Circuit of a high-quality UMZCH using field-effect transistors. Wideband UMZ with low distortion. Switching power supply
A long time ago, two years ago, I purchased an old Soviet speaker 35GD-1. Despite its initial poor condition, I restored it, painted it a beautiful blue and even made a box for it out of plywood. A large box with two bass reflexes greatly improved its acoustic qualities. The only thing left is a good amplifier that will drive this speaker. I decided to do something different from what most people do - buy a ready-made class D amplifier from China and install it. I decided to make an amplifier myself, but not some generally accepted one on the TDA7294 chip, and not on a chip at all, and not even the legendary Lanzar, but a very rare amplifier on field-effect transistors. And there is very little information on the Internet about field amplifiers, so I became interested in what it is and how it sounds.
Assembly
This amplifier has 4 pairs of output transistors. 1 pair – 100 Watt of output power, 2 pairs – 200 Watt, 3 – 300 Watt and 4, respectively, 400 Watt. I don’t need all 400 watts yet, but I decided to install all 4 pairs in order to distribute the heating and reduce the power dissipated by each transistor.
The diagram looks like this:
The diagram shows exactly the values of the components that I have installed, the diagram has been tested and works properly. I am attaching the printed circuit board. Lay6 format board.
Attention! All power paths must be tinned with a thick layer of solder, since a very large current will flow through them. We solder carefully, without snot, and wash off the flux. Power transistors must be installed on the heat sink. The advantage of this design is that the transistors do not need to be isolated from the radiator, but can be molded together. Agree, this saves a lot on mica heat-conducting spacers, because it would take 8 of them for 8 transistors (surprisingly, but true)! The heatsink is the common drain of all 8 transistors and the audio output of the amplifier, so when installing it in the case, do not forget to somehow isolate it from the case. Despite the fact that there is no need to install mica gaskets between the transistor flanges and the radiator, this place must be coated with thermal paste.
Attention! It’s better to check everything right away before installing the transistors on the radiator. If you screw the transistors to the heatsink, and there are any snot or unsoldered contacts on the board, it will be unpleasant to unscrew the transistors again and get smeared with thermal paste. So check everything at once.
Bipolar transistors: T1 – BD139, T2 – BD140. It also needs to be screwed to the radiator. They don't get very hot, but they still get hot. They also may not be isolated from heat sinks.
So, let's proceed directly to the assembly. The parts are located on the board as follows:
Now I am attaching photos of the different stages of assembling the amplifier. First, we cut out a piece of PCB according to the size of the board.
Then we put the image of the board on the PCB and drill holes for the radio components. Sand and degrease. We take a permanent marker, stock up on a fair amount of patience and draw paths (I don’t know how to do LUT, so I’m struggling).
We arm ourselves with a soldering iron, take flux, solder and tin.
We wash off the remaining flux, take a multimeter and check for short circuits between tracks where there should not be one. If everything is normal, we proceed to installing the parts.
Possible replacements.
First of all I will attach a list of parts:
C1 = 1u
C2, C3 = 820p
C4, C5 = 470u
C6, C7 = 1u
C8, C9 = 1000u
C10, C11 = 220n
D1, D2 = 15V
D3, D4 = 1N4148
OP1 = KR54UD1A
R1, R32 = 47k
R2 = 1k
R3 = 2k
R4 = 2k
R5 = 5k
R6, R7 = 33
R8, R9 = 820
R10-R17 = 39
R18, R19 = 220
R20, R21 = 22k
R22, R23 = 2.7k
R24-R31 = 0.22
T1 = BD139
T2 = BD140
T3 = IRFP9240
T4 = IRFP240
T5 = IRFP9240
T6 = IRFP240
T7 = IRFP9240
T8 = IRFP240
T9 = IRFP9240
T10 = IRFP240
The first thing you can do is replace the operational amplifier with any other one, even imported, with a similar pin arrangement. Capacitor C3 is needed to suppress the self-excitation of the amplifier. You can put more, which is what I did later. Any 15 V zener diodes with a power of 1 W or more. Resistors R22, R23 can be installed based on the calculation R=(Upit.-15)/Ist., where Upit. – supply voltage, Ist. – stabilization current of the zener diode. Resistors R2, R32 are responsible for the gain. With these ratings, it is somewhere around 30 - 33. Capacitors C8, C9 - filter capacitances - can be set from 560 to 2200 µF with a voltage not lower than Upit. * 1.2 so as not to operate them at their maximum capabilities. Transistors T1, T2 - any complementary pair of medium power, with a current of 1 A, for example our KT814-815, KT816-817 or imported BD136-135, BD138-137, 2SC4793-2SA1837. Source resistors R24-R31 can be set to 2 W, although it is undesirable, with a resistance from 0.1 to 0.33 ohms. It is not advisable to change power switches, although IRF640-IRF9640 or IRF630-IRF9630 are also possible; it is possible to use transistors with similar passing currents, gate capacitances and, of course, the same pin arrangement, although if you solder on wires, this does not matter. There seems to be nothing more to change here.
First launch and setup.
The first start-up of the amplifier is carried out through a safety lamp into a 220 V network break. Be sure to short-circuit the input to ground and do not connect the load. At the moment of switching on, the lamp should flash and go out, and go out completely: the spiral should not glow at all. Turn it on, hold it for 20 seconds, then turn it off. We check to see if anything is heating up (although if the lamp is not on, it is unlikely that anything is heating up). If nothing really heats up, turn it on again and measure the constant voltage at the output: it should be in the range of 50 - 70 mV. For example, I have 61.5 mV. If everything is within normal limits, connect the load, apply a signal to the input and listen to music. There should be no interference, extraneous hums, etc. If none of this is present, proceed to setup.
Setting up this whole thing is extremely simple. It is only necessary to set the quiescent current of the output transistors by rotating the trimmer resistor slider. It should be approximately 60 - 70 mA for each transistor. This is done in the same way as on Lanzar. The quiescent current is calculated using the formula I = Up./R, where Up. is the voltage drop across one of the resistors R24 - R31, and R is the resistance of this resistor. From this formula we derive the voltage drop across the resistor required to set such a quiescent current. Upd. = I*R. For example, in my case it = 0.07*0.22 = somewhere around 15 mV. The quiescent current is set on a “warm” amplifier, that is, the radiator must be warm, the amplifier must play for several minutes. The amplifier has warmed up, turn off the load, short-circuit the input to common, take a multimeter and carry out the previously described operation.
Characteristics and features:
Supply voltage – 30-80 V
Operating temperature – up to 100-120 degrees.
Load resistance – 2-8 Ohm
Amplifier power – 400 W/4 Ohm
SOI – 0.02-0.04% at a power of 350-380 W
Gain factor – 30-33
Reproducible frequency range – 5-100000 Hz
The last point is worth dwelling on in more detail. Using this amplifier with noisy tone blocks such as the TDA1524 may result in seemingly unreasonable power consumption by the amplifier. In fact, this amplifier reproduces interference frequencies that are inaudible to our ears. It may seem that this is self-excitation, but most likely it is just interference. Here it is worth distinguishing between interference that is not audible to the ear and real self-excitation. I encountered this problem myself. Initially, the TL071 opamp was used as a preamplifier. This is a very good high-frequency imported op-amp with a low-noise output using field-effect transistors. It can operate at frequencies up to 4 MHz - this is sufficient for reproducing interference frequencies and for self-excitation. What to do? One good person, many thanks to him, advised me to replace the opamp with another one, less sensitive and reproducing a smaller frequency range, which simply cannot operate at the self-excitation frequency. So I bought our domestic KR544UD1A, installed it and... nothing has changed. All this gave me the idea that the variable resistors of the tone unit were making noise. The resistor motors rustle a little, which causes interference. I removed the tone block and the noise disappeared. So it's not self-stimulation. With this amplifier you need to install a low-noise passive tone block and a transistor preamplifier in order to avoid the above.
To date, many versions of UMZCH with output stages based on field-effect transistors have been developed. The attractiveness of these transistors as powerful amplifying devices has been repeatedly noted by various authors. At audio frequencies, field-effect transistors (FETs) act as current amplifiers, so the load on the pre-stages is negligible and the insulated gate FET output stage can be directly connected to the pre-amplifier stage operating in class A linear mode.
When using powerful PTs, the nature of nonlinear distortions changes (fewer higher harmonics than when using bipolar transistors), dynamic distortions are reduced, and the level of intermodulation distortions is significantly lower. However, due to a lower transconductance than that of bipolar transistors, the nonlinear distortion of the source follower turns out to be large, since the transconductance depends on the level of the input signal.
The output stage on powerful PTs, where they can withstand a short circuit in the load circuit, has the property of thermal stabilization. Some disadvantage of such a cascade is the lower utilization of the supply voltage, and therefore it is necessary to use a more efficient heat sink.
The main advantages of powerful PTs include the low order of nonlinearity of their pass-through characteristics, which brings the sound features of PT amplifiers and tube amplifiers closer together, as well as a high power gain for signals in the audio frequency range.
Among the latest publications in the journal about UMZCH with powerful PTs, articles can be noted. The undoubted advantage of the amplifier is the low level of distortion, and the disadvantage is low power (15 W). The amplifier has more power, sufficient for residential use, and an acceptable level of distortion, but appears to be relatively complex to manufacture and configure. Hereinafter we are talking about UMZCHs intended for use with household speakers with a power of up to 100 W.
UMZCH parameters, focused on compliance with international IEC recommendations, determine the minimum requirements for hi-fi equipment. They are quite justified both from the psychophysiological side of human perception of distortion, and from the actually achievable distortion of audio signals in acoustic systems (AS), on which the UMZCH actually works.
In accordance with the requirements of IEC 581-7 for hi-fi speakers, the total harmonic distortion factor should not exceed 2% in the frequency range 250 ... 1000 Hz and 1% in the range above 2 kHz at a sound pressure level of 90 dB at a distance of 1 m. The characteristic sensitivity of household speakers is 86 dB/W/m, this corresponds to an UMZCH output power of only 2.5 W. Taking into account the peak factor of music programs, taken equal to three (as for Gaussian noise), the output power of the UMZCH should be about 20 W. In a stereophonic system, the sound pressure at the low frequencies approximately doubles, which allows the listener to move away from the speaker by 2 m. At a distance of 3 m, the power of a stereo amplifier of 2x45 W is quite sufficient.
It has been repeatedly noted that distortions in UMZCHs on field-effect transistors are caused mainly by the second and third harmonics (as in working speakers). If we assume that the causes of nonlinear distortions in the speakers and the UMZCH are independent, then the resulting harmonic coefficient for sound pressure is determined as the square root of the sum of the squares of the harmonic coefficients of the UMZCH and the speaker. In this case, if the total harmonic distortion coefficient in the UMZCH is three times lower than the distortion in the speakers (i.e., does not exceed 0.3%), then it can be neglected.
The range of effectively reproduced frequencies of the UMZCH should be no longer audible to humans - 20...20,000 Hz. As for the rate of rise of the output voltage of the UMZCH, in accordance with the results obtained in the author’s work, a speed of 7 V/μs is sufficient for a power of 50 W when operating at a load of 4 Ohms and 10 V/μs when operating at a load of 8 Ohms.
The basis for the proposed UMZCH was an amplifier in which a high-speed op-amp with tracking power was used to “drive” the output stage in the form of composite repeaters on bipolar transistors. Tracking power was also used for the output stage bias circuit.
The following changes have been made to the amplifier: the output stage based on complementary pairs of bipolar transistors has been replaced by a cascade with a quasi-complementary structure using inexpensive IRFZ44 insulated gate PTs and the depth of the total SOS is limited to 18 dB. The circuit diagram of the amplifier is shown in Fig. 1.
The KR544UD2A op-amp with high input impedance and increased speed was used as a pre-amplifier. It contains an input differential stage on a PT with a p-n junction and an output push-pull voltage follower. Internal frequency equalization elements provide stability in various feedback modes, including voltage follower.
The input signal comes through the low-pass filter RnC 1 with a cutoff frequency of about 70 kHz (here the internal resistance of the signal source = 22 kOhm). which is used to limit the spectrum of the signal entering the power amplifier input. Circuit R1C1 ensures the stability of the UMZCH when the value of RM changes from zero to infinity. To the non-inverting input of op-amp DA1, the signal passes through a high-pass filter built on elements C2, R2 with a cutoff frequency of 0.7 Hz, which serves to separate the signal from the constant component. Local OOS for the operational amplifier is made on elements R5, R3, SZ and provides a gain of 43 dB.
The voltage stabilizer for the bipolar supply of op-amp DA1 is made on elements R4, C4, VDI and R6, Sat. VD2 respectively. The stabilization voltage is chosen to be 16 V. Resistor R8 together with resistors R4, R6 form a divider of the output voltage of the UMZCH to supply “tracking” power to the op-amp, the swing of which should not exceed the limit values of the common-mode input voltage of the op-amp, i.e. +/-10 V "Tracking" power allows you to significantly increase the range of the op-amp's output signal.
As is known, for the operation of a field-effect transistor with an insulated gate, in contrast to a bipolar one, a bias of about 4 V is required. For this, in the circuit shown in Fig. 1, for transistor VT3, a signal level shift circuit is used on elements R10, R11 and УУЗ.У04 to 4.5 V. The signal from the output of the op-amp through the circuit VD3VD4C8 and resistor R15 is supplied to the gate of transistor VT3, the constant voltage on which relative to the common wire is +4, 5 V.
The electronic analogue of the zener diode on elements VT1, VD5, VD6, Rl2o6ecne4H shifts the voltage by -1.5 V relative to the op-amp output to ensure the required operating mode of transistor VT2. The signal from the output of the op-amp through circuit VT1C9 also goes to the base of transistor VT2, which is connected according to a common emitter circuit, which inverts the signal.
On R17 elements. VD7, C12, R18 an adjustable level shift circuit is assembled, which allows you to set the required bias for transistor VT4 and thereby set the quiescent current of the final stage. The capacitor SY provides “tracking power” to the level shift circuit by supplying the UMZCH output voltage to the connection point of resistors R10, R11 to stabilize the current in this circuit. The connection of transistors VT2 and VT4 forms a virtual field-effect transistor with a p-type channel. i.e., a quasi-complementary pair is formed with the output transistor VT3 (with an n-type channel).
Circuit C11R16 increases the stability of the amplifier in the ultrasonic frequency range. Ceramic capacitors C13. C14. installed in close proximity to the output transistors serve the same purpose. Protection of the UMZCH from overloads during short circuits in the load is provided by fuses FU1-FU3. since IRFZ44 field-effect transistors have a maximum drain current of 42 A and can withstand overloads until the fuses blow.
To reduce the DC voltage at the output of the UMZCH, as well as to reduce nonlinear distortions, a general OOS has been introduced on elements R7, C7. R3, NW. The AC OOS depth is limited to 18.8 dB, which stabilizes the harmonic distortion coefficient in the audio frequency range. For direct current, the op-amp, together with the output transistors, operates in the voltage follower mode, providing a constant component of the UMZCH output voltage of no more than a few millivolts.
The figure shows the circuit of a 50 W amplifier with MOSFET output transistors.
The first stage of the amplifier is a differential amplifier using transistors VT1 VT2.
The second amplifier stage consists of transistors VT3 VT4. The final stage of the amplifier consists of MOSFETs IRF530 and IRF9530. The amplifier output is connected through coil L1 to an 8 Ohm load.
The chain consisting of R15 and C5 is designed to reduce noise levels. Capacitors C6 and C7 are power filters. Resistance R6 is designed to regulate the quiescent current.
Note:
Use bipolar power supply +/-35V
L1 consists of 12 turns of insulated copper wire with a diameter of 1 mm.
C6 and C7 should be rated at 50V, the remaining electrolytic capacitors at 16V.
A heatsink for the MOSFETs is required. Dimensions 20x10x10 cm made of aluminum.
Source - http://www.circuitstoday.com/mosfet-amplifier-circuits
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