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INDUSTRIAL APPLICABILITY The present invention is applied to audio equipment having a
headphone jack such as a radio, a tape recorder, and the like. The structure of the prior art and
the problems thereof-Fig. 1 shows an example of a conventional acoustic amplification circuit
comprising a speaker and a headphone jack. In the figure, (1) is an R channel amplifier, α1 is an
R channel amplifier, (6 and 10 days are speakers, (9) is a headphone jack and house, and αγj is
an L channel volume resistance and an R channel volume resistance, When the headphone plug
is not connected to the jack (9). Changeover switch (5 ', Q4 + movable contacts (5c), (llIc) are
connected to the speaker side contacts (5a), (111a), the headphone plug is inserted into the jack
(9), the output is from the speaker to the headphone Is configured to be switched to one. Also,
the headphone circuit is designed to reduce the output of the amplifiers (1) and 00 to an
appropriate volume level by a dump resistor ('Q (8)) and to add it to the headphones. In such a
conventional circuit, especially in a small set, the waveform at the maximum output when the
input is fixed becomes close to a square wave in order to obtain the volume feeling at the
maximum output because the output is small. It is designed. Therefore, when listening with
headphones, it is sufficient to narrow down the volume α6 and μ71 sufficiently, but if you raise
the volume a little, depending on the source of the input signal (especially, announcements of AM
broadcasting, songs with a lot of bass, etc.) The output waveform is clipped, and the harmonics of
it are heard as irritating and irritating sounds. The noise is not very noticeable when listening to
the built-in speaker, but it becomes noticeable when listening to the headphone ヘ ッ ド ホ ン
Purpose of the Invention The present invention relates to the output of a small amplifier as a
headphone The purpose is to eliminate the above-mentioned noise and noise when listening to
the music. The present invention provides a negative feedback circuit that reduces the output of
the amplifier in conjunction with the connection of the output of the amplifier to the headphones.
Description of the Embodiment FIG. 2 shows an example of a stereo amplification circuit
according to the present invention. In the figure, the same reference numerals as in FIG. 1 denote
the same parts as in FIG. The main difference from FIG. 1 is that a negative feedback circuit is
provided to feed back via the headphone jack (9) and the amplifier (1), 00 via the resistor α8),
09), and the headphone jack (9) The point is that the headphone jack (9) is connected to the
switch (5) interlocked with the operation of attaching and detaching the plug, and one of the
fixed contacts (5b) and (14b) of the α sensor.
In the circuit of FIG. 2, when the headphone plug is pulled out from the jack (9), the movable
contacts of the switches (5 :, (14)) in which the outputs of the amplifiers (1) and (10 are
interlocked with the headphone jack (9) (5c), (111c), fixed contact f5a, (1'la) k "r speaker (6) α5)
is output as an audio output. When listening with headphones, when the headphone plug is
inserted into the headphone jack (9), the movable contacts (5c) and (llic) of the switch are
switched to the fixed contacts (5b) and (14b) in conjunction with this. At the same time as the
audio output is output to the headphones, at the same time part of the output is resistance α8.1.
The signal is negatively fed back to the input of the amplifier through (19), and the output of the
amplifier (1), 0α decreases. Therefore, the output waveform can not be clipped against a
constant input signal. In consideration of the input signal of each input 1, select the feedback
resistance 08), α, resistance (A), and υ so that the output waveform is not distorted (there is no
problem if the distortion is 5 or less). As a result, it is possible to add an audio signal with less
distortion to the headphones via the resistors (7 ′) and (8). It is billy in the voice to hear in. You
can hear the noise without the noise. In the above embodiment, although the stereo amplifier
circuit has been described, it is needless to say that the present invention I31: I is applicable only
to the stereo circuit <and also to the monophonic amplifier circuit. According to the present
invention, it is possible to listen to music or voice regardless of the position of the sliding
terminal of the volume resistance and for any input signal source. This is very effective especially
for small amplifiers.
Brief description of the drawings
FIG. 1 is a circuit diagram of an example of a conventional amplifier circuit including a speaker
and a headphone jack, and FIG. 2 is a circuit of an embodiment of the present invention.
(1 bit, α 0 ··· amplifier, t 5:, a 4 J ··· switch, (6), α eye · · · speaker, (7 j (8) · dump resistance, (9) ·
headphone jack , Qt &, 07 no--volume resistance, α 8 !, (tQ, protection η ... feedback resistance.
Figure 1
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