UMZCH without general feedback. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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The proposed UMZCH is built without general feedback. Repeatedly, comparing the listening method, the quality of different transistor UMZCH with OOS, I had to think about how to improve their ability to transmit a completely stage image, and make the localization of sources more natural. The result of searches in this direction was the UMZCH circuit solutions, in which either there is no environmental protection, or it is local. In my opinion, there are two main reasons for the violation of the naturalness of the musical image.

Firstly, this is the introduction of phase distortions into the signal and the expansion of the spectrum of distortions in the UMZCH from 00С - to transmit sound brighter or softer, the balance between harmonics is important.

Secondly, the control of the voltage of the signal supplied to the loudspeaker is presented as "violence" on the speaker system. Indeed, initially, when recording phonograms, the sound is perceived as a level of pressure, i.e. as power in its integral calculus. And accordingly, when playing phonograms, the amplifier must transmit the signal power, and not just the instantaneous values ​​of current or voltage. Under this condition, less distortion is introduced into the output signal, which has a very favorable effect on the accuracy of the transfer of the stage image. I stopped measuring the technical parameters of my UMZCH five years ago, because with repeated listening to custom-made amplifiers, no one gave preference to one or another technical parameter. The main criterion is a subjective assessment of the properties of each amplifier, and the fact that amplifiers can differ significantly by this criterion is known, perhaps, to everyone! So, taking into account subjective assessments of the properties of the UMZCH, the proposed option will be an excellent replacement for many industrial amplifiers. The repeatability of the design was tested on four samples of a similar stereo amplifier.

Main technical parameters

The maximum power on a 4 ohm load is limited by the current protection device. For control listening with this amplifier, a DENON DVD 700 CD player and a Monitor Audio Silver 81 speaker system were used, an ARCAM "Diva A-75S" amplifier was used in the control path. When playing the soundtrack from the CD "Dark Side of the Moon" (Pink Floyd), the phantom helicopter rose a meter above the speaker system and flew over it, and not from one speaker to another, as is usually the case with most amplifiers. Stage and musical images of live recordings are also transmitted quite naturally.

(click to enlarge)

About the amplifier circuit

The diagram of one UMZCH channel is shown in fig. 1. Input stage - differential on transistors VT1, VT5 and VT2, VT6 with stable current sources on VT3, VT4. This is followed by a voltage amplifier with transistors VT7, VT9, VT11 and VT8, VT10, VT12, the peculiarity of which is that the transistors do not saturate at the maximum amplitude of the output voltage due to the diodes VD3, VD4. In the amplitude limitation mode for the output signal voltage, the base current of the voltage amplifier transistors VT7, VT9, VT11 and VT8, VT10, VT12 is limited, and they operate in a mode that excludes saturation mode. This ensures that there is no delay in exiting the limitation of the output signal by the supply voltage. The transistors are connected in parallel to increase the gate drive current of the output transistors. This made it possible to obtain the maximum amplitude corresponding to 8 V eff from the UMZCH output at a load of 30 ohms. undistorted sinusoidal signal up to 200 kHz (maximum generator frequency). A feedback signal acts from the output of the voltage amplifier through the R15R17R18 divider.

As can be seen from the diagram, there are no corrective capacitors in the amplifier. This became possible because the output stage is excluded from the feedback loop and the stability of the amplifier has increased dramatically. The output stage is a voltage follower made on complementary field-effect transistors from HITACHI. It features the same output impedance and symmetrical harmonic distortion for the positive and negative signal. Most complementary transistors differ in many ways, including dynamic parameters. Therefore, in amplifiers without a common feedback, a pronounced asymmetry of nonlinearity occurs, especially at high frequencies; at the lowest, it is associated with different thermodynamic properties of transistors.

In contrast, a single-ended amplifier has a nearly equal harmonic spectrum in both parts of the amplitude response (negative and positive signal voltage), although this parameter is often numerically more than 1% - and still sounds good! In this amplifier, I applied a circuit design solution in which two transistors of different conductivity with the same drain current work in a push-pull output stage at any time. It made it possible to bring together the spectra of harmonics for signals in different parts of the amplitude characteristic, and this was achieved without a cascade bridge circuit. A little more about implicit subjective preferences. An amplifier with a common OOS perceives reactive load disturbances, as well as external acoustic effects on the moving system of the loudspeaker head, controlling the output voltage.

In practice, sound reproduction with such an amplifier is often expressed by the "empty" space of the sound stage between the loudspeakers and its center. A simple experiment was carried out. Resistors with a resistance approximately equal to half of their impedance were connected in series with the AC speakers, after which they listened with an amplifier with a general OOS of those phonograms where "dips" in the sound stage are most clearly manifested. The result of listening confirmed the assumption: such an effect is much more noticeable with an amplifier with a common feedback without additional resistors.

The proposed output stage provides a low damping factor K_д=R_н/R_O, here the output impedance is 2 ohms. This became possible due to the serial connection of the transistors of the output stage, and as a result, the slope of the equivalent powerful transistor decreased by half. Such an output impedance, in the absence of a common OOS, made it possible to improve the localization of virtual sound sources, and for this it is necessary to convey the phase of the signal as correctly as possible. In this case, the slew rate of the output signal is important. Based on the measurement of real parameters of high-frequency dynamic heads, the following results were obtained: R_к\u4.5d 12.2-XNUMX ohms; L_k=0.16-0.33 mH. For the highest frequency head, specific values ​​correspond to the time constant t=L_k/R_k\u0.00027d 12.2 H / 0.000022 Ohm \uXNUMXd XNUMX s, and the cutoff frequency of such a converter is f_Wed=ω/2π=7191 Hz. Above this frequency, the dynamic head works as a low-pass filter and introduces noticeable phase distortions into the transmitted signal.

Speaker developers pay special attention to the selection of dynamic heads and filters with the same parameters. In order for the amplifier not to have a significant effect on the frequency properties of the sound reproduction path, its maximum operating frequency must exceed the cutoff frequency of the HF head by an order of magnitude - in this case, 71910 Hz, and the slew rate SR = 29,3 V / μs at 65 V. Let's calculate the required maximum output slew rate for amplifier option with maximum output voltage of 65 V and maximum operating frequency of 24100 Hz (low-pass cutoff frequency of Audio CD players with DAC without upsampling): SR'=2πf_MaxU_load\u2d 3.14 * 24100 * 65 * 9.8 \u2d 3.14 V / μs. The amplifier provides the output signal slew rate not less than SR=200000*30*1.41*(53*20)=200 V/µs. Thus, the amplifier is able to work with speakers that have an extended frequency response (above XNUMX kHz). The high temperature stability of the output stage does not require the use of measures for additional stabilization of the quiescent current; with an active load, its frequency response is linear up to XNUMX kHz.

The scheme of the AC protection node is classic and repeatedly proven. When power is applied, there is a delay for connecting the load by 10 s (it can be changed by selecting resistors R32, R33). If there is a constant component at the amplifier output of more than ±0,6 V, the load is disconnected by opening the relay contacts. When the power is turned off, the AC is turned off within 0,2 s. Protection against current overloads in the amplifier is based on limiting the drain current of powerful transistors by limiting the voltage at the gates with zener diodes and diodes VD13, VD14 and VD15, VD16; thus, the maximum current through the output transistors does not exceed 7 A. It should be noted that these elements can introduce distortion at frequencies above 100 kHz, so it is not recommended to install them unnecessarily. The description of the transistors indicates the presence of a built-in two-anode zener diode in the gate-source circuit at 15 V, this allows you to protect the gate from breakdown at higher amplitudes of the control voltage.

Amplifier Open Circuit Voltage Gain K_u=1+(R17/2R15)=51(34 дБ).

(click to enlarge)

Amplifier design

Structurally, the amplifier is made on a printed circuit board with dimensions of 160x100 mm. PCB drawings and component locations are shown in Fig. 2. The board contains all the elements of the amplifier and the rectifier of the power supply. The wires of the power and load circuits, as well as the input circuit, are connected to it. The transistors are pressed against the heat sink directly through the board. This solution has been used by me in my designs for more than 10 years. This allows you to keep all communications minimal. As a heat sink, any flat metal surface is used, for example, cases; the location and fastening of the board itself is also not difficult. It should be noted that the common wire of the input circuit is not connected on the board to the common wire of the power supply. This is done in order to be able to connect the common wires of the corresponding circuits at a common point (star) of a multi-channel system in order to reduce the level of interference. In the absence of such a need, it is possible to make a floating jumper between the output point to contact X2 and the adjacent conductor of the common wire.

The preamplifier is powered from a separate source with a voltage exceeding the supply voltage of the output stage by 10 ... 25 V. This provides a more complete use of the voltage and eliminates the penetration of the output signal into other stages along the power circuits. In the output stage, you can leave one transistor each, in which case the output impedance of the amplifier will become 1 Ohm, while you should either reduce the number of diodes in the bias circuit to three or four, or connect two transistors in parallel to the arm - then the output impedance of the amplifier will become 0,5 .14 Ohm and the maximum output current will increase to 8 A. In this case, again, the number of diodes in the bias circuit should be reduced to three or four. For cases of parallel or serial connection of output transistors, there are places for jumpers on the board from the mounting side, they are simply closed by soldering in accordance with the selected connection scheme. The supply voltage of the output stage for operation on a load of 2 ohms - no more than 70x190 V with a load power of up to 4 W; for 2 ohms - 40x100 V at power up to 4 watts. When two transistors are connected in parallel in the arm, the power at a load of 350 ohms reaches 2 W at a supply voltage of 65x2 V. The maximum values ​​\u40b\u2bof the AC voltage from the windings of the mains transformer indicated in the diagram correspond to voltages of 70xXNUMX V for powering the output stage and a little less than XNUMXxXNUMX V - for preliminary stages.

When field-effect transistors were connected in series, the subjective listening result was the most favorable, and it was noted that the sound character had features inherent in tube amplifiers. Paralleling power transistors is especially useful in a subwoofer amplifier with a closed-type loudspeaker with a low delay time - the sound of such a subwoofer neatly complements the sound stage. I used one transistor per arm only in my car amplifiers; their high quality allowed them to take four prizes (two of them first) at the car audio competition in 1992. After assembling the board, you should set the quiescent current of the output stage by a set of the required number of diodes in the VD5-VD12 circuit. To do this, it is enough to supply power to the preliminary and output stages.

The output stage transistors should be pressed against the heat sink through heat-conducting electrical insulating material. Two diodes can be left in the bias circuit for the simple and parallel stages, and four for the series. After that, by increasing their number, the selected quiescent current is set. To display the nuances of reproduction, I recommend choosing a quiescent current in the range of 200 ... 500 mA, it depends on the area of ​​\u100b\u0,6bthe heat sink used and the efficiency of its cooling. No additional measures to stabilize the quiescent current are required. The dead point to the change in the temperature of the crystal is at a quiescent current of about XNUMX mA and a gate-source voltage of XNUMX V. After setting the quiescent current, it is necessary to minimize the DC voltage at the output of the amplifier.

Since there is no capacitor in the feedback circuit, the gain for AC and DC voltages is equal. The consequence of this may be a small DC voltage at the output of the amplifier. Practice has shown that only a positive voltage of up to 1,5 V is found at the output of such an amplifier. To adjust the mode, turn off the fuses in the power supply circuit of the output stage and apply power to the preamplifier. The cascade is balanced by choosing the connection point of the top resistor R17 according to the output circuit with the diodes VD5-VD12 of the bias circuit: the lower the diode jumper point is selected according to the circuit, the greater the compensation of the constant component. By measuring the voltage on the collectors of transistors VT11 and VT12 with a multimeter relative to the common wire, they achieve their equality in absolute value. For such a setting, the board provides for the installation of floating jumpers, when the selected conductors are closed with a drop of solder in the desired circuit (this can only be done when the board is removed from the heat sink). But the R17 resistor can also be soldered from the side of the parts directly to the output of one of the diodes without removing the board from the heat sink, and the quiescent current can be adjusted by closing the diodes on the board with pieces of wire from the side of the elements. This completes the amplifier adjustment.

At the input of the amplifier, you can install a 16 μF isolation capacitor C1 (shown only on the board), for example, the K73-17 group, but this is usually not required in stationary music centers. The relay installed on the printed circuit board is WJ113A, WJ113-2C for a voltage of 12 or 24 V or another similar design for a current of at least 16 A, for example, from TTI. Diodes in the bias circuit can be set to any high frequency. Domestic zener diodes are also applicable, for example, KS215Zh, KS218Zh, KS515G, KS509A-KS509V.

All parts used in the amplifier (except for output transistors) are freely sold in many companies selling radio components. Documentation for output transistors in PDF format can be easily found on the Internet on the websites of domestic companies selling radio components.

Author: A. Grigoriev, Tomsk. Radio No. 1, 2007; Publication: cxem.net

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