Low frequency amplifier circuits.

Audio Frequency (AF) Amplifiers

Actually, there is an overwhelming number of such circuits on the Internet. They vary in class, implementation, and parameters—ranging from simple designs to complex AF amplifiers with multi-loop negative feedback to minimize distortion. Building a "Hi-Fi" class AF amplifier for a ham radio receiver makes no sense for one simple reason: the signal source (the received airwaves) is far from a "musical" audio spectrum. On the contrary, it is a noisy, "dirty" audio spectrum because it reproduces "live" on-air signals. Received stations can experience fading and associated distortions, as well as high signal levels that cause noticeable clipping. Therefore, amplifying such signals with a "Hi-Fi" class amplifier is absurd, to say the least!

All circuits in this section are my own designs, based on knowledge and information gathered from public Internet sources. They are quite simple, easy to tune, use widely available components, and are inexpensive to build. A "reasonable" output power for such AF amplifiers should not exceed 2W into a 4-8 Ohm load. This amount of power is more than enough to hear even the weakest on-air signals through speakers without straining your hearing.

I would also like to add the following: ready-made AF amplifier ICs (Integrated Circuits) can serve perfectly as audio amplifiers. Examples include the LM386, TDA1013, TDA7052, TDA2003, TDA2030, TBA810, TA7368, and others. These amplifier ICs can be powered by anywhere from +3-5V up to +30V, utilizing either single or dual power supplies.

The most popular and widely used AF amplifier IC in amateur radio equipment today is the LM386, which delivers an output power of up to 1...1.5 watts. It is used in 9 out of 10 receiver designs. Despite a relatively noticeable distortion rate of 1-2%, the audio quality and overall performance of this IC are quite decent. Additionally, this IC does not require a heatsink.

Another excellent solution, particularly because it requires almost zero external components, is the TA4425 amplifier IC from Toshiba. This IC also boasts a low distortion percentage and needs practically no peripheral circuitry. Its only drawback (which in some cases is an advantage) is its fixed, non-adjustable voltage gain of about 50 dB (300 times voltage amplification), which can be excessive in many scenarios. However, in other cases, such as in direct conversion receivers, this fixed gain allows you to significantly reduce the gain in the preamplifier stages, thereby avoiding the "microphonic effect" caused by excessive overall gain in the audio path.

Below are various AF amplifier circuits suitable for use in both direct conversion receivers and standard superheterodyne receivers.

Standard NPN transistor AF amplifier.

The circuit above is a standard amplifier using an NPN transistor. The voltage gain is adjusted by selecting the value of resistor R2*.

LM386 in a paraphase configuration.

The circuit above shows an LM386 used in a paraphase (differential) configuration. This setup increases the overall gain. The first stage is a 0/180-degree phase splitter amplifier. The emitter and collector of the transistor provide opposite-polarity (0/180 degree) signals, which are then differentially amplified by the LM386 IC.

Power-boosted AF amplifier with discrete transistors.

The circuit above is a "power-boosted" version of an AF amplifier. Complementary transistors are connected three in parallel. Tuning this amplifier is standard: set the quiescent current to 5-6 mA and adjust the voltage at the left plate of the electrolytic capacitor (going to the speaker) to exactly half of the supply voltage (Vcc/2).

Bridge AF amplifier using TDA2822.

The circuit above demonstrates how to build a "bridge" (BTL) AF amplifier using a single TDA2822 IC. The first stage acts as a phase splitter. The 0 and 180-degree AF signals are fed into separate internal amplifiers within the TDA2822. Each amplifier boosts its own "half-wave," and these half-wave signals are then summed directly at the speaker.

Alternative bridge amplifier circuit.

The circuit above is functionally similar to the schematic shown in Figure 1.4.

Ageev's AF amplifier.

The circuit above was designed by author Ageev. This amplifier is very simple to assemble and easy to tune.

Electronic gain control with TDA7052A.

The circuit above illustrates an option for "electronic" gain (volume) control using the TDA7052A IC. Please pay attention: the part number must end with the letter "A". Without this letter, the electronic adjustment feature will not work, as that functionality is simply absent in the standard chip version.

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