The simplest stereophonic UMZCH class D

Recently, UMZCH class D. About one of the ICs for the UMZCH class D PAM8403 described in this article.

Sound amplifier based on PAM8403

For small-sized UMZCH, it is necessary to solve several mutually exclusive problems. Such UMZCH should have:

  • small dimensions;
  • low supply voltage (3…6 AT);
  • high efficiency;
  • low output impedance.

The block diagram of the classical UMZCH class D is shown in Fig. 1. It consists of a pulse width modulator (Prosperity), output stage (powerful keys) and low pass filter (FNC).

rice. 1

PWM converts analog audio signal to high frequency pulses, whose duration depends on the instantaneous voltage value of the analog signal at the time of conversion. The signal received at the PWM output has a low frequency component, which in shape repeats the sound signal at the input, which at the output will have a significantly higher amplitude and power. LF component is fed to the load (dynamic head) VA through low-pass filter and blocking capacitor CR Low-pass filter cuts off the high-frequency pulse component of the PWM signal, and CR – constant component. Moreover, the choke in the low-pass filter has a high inductance, and hence large dimensions.

UMZCH class D with the so-called bridge output are most suitable for use in small-sized equipment. This allows you to double the signal swing at the load and get rid of the blocking capacitor at the output.. Note, that the standard UMZCH class d (even pavements) not spared one more drawback – the presence of a choke in the low-pass filter. This choke – the most expensive and dimensional UMZCH part.

therefore, in small-sized equipment in recent years, bridge UMZCH of class D have been used without a low-pass filter at the output. In such UMZCH on the direct and inverse outputs of the low frequency sound signals will be antiphase, and high frequency pulse signals are in phase. In this case, the difference sound signal at the load will be of double amplitude, and the difference between the high-frequency pulse signal at the outputs will be close to zero.

In such a monophonic UMZCH, there are two pulse-width modulators in pairs that control the output keys in parallel. Square wave signals in quiescent mode at the output of the circuit will be in phase or will have a small phase shift (Fig. 2).

rice. 2

As a result, the dynamic head at rest, at worst, antiphase symmetric pulses of short duration arrive. To smooth them out, the inductance of the dynamic head is usually sufficient and, sometimes, a small capacitor is installed, which is connected in parallel with the load.

In the amplification mode of the input LF sound signal, PWMs work in antiphase, ie. if the pulse duration at the output of one PWM increases, then at the output of another decreases and vice versa (Figure 3).

rice. 3

This leads to impulse asymmetry., applied to the load, (a), so, to the appearance of current in the dynamic head.

One of the most popular microcircuits for stereophonic bridge UMZCH class D without LPF – this is PAM8403, which at supply voltage only 5 Used in 3-watt stereo UMZCH.

Main settings PAM8403:

  • supply voltage 2.5…5.5 AT;
  • voltage gain 24 dB;
  • PWM conversion frequency 260 kHz;
  • the load of each of the UMZCH channels on the PAM8403 is the dynamic heads with the voice coil impedance 4 or 8 ohm.

The PAM8403 is available in a SOP-16 SMD package.

The functional diagram of the considered microcircuit is shown in Fig. 4.

rice. 4

A schematic diagram of a stereophonic UMZCH on a PAM8403 microcircuit suitable for self-assembly is shown in Fig. 5.

rice. 5

The disadvantages of class D amplifiers include high harmonic distortion and increased noise level..

The UMZCH in question is assembled on a double-sided printed circuit board with a size of 20×20 mm using only SMD parts (capacitors and resistors of standard size 0805).

Besides, to reduce the radiation of high-frequency electromagnetic interference with this UMZCH, the manufacturer recommends wearing ferrite "beads" (Ferrite Beads) on wires, going to the dynamic heads and install small capacitors (220 pF) So, as shown in fig. 6.

rice. 6

"Beads" (Ferrite Beads) can be removed from the old switching power supply board. It is important that they are the same.

To electrically connect the printed tracks on different sides of the board, use thin tinned copper jumpers, passed and soldered through the corresponding holes in the board.

This UMZCH can be used not only in various radio amateur designs, but when repairing a variety of small-sized equipment, by the so-called "implantation" method, the essence of which is, that a faulty UMZCH device is turned off, its parts are dismantled, and instead of it and in its place, a ready-made board is installed and connected.

Author: Igor Bezverhny, g. Kiev
A source: Amateur radio №7-8 / 2020

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