How to connect an adjustable impedance to a stereo. Electronic volume control with remote control

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. Rear side of the device

The volume is increased by a button or remotely from the remote control (infrared control). Almost any home remote control will work.

The device diagram is shown in Figure 3.

Fig. 3. Electrical schematic diagram

Sound level switching is based on 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 a zero sound level. The rest of the outputs Q1-Q9 are connected to resistors R4-R12, which have different resistances.
Let me remind you that the microcircuit at the same time gives out a signal high level only at 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 according to the diagram), so that with each switching of the microcircuit to the base of the transistor VT2, more and more more current gradually opening the transistor.
A signal from an external ULF or sound source is fed to the collector of this transistor.
So, by switching the counter microcircuit, we, in fact, change the collector-emitter resistance and thereby change the volume of the sound coming to the speaker.
The resistances of the resistors depend on the gain of the transistor (h21e). For example, when using the 2N3904, the resistance of the resistor R4 can be about 3 kOhm in order to slightly "open" the transistor, while the sound will be at the quietest level. And the resistance R12 should be the smallest of the entire group (about 50 ohms) in order to provide the saturation mode and the maximum collector-emitter bandwidth, respectively, the maximum volume of this regulator.
It is difficult for me to indicate specific ratings of R4-R12, since it still very much depends on the power of the audio signal applied to the transistor, as well as on the power supply. It is best to use multiturn trimmers and tune the steps "by ear".

The lower part of the diagram shows an indication unit based on the K176ID2 (DD2) decoder. It is designed to control a seven-segment indicator.
A binary code is fed to the inputs of the decoder, therefore an encoder is built on the VD1-VD15 diodes, which converts the decimal signal from CD4017 into a binary code understandable for K176ID2. Such a diode circuit may seem strange and archaic, but it is quite workable. Diodes should be selected with low voltage drop, such as Schottky diodes. But in my case, ordinary silicon 1N4001 was 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 a binary code. Next, DD2 will accept a binary code and the required number will be displayed on the seven-segment indicator, showing 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 scheme uses the second type. Resistor R17 limits the segment current.
Resistors R13-R16 pull down the decoder inputs to minus for stable operation.

Now let's look at the top left side of the diagram. The DIP 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 tact button. Capacitor C3 eliminates contact bounce. Resistor R2 pulls down the CLK input to minus, preventing false alarms.
After power is applied, the HL1 LED is on, and the indicator shows zero - this is the silent mode (Figure 4, top).

Fig. 4. Display of levels on the indicator

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

If you set the switch to the lower (according to the diagram) position, then 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, which unlocks the 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 remotes (from a TV, DVD, etc.) work. When you press any button on the remote control, the volume will be controlled.

The circuit contains a latching button SB2. As long as it is pressed, the RST reset pin is connected to the power supply minus and the counter will toggle. Using this button, you can reset the counter and the volume level to zero, and if you leave it in the off position, the reset pin will not be pulled down 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, clock button and TSOP receiver with harness are brought to a separate board

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

Fig. 6. Device connection

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

Disadvantages of the device.
1) Volume control occurs only up the level, i.e. only louder. You will not be able to subtract immediately, you will have to reach the 9th level and then return to the initial level again.
2) Sound quality deteriorates slightly. Distortion is greatest 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, since the emitters of both transistors are combined to negative power, resulting in a "mono" sound.

Improvement of the circuit.
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 an emitter and a light receiver connected by optical communication. They are galvanically isolated, which means that the control circuit should not interfere with sound signal passing through the photoresistor. The photoresistor under the influence of the light of the emitter (LED, etc.) 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 Volts. When the voltage rises, the resistance of the current-limiting resistor R17 should be increased so that the seven-segment indicator HG1 does not fail, and the resistance R1 should also be increased to protect the TSOP receiver. But I do not recommend exceeding the supply voltage above 7 Volts.

There is a video for this article that describes the principle of operation, shows the structure assembled on the board, and a test of this device.

List of radioelements

Perhaps I'll start with a quote: "The task of regulating the signal level - in other words," loudness "- is one of the difficult problems in the circuitry of sound equipment." Here the author, greatly simplifying the problem, equates concepts such as "signal level" and "loudness", and then describes his level control. Signal level is a concept from the field of audio amplifiers (and not only) frequency amplifiers. The terms "level control" or "gain control" are used here. And loudness is a concept from the field of physiological acoustics, where "loudness", "loudness level", etc. are used.
The concept of "loudness" is much more complicated than the term "signal level" used by audio engineers and sound engineers and denoting the amount of voltage (in volts or decibels) at different points in the sound reinforcement path. Level controls, unlike volume controls, are not frequency dependent devices. There is even such a thing as a "loud-compensated volume control" (smells of tautology!), Meaning a control that takes into account the properties of hearing. It is worth mentioning the term "physiological volume control", similar to the one just named. Undoubtedly, the volume controls in hi-fi equipment are, as a rule, subtle-pensioned, or physiological. We will not consider the equipment of the "high end" (Hi-End), since any whims of snobs are fulfilled there for a lot of money. Luxury obliges!
It is known that the sensitivity of the human ear depends on the frequency, and therefore the same perceived loudness of sound at different frequencies corresponds to different levels of sound pressure. Graphically, this dependence is illustrated by "curves of equal loudness" (Fig. 1). To provide high quality reproduction of a particular sound program, it is necessary, focusing on the curves of equal loudness, to compensate for the corresponding differences in the sensitivity of hearing. Loudness volume controls are designed to perform this task.

However, designing such a regulator is far from easy. The point is that the shape of equal loudness curves is ambiguous. It depends on a number of factors, in particular, on the acoustic properties of the listening room, on the presence of masking noise, on the hearing characteristics of the listener himself, etc. As a result, the required frequency response family of the compensated volume control is also ambiguous. Still, good results, according to listeners, can be obtained using the standard curves of equal loudness of pure tones for a plane sound wave. But they need to be adjusted, guided by the considerations below.
When listening to music programs, the volume level usually does not exceed 90 phon and can be reduced by the listener to the hearing threshold or to the noise level in the room. For definiteness, the range of volume control at frequencies of 1 ... 2 kHz is taken equal to 80 dB. We will assume that the frequency response of the regulator is linear, and the musical program is balanced in timbre in the position of the regulator corresponding to the maximum volume (80 phon). The transition from this volume level to another, for example, 60 phon, requires correction of the frequency response of the regulator.
To obtain the corrected dependence in Fig. 1, we draw a horizontal line through the division of 80 dB on the L-axis (shown by the dashed line). Then we measure the distance from this straight line to several n points lying on a curve of equal loudness 80 phon. Further, these distances are laid down from the corresponding points on the curve of equal loudness 60 background. Through the new coordinates obtained in this way, we draw a curve that will be the corrected frequency response of the regulator in a position corresponding to a volume level of 60 phon.

Likewise, relative to a curve of equal loudness 80 phon. the corrected frequency response is plotted at volume levels of 40.20 and 0 (3) background, and the frequency response family of the volume control required for correct loudness is obtained. In the range of volume level change of 80 dB, it is shown in Fig. 2 (solid bold lines).
Now it is necessary to build a loudness-compensated volume control, the frequency response family of which approaches the required one in the best way. In the frequency range below 2 kHz, the curve corresponding to the minimum gain can be approximated by the frequency response of an RC circuit. shown in Fig. This characteristic to the left of the inflection frequency f1 (Fig. 3b) has a slope of 6 dB per octave. If the resistor R2 of this circuit is made variable, and its minimum resistance is chosen much less than R1. then when regulating the resistance R2, along with a change in the transmission coefficient of the circuit, the frequency of inflection of its frequency response will also change. As can be seen from Fig. 2, taking into account the approximation within 3 dB, the inflection frequency must move in the process of regulation along the LP line in order to provide the desired loudness. In this case, the range of change in resistance R2 cannot be more than 100, since fа / fв should change by 80 dB (10,000 times). The resistance R2 should change by the same factor.

It is quite obvious that by changing the resistance of only one resistor R2, it will not be possible to achieve such a shift in the inflection frequency and change in the transfer coefficient. However, by increasing the number of series-connected RC-circuits and at the same time decreasing the adjustment limits of the resistor R2 in each of them. this problem can be solved. Already two such RC-circuits (the time constant of the second circuit should be 20 ... 40 times greater than the first) allow you to get a completely acceptable result: the deviation of the curves of the real AFC family (dashed lines in Fig. 2) from the required (solid line) does not exceed 3dB.
At frequencies above 2 kHz, a decrease in loudness from 80 to 60 phon is accompanied by the appearance of an inflection on the 60 phon curve at a frequency of 5 kHz with a slope of 3 dB per octave. With a further decrease in volume down to the threshold of the auditory sensation (level 3 background), the inflection frequency shifts from 5 to 3 kHz, while the slope of the curves practically does not change. In this frequency range, the background curve 3 can be approximated by the frequency response of the RC circuit shown in Fig. 4a. The values ​​of the resistors R1 and R2 here are the same as in the RC circuit. shown in Fig. A change in resistance R2 does not lead to a shift in the inflection frequency f2 (Fig. 4b).
So that an increase in volume from 60 to 80 phon is not accompanied by a rise in higher sound frequencies, the RC circuit must provide frequency compensation at the maximum transmission coefficient, which can be achieved by shunting the resistor R2 with a capacitor C2 of such a capacity, at which the equality of the time constants T2 = R1C1 and x3 = R2-C2. In this case, the decrease in resistance R2 required to control the loudness will be accompanied by a decrease in the time constant ts and a shift in the cutoff frequency of the RC circuit (f3 = 1 / 2nR2-C2) to a higher frequency region, and the inflection frequency f2 will remain unchanged, which will ensure the required correspondence The frequency response of the RC circuit with curves of equal loudness in the nе frequency range above 2 kHz.

An example of a practical implementation of a loudness control is shown in Fig. 5 (4, 5]. The resistances of the resistors and capacitors included in it can be calculated using the following relations:
R1 = R3 = R:
R4min = R5min = 0.01R;
R4max = R5max = 10R;
R1C2 = R3C3 = 20MKC;
R4minC4 = 4000 μs;
R5minC5 = 100 μs;
R5maxC6 = 20 μs. Resistance R can be selected within 103..106 Ohm. In Fig. 5 R = 510 kΩ. R5minC5 = 2000 μs (4000); R4minC4 = 100 μs.
To avoid bypassing the R5-C5 circuit. The amplifier 34 connected to the output of the regulator must have a large input impedance and a low input capacitance. It, in particular, can be performed according to the voltage follower circuit on an op-amp with field-effect transistors at the input. The output impedance of the amplifier connected in front of the regulator should be 20 times less than the resistance R2. The variable resistors of the loudness control must be doubled. In our case, their functions are performed by the photoresistors R4, R5, and the resistor R10 serves as the regulator. changing the current through the HL1 incandescent lamp. The photoresistors SFZ-1 used in the volume control have a high response rate (time constant is less than 0.06 s) and the required resistance variation range. Incandescent lamp (subminiature) - NSM (6.3 Vx20 mA). the current through it varies within 6 ... 18 mA. The photoresistors are placed close to the incandescent lamp, and the entire regulator is housed in a light-tight metal shield.
Figure 5 shows a two-channel control for a stereo amplifier. In it, it is necessary to select in pairs the photoresistors in different channels so that when changing in the range from 104 to 106 Ohm, their resistances differ by no more than 20%. Otherwise, channel imbalance will be noticeable during volume changes.
Stereo balance is regulated by resistor R9 within ± 6 dB. Capacitors C7, SV eliminate rustles and crackles created by variable resistors.
Variable resistor R10 must have a linear control characteristic. Fixed resistors - with a resistance deviation from the nominal value of no more than ± 5%. Capacitors C1. С4, С5 - paper MBM, the rest - ceramic. The capacitance of the capacitor C6 depends on the mounting capacitance and the input capacitance of the amplifier connected to the output of the volume control. Incandescent lamps must be powered from a stabilized power source.
Adjusting the regulator comes down to ensuring the linearity of the frequency response at K „= 0 dB (by selecting C6) and checking the identity of the family of its frequency response in different channels stereo amplifier at different volume levels.

Another example of a regulator is shown in Figure 6. It uses double variable resistors with linear relationship resistance from the angle of rotation of the axis (group "A"). For a stereo regulator, you need to use two double variable resistors. Such a solution does not cause any special problems with the balance adjustment, if the volume level scales are applied on the panel where both resistors are installed.
The attempt to use a quad resistor runs into great difficulties; firstly, it is a very rare "bird" in our area, secondly, its resistors have large variations in resistance, and thirdly, an additional balance regulator is required, which does not simplify the entire structure. The spread of the resistances of the doubled resistors is quite acceptable for this circuit. If the double resistors have a different resistance, then the capacitances of the capacitors must be recalculated according to the given ratios. Resistors R3 and R5 serve to stop the bass boost outside the audio n-range.
With the upper position of the variable resistor sliders, the gain of the regulator is -6 dB. The adjustment range at a frequency of 2 kHz is 80 ... 85 dB. Deviation from the required AMX is no more than ± 2 dB. if the load resistance of the regulator is more than 1 MΩ, and the load capacitance is less than 50 pF. Capacitors C1. SZ. C5 - film, the rest - mica. Adjustment of the regulator - yes, no adjustment!
And finally, I will say that if you listen only to loud music, then it is enough to have a level regulator with a regulation range of 10 ... 15 dB. But if you want to feel the charm of the quiet music, as if coming from the nearest park, then build this volume control, you will not regret it!
Literature
1. A. Nikitin. Volume control in Hi-Fi equipment. - Radio hobby, 2002. No. 2, P.63.
2. Terekhov P. About volume control. - Radio, 1982, No. 9, P.42.
3. Zwicker E. .. Feldkeller R. Ear as a receiver of information. - M .: Communication. 1971.
4. I. Pugachev. Loudness volume control. - Radio, 1988. N911.C.35.
5 USSR author's certificate No. 1390776. - Bulletin "Discoveries, inventions ...". 1988, no. 15.

I. Pugachev, Minsk.

In this part of the article, we will talk about the aspects of matching Nikitin's volume control with an amplifier.
To obtain the declared parameters, reduce distortion and ensure smooth volume control, Nikitin's regulator must be matched to input impedance amplifier!

Let's consider in order:

1. General issues of regulator approval.
2. Coordination of the regulator with circuits on the op-amp and transistors.
3. Coordination of the regulator with tube stages.

1. General questions of approval.

To consider the general nuances of matching Nikitin's volume control with amplifiers, let us refer to the article " Distortions arising in the stages on the op-amp when regulating the signal level ", author V.A. Svintenok.

I will not cite it in its entirety (anyone interested will easily find it on the Internet). In it, the author, having conducted not entirely correct and incomplete experiments, confirmed the well-known fact that amplifiers in an inverting connection sound better and have less distortion than amplifiers in a non-inverting connection. This feature has long been noticed and tried to explain Douglas Selfie and Nikolay Sukhov(the author of the very "high fidelity amplifier"). The latter came to the conclusion that a similar effect is caused by the fact that in a non-inverting connection transition b-e the input transistor is outside the common negative circuit feedback, due to which the Miller capacity is not compensated. Accordingly, for an amplifier with field-effect transistors at the input, this effect is either much weaker or not observed at all.

So, Nikitin also took part in the experiments described in the article. Sometimes, however, it is not entirely correct. It is not clear why it was necessary to take the characteristics of an unloaded regulator ??? I repeat once again that to ensure the declared parameters (adjustment step, adjustment uniformity, adjustment range, etc.), the regulator must be matched to the load!!!

Note: in this article Nikitin is more often referred to as « ladder type ".

So, the most interesting and useful conclusions from the article:

... As shown above, the non-inverting inclusion of an op-amp with resistors at the inputs does not allow the maximum potential of most microcircuits for nonlinear distortion to be realized. Inverting inclusion gives the series best characteristics: less nonlinear distortion, shorter and "softer" spectrum of distortion, no "threshold" (a sharp increase in the higher harmonics in the spectrum), the distortion and the spectrum are not affected by the internal resistance of the signal source.

A standard construction of a level controller with a buffer follower in inverting connection is shown in Fig. 15. In practice, such a scheme is used quite rarely and this is due to the following. To keep the input impedance of the circuit at the same resistance valueRп and the law of change in resistance from the angle of rotation of the potentiometer knob is necessary for the resistors of the circuit to satisfy the conditionR>Rп (3 or more times). To get an acceptable input impedance of the circuit, you have to choose resistors high enough.R. This in turn leads to an increased noise level in the circuit.

However, consider this circuit as a starting point for this type of wiring.

For the circuit shown in Fig. 15, the maximum distortion will be in the upper position of the potentiometer sliderRп and correspond to the repeater in the inverting connection. Further, as the signal level at the output of the potentiometer decreases, the distortion at the output of the op-amp will proportionally begin to decrease. In this connection, it is enough to describe the behavior of the active element in the regulator by describing it at one point - at the point of observation of maximum distortions.

Table 10 shows the harmonic distortions for the input voltage of 2 and 4 volts for the inverter assembled according to the diagram in Fig. 15 with the nominal resistorsR = 5kOhm and with the controller transmission coefficient Kp = -1.

Table 10.

Analyzing the data given in Table 10, one can notice that the choice of microcircuits for building signal level controllers with low distortion is much wider.

Best ICs in this inclusionLME49860, LME49710 andAD8066... In addition to excellent nonlinear distortion characteristics, they also have an excellent distortion spectrum: 2-3 harmonics at an input voltage of four volts.

Excellent characteristics andJRC2114, OP275 andNE5532... The spectra of the first two microcircuits contain 4 - 5 harmonics at an input voltage of 4 volts, butNE5532 is long, with a dip. It is best used when the input voltage is less than four volts.

Good spectra (four harmonics) at an input voltage of 4 volts and yAD826, THS4062, LT1220... MicrocircuitsOPA2134, AD5599 andAD8620 it is better to use at an input voltage of two or less volts. HaveLM6171 v inverting distortion is significantly higher, and the nature and behavior of the spectrum from the supply voltage is the same as in the non-inverting connection.

As mentioned above, in practice, it is problematic to realize the high distortion potential of this type of regulator due to the inherent disadvantages of this inclusion. So, to obtain an input resistance close to 10 kOhm, it is necessary to select rather high-resistance resistors (more than 30 kOhm) in the inverter circuit, which will lead to a significant increase in the noise of the regulator and reduce the number of microcircuits capable of working at a sufficiently high quality level in this connection. To a large extent, these problems can be solved if a ladder-type signal level control is used in this inclusion ...

... to do this, you need pull-up resistor disconnect the regulator from the common wire and connect to the inverting input of the op-amp, as shown in Fig. 16.

All the advantages of this regulator are preserved in this inclusion. With a controller gain of 0dB, the circuit is a unity gain inverter with an input impedance of 10kΩ. The maximum distortions of such a regulator correspond to the maximum signal at the input of the inverter and will correspond to the data values ​​given in Table 10. At the input of the regulator, you can turn onRC circuit for limiting high frequencies without fear of increasing harmonic distortion. As the voltage decreases, distortion will also decrease, which is a normal and natural property of the regulator in this inclusion.

The maximum signal attenuation and frequency response are determined by the maximum attenuation of the regulator and its frequency response

Running a little ahead, we can say that this is one of better solutions allowing to obtain the minimum achievable nonlinear distortion with a "soft" and short spectrum. In this inclusion, distortions are achievable that do not exceed the level of units of a hundred thousandths at 4 volts at the input with a monotonic decrease in distortion as the attenuation coefficient of the regulator increases.

The only “not strong” part of the regulator is noise. They will be determined by resistors (equivalent value no more than 6 kOhm) and the inverter noise transfer ratio (equal to two) ...

It should also be noted that in the course of experiments at non-inverting Turning on the amplifier, the author revealed an increase in distortion with an increase in the mounting capacity of the regulator. Therefore, when assembling the circuit in this version, special attention should be paid to the elements of the regulator, their location and installation method!

2. Coordination of Nikitin's volume control with circuits based on op-amp and transistors.

An example of matching Nikitin's volume control with non-inverting amplifier:

click-to-zoom

Here, the input impedance of the amplifier is determined by the value of the resistor R11. To match the volume control, its nominal value is 10 kOhm. If you need to get more gain from the op-amp, you can increase the value of the resistor R12.

Let me remind you that in this circuit the potential of the operational amplifier (in terms of parameters and sound quality) is not fully realized and the circuit is quite sensitive to the capacity (quality) of the installation. Therefore, it is recommended to use it only if absolutely necessary.

When using op-amp in inverting switching on the above disadvantages are eliminated:

click-to-zoom

Here the input impedance of the amplifier is determined by the value of the resistor R11. For agreement with Nikitin's volume control, its value was chosen 10 kOhm.

Attention! In the above diagrams, the resistor values ​​are indicated to match the Nikitin volume control with the load. 10kohm... If the regulator is designed for a different load (for example, using the table from) the values ​​of the indicated resistors need to change on the appropriate.

An example of matching a regulator with a real amplifier:

the figure shows the input stage of the modernized power amplifier by V.Korol:

The cascade is made according to push-pull scheme, and with identical parameters complementary transistors T1 and T2 due to mutual compensation of base currents, the input resistance of such a stage will be determined mainly by the value of the resistor R1.

To match such an amplifier with Nikitin's volume control (by 10 kOhm), it is enough to install a 10 kOhm resistor R1:

click-to-zoom

3. Coordination of Nikitin's volume control with tube stages.

I suspect that some readers may find the regulator's input impedance (10kΩ) relatively low. Although in most modern devices ( sound cards, CD / DVD players) at the output there are buffers that allow you to connect a load of at least 2 kOhm, however ...

Suddenly someone wants to load tube stage to this regulator.

In this case, if there is no cathode follower at the output, to match the relatively low input impedance of the regulator with the high output impedance of the circuit (resistive tube stage or SRPP), you can use the one proposed by Zyzyuk (it must be turned on between the output of the tube stage and the volume control):

Setting up the circuit (performed with a short-circuited input - connect the free output C1 to the "common" wire of the circuit):

1. resistor R4 sets the quiescent current VT2 equal to 35mA.
2. resistor R1 is set to "0" constant voltage at the output of the circuit.

At the specified currents and voltages, no heat sinks are required for the transistors.

And even better would be to use "", picking up the input and output resistance.

Good luck with your creativity, high-quality sound and working schemes!