Headphone amplifiers. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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A headphone amplifier with an impedance of about 100 ohms can be made on just two CMOS logic chips with a minimum of additional parts. The sound quality achieved with such amplifiers, the author assesses as quite high.

Headphone amplifiers (hereinafter UGT) receive little attention on the pages of amateur radio literature. Connecting them is considered a trivial task. In audio frequency power amplifiers, they are usually connected to the outputs instead of the speaker system (AC) through a special divider or quenching resistor. In this case, the signal passes through the entire amplification path, which is by no means optimal for amplifying power up to hundreds of milliwatts. CD players or cassette decks typically use low cost chips covered by the general OOS. This is common and audible when compared to high quality discrete amplifiers.

UGT can have a much "shorter" path, consisting of only one amplification stage; in this case, the headphone is connected to its output directly without a quenching resistor, which improves its damping.

On fig. 1 shows the UGT circuit, in which logical elements 2I-NOT and 2OR-NOT of CMOS chips (DD1 and DD2) connected in parallel are used. There are four CMOS transistors in the 2I-NOT element, two of which are connected in parallel (in the upper arm), and the other two are connected in series (in the lower arm).

(click to enlarge)

In the 2OR-NOT element, the connections of transistors in the arms are made the other way around (Fig. 2). If you connect the elements 2OR-NOT and 2I-NOT in parallel, covering them with a feedback circuit, you get a symmetrical push-pull amplifying stage. In its lower arm, two transistors with an n-type channel will be connected in parallel, and two with a p-type channel in its upper arm (in this case, the current through the arm, where the transistors are connected in series, is very small). In the described UGT, two 2I-NOT and two 2OR-NOT elements are used in each amplification channel, as a result, we have four transistors connected in parallel in each arm, which ensures an acceptable load capacity of the device.

The amplifier circuit is simple. It works stably, without excitation, and with proper installation, it practically does not need to be adjusted. The gain of the device is set by the ratio of the resistances of the resistors R2 and R3 (in this case it is equal to 3). The input impedance of this amplifier is almost equal to the resistance of the resistor R2, and this should be taken into account in relation to the output impedance of the signal source. Coupling capacitors C1 and C2 separate the inputs of the amplifier stages of the left and right channels in terms of the DC component, and capacitors C3 and C4 separate the outputs from the headphones.

Coupling capacitors limit the bandwidth from below, and also introduce their own nonlinear distortions due to absorption processes in the dielectric. Using a bipolar power supply of microcircuits, it is possible to exclude all isolation capacitors (C1-C4 in Fig. 1). The power supply rectifier for this power supply option can be assembled according to the diagram shown in Fig. 3.

These amplifiers require a stabilized power supply because they are relatively sensitive to ripple. For 4000V series CMOS microcircuits, the supply voltage is not more than 18 V; for the K561 series, it is limited to 15 V. It must be borne in mind that a higher supply voltage of microcircuits corresponds to a higher value of through current (quiescent current in linear mode). With this feature in mind, integral stabilizers are chosen. The power transformer must provide a rectified voltage a few volts more than the output of the stabilizer DA1 at a current of at least 0,3 A.

For both versions of the UGT, the printed circuit board was not developed; they were assembled on a breadboard. Microcircuits 4011 and 4001 can be replaced by Russian ones: K561LA7 and K561LE5 or K176LA7 and K176LE5, respectively, given the peculiarities of their power supply. Both "complementary" microcircuits are mounted in two layers (case to case), soldering together the corresponding conclusions. Other parts are mounted on the reverse side of the board, and the board itself is installed in the case so that the microcircuits are tightly pressed against one of the walls (it is useful to use heat-conducting paste during installation), which provides additional heat removal. It is better to fix the voltage regulator chip on a heat sink with an area of ​​at least 20 cm2.

When you turn on the amplifier for the first time, you need to check the current consumption; at a voltage of 15 V, it should not exceed 100 mA. Then you need to make sure that the constant voltage at the output is equal to half the supply voltage (in the UGT, according to the circuit in Fig. 3, it is close to zero relative to the common wire).

Both UGTs provide an output power of about 150 mW into a 120 ohm load. With this power value at the UGT output, the level of harmonic distortion of a sinusoidal signal with a frequency of 1 kHz is relatively high - about 0,45% (low order harmonics). However, how much these simple amplifiers benefit when compared with UGTs in CD players and tape decks, anyone who has repeated such an amplifier will be able to appreciate. According to the author, the difference is significant.

Before using the UGT, assembled according to the scheme in Fig. 3, you need to make sure that there is no constant voltage component at the outputs of the signal source, otherwise you will have to introduce isolation capacitors at the input of the amplifier.

Connection of low-resistance (less than 80 Ohm) headphones to the described amplifier is possible through current-limiting resistors with a resistance of 100 Ohm.

If the quiescent current of such an amplifier exceeds the value indicated by the author, then to reduce it, the supply voltage of the microcircuits should be reduced.

Author: M. Sapozhnikov, Ganei Aviv, Israel

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