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AF Amplifier with Up/Down Volume Setting Schematic Circuit Diagram

Introduction to Philips Semiconductors TDA8551

The Philips Semiconductors TDA8551 is a compact audio amplifier featuring a built-in volume control. Operating at +5 V, it delivers a nominal output power exceeding one watt into 8 ohms. Additionally, it can function within a supply voltage range of +2.7 to +5.5 V, albeit with reduced output power proportional to the voltage.

AF Amplifier with Up Down Volume Setting Schematic Circuit Diagram 1

Volume Control and Adjustment

The TDA8551 offers versatile volume control, with the ability to modify the output volume across a range from -60 dB to +20 dB, encompassing 64 distinct steps. This control is achieved using a pair of up and down pushbuttons. The shared UP/DOWN input, which serves both switches, has three operational states. When left ‘floating,’ meaning that both switches are open, the volume remains constant. A pulse to ground reduces the volume by 1.25 dB, while a positive pulse elevates it by the same increment. Upon power-up, the internal counter defaults to the -20 dB setting.

Mute and Standby Functionality

An additional input labeled MODE allows the amplifier to transition between various states, including operational, mute, and standby. When this input is connected to ground, the amplifier operates normally. Applying +5 V to this pin activates standby mode, effectively reducing the current consumption from its typical operational level of 6 mA to less than 10 µA. The MODE input can also function as a mute input by applying a voltage ranging from 1 to 3.6 V. This voltage can be sourced through a connection to the SCR pin, positioned at half of the operating voltage and linked to a filter capacitor.

AF Amplifier with Up Down Volume Setting Schematic Circuit Diagram 2

Speaker and Headphone Connections

The loudspeaker is configured in a floating arrangement, connected between the two outputs of the bridge amplifier within the TDA8551. This setup allows for the attainment of the desired output power levels, even when dealing with a low supply voltage. In cases where headphone applications require less output power, headphones can be connected between the ground and one of the amplifier outputs through an electrolytic coupling capacitor. By employing two TDA8551 ICs, a stereo headphone amplifier can be assembled following this approach. The TDA8551 is available in a DIP8 package, and the Surface Mount Device (SMD) variant is denoted as the TDA8551T, packaged in an SO8 format. The datasheet can be acquired from


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