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Remote-controlled Preamp with Digital Pot Schematic Circuit Diagram

High-Quality Preamplifier with Digital Potentiometer

Presented here is a straightforward yet superior preamplifier employing a DS1882 digital potentiometer, a component tailored for audio applications. Controlled through an I2C interface by an R8C/13 microcontroller, this design offers noteworthy advantages in terms of remote control and the absence of mechanical components.

Remote-controlled Preamp with Digital Pot Schematic Circuit Diagram

The circuit is controlled by two buttons (volume up and volume down) and an infrared receiver connected to the microcontroller. The software in the microcontroller, written in C, is designed to interpret RC5 codes and supports the following commands:

  • volume up;
  • volume down;
  • mute.

Expanding Control Possibilities

Additional commands can certainly be incorporated into the circuit. The audio signal enters through phono sockets and passes through coupling capacitors before reaching the digital potentiometer. These potentiometers, set up as voltage dividers with a total resistance of 45 kΩ, allow adjustment of the wiper position through the I2C interface.

Signal Buffering for Enhanced Output

At the potentiometer outputs, two operational amplifiers operate in a non-inverting configuration, buffering the high-impedance attenuated signal. These amplifiers offer a gain of 5.7. The capacitors within the feedback network are sized to ensure a signal bandwidth of approximately 150 kHz when the output is not loaded.

Optimizing Output Coupling Capacitors

The choice of output coupling capacitors depends on the input impedance (Rin) of the subsequent power amplifier stage. As a general guideline, a capacitance value of C=1/(100Rin) is suitable. The circuit diagram illustrates a 10 µF value, which is typically sufficient in most cases. In specific situations, connecting the outputs to ground via high-value resistors can establish a definite DC level.

Decoupling and Component Substitutions

To maintain stability, ±5 V supply voltages for the opamps and DS1882 are decoupled using 100 nF capacitors. The NE5532 opamp, a more budget-friendly alternative, can be utilized instead of the specified device without noticeable signal degradation. Additionally, all unused pins on the microcontroller are grounded for proper functioning.

Utilizing Serial Debugging Interface and Boot Code

The R8C microcontroller comes equipped with a serial debugging interface and boot code, enabling program downloads into its flash ROM. The serial connections are accessible at K1. To establish a connection with a PC, an RS232-to-TTL level adaptor (typically incorporating a MAX232) is necessary. Alternatively, a USB-TTL cable can be used for connection via a USB port. TxD from the PC should link to RXD1 on the R8C, and RxD on the PC should connect to TxD1 on the R8C. For programming, J2 must be fitted, grounding pin 28 (MODE) on the R8C. Power can then be applied to the circuit for a power-on reset, or reset button S4 can be pressed. Programming can be accomplished using the FlashSTA program, which can be downloaded for free from the accompanying web pages, along with the microcontroller firmware.

Expanding with Input Selection Switch

An option for expansion involves adding an input selection switch, achievable using an analog switch IC. This IC can be controlled via the existing I2C bus, offering enhanced functionality.

Understanding RC5 Remote Control Code Structure

The RC5 remote control code structure has been previously detailed in Elektor, available as a free ‘RC5 Code’ download. This protocol defines a five-bit address for the type of remotely controlled device, such as a television or VCR. In the author’s setup, the preamplifier was controlled using the remote from a Hauppauge TV card, and the firmware was configured with the address designated for TVs (‘00000’). If a different remote control is used, the firmware address must be adjusted accordingly. The address, represented in the ‘preamp.h’ file as ‘#define IR_DEV_ADDRESS 341’, translates to 341 in decimal, following a specific Manchester coding procedure based on the binary address ‘00000’, resulting in ‘0101010101’.

Implementing Timer Module and Addressing Infrared Sensor Challenges

The R8C incorporates a timer module for generating the RC5 signal, triggered by an interrupt. It is essential to note that the infrared sensor might face reliability issues if placed near fluorescent or low-energy light bulbs, as these sources emit significant infrared light, potentially interfering with the sensor’s operation.

[1] www.elektor.com/080213
[2] www.elektor.com/090976
[3] www.elektor.com/071149

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