This block (all components are already soldered on printed circuit board) is specially designed for use both at home, as part of a home audio-video complex, and in a car (when generating a signal for car amplifier power).
The proposed unit will allow the radio amateur to assemble a simple and reliable pre-stereo adjustable amplifier with balanced inputs, which has a low level of self-noise, maximum functionality and wide range supply voltages. A preamplifier is installed between the linear or power output of the signal source and the input of the power amplifier to adjust the level of the desired signal at its output.

The pre-stereo adjustable amplifier is made on eight operational amplifiers DA1.1… DA1.4 and DA2.1… DA2.4. An artificial midpoint is made on the resistive divider R26, R27 and capacitor C8.
Op-amp DA1.1…DA1.4 has two balanced input blocks. The adders are made on op-amps DA2.1 and DA2.2. This design allows you to use almost any source (linear output(s), PA output(s)) to pick up a useful signal. Op-amps DA2.3 and DA2.4 are used to make amplifiers with variable gain in the range of 20 dB. Contacts X9 (+ supply voltage), X10 (- supply voltage) are supplied with supply voltage.

When using BM2118 after preamp, the signal from the line output should be applied to pins X1 (left), X5 (right), while X3 and X7 are connected to the ground of the power supply.

When using BM2118 after power amplifier, the signal from the output of the power amplifier should be applied to pins X2 (left), X6 (right), while X3 and X7 are connected to the ground of the power supply.

When using BM2118 after preamplifier with differential outputs the signal from the linear output should be applied to contacts X1, X3 (left), X5, X7 (right).

When using BM2118, the signal from the output of the power amplifier should be applied to pins X2, X4 (left), X6, X8 (right).

The useful amplified/suppressed signal is removed from contacts X11, X13 (left) and X12, X13 (right) for subsequent processing or supply to the PA. Potentiometer R17 adjusts the output signal level.
Attention! If there is a possible unbalance of the gains of the left and right channels (since the tuning is carried out by a dual resistor), it is necessary to select the values ​​of resistors R18 and R21.

Contacts used when connecting BM2118 after preamp
X1 - left channel signal input
X5 - right channel signal input
X3 - common wire
X7 - common wire

after power amplifier
X2 - left channel signal input
X6 - right channel signal input
X3 - common wire
X7 - common wire

Contacts used when connecting BM2118 after preamplifier with differential outputs
X1, X3 - left channel signal input
X5, X7 - right channel signal input
There is no common wire

Contacts used when connecting BM2118 after a power amplifier with differential outputs
X2, X4 - left channel signal input
X6, X8 - right channel signal input
There is no common wire.

NM2118 - Pre-stereo adjustable amplifier with balanced inputs buy in Master Kit. Driver, programs, diagram, reviews, instructions, do it yourself, DIY

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NM2118
Stereo pre-amplifier with balanced inputs

BM2118

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The set has been discontinued. Use functional analogue BM2118

This device is specially designed for use both at home, as part of a home audio-video complex, and in a car (when generating a signal for a car power amplifier). The proposed assembly kit will allow the radio amateur to assemble a simple and reliable pre-stereo adjustable amplifier with balanced inputs, which has a low self-noise level, maximum functionality and a wide range of supply voltages.

Specifications

Peculiarities

• A preamplifier is installed between the linear or power output of the signal source and the input of the power amplifier to adjust the level of the useful signal at its output!

Additional information

The pre-stereo adjustable amplifier is made of eight operational amplifiers DA1.1 - DA1.4 and DA2.1 - DA2.4. An artificial midpoint is made on the resistive divider R26, R27 and capacitor C8. Op-amps DA1.1 - DA1.4 have two balanced input blocks. The adders are made on op-amps DA2.1 and DA2.2. This design allows you to use almost any source (linear output(s), PA output(s)) to pick up a useful signal. The op-amps DA2.3 and DA2.4 are used as amplifiers with variable gain in the range of +/-20 dB. Contacts X9 (plus supply voltage), X10 (minus supply voltage) are supplied with supply voltage. When using a low-power source (linear output, etc.) with differential outputs, the input signal to the processing unit is supplied relative to contacts X1, X3 (left) and X5, X7 (right). When using a powerful source (PA output, etc.) with differential outputs, the input signal to the processing unit is supplied relative to contacts X2, X4 (left) and X6, X8 (right). When using a low-power source (linear output, etc.) with conventional potential outputs, the input signal to the processing unit is supplied relative to contacts X1, X3 (left) and X5, X7 (right), and X3 and X7 must be connected to the ground of the power source. When using a powerful source (PA output, etc.) with conventional potential outputs, the input signal to the processing unit is supplied relative to contacts X2, X4 (left) and X6, X8 (right), and X3 and X7 must be connected to the ground of the power source. The useful amplified or suppressed signal is removed from contacts X11, X13 (left) and X12, X13 (right) for subsequent processing or supply to the PA. Potentiometer R17 adjusts the output signal level. !Design. Structurally preamplifier made on a printed circuit board made of foil fiberglass with dimensions of 54x40 mm. The design provides for installation of the board into the case; for this purpose, there are mounting holes with a diameter of 3 mm at the edges of the board.

capacitors C 2 and C 3 – blocking (provide short circuit to the microwave and breaking the circuit DC). Matching the transistor and tuning it to a given frequency range is carried out using capacitors C 1, C 4 and sections of microstrip lines with lengths l 1, l 2, l 3 and l 4, approximately equal to a quarter of the wavelength at the central frequency of the operating range. Capacitors C 1 and C 4 also act as separating capacitors (provide DC decoupling of the power circuit from the input and output lines). At the input, the L-shaped SC1 is formed by segments of microstrip lines of length l 1, l 2, similarly at the output, SC2 is formed by segments of microstrip lines l 3, l 4. Short-circuited to the microwave (via C 2 and C 3) loops l 1 and l 4 simultaneously serve to supply power to the electrodes of the transistor. STs1 and STs2 provide matching of microstrip lines of standard wave impedance W, which are connected to the transistor, with the input and output resistances of the transistor. R 4 is a stabilizing resistor to prevent self-excitation of the amplifier.

The noise figure of the amplifier is greater than the minimum noise figure of the transistor on which the amplifier is implemented. This is due to the impossibility of accurately implementing the optimal source impedance in the frequency range. In addition, losses in the circuits that are connected before the transistor also contribute to the increase in the noise figure of the stage. In an amplifier with a gain bandwidth of 10–50%, the noise figure usually exceeds the transistor noise figure by no more than a few tenths of a decibel.

16.4. Operating principle of a balanced amplifier

The contradiction between power matching and noise mismatch, which occurs in a single-stage amplifier circuit, is overcome in a balanced amplifier. A balanced amplifier (Fig. 16.6) consists of two quadrature bridges and two identical active elements. Matched loads are connected to one of the bridge arms.

The passage of a microwave signal through a balanced amplifier is shown in Fig. 16.6. The signal with amplitude A, which is supplied to arm 1 of the input bridge, is divided into two equal parts in power in arms 2 and 3. Moreover, according to the properties of the quadrature bridge, if the phase of the signal that enters arm 2 is equal to (− ϕ) relative to the input signal in arm 1, then the phase of the incoming signal

in arm 3 is equal to (− ϕ − 90° ). From the outputs of the input bridge, signals are sent to

amplifying elements with identical transmission coefficients Ge − j Φ . The amplified signals enter arms 3 and 2 of the output bridge. Since the bridge is symmetrical, when the signal passes from arm 3 to arm 4 and from arm 2 to arm 1, the phase changes to (− ϕ). And when passing from arm 3 to arm 1 and from

arm 2 to arm 4, the phase changes by (− ϕ − 90°). As a result, there are 4 outputs in the arm -

In this bridge, oscillations are not excited, since the signals that came from shoulders 2 and 3 are in antiphase. In arm 1 we have an amplified signal with an additional phase shift (− 2ϕ − Φ − 90°).

Real amplification elements are not perfectly matched with the microwave path at the input and output. Therefore, reflected signals occur. Let's consider the passage of signals reflected from the inputs of amplifying elements. These signals of equal amplitude are divided in half and summed in arm 1 of the input bridge in antiphase, and in arm 4 in phase. Similarly, signals that are reflected from the outputs of the amplifying elements are absorbed in the load of arm 4 of the output bridge. Thus, with ideal bridge characteristics and identical reinforcing elements balanced amplifier is completely matched between input and output. In real designs, the use of a balanced circuit can significantly reduce the standing wave ratio at the input and output of the amplifier.

Compared to a single-stage amplifier circuit, a balanced circuit has the following main advantages:

The maximum output power increases by 2 times, regardless of the limitation by the type of AE used or supply voltage;

low SWR input and output of the amplifier.

Disadvantages of a balanced circuit:

2 times more circuit elements, which increases weight and size indicators, greater manufacturing complexity and the likelihood of failure;

twice the current consumption.

Security questions

20. What is the functional purpose of the amplifier?

21. What does power gain show?

22. How is the default operating frequency range of an amplifier determined?

23. What parameters quantitatively characterize the linear distortion of an amplifier?

24. What parameters characterize the matching of the amplifier with the microwave path?

25. What is the dynamic range of an amplifier?

26. How does amplifier nonlinear distortion manifest itself?

27. What parameter is used to estimate the level of third-order intermodulation products?

28. How is the efficiency of a microwave amplifier assessed?

29. What is the noise figure of an amplifier?

30. What characterizes the noise temperature of an amplifier?

31. How is the noise figure of a cascade amplifier estimated?

32. By what main criteria are microwave amplifiers classified?

33. How are the types of microwave amplifiers classified according to fundamental parameters?

34. How are the types of microwave amplifiers classified by purpose?

35. How are the types of microwave amplifiers classified by design?

36. How are types of microwave amplifiers classified according to AE type?

37. What amplifiers are called low noise amplifiers (LNA)?

38. Which type of microwave amplifier has the highest sensitivity?

39. What are the advantages of transistor microwave amplifiers?

40. What are the main types of microwave vacuum amplifiers that are widely used in practice?

41. What structural elements does the single-stage circuit contain? transistor amplifier Microwaves, what is their purpose?

42. Why is microwave amplifier stability analysis important?

43. What is the difference between extreme gain and mismatch modes?

44. How can we explain that with ideal bridge characteristics and identical AE characteristics, the balanced amplifier is completely matched across its inputs?

45. What are the advantages and disadvantages of a balanced circuit compared to a single stage amplifier circuit?

High-end devices are usually equipped with balanced connectors. This connection has many advantages, but is not cheap. Modern microcircuits help find a compromise solution.

It is generally accepted that balanced inputs and outputs are attributes of purely High-End technology. XLR connectors can be seen in amplifiers and CD players from audiophile companies Accuphase, Burmester, Krell, Mark Levinson, Luxman, etc. Symmetrical lines are widely used in professional audio equipment, in circuits for amplifying small signals and, if necessary, transmitting signals over long distances. As a rule, the organization of balanced lines involves at great expense, since it requires either high-quality transformers, or differential amplifiers and cunning inverters capable of operating a reactive load. How can you make such a connection yourself?

Before we try to answer this question, let's remember where they came from and why we need balance lines at all. Their main advantage is the ability to suppress common-mode interference, i.e. any interference from outside with very long length cable. This is important, but at home, where the distance between components does not exceed a few tens of centimeters (unless you place monoblocks next to speakers), another advantage of a symmetrical connection is more attractive. The fact is that the standard for it was initially developed for sound studios, where the line resistance is 600 Ohms. Such a small value makes the cable inductance and capacitance uncritical, and this is exactly what we need. Most high-end devices allow both balanced connection via XLR sockets and regular connection via RCA. So, many years of listening experience show that in the first case, the detail of the sound picture and spatial characteristics improve, the sound becomes more accurate and organized. Balanced lines will also be very useful where the signal is initially balanced, for example in single-bit DACs (see Andrey Markitanov’s publications in the previous and current issues) or phono preamplifiers with a differential input. There is simply no point in converting the signal to single-ended, and then, in the amplifier, again to push-pull. By the way, about amplifiers. Anyone who has ever tried to make a tube push-pull knows how difficult it is to assemble a good bass reflex - with low distortion on large signals, with the same frequency response and output impedance across the shoulders, good symmetry at any levels...

In most cases, transformers are used to solve these problems, and it is this circumstance that has long held back the use of balanced lines in home appliances, and here’s why. The transformer, being an absolutely simple and understandable device, is very labor-intensive and difficult to manufacture. You need good iron for the core, preferably with the addition of nickel or manganese, sectioned winding and careful laying of the wire, slightly thicker than a human hair. Otherwise, the trance will cut off frequencies at the edges of the audio range and twist the phase of the signal as it pleases. Add to this the need for double or even triple shielding and completely manual work, and it becomes clear that the price of a decent product will be very high. For example, a pair of Tango NC-22 interstages costs about $400 according to the catalogue! Trying to use miniature transics from microphones or taken out of decommissioned mixing consoles will lead to nothing - at best you will get a band of 80 Hz - 13 kHz plus large distortions at amplitudes greater than 0.775 V. Is it possible to find a way out?

Rice. 1. Balanced line with transmission coefficient K=1. The cable length can reach 150 m without compromising quality.

It is possible, if we remember that, in essence, transformers are impedance converters, i.e. devices that allow optimal matching of the signal source with the consumer. If there are two identical windings, they are capable of converting a single-cycle signal into a push-pull signal. Such a need, as already mentioned, arises quite often, so many semiconductor manufacturers began to produce specialized amplifiers - linear drivers and receivers. In the specifications for such microcircuits Burr-Brown and Analog Devices it is written: “transformer like driver”. Since the chips of these companies have almost identical circuit design and parameters, let’s consider the construction of a balanced line using the example of the SSM2142 driver and the SSM2143 receiver from Analog Devices (Fig. 1). Almost exact analogs from Burr-Brown are INA137 and DRV134. Their main feature is the high accuracy of the “built-in” resistances that set the gain. It is achieved by laser adjustment during the manufacturing of microcircuits and is 0.005%, which is absolutely unattainable when building a circuit using discrete elements. Thanks to this, common mode noise suppression averages 100 dB, and distortion in the frequency range 10 Hz - 100 kHz does not exceed 0.0008%. It is noteworthy that both chips provide all the declared characteristics with input signal amplitudes up to 10 V.

Rice. 2. Simplified diagram of the SSM2142 driver. This tiny chip is capable of driving the pre-terminal stage of an amplifier.

The SSM2142 driver (Fig. 2) costs about 200 rubles. retail. It has a gain of 2 (6 dB) and creates two absolutely identical clones of the output signal with an amplitude of up to 10 V at a load resistance of 600 Ohms. This microcircuit is insensitive to its reactive component and is capable of “pumping” a line up to 150 m long while maintaining the ideal meander shape. With its help, many design ideas can be easily realized. For example, by replacing a conventional op-amp at the output of a CD player with it, you can easily obtain a high-quality balanced output. The author successfully used SSM2142 in a bass reflex tube amplifier, applying a signal from its outputs directly to the grids of the 6N7S triode driver. Where else will you find a bass reflex with a linear frequency response up to 1 MHz, absolutely symmetrical and with an output impedance of 50 Ohms? The chip easily swings the lamp even when grid currents arise and does not care about the Miller capacitance. True, there are limitations. It must be remembered that the output circuits of the SSM2142 should not contain any correction circuits or resistive dividers, otherwise the symmetry may be broken. When using a driver in a power amplifier, it should be avoided to be covered by a common feedback loop, since excitation at microwave frequencies is possible. The SSM2142 performs best in triode circuits where the OS is not needed at all. Another personally tested application of the chip is organizing a balanced output for the GZ-102 generator to capture the characteristics of a tube preamp with XLR inputs. Here, any attempt to cheat by shorting one of the lines to ground or bypassing the input transformer led to unreliable measurement results.

Rice. 3. In the SSM2143 receiver, the resistances are adjusted with an accuracy of 0.005%.

Sometimes there is a need for reverse conversion, for example, for all kinds of adjustments and corrections. In a symmetrical stereo path, this will require precision quad potentiometers and twice as many parts, which are very expensive. Therefore, a balanced input signal is often converted to single-ended, corrected, and returned to its original form at the output. It happens that you need to correctly connect a single-ended tube circuit to the balanced line. In both cases, the SSM2143 receiver can help out (Fig. 3), which, having received a symmetrical signal, converts it into a normal one with minimal distortion and noise, and the coefficient can be 0.5 or 2, depending on how the connections are connected microcircuit legs. It also has another useful option - the ability to obtain a constant voltage from -10 to +10 V at the output. Thus, it is easy to set the bias on the lamp grid by connecting the cathode directly to ground. To do this, you will additionally need a low-noise op-amp, for example OP27 (Fig. 4). The SSM2143 easily handles load capacitance up to 300 pF and delivers current up to 20 mA. One chip costs about 130 rubles.

Rice. 4. This is how you can get constant voltage at the output of SSM2143 in the range of ±10 V.

Now a general note for all types of such microcircuits. Experience has shown that their sound very much depends on proper nutrition, and saving here is at a loss. All characteristics of drivers and receivers are stated at a supply voltage of ±18 V, but it makes sense to assemble two complementary 15-volt stabilizers, either on LM317/337 or on discrete elements. Best results were obtained when powered by parallel push-pull stabilizers (if you are interested, we will publish the circuit). Try germanium diodes in the rectifier and Black Gate capacitors in the filter, you won’t regret it. Each leg of the chip, to which “+” or “-” is applied, must be shunted with 0.1 - 0.47 μF ceramics and a tantalum droplet capacitor.

It probably makes sense to dwell on ethical issues. Many “tube” audiophiles do not even allow the thought of using microcircuits in High End circuits. I agree, transformers are much more musical, since they do not produce an endless “tail” of higher-order harmonics. But in this case, you have to pay too much for the purity of the idea. Of course, if you have 300 - 400 extra bucks in your pocket, I wholeheartedly recommend spending it on Tango, Tamura or MagneQuest products. If you have some experience, you can wind the trance yourself. And if you don't have either, then a couple of tiny chips in a DIP8 package can be very useful. At least in most cases they sound very decent, and their inclusion even in the most “polished” path does not cause any noticeable degradation. In addition, you save on cables, which is confirmed by experiment - replacing the expensive symmetrical XLO cable with a regular studio twisted pair cable 6 m long was absolutely not noticeable.