Self powered audio compressor circuit
View Patent ↗The application relates to a compressor circuit for a listening device, the compressor circuit comprising a pair of input terminals and a pair of output terminals, the compressor circuit being adapted for receiving an electric input signal representing an audio signal with an input voltage swing V ipp at the input terminals and for providing a possibly compressed version of the input signal as an output signal with an output voltage swing V opp at the output terminals, the compressor circuit comprising. The application further relates to a listening device and to the use of the compressor circuit. The object of the present application is to provide a compressor circuit with a proper functioning also at relatively low input levels. The problem is solved in that an AC to DC voltage converter for generating a DC-control voltage based on the electric input signal or an AC signal derived therefrom, and a voltage controlled resistance for providing an output voltage swing V opp controlled by the DC-control voltage, wherein the compressor circuit is energized by the electric input signal. An advantage thereof is that a compressor circuit facilitating a flexible adjustment of the maximum output level is provided. The invention may e.g. be used in portable and non-powered listening devices, e.g. headsets or active ear protection devices.
1. A compressor circuit for a listening device, the compressor circuit comprising:
a pair of input terminals; and
a pair of output terminals, wherein
the compressor circuit is configured
to receive an electric input signal representing an audio signal with an input voltage swing V ipp at the input terminals and
to provide a possibly compressed version of the input signal as an output signal with an output voltage swing V opp at the output terminals, and
the compressor circuit further comprises
an AC to DC voltage converter for generating a DC-control voltage based on the electric input signal or an AC signal derived therefrom; and
a voltage controlled resistance for providing the output voltage swing V opp controlled by the DC-control voltage,
wherein the compressor circuit is energized by the electric input signal, and
wherein the voltage controlled resistance comprises a first FET transistor and a second FET transistor, each FET transistor including a source terminal, a gate terminal, and a drain terminal, and
wherein the source terminal of the first FET transistor is directly electrically connected in series to the source terminal of the second FET transistor.
2. The compressor circuit according to claim 1 , further comprising:
a drop resistance for providing a minimum input resistance of the compressor circuit.
3. The compressor circuit according to claim 1 , further comprising:
an AC voltage control unit for controlling voltage level of the electric input signal.
4. The compressor circuit according to claim 3 , wherein
the AC voltage control unit provides a predefined multiplication factor M, and
the output voltage from the AC voltage control unit is a factor M times the input voltage to the AC voltage control unit.
5. The compressor circuit according to claim 4 , wherein
the multiplication factor M is set between a maximum and a minimum value in predefined steps.
6. The compressor circuit according to claim 3 , wherein
the AC voltage control unit comprises a transformer.
7. The compressor circuit according to claim 3 , wherein
the AC voltage control unit comprises a diode circuit.
8. The compressor circuit according to claim 1 , wherein
the AC to DC voltage converter comprises a diode circuit.
9. The compressor circuit according to claim 1 , wherein
the gate terminal of the first FET transistor is AC coupled to the drain terminal of the first FET transistor, and
the gate terminal of the second FET transistor is AC coupled to the drain terminal of the second FET transistor.
10. The compressor circuit according to claim 1 , wherein
the output voltage swing V opp from the compressor circuit is varying with a frequency of the audio signal represented by the electric input signal in a predetermined manner.
11. The compressor circuit according to claim 10 , further comprising:
a passive modification circuit including a combination of resistors and capacitors and/or inductors in the control path, the control path including the circuitry for generating the DC-control voltage for controlling the voltage controlled resistance.
12. A listening device comprising a compressor circuit according to claim 1 and a speaker unit electrically connected to the compressor circuit.
13. A listening device according to claim 12 adapted to limit the output of the speaker unit to a predefined maximum sound pressure level.
14. A stereo headset comprising first and second listening devices according to claim 12 .
15. A stereo headset according to claim 14 adapted to receive separate audio signals from an audio delivery device to said first and second listening devices.
16. A stereo headset according to claim 15 comprising circuitry adapted to provide that the output levels in the respective earphones controlled by the respective compressor circuits of the first and second listening devices are correlated, so that a specific compression in one of the earphones results in a corresponding compression at the other earphone.
17. A stereo headset according to claim 16 adapted to provide that the earphone presently experiencing the highest level determines the output level of both earphones.
18. A stereo headset according to claim 14 adapted to limit the output of any of the speakers of the system to a predefined maximum sound pressure level.
19. The compressor circuit according to claim 1 , wherein
the gate terminal of the first FET transistor is directly electrically connected to the gate terminal of the second FET transistor.
20. The compressor circuit according to claim 19 , wherein
the drain electrode of the first FET transistor is directly electrically connected to a first output terminal of the pair of output terminals,
the drain electrode of the second FET transistor is directly electrically connected to a second output terminal of the pair of output terminals,
the source terminal of the first FET transistor is not directly electrically connected to either of the pair of output terminals, and
the source terminal of the second FET transistor is not directly electrically connected to either of the pair of output terminals.