category Audio circuits materials in category * Subcategory Schemes of devices for switching and indicating audio signals and preamplifiers
Compared to electromechanical ones, electronic input signal switches are more reliable, have smaller dimensions and weight, and are more convenient to operate.
Along with all the listed advantages, the switch offered to radio amateurs is distinguished by its simplicity of circuit design and original indication of the connected input.
The nonlinear distortion it introduces into the input signal with a load of at least 1 MOhm and an input signal of up to 0.5 V is about 0.001%. Inputs are switched with just one button.
Audio signal input switch circuit
The switch works as follows:
When the power is turned on, counters DD1 and DD2 are reset, during which all (except output 0) outputs of counter DD2 are set to a logical zero level. At output 0, the level of logical one is set.” This voltage opens the corresponding keys of switches DD3 and DD4, signals from the inputs In1 pass to the output of the switch.
The HG1 indicator indicates this state as 0, which corresponds to the connection of the first input. When you press the input signal selection button SB1 once, a pulse is sent to the input of counters DD1 and DD2, at which 1 lights up on the HG1 indicator, and the level of logical one from output 0 of counter DD2 shifts to output 1." The voltage that appears at this output opens the corresponding keys switches DD3, DD4, after which its second inputs Bx2 are connected to the output of the switch.
Similar processes accompany pressing the key a second and third time, during which the third and fourth inputs are connected. When you press the SB1 button for the fourth time, the counters DD1 and DD2 are reset again, i.e. the first inputs are again connected to the load, the HG1 indicator indicates 0 and the process is repeated from the very beginning.
The switch can also use the method of indicating connected inputs using LEDs HL1 - HL4 (part of the circuit outlined by a dash-dotted line), while the need for the DD1 chip and the HG1 indicator disappears.
When installing, instead of the K176IE8 microcircuit, you can use K561IE8, K561IE9. The K561KTZ microcircuit will completely replace the K176KT1, but at the same time nonlinear distortions will increase approximately five times.
A switch is a device that allows you to switch (turn on or switch) electrical signals. An analog switch is designed for switching analog, i.e., signals that vary in amplitude over time.
I will note; that analog switches can be successfully used for switching digital signals.
Typically, the on/off state of an analog switch is controlled by applying a control signal to the control input. To simplify the switching process, digital signals are used for these purposes:
♦ logical one - the key is turned on;
♦ logical zero - disabled.
Most often, the level of a logical unit corresponds to a range of control voltages ranging from 2/3 to 1 of the supply voltage of the switch microcircuit; the level of a logical zero corresponds to a zone of control voltages ranging from 0 to 1/3 of the supply voltage. The entire intermediate region of the control voltage range (from 1/3 to 2/3 of the supply voltage) corresponds to the uncertainty zone. Since the switching process is, albeit implicitly, threshold in nature, an analog switch can be considered in relation to the control input as the simplest.
The main characteristics of analog switches are:
The disadvantages of the switch include the fact that the limit
When the generator is turned on, both key elements of the microcircuit are open. C2 is charged through R5 to the voltage at which the DA1.1 switch turns on. The resistive divider R1-R3 is supplied with supply voltage; C1 is charged through R4, R3 and part of the potentiometer R2. When the voltage on its positive plate reaches the switch-on voltage of switch DA1.2, both capacitors will be discharged, and the process of their charge-discharge will be repeated periodically.
To check the serviceability of the light indication elements, you must briefly press the SA1 “Test” button.
When working on an inductive load (electromagnets, windings, etc.), to protect the output transistors of the microcircuit, pin 9 of the microcircuit should be connected to the power bus, as shown in Fig. 23.26.
Rice. 23.24. Structural Fig. 23.26. turning on the microcircuit
ULN2003A (ILN2003A) microcircuits (JLN2003A when operating on an inductive load
The UDN2580A contains 8 keys (Figure 23.27). It is capable of operating resistive and inductive loads with a supply voltage of 50 V and a maximum load current of up to 500 mA.
Rice. 23.27. Pinout and equivalent chip UDN2580A
UDN6118A (Fig. 23.28) is designed for 8-channel switched active load control at a maximum voltage of up to 70(85) V and a current of up to 25(40) mA. One of the areas of application of this chip is matching low-voltage logic levels with high-voltage loads, in particular, vacuum fluorescent displays. The input voltage sufficient to turn on the load is from 2.4 to 15 V.
They coincide with the UDN2580A microcircuits in pinout, and in their internal structure with the UDN6118A microcircuits, other microcircuits in this series are UDN2981 - UDN2984.
Rice. 23.29. Structure and pinout of the ADG408 analog multiplexer chip
Rice. 23.28. Pinout and equivalent chip UDN6118A
Analog multiplexers ADG408!ADG409 from Analog Device can be classified as digitally controlled multi-channel electronic switches. The first of the multiplexers (ADG408) is capable of switching a single input (output) to 8 outputs (inputs), Fig. 23.29. The second (ADG409) - switches 2 inputs (outputs) to 4 outputs (inputs), fig. 23.30.
The maximum closed switch does not exceed 100 Ohms and the supply voltage of the microcircuit.
The microcircuits can be powered from a bi- or unipolar power source with a voltage of up to ±25 V; accordingly, the switched signals must be within these ranges in sign and amplitude. Multiplexers are characterized by low current consumption - up to 75 μA. The maximum frequency of switched signals is 1 MHz.
The load resistance is at least 4.7 kOhm with its capacity up to 100 ηF.
Shustov M. A., Circuitry. 500 devices on analog chips. - St. Petersburg: Science and Technology, 2013. -352 p.
In amateur practice, there is often a need to periodically connect various sound sources to one final amplifier. Rearranging the connectors every time is a tedious task. It is much more convenient to connect the desired sound source by simply turning the switch knob of a stereo electronic signal switch, which can be assembled from the proposed set. It is intended both for use as part of an amateur low-frequency amplifier (for example, sets NM2011 or NM2012 - ULF, sets NM2111 or NM2112 - tone and volume control unit), and for independent use in various low-frequency amplification devices.
Specifications
Supply voltage [V].................................................. ...................6-23
Current consumption no more than [mA].................................................. ....................5
Frequency band [kHz]................................................... ....................0.02-1000
Noise voltage [µV].................................................. ............................5
Maximum input signal level (rms) [V]....................5
Input impedance not less than [kOhm].............................................100
Output resistance no more than [Ohm]............................................400
Harmonic coefficient no more than [%]................................................... ....0.03
Transition attenuation between inputs not less than [dB]...................75
Description of the operation of the electronic switch
The switch board assembly is shown in Fig. 1. The electrical circuit of a stereo electronic switch (Fig. 2) is based on the TDA1029 microcircuit, which is a stereo four-channel analog multiplexer. The finished device has five stereo inputs and one output.
Signals supplied to input IN1 go directly to the chip. This makes it possible to use its full frequency range, exceeding 1 MHz. However, recommend it
can only be used for a limited set of signal sources. This is due to the fact that due to the excessively wide bandwidth of the microcircuit, interference may occur from radio stations operating in the long-wave range and generators operating in the ultrasonic frequency range (25...100 kHz). To weaken the effect of possible interference, first-order low-frequency filters (LPF) (R1...R6, C9...C14) are introduced into the input circuits IN2...IN4 of the switch circuit. By selecting filter characteristics, you can set the required bandwidth of the corresponding input.
Inputs IN/OUT4 and IN5 are multifunctional. The IN/OUT4 input can work as a regular, universal input, equivalent to the IN2 and IN3 inputs. If it is necessary to use a microphone, a microphone amplifier is additionally connected to the switch (not included in this set), the output of which is connected to input IN5 (labeled on the board as “input microphone amplifier”). In this case, the IN/OUT4 connector serves as an additional output for a microphone amplifier, for example for recording. If it is necessary to set sound blocking, the three contacts of input IN5 must be connected with a jumper and, by installing switch SA2, use it as a quick sound blocking (“Mute” mode). As can be seen from Fig. 2, pressing SA2 will immediately switch to the blocked input IN4. and the sound will stop. When you release the button, the source that was there before will immediately connect. But in this case, the device can switch only three inputs.
Switch SA1 can be of any type and is installed on the front panel of the amplifier. In Fig. 2 shows the position of the arrow
selector SA1 when selecting a signal source connected to the first input (IN1). LEDs VD1...VD4 are also installed on the front panel and serve to indicate the enabled channel. When the SA2 button is installed, the VD4 LED indicates the “Mute” mode.
Switching of multiplexer channels occurs when a code combination is applied to the control pins of the microcircuit. The correspondence between the code supplied to the control outputs and the enabled channel is given in Table. 1.
The input voltage from stereo signal sources is supplied to inputs 1...8 of the TDA1029 microcircuit. In addition, a bias voltage is supplied from pin 10 to inputs 1...8 of the microcircuit through resistors R7...R14.
Capacitors Cl...C8 are separation capacitors. They are designed to decouple input circuits using a constant component.
RCA connector blocks (“tulip”) are used as input connectors IN1...IN4.
Switch assembly
Before assembling the board, read the recommendations given at the beginning of this book. To avoid failure of radio elements, try to follow generally accepted installation rules. The list of all elements included in the set is presented in Table. 2. The arrangement of elements on the board is shown in Fig. 3.
When connecting the switch to an existing ULF, it is advisable to carry out the interconnect installation using shielded wire to reduce the influence of interference. If this is not possible, it is necessary to use a larger cross-section mounting wire for the common bus.
A correctly assembled device does not require configuration. Good luck to you!
Input selector for amplifier on relay (DIY).
To switch multiple input signals to a power amplifier without constantly tugging at the cords, various types of selectors are used. Below is a schematic diagram of such a selector; 12-volt relays are used as switching elements. The circuit is capable of switching 4 stereo audio sources. RCA and relay input connectors are located on one small board, which reduces noise and uses fewer shielded cables. The selection of inputs is carried out by a miniature flip switch with 4 positions. The board also contains a rectifier and a filter capacitance for the power supply. The schematic diagram of the selector is shown below:
The power connector is supplied with an alternating voltage of 9...12 Volts from a step-down transformer. In the diagram after the rectifier we see resistor R* marked 0R or more. This resistance is needed to limit the current when using transformers with a higher voltage than 9 Volts. When supplying an alternating voltage of 9 Volts, simply install a jumper. When applying 12 Volts after the rectifier and smoothing capacitance, the result will be 16.92 Volts, and this is already too much for a 12-volt relay; we install a current-limiting resistor. We estimate the nominal value using the formula: 16.92-12 / relay winding current.
The board configuration looks like this:
In the figure, the yellow dot under the resistor R* indicates the location of the droshk cut in the case of using a current-limiting resistor.
Printed circuit board for relay input signal selector in LAY6 format:
Photo view of the LAY6 format selector board:
RCA stereo connector – 4 pcs.
Relay 12 Volt HK19F-DC12V-SHG – 4 pcs.
Link to product page
4-position switch - 1 pc.
5Pin (2.54mm) connector for connecting a biscuit switch – 1 pc.
2Pin connector with bolt clamp (power connection) – 1 pc.
3Pin connector (connecting the selector output to the amplifier input) – 1 pc.
Imported diode assembly type W04, W06 – 1 pc.
You can also install diode assemblies like DB102, DB103 or similar on the board.
Electrolyte capacitor 470...1000mF/25-35V – 1 pc.
Diode 1N4001 (in parallel to the relay windings) – 4 pcs.
LED 5mm – 4 pcs.
Resistors in the LED circuit 1 kOhm – 4 pcs.
Current limiting resistor 200R 0.25W – 1 pc.
Connectors Input1 – Input4 - 3Pin 2.54mm – 4 pcs. This is if you do not use standard RCA input connectors, but external ones, which are installed not on the selector board, but on the amplifier body.
And one more Vcc connector is for supplying a constant supply voltage to the board; in this case, the variable is not connected, and the diode assembly does not need to be soldered.
Surely many radio amateurs, especially the older generation, still have “hard” logic chips like the K155, KR1533, K561 and similar series gathering dust in their bins. Many began their acquaintance with digital technology with them. In the era of microcontrollers, such microcircuits are used less and less, and not everyone will raise their hand to throw away such a “rarity”...
Let's try to find at least some use for them, and in the context of our publication, of course, we'll try to integrate them into audio equipment.
Proposed design amplifier input selector allows you to use a convenient and fashionable encoder to switch the inputs of your device, as well as select which one will be activated when the power is turned on (the encoder must have a button-press function). It turned out to be a funny scheme, however.
In industrial devices it looks something like this:
Now you can also equip your amplifier with such a fashionable switch.
Pros of the device:
- quite convenient switching of inputs with various options for indicating the active input
- low cost and availability of components,
- absence of clock signals (true audiophiles can safely build this selector into their tube amplifiers - the circuit generates pulses only at the moment of switching inputs.)
- the ability to select and, if necessary, quickly change the input that will be activated when the amplifier is turned on.
- the number of switched inputs can be changed from 2 to 10.
To be fair, we also note the disadvantages of the device:
- irrational use of the memory chip. Only one cell is involved in the work. Although, given the current cost of such microcircuits, this drawback can be considered insignificant.
- no remote control.
- relative difficulty. On a microcontroller everything would be much simpler, although it’s not a fact that it’s cheaper.
- increased energy consumption. Depends on the series of chips used. Compared to the overall power consumption of a tube amplifier, this disadvantage is also very relative.
The schematic diagram of the device is shown in the figure:
Click to enlarge
The IC7 chip contains a debounce suppressor for the encoder contacts. Elements IC8A, IC8B, IC1a, IC1C form counting pulses in one channel when the encoder is rotated in the appropriate direction, blocking the second channel to prevent false positives. Counting pulses are sent to the reverse counter IC3, which is the “heart” of this device.
From the counter outputs, the binary code of the selected input is sent to the decoder - microcircuit IC6. From the decoder outputs, signals through buffer stages (not shown in the diagram) are used to control relays or electronic switches that directly switch the amplifier inputs.
Also, signals from pins 1 and 10 are used to block the count when the first or last input is reached. In the version shown in the diagram, the selector is capable of switching 9 inputs. If you need fewer, for example 4 inputs, then pin 6 of IC1B should be connected to pin 4 of IC6.
From the outputs of the binary counter (by the way, if there are less than 10 inputs, then a BCD counter can also be used), the binary code of the selected input is also sent to the bidirectional buffer IC5. When you press the valcoder button through the contact bounce suppressor on element IC8C, elements IC2a IC2B generate control signals to write the active input code to the non-volatile memory EEPROM IC4 in a cell with address zero.
When the power is turned on, the memory chip puts on the data bus the value written in the zero memory cell. This value is loaded via asynchronous inputs into the counter IC3 using a pulse generated by the circuit R6, R7, C6. This is how the selected input is activated.
There are two ways to organize the active input indication.
The first way is to connect LEDs to the outputs of the decoder IC6. Then you get the option as shown in the first picture (see above).
The second method is more advanced. A seven-segment LED indicator that will show number selected input.
Since high performance is not required from the circuit, the device can use digital microcircuits of different series, which will determine the power consumption.
Domestic analogues of the used microcircuits:
- IC1, IC2, IC7, IC8 - 4093 - K561TL1 and similar
- IC3 - 74HC193 - KxxxIE6, KxxxIE7
- IC5 - 74HC245 - KxxxAP6 (AP4 or AP5 with circuit change)
- IC6 - 74HC42 - KxxxID6 (other decoders can be used depending on the required number of switched inputs)
The article was prepared based on materials from the Elector magazine.
Free translation by the Editor-in-Chief of RadioGazeta.
Happy creativity!
