each input signal is introduced by a linear element; and the feedback signal is split so as to be introduced to each constant current feedback path via a respective non-linear element.