A little history

This design appeared after I assembled the famous OM 2.5 amplifier. Naturally, the question arose of choosing a volume control, protection and other service functions. Of course, I also wanted to have a digital input and remote control, but this already seemed like a completely inaccessible space. I had never done any programming of controllers or design of electronic circuits before. However, as they say, the road can be mastered by those who walk, and the Atmega16 controller with a PGA2311 volume control chip settled on the breadboard. As a result, I was so fascinated by the process that it was very difficult to finish the project. While there was free memory and controller legs, ideas appeared to expand functions and add new modules. The boards for all modules were initially laid out in DipTrace and made by hand using photoresist. Then I tried to order some of the boards from production. Therefore, in the photo there is a hodgepodge of blue homemade and green factory circuit boards. So, in this article I tried to describe what I ended up with.

System functions.

• Soft start, delay adjustable from 0 to 30 seconds.
• Speaker turn-on delay, adjustable from 0 to 30 seconds.
• NEC standard remote control with remote control configuration from the menu system
• Speaker switching using protection boards: zones A/B (button, remote control), left/right (remote control) or simply on/off.
• Input selector control for 4 inputs (buttons, remote control)
• Volume and balance control using a PGA23XX chip or Nikitin relay RG (encoder, remote control)
• Matyushkin tone block control with relay adjustment of bass and treble (encoder, remote control)
• Control - transmission of stop/start/rewind/track commands (remote control)
• Thermal control on digital sensor LM75, one or two channels, shutdown when overheating, turn on fans
• Power buttons, speaker switching, four input selector buttons and Mute
• Adjusting the brightness of the screen backlight (remote control)
• Screen savers: screen blanking, level indicator and spectrum analyzer

System composition and configuration.

The system consists of a controller with a 4x20 character display mounted on the front panel and several execution modules. The display is installed parallel to the controller board on four racks and connected to it with PLS-PBS connectors, resulting in a fairly compact “sandwich” 12mm high. All connections are made around the perimeter of the controller board using XH corner connectors.

The modules carry out the necessary adjustments/switching and are installed in the amplifier housing taking into account minimizing the length of the signal circuits:

• Volume control based on PGA23XX with an input selector for 4 inputs and a connector for connecting a PCM2705 USB DAC
• Nikitin volume control
• Input selector for 4 inputs (for use with Nikitin RG)
• Matyushkin tone control with relay adjustment of bass and treble
• AC protection from direct voltage with switching of two zones A/B
• Thermal sensors
• Standby power supply with input filter and soft start control

The configuration of the modules used is determined by a dip switch on the controller board. It is read when power is supplied to the controller and determines the algorithm for further operation of the system:

The volume controls, tone control and input selector are connected to the SPI controller bus in series; for this purpose, the module boards have Control IN and Control Out connectors. When using the Nikitin RG, two such modules can be connected to adjust the balance. This allows you to flexibly configure the control system for a specific device. The range and step of volume control for PGA23xx and RG Nikitin can differ significantly, so they are set in the system setup menu. Important - the firmware does not check the entered values ​​for adequacy, so you should not set the maximum volume of +32db for the Nikitin RG. All possible options for connecting modules to the SPI bus:

• controller ->
• controller -> TB Matyushkin -> RG on PGA23XX with input selector and DAC
• controller -> RG Nikitin -> input selector
• controller -> RG Nikitin -> RG Nikitin -> input selector
• controller -> TB Matyushkina -> RG Nikitin -> input selector
• controller -> TB Matyushkina -> RG Nikitin -> RG Nikitin -> input selector

Thermal sensors are connected to the controller via the I2C bus. Their presence and quantity are also set by a dip switch. There are three options: thermal control is disabled, one sensor is used, or two sensors are used for each amplifier channel. If temperature control is enabled, you can set the maximum temperature at which the device will turn off. The temperatures for turning the fan on and off are also adjusted. When using two thermal sensors, it is possible to organize independent airflow for each channel.

Indication.

All information is displayed on a 4x20 character display on the well-known HD44780 controller. The first line displays the status of the AC switch. The same line displays the temperature of the radiators, obtained from thermal sensors, when it exceeds the temperature when the fan is turned on. The second line displays the attenuation of the RG in decibels. The third line is the state of balance. When adjusting the bass or treble, their status is also displayed in this line instead of the balance. The last line displays the input names and the current input.

Another indication organ is the LED. It lights up when the system is connected to the network and is in standby mode. When turned on, it goes out and blinks to indicate that commands are being received from the remote control.

If no controls are used for a certain period of time, the screen may switch to screensaver mode. The simplest one is to reduce the brightness of the screen backlight. If you connect an input or output audio signal to the corresponding inputs of the controller, you can use the “Level Indicator” or “Spectrum Analyzer” screensavers based on the Fourier transform.

Control.

For control, pushbuttons are used that close the corresponding controller inputs to ground, an encoder with a button, and a remote control with the NEC protocol. The encoder controls the volume control. When you press its button, the encoder sequentially switches to adjusting the balance/bass tone/treble tone. At the same time, symbols corresponding to the current mode flash on the screen. Only a minimal set of commands is implemented on the buttons and encoder; the full functionality of 26 commands is available only from the remote control. Some functions, such as changing the volume, support receiving auto-repeat commands from the remote control (when the remote control button is held down). For functions such as On/Off, auto-repeat is intentionally disabled - to repeat the command, you must press the remote control button again.

The minimum required kit to start and configure the system is a power button, an encoder and a remote control. When power is supplied to the controller, it will be in standby mode. A long press on the power button (from 2 seconds) puts the controller into setup mode. In this case, only the screen turns on, the soft start relays remain turned off. Navigate through the setup menu and change parameter values ​​by rotating the encoder. To select menu items, enter editing and confirm the selection, you must press the encoder button.

You can simply enter the remote control command codes in the corresponding setup submenu if you know them. But it’s easier to read them from the existing remote control. To do this, you need to enter the editing code of the desired command and press the corresponding button on the remote control. If the controller was able to accept the command, it will blink the standby LED and enter the code in the edit field. To confirm the code, all you have to do is press the encoder. All configurable parameters and commands are shown in the table below:

Controller circuit

Power is supplied through the protective diode D1 and the 5V stabilizer U1. Keys Q1 and Q2 control the soft start relay. R9 adjusts the contrast of the display; for a blue-backlit screen on the third leg of the X9 connector, you need to set the voltage to about 0.85-0.9V. Q3 is a PWM key for adjusting the brightness of the display backlight.

All buttons and the S1 configuration dip switch are connected to the controller via the I2C bus using PCF8574 port expanders (U3, U4). Pressing any button causes an interruption on the PB2 leg of the Atmega and, as a result, polling U3 for the code of the button pressed. The encoder (x6) and IR receiver (PH1) are also connected to the controller legs, which support external interrupts - PD2 and PD3.

Operational amplifier U5 is used to supply the analog signal of the right and left channels to the ADC inputs. Based on the data received from the ADC, the functions of a level indicator and a spectrum analyzer are implemented. The ADC inputs operate with a signal in the range of 0-5V, so the audio signal needs to be amplified/attenuated to an amplitude of 2.5V and a DC component of 2.5V added. The gain is determined by R15/R19 and R16/R20. R17 and R18 provide the required 2.5V bias. U5 must be Rail to Rail in input and output and operate with a 5V supply. When adjusting resistors R13, R14, it is necessary to achieve the maximum possible amplitude of the analog signal on PA6, PA7 (U2) without signs of a clip.

Firmware, Fuses, Modeling

Connector X2 is used for firmware. When flashing the controller, be sure to disconnect any modules from connector X3. After flashing the program, the Eeprom data file must be uploaded. When installing fuses, you need to disable the JTAG debugger (JTAGEN) and set the frequency to 8 MHz (CKSEL0, CKSEL1, CKSEL2, CKSEL3), everything else is default.

A model of the controller in Proteus 8 is attached to the article. With its help, you can familiarize yourself with the controller, test functions, indication, control signals, without assembling the device. I could not find the model of the LM75 digital thermometer, so another similar sensor and firmware are used taking into account this replacement. To emulate the NEC remote control, a simple model and firmware were made; I found the encoder emulator model in an open project. The firmware for these models is included with the Proteus file.

Thermal sensor

Thermal sensors are pressed against the radiators with the side with the microcircuit. On the other side of the board, jumpers set the addresses of the sensors on the I 2 C bus. The address of the left channel is 000, the right one is 001. If one sensor is used, the address of the left channel is set. An important limitation is that the outputs for switching on the OS blower are low-current and can pass a current of up to 100 µA. This must be taken into account when connecting keys that control fans to the controller.

Nikitin volume control

A circuit is used that is inverse to the original one - when the relays are turned off, the weakening of the regulator is maximum. Shift register U1 receives volume data from the controller (X9). Its outputs are amplified by Darlington switches with protective diodes U2, because The 74HC595 register cannot supply the required current to all relays. In addition, thanks to ULN2003A, you can use a relay that does not have to be 5V. The relay windings can be powered from the controller board, but it is better to power them from a separate source; connector X11 is provided for this. If relays with windings greater than 5V are used, external power is the only option. The choice of power source is determined by jumpers J1 and J2.

When installing all relays, an attenuation of up to -128 db and a control step of 1 db are provided. If the attenuation of -64db is sufficient, the K7 relay does not need to be installed. In this case, the output signal is removed from connectors X6, X8. You can increase the control step to 2db; to do this, it is enough not to install relay K1 and apply the input signal to connectors X2, X4.

Resistors R15 and R16 are needed to match the output impedance of the regulator with the input impedance of the amplifier. R15 is set if -64db output is used, R16 - for -128db output. The resistor value is determined based on the output resistance of the RG of 10 kOhm and the value of the input load resistance. If an input selector is not used, it is necessary to install resistors R20, R21, R22 to connect the digital and analog grounds. If there is an input selector, it is better to connect the grounds on its board.

The control circuit for the input selector is similar to the Nikitin RG, but with some simplifications. Since only one relay is turned on at any time, the current in register U1 is sufficient, and it was decided to abandon the ULN2003. Therefore, only 5V relays can be used in the input selector. When using conventional relays, jumper J1 is soldered. Jumper J2 is made for experiments with bistable relays for the future.

A Nikitin RG can be installed on the input selector. In this case, the analog inputs/outputs and the control bus are connected using PLS-PBS connectors. For this purpose, the selector has two outputs per channel, corresponding to the inputs of the Nikitin RG with a control step of 1db and 2db. R1, R2, R3 connect analog and digital ground. The jumper on the J3 board allows you to connect the grounds to the device body through a metallized mounting hole on the board.

In the original TB Matyushkin circuit, high frequencies are regulated by a variable resistor. This did not fit into the concept of my design, so the resistor was replaced with a relay divider. But it was necessary to reduce the number of relays in order to adjust the bass, treble and enable direct into the 7 legs of the ULN2003. I borrowed the switching diagram for three relays, instead of four, from. To minimize the board, 63V Epcos Mylar capacitors with a pin pitch of 5mm were used.

The relay switching control circuit is completely similar to Nikitin's RG. The only addition is the X4 Direct output for an external tone block bypass relay. The Direct relay turns on when all timbres are set to 0. The controller does not yet provide an additional command for turning on Direct, but it is not difficult to add.

This is the first module from which the development of the controller began. PGA2311 (U2) control consists of two eight-bit shift registers connected in series. Each register controls the volume of its channel. The chip has a data output, to which another regular register U3 was connected. It controls four input relays. The remaining four legs of the register transmit commands to the USB DAC through a 3V divider - play/pause, stop, rewind right/left, prev/next. track. This makes it possible to control the playback of playlists on a computer from the amplifier’s remote control, which is quite convenient. Analogue and digital power supply are separate and are supplied from three stabilizers - U4, U5, U6. The board has diode bridges and filters; you just need to connect a transformer. Instead of the PGA2311, the PGA2310 microcircuit can be used; to do this, it is enough to replace the stabilizers U4 and U5 with similar ones with an output voltage of 12V. An important feature is that digital and analog power must be supplied synchronously. The design of the module involves installation on the rear wall of the amplifier.

Instead of the first analog input, you can install a PCM2706 USB DAC. I posted all the materials on it on . In this case, instead of the X1 RS-813 connector, a connector for 3 RS-613 inputs is installed. An additional filter for the DAC is made on the operational amplifier U1. In addition, it amplifies the DAC output to standard 1.2V.

Measurements

The quality of the modules after assembly was checked using measurements by the program. EMU-0404 was used as a sound card. Thanks to this, I was able to detect and correct some errors in the board layout. I will not clutter the article with pictures with measurement results; they are attached to the project files. In general, we can say that the noise and harmonics of the modules are at the limit of the measuring capabilities of the EMU-0404.

List of radioelements

Most often, in cascades of volume controls of high-quality sound reproducing equipment, variable resistors are used directly as regulators, allowing the signal gain to be gradually or smoothly changed. However, often in tube LF amplifiers step volume controls are used, made using fixed resistors and switches.

The simplest and most common circuit solution for a tube ULF volume control when choosing smooth control is to introduce a potentiometer with a variable voltage division coefficient into the input circuit, into the interstage circuit or into the negative feedback circuit of the amplifier. By moving the slider of this potentiometer, the volume is directly adjusted. In this case, it is recommended to use variable resistors with a so-called logarithmic characteristic (type B characteristic) as an adjustment potentiometer to ensure a uniform change in the volume of the reproduced signal at different input signal levels.

If desired, the volume control with smooth adjustment can be replaced with a regulator with step adjustment. To do this, it is enough to make an appropriate replacement of the regulating element, that is, instead of a potentiometer, install a chain of series-connected constant resistors, the number of which and the ratio of their values ​​determine the range and law of regulation.

When choosing a volume control circuit, one should not forget that the human ear has different sensitivity to signals of different frequencies and volumes. In practice, this phenomenon manifests itself in the fact that when the volume of the reproduced sound signal decreases, the listener gets the impression of a change in sound timbre, which is expressed in an apparently significantly greater decrease in the relative volume of the components of lower and higher frequencies compared to mid-frequency signals. Therefore, in high-quality sound-reproducing equipment, fine-compensated volume controls are used, in which, when the volume is reduced, the necessary rise in the components of lower and higher frequencies is carried out to ensure equal loudness of perception. As the volume increases, the required rise in the edge frequency components decreases. The basis of fine-tuned volume controls is usually potentiometers with one or two taps, to which the corresponding RC circuits are connected.

Typically, the volume control is used to change the level of the ULF output signal with minimal introduced distortion. In this case, most often a variable resistor is used as such a regulator, connected either at the input of the amplifier or between the preliminary and final stages. Instead of a variable resistor, as already noted, a step regulator can be used, made on the basis of a switch and a cassette of resistors with different resistances. Simplified circuit diagrams of the simplest volume controls are shown in Fig. 1.

Fig.1. Simplified circuit diagrams of volume controls

To prevent the possibility of overloading the first amplifier tube with a large amplitude of the input signal, the volume control connection diagram shown in Fig. 1, a. In this case, the variable resistor is used directly as a load of the previous device. If the maximum amplitude of the input signal is small, a variable volume control resistor can be installed in the control grid circuit of one of the subsequent amplification stages, as shown in Fig. 1, b. The advantage of this connection is that it reduces the impact of external interference, since a useful signal is supplied to the regulator, already amplified to the required level.

The volume level in tube ULFs can also be adjusted using special cascades, which provide a change in the slope of the lamp characteristic. The principle of operation of such volume controls is based on the fact that when a lamp with a high internal resistance is used in the amplifier stage, the gain of such a stage will be proportional to the steepness of its characteristic (S). Therefore, when using a lamp with a variable slope characteristic, to change the gain of the cascade, it is enough to move the operating point to an area with a different slope value. Changing the position of the operating point and, accordingly, the gain can be done in different ways, for example, by changing the value of the bias voltage or the voltage on the lamp screen grid. Simplified circuit diagrams of such volume controls are shown in Fig. 2.

Fig.2. Simplified circuit diagrams of volume controls with changing the slope of the lamp characteristic

It should be noted that the volume controls considered, which use the principle of changing the slope of the lamp characteristic, can only be used in the first stages of the ULF at relatively small amplitudes of the input signal (no more than 200 mV). At higher input signal levels, significant nonlinear distortion may occur due to the curvilinearity of the dynamic response.

To adjust the volume in low-frequency tube amplifiers, regulators are often used that provide compensation for low frequencies at low input signal levels. A schematic diagram of one of these regulators is shown in Fig. 3.

Fig.3. Schematic diagram of a volume control with low frequency compensation at low input signal levels

An input signal with a fixed increase in the level of the lower frequencies of the reproduced range is supplied to the input of the cascade. This level is determined by the resistance values ​​of resistors R1, R2 and R3, which form the input divider, as well as the value of the capacitance of capacitor C2. From the output of the regulator, a feedback signal is supplied to the lamp grid circuit through a divider formed by elements R7 and C2. The higher the volume level, the greater the feedback. The resistance value of resistor R7 determines the ratio of the attenuation of low frequencies in the feedback circuit to the rise of these frequencies in the input circuit. Ideally, by selecting the resistance of resistor R7, it should be ensured that the attenuation of low frequencies in the feedback circuit is equal to their increase in the input circuit. In this case, the shape of the frequency response of the signal at the output of the stage will be close to linear. Shown in Fig. 3 element ratings are designed to use one of the triodes of the 6N2P lamp.

When the signal volume is reduced using potentiometer R6, the feedback value also decreases, but the fixed increase in low frequencies remains the same. As a result, the level of low frequencies in the output signal increases. At very low volume values, there is practically no feedback, and the cascade characteristic is determined only by the parameters of the chain R1, R3 and C2. At the same time, the rise in lower frequencies is maximum.

One of the disadvantages of this circuit is that the triode is connected before the volume control, so with a very strong input signal it can be overloaded. However, the signal from the input is fed to the control grid of the lamp through a divider, which, even at a frequency of 50 Hz, provides an attenuation of more than 4 times. As a result, this circuit can operate without distortion at an input signal level of up to 4-5 V. It should also be noted that the circuit in question is sensitive to the level of anode voltage filtering, so the use of the R8C5 filter in the lamp anode power circuit is mandatory.

When designing a tube ULF, radio amateurs often set themselves the task of including a cascade, with which they can adjust the volume remotely. The use of remote consoles with potentiometers placed in them in conventional regulators can hardly be considered a good solution, since most often such consoles are connected to the amplifier using long cables, which leads to very significant distortions. However, there are a variety of circuit solutions that provide volume control at a distance, for example, by changing the DC control voltage, with virtually no distortion. A schematic diagram of one of the options for a volume control with remote control is shown in Fig. 4.

Fig.4. Schematic diagram of a volume control with remote control

A distinctive feature of the regulator in question is the inclusion, instead of the cathode resistor of the amplifier stage triode, of another triode, which acts as a regulating element. When the value of the constant negative voltage supplied to the grid of the second triode changes, the value of its resistance changes. As a result, the depth of negative feedback for the first triode changes. So, for example, as the internal resistance of the second triode increases, the negative coupling increases, and the gain of the first triode decreases. In this circuit, an imported double triode of the ECC82 type can be replaced, for example, with a domestic 6N1P lamp.

In high-quality tube sound-reproducing equipment, volume controls with loudness compensation are widely used. The need to use such volume controls is explained by the fact that the sensitivity of the human ear changes depending on the frequency and volume of the perceived sound signal. For example, better sensitivity corresponds to the perception of mid-frequency components compared to higher and especially lower frequency components. Therefore, when the volume is reduced, the listener has a subjective feeling that the level of the components of the higher and lower frequencies of the reproduced range is simultaneously decreasing. As a result of research carried out in this area, certain dependencies were drawn up, which were called curves of equal loudness.

So that at different volume levels all frequency components of the reproduced signal are perceived equally, high-quality sound-reproducing equipment uses volume controls, in which, as the volume decreases, the necessary rise in the components of lower and higher frequencies is carried out, and with an increase in volume, the rise in the components of the boundary frequencies decreases. Such regulators are called loud-compensated or frequency-dependent. Naturally, developers strive to ensure that the characteristics of thin-compensated volume controls are as close as possible to equal volume curves.

The simplest option for constructing a frequency-dependent volume control is to combine the volume control itself and the tone control using paired variable resistors. Schematic diagrams of such volume controls are shown in Fig. 5, a and 5, b. Often, high-volume volume controls use potentiometers with one or two taps, to which the corresponding RC circuits are connected. A schematic diagram of one of the variants of such a volume control is shown in Fig. 5, c.

Fig.5. Schematic diagrams of simple loudspeaker volume controls

The current-compensated volume control can also have step adjustment. The advantages of such regulators, in addition to the absence of a potentiometer of the appropriate design, include the ability to select a significantly wider adjustment range. A schematic diagram of one of the options for the input stage of a tube ULF with such a regulator is shown in Fig. 6.

Fig.6. Schematic diagram of a thin-compensated volume control with step adjustment

Loudness compensation in volume controls can also be implemented using special filters. The schematic diagram of the regulator with a loudness filter is shown in Fig. 7.

Fig.7. Schematic diagram of a volume control with a loudness filter

In the circuit under consideration, the loudness filter is a double T-bridge, the transmission coefficient of which for the components of the middle frequencies of the reproduced range is less than the transmission coefficient for the components of lower and higher frequencies. In maximum volume mode, the potentiometer R4 slider should be in the upper position in the circuit, while the filter is short-circuited and does not affect the shape of the frequency response. To decrease the volume, the slider of potentiometer R4 should be moved down, which reduces the shunting effect of the upper part of this potentiometer on the filter. As a result, components of certain frequencies begin to pass through the filter in accordance with its frequency response. Since the components of the middle frequencies are attenuated by this filter to a greater extent than the components of the extreme frequencies, the change in the frequency response of the amplifier occurs according to a dependence close to equal volume curves. Potentiometer R4 must have a logarithmic characteristic (type B).

In this article we will look at the circuit of an electronic volume control with remote control and digital level indication.

Fig.1. Front of the device

Fig.2. Back of the device

The volume is increased by button or remotely from the remote control (infrared control). Almost any home control panel is suitable.

The device diagram is shown in Figure 3.

Fig.3. Electrical circuit diagram

Audio level switching is based on the CD4017 decimal counter (DD1). This microcircuit has 10 outputs Q0-Q9. After power is applied to the circuit, a logical one is immediately present at the Q0 output, the HL1 LED lights up, indicating zero sound level. Resistors R4-R12, which have different resistances, are connected to the remaining outputs Q1-Q9.
Let me remind you that the microcircuit at the same time produces a high-level signal at only one of its outputs, and sequential switching between them occurs when a short pulse is applied to the input (pin 14).
Based on this, the resistances in the group of resistors R4-R12 are selected in descending order (from top to bottom in the circuit), so that each time the microcircuit is switched, more and more current flows to the base of transistor VT2, gradually opening the transistor.
A signal from an external VLF or sound source is supplied to the collector of this transistor.
So, by switching the counter chip, we essentially change the collector-emitter resistance and thereby change the volume of the sound entering the speaker.
The resistance of the resistors depends on the gain of the transistor (h21e). For example, when using 2N3904, the resistance of resistor R4 can be about 3 kOhm in order to “open” the transistor a little, while the sound will be at the quietest level. And resistance R12 should be the smallest of the entire group (about 50 Ohms) to ensure saturation mode and maximum collector-emitter throughput, respectively, the maximum volume of this regulator.
It is difficult for me to indicate the specific ratings of R4-R12, since this still very much depends on the power of the audio signal supplied to the transistor, as well as on the power supply. It is best to use multi-turn trimming resistors and adjust the stages “by ear”.

At the bottom of the diagram there is an indication unit based on the K176ID2 (DD2) decoder. It is designed to control a seven-segment indicator.
A binary code is supplied to the decoder inputs, so an encoder is built on diodes VD1-VD15, which converts the decimal signal from CD4017 into a binary code understandable for K176ID2. This diode circuit may seem strange and archaic, but it is quite functional. Diodes should be selected with a low voltage drop, such as Schottky diodes. But in my case, ordinary silicon 1N4001 were used, they can be seen in Figure 2.
So, the signal from the counter output goes not only to the base of the transistor, but also to the diode converter, turning into binary code. Next, DD2 will accept a binary code and the seven-segment indicator will display the required number indicating the sound level.
The K176ID2 microcircuit is convenient in that it allows the use of indicators with both a common cathode and a common anode. The second type is used in the scheme. Resistor R17 limits the current of the segments.
Resistors R13-R16 pull the decoder inputs to minus for stable operation.

Now let's look at the top left part of the diagram. The two-position switch SA1 sets the volume control mode. In the upper (according to the diagram) position of the SA1 key, the volume is changed manually by pressing the SB1 clock button. Capacitor C3 eliminates contact bounce. Resistor R2 pulls the CLK input to negative, preventing false alarms.
After power is applied, the HL1 LED lights up, and the indicator shows zero - this is silent mode (Figure 4, top).

Fig.4. Displaying levels on the indicator

By pressing the clock button, the speaker volume increases in small jumps from level 1 to level 9, the next press again activates the silent mode.

If you set the switch to the down position (according to the diagram), the DD1 input is connected to an infrared remote control circuit based on a TSOP receiver. When an external IR signal arrives at the TSOP receiver, a negative voltage appears at its output, unlocking transistor VT1. This transistor is any low-power PNP structure, for example KT361 or 2N3906.
I recommend choosing an IR receiver (IF1) with an operating frequency of 36 kHz, since it is at this frequency that most remote controls (for TV, DVD, etc.) operate. When you press any button on the remote control, the volume will be controlled.

The circuit contains a locking button SB2. While it is pressed, the reset pin RST is connected to the power supply minus and the counter will switch. Using this button, you can reset the counter and volume level to zero, and if you leave it in the off position, the reset pin will not be pulled to minus and the counter Not will receive signals from the remote control, and Not will respond to pressing the SB1 button.

Fig.5. Switches, a tact button and a TSOP receiver with wiring are located on a separate board

I supply the audio signal to the regulator transistor from an amplifier on a PAM8403 chip. The VT2 collector is connected to the positive output of one of the amplifier channels (R), and its emitter is connected to the positive terminal of the speaker (red wire in the photo). The negative terminal of the column (black and red) is connected to the negative of the channel being used. The sound source in my case is a mini mp3 player.

Fig.6. Connecting the device

Why are trim resistors used?
I would like to draw your attention to the photo of the back of the device (Fig. 2). There you can see that there are three 100 kOhm trimming resistors R4, R5, R6. I only implemented three volume levels because the remaining resistors (R7-R12) did not fit on the board. Trimmer resistors allow you to adjust volume levels for different sound sources, because they differ in audio signal strength.

Disadvantages of the device.
1) Volume control occurs only up in level, i.e. only louder. You won’t be able to decrease it right away; you will have to reach level 9 and then return to the initial level again.
2) The sound quality deteriorates slightly. The greatest distortion is present at quiet levels.
3) Does not control the stereo signal. The introduction of a second transistor for one more channel does not solve the problem, because The emitters of both transistors are combined at the minus power supply, which results in a “mono” sound.

Improvement of the scheme.
You can use a resistor optocoupler instead of a transistor. A fragment of the circuit is shown in Figure 7.

Fig.7. A fragment of the same circuit with an optocoupler

A resistor optocoupler consists of a light emitter and a light receiver connected by optical communication. They are galvanically isolated, which means the control circuit should not interfere with the audio signal passing through the photoresistor. The photoresistor, under the influence of light from the emitter (LED or the like), will change its resistance and the volume will change. The optocoupler elements are galvanically isolated, which means that two or more audio signal channels can be controlled (Fig. 8).

Fig.8. Control of two channels using resistor optocouplers

Resistors R4-R12 are selected individually.

The device can be powered from USB 5 Volt. As the voltage increases, the resistance of the current-limiting resistor R17 should be increased so that the seven-segment indicator HG1 does not fail, and the resistance of R1 should also be increased to protect the TSOP receiver. But I do not recommend exceeding the supply voltage above 7 Volts.

This article includes a video that outlines the principle of operation, shows the design assembled on the board, and tests this device.

List of radioelements

An excellent solution for modding a bass amplifier!

Are available

Cheaper as a set

The device is designed to adjust the volume or other parameters of the audio system. It is a 2-channel digital variable resistor 10 kOhm (AD8403, 255 adjustment steps), tunable using a rotary encoder or IR remote control. IR remote control is not included.

Specifications

Additional Information

The selection of an adjustable parameter is made by pressing or rotating the valcoder button, IR remote control, the status is displayed on LEDs LED1...LED20

The overall volume is adjusted using the remote control buttons VOL >,

Balance left - right is adjusted using the remote control buttons |, | or the encoder, and the LEDs LED17, LED18 and LED19, LED20 light up alternately.

The forward-backward balance is adjusted using the remote control buttons TU-, TU+ or the encoder, while the LEDs LED17, LED20 and LED18, LED19 light up alternately.

After 30, the device goes into volume control mode.

When you press the MUTE button on the remote control, the volume of the channels gradually decreases, while the LEDs LED17...LED20 begin to blink. Pressing the MUTE or any other button again returns the volume to its original state.

Additional settings are made by shorting the corresponding pins of the JD1 connector (the first pin is square):

1-3 – reset all settings (default values)

3-6 – remote control button override mode (wait for sequential pressing of volume down and up, balance left and right, balance back and forth, MUTE)

1-4 / 1-5 – decrease the sensitivity of the encoder and remote control, respectively

4-6 / 5-6 – increasing the sensitivity of the encoder and remote control, respectively

All set parameters are saved in the non-volatile memory of the device.

Articles

Scheme

Connection sketch

Contents of delivery

• Electronic resistor assembly - 1 pc. PC.
• Instructions - 1 pc. PC.

Questions and answers

• Hello! Is it possible to order MP1231 Audio regulator 2 channels by mail?
  • Good afternoon, Andrey Petrovich! We will start working with Russian Post presumably in March. In the meantime, we use delivery by SPSR service.
• where can I get the firmware?
  • Unfortunately, the firmware is not distributed.
• Is the remote control included?