29.06.2020

Umzch pop 2 x 200 watts. Schematic diagram of the amplifier on STK4050

The article describes a powerful tube UMZCH, built on 6H2P, 6N1P, 6P45S finger-type lamps, the circuit of which the author combined from several tube amplifiers with an output power of 25...50 W, operating on base lamps.
The circuit diagram of the amplifier is shown in Fig. 1, the connection diagram of the output transformer windings is shown in Figure 2, the circuit diagram of the power supply is in Figure 3. The winding data of the power transformer is given in the table.

Technical characteristics of UMZCH
Output power. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2*200W
Power consumption:
idle mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 W
Work mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 W
Frequency range. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.01…40 kHz
Nonlinear distortion factor. . . . . . . . . . . . . . . . . . . . . 0.5%
Amplitude of the input signal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 V
Depth of tone control. . . . . . . . . . . . . . . . . . . . . . . . . ±15dB
Load resistance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 ohm

It is better to use imported small-sized electrolytic capacitors with a voltage not lower than that specified in the diagram. The tone block capacitors are any, and the galvanic isolation and protection of network interference is 0.1 µF * 630 V. Indicators from old reel-to-reel tape recorders (Ilet, Jupiter) are used. The output tubes of the amplifier are installed horizontally, but pins 3 and 8 of the 6P45S lamp must be vertical to avoid interelectrode short circuits. Cooling of the power and output parts of the amplifier is forced. The output tubes, transformers and power transformer must be shielded from the rest of the amplifier components, as shown in the photo.
The TC180 power transformer from tube TV on two coils. All its windings are removed and wound according to the data shown in Fig. 2. Winding an output transformer is complex; maintaining the number of turns and inter-winding connections is very important. Windings 2, 3, 5, 6 are wound in three layers and brought out to the existing coil terminals. Windings 1, 4, 7 are single-layer. They are output to only two pins, as they are connected in parallel. Winding 8 is wound last and output to the two remaining terminals. After assembling the transformer, you need to connect the windings to each other (Fig. 2).

The insulation between the layers of windings 3, 5, 6 is taken from large non-polar capacitors. It fits exactly as long as you remove the foil first. Between windings high voltage and load windings use standard TC180 insulation. The windings are wound tightly turn to turn. The insulation between the layers is also placed tightly, this is necessary to avoid vibration of the turns with sound frequency and so that all the windings are included.

Power transformer type ST-270 - from a color tube TV. The mains winding is factory-made; the 110 V winding can also be used from the factory, since it is wound immediately after the screen. All other windings are removed and wound according to the table data.

The diodes and capacitors of the power supply are installed on a textolite board between the amplifiers. Resistors and diodes D1–D4 are soldered to the combs. The inductor Dr1 is wound on a magnetic core Ш10*20 and contains 600 turns of PEL:1 wire with a diameter of 0.25 mm. The anode voltage is rectified by three bridges on diodes D7–D18 connected in series. The incandescent power supply of 6H2P lamps is constant, rectified by diodes D5, D6, the 6N1P lamps are alternating with a positive potential taken from the anode voltage of +355 V.
The 6P45S output lamps are heated alternating voltage 6.3 V, separate for each pair.
Cooling fans are four-inch from a computer with a voltage of 220 V. Switch S2 switches fans Ed1 and Ed2 to a voltage of 127 V to reduce speed when operating in cold conditions. Capacitor 0.047 uF * 630 V eliminates clicking when turned off.

Setup.

Resistor R1 sets the balance of the output lamps, achieving zero readings on the voltmeter (shown as a dotted line in Fig. 1) connected between capacitors C1, C2. The voltmeter scale is 3 V. Resistor R2 adjusts the bias voltage to
output lamps. Before setting, you must set R2 to its highest position. Using resistor R3, the output signal level indicator is adjusted. When the amplifier is self-excited, the winding terminals feedback need to be swapped.

Literature - RA 1‘2006\

You can build an amplifier using transistors, but it is much easier and faster to build an amplifier based on a hybrid integrated circuit STK40xx series manufactured by Sanyo. The amplifier produces high sound quality and low level noise.

Maximum output power amplifier, for example on STK4050 - 200 W!

The sound has good quality. The amplifier can be used in home theaters, computers, etc. Can also be used as a subwoofer amplifier. For the stereo option, you need to assemble two such amplifiers. Load resistance 8 ohms. The microcircuit must be installed on a good heatsink through heat-conducting paste. The power and output traces of the printed circuit board must have a maximum width.

Basic specifications STK4050:

Extremely permissible voltage supply +/- 95 V
Rated supply voltage +/- 65 V
Rated output power 200W
Power dissipation (P out = 200 W) 130 W
Harmonic coefficient (P ed. = 200 W) 0.3%
Nominal load impedance 8 ohms
Input impedance 55 kOhm (P out = 1 W, F = 1 kHz)
Frequency response (+0, -3 dB) 20 Hz - 50 kHz
Voltage gain 40 dB
Sensitivity 350 mV

Schematic diagram of the amplifier on STK4050

Peculiarities STK4050:

Compact and slim body
STK series has 18 contacts maximum power per channel from 120 to 200 W
Simple heat sink design
Current application of mirror circuitry reduces distortion to 0.08%
Switching off the load due to thermal protection and short circuit, as well as noise suppression when turning on / off the power

Power supply and internal circuit STK4050

Amplifier PCB

Finalization of the circuit on STK4050

GIS STK40XX characteristics table

P O P U L A R N O E:

How to provide loudspeaker communication to, say, two points located at a considerable distance from each other? A similar task arises in school, pioneer camp, small village or far away rooms of the house. And in all such cases, an intercom comes to the rescue.

Lightsaber(eng. Lightsaber) is fantastic weapon many people know from the science fiction saga " Star Wars" It can be found in science fiction films and stories.

The basis of the amplifier is the K140UD708 chip, pre-stage built on domestic transistors of the KT814/KT815 series, preferably with the letter G, the transistors can be replaced with other complementary pairs, which are similar in their parameters to those indicated. This circuit belongs to the class of fairly expensive amplifiers; stereo versions of such amplifiers cost from $200, but assembly will cost tens of times less, therefore, if your hands grow from the right place, then you can assemble it yourself! Schematic diagram and a 200-watt audio power amplifier circuit board in the pictures below:

Resistors:

R1 R11 =1k
R2 = 36k
R3 = 240 from
R4 R5 = 330 from
R6 R7 = 20k
R8R9 = 3.3k0.5w
R10 = 27om2w
R12 R13 R14 R15 = 0.22from 5w
R16 = 10k

Capacitors:

C1 = O.ZZtk
C2=180r
SZ C4= 10mk25v
С5 С6 = 0.1 tk
С7 = 0.1tk
С8 = 0.22tk
C9-C10 = 56r

Zener diodes:

VD1 VD2 = KC515A

Transistors:

VT1 = KT815G
VT2 = KT814G
VT3VT5 = 2SA1943
VT4 VT6 =2SA1943

The output stage transistors can be replaced with domestic KT8101A and KT8102A, you can increase their number in order to increase the output power of the amplifier, but do not forget to increase the supply voltage. It is important that the transistors are identical in characteristics and have different structures.

The output transistors are installed on a heat sink; it is advisable to select a larger one, and be sure to not forget about the insulating gaskets of the transistors. At the specified supply voltage, our amplifier is capable of delivering up to 200 watts of pure power to the load; by adding two more pairs of output transistors, the power can be increased to 450 watts, but then the amplifier supply voltage also needs to be increased to +/-70 volts.

Main advantages:

Stunning, focused and detailed sound
Heartfelt vocals that create the impression of communication with the performer
Highest thermal stability even when operating at full power. The output transistors operate in class B, so they are not subject to self-heating.
Power up to 200 W with simplicity and VERY cheap implementation.

This story began after reading the publication and discussing it for more than a year on the Vlab and Ussr Hi-Fi forums. Since then, it has become obvious that without the original article, from which it was compiled, further improvement of the Gumel amplifier will turn into pulling out the tonsils through... well, you understand me. I was able to find this article. Below is a scan of the original and my translation.

The basic principle of a current-controlled amplifier was first described in The Elector (see Elector #8 and 21). To briefly summarize, his circuit uses the effect of the four passive components (bridge) R2, R3, L and C, shown in Fig. 1, due to which the non-linear characteristic of the output stage becomes unimportant. Thus, it became possible to use a class B output stage (i.e., bias at the bases of the output transistors below the cutoff potential, so their quiescent current is zero) with all its advantages and without its inherent disadvantages (transient distortion) in this design.

The circuit shown in Fig. 2 functionally implements the principle of current control described above. According to the author, this UMZCH allows you to get 100 W when operating at a 4 Ohm load, while Kg at a frequency of 1 kHz is stated to be 0.006% with a power of 60 W. If there is equipment that allows you to produce precise measurements Kg, C3 can be replaced with a 22 pF variable capacitor, and the latter is tuned to minimize distortion.

The circuit also contains an innovation in the form of an equivalent load (R9).

The output stage is controlled (via transistors T2 and T5) by transistors T1 and T4, connected in series to the positive and negative arms of the op-amp power supply, respectively. This also improves the slew rate of the 741 op amp (meaning LM741 and clones). If, however, a faster op amp is used (such as the LF357), then the ratings of R4 and R7 must be changed to provide enough quiescent current for the op amp to keep the output transistors off.

Graham Schmidt (Germany)

Despite the fact that the idea itself, in its development, undoubtedly makes it possible to obtain the highest parameters at scanty circuitry and monetary costs, the elementary basis used by Schmidt is undoubtedly behind the times today. Today, high-precision op-amps with impressive operating speed and slew rate, powerful low-noise transistors that require almost no pairing, high-frequency diodes with a low opening threshold and inexpensive zener diodes, the voltage accuracy of which is no worse than a fraction of a percent and weakly depends on temperature, have become available. Moreover, these components are now relatively cheap and available.

Based on these facts, continuous experiments and searches, changing circuits and boards, an optimal combination of ratings and parameters of the device was obtained, the diagram of which is given below:

OU. The common TL071 was chosen as a musical, high-speed op-amp with a low bias voltage, which is very critical in this circuit, because without C1 this UMZCH can actually work as an amplifier direct current, since it does not contain capacitance in the OOS circuit. The best TL071 I've ever handled was an op amp made by Texas Instruments ® . The output offset without calibration was no more than 3 mV. For normal operation of the UMZCH, it is necessary that the output offset does not exceed 30 mV. But, since it is not always possible to get op-amps from elite companies, such as TI (Texas Instruments ®), NS (National Semiconductors ®) and AD (Analogue Devices ®), with a low bias voltage, the board has a place for installing a tuning resistor (the value is taken from the datasheet) format CA-6V or similar.

Possible replacements (from most preferred to least):

Elite op-amps Burr-Brown, etc., TL071 produced by “low” brands such as ST, KR544UD2A, KR544UD1A, KR140UD608, KR574, etc.

Replacing the OS will also entail a change in the parameters of the LLC and local environmental protection. Capacity C2 set to compensate for the drop in gain with increasing frequency for the 741 op amp. For the TL071, this unevenness appears far beyond the audio range and therefore does not require correction. One of the Vlab forum members completely excluded this capacitor. I suggest setting a capacitance of about 500 - 1000 pF for circuit stability and a jumper J.P.1 , which allows you to disable this correction.

Zener diodes were installed in the base dividers of the Emitter Follower (EF) transistors formed by VT1 and VT2. Together with resistors R5 and R6 with a power of 0.5 W, the zener diodes form parametric stabilizers that allow you to change UMZCH power supply within wide limits, without recalculating resistive dividers. For the best result, it is advisable to select zener diodes in pairs according to the stabilization voltage within 12 - 13 V, but always the same. A voltage of 15 V is unacceptable, because then the op-amp in this circuit may fail or go into an extremely nonlinear mode.

My design uses 1N4742A, as a variant of BZX55C12 or domestic ones, but they require selection, because their spread is greater.

Diodes also answer modern trends. Along with resistors R15 And R16 diodes D1 And D2 perform the functions of pre-output thermostabilization ( VT3 , VT4 ) cascade, and also prevent the flow of quiescent current through the output transistors ( VT5 , VT6 ) cascade even with significant heating of the device.

Protection diodes D3 And D4 1N4007 are provided, but they are installed only if the output super-betta transistors do not have built-in ones. In my case, TIP142/147 has these diodes. When installing transistors type 2SC5200,2SA1943 diodes D1 , D2 should be germanium pulse type D311 or low-power Schottky diodes, it is important that the voltage drop across the forward junction of the diode is 0.25 - 0.3 V.

Diodes D6 And D7 , included in forward bias, in combination with capacitors C4..C7 prevent the penetration of noise into the op-amp power stage, which arises due to the high consumption of the output stage at high power.

Transistors. The output stage was left unchanged; its characteristics do not matter. Popular high-frequency transistors BC546/556 were installed in the ED. Limiting resistors were included in the emitter circuits of the pre-output stage R15 , R16 , helping to stabilize the quiescent current. In addition, it is convenient to measure the quiescent current using the voltage across these resistors. Its size is 20 mA. That. the voltage across the resistors should be 15 * 0.02 = 0.3 V.

The transistors of the pre-output stage were selected according to their sound. All considered options sounded approximately the same in midrange and treble, but the TIP31C/32C manufactured by Fairchild Semiconductors ® (Beware of counterfeits!!!) gave not only an excellent vocal picture and detail, but also the most collected and dense bass. For the purpose of thermal stability, in addition to the measures described above, VT3 And VT4 spaced at different ends of the board and each installed on a separate small plate heat sink with a surface area of about 30 cm 2 .

Resistors C1-4 (carbon) or MLT (metal film). All, except those indicated separately, at 0.125 - 0.25 W.

Capacitors C12, C3– K10-17b; C1, C4, C6, C8, C10– K73-17; C2 – K73-9.

The rest are electrolytes, better than well-known Japanese companies - Rubycon, Mitsumi, Matsushita (Panasonic), Samsung, Sanyo, Jamicon.

Settings

The setup is performed with the output stage transistors turned off. VT5 And VT6 soldered last.

Coil made on a mandrel d=7 mm in two layers and contains 9+7 turns of copper wire with a diameter of 0.8 mm in varnish or epoxy insulation. Impregnated with Moment glue or paraffin for rigidity. The final result largely depends on the accuracy and quality of the coil.

Balancing. To check, first install R7 And R8 By 180 Ohm. Connect the power to the amplifier through powerful wirewound resistors (at least 5 W) with a resistance of approximately 50 - 100 Ohms each. This will avoid possible breakdowns, overheating, power supply overload and other problems. Plate heat sinks are installed on the pre-output transistors. The input is short-circuited to ground.

Now we supply power to the amplifier and measure constant pressure at his exit. If it is less 30 mV, then you are lucky and you don’t need to calibrate the op-amp. Otherwise, a trimming resistor is installed in the board and with its help the output voltage is set to zero. The value and connection circuit of the trimming resistor are selected based on the technical documentation for the microcircuit.

The quiescent current of the pre-output stage is 20 mA. It is installed by selecting resistors R7, R8 until a voltage of 300 mV is obtained on resistors R15, R16. All these resistors must be matched in pairs with the greatest possible accuracy. Start with 180 ohms. For different op-amps and transistors, the ratings can vary from 180 to 330 Ohms. The greater the resistance of resistors R7, R8, the higher the quiescent current of the pre-output stage.

Now install the output transistors. They are attached to a heat sink with an area of about 300 cm2 through mica with thermal paste on screws with insulating bushings. Check the quiescent current again.

Bridge balance. This step can only be performed if you have an oscilloscope and a generator (can be done from a computer). Must be submitted at the entrance 15-20 kHz sinusoid. First, set a small level and look at the area near the axis. If there are noticeable “deflections” of the sinusoid, then adjustment is needed. For this instead C3 a trimming capacitor of approximately 30 pF is installed. By changing it, one achieves the disappearance of the area of “undercompensation”.

Check the output zero again. The setup is complete!

Printed circuit board made of one-sided foil PCB with a thickness of 1.5 mm. Board size 90x60 mm. Below is a layout of elements and a drawing of a printed circuit board for laser ironing technology (LUT). C5 And C7 installed under the board on the foil side. D6 And D7 – vertically.

Amplifier quiescent current I xx: 20 – 30 mA

Output stage quiescent current: 0 mA

Reproducible frequency band at –3 dB level: 5 – 100000 Hz

Sound on this moment surpasses everything I have heard. Namely: Quad 405 (Hungary), SAR Whisper, TDA7294 circuit development, TDA2050, TDA2050 ITUN, LM1875, LM1875 ITUN, LM3886, LM3886 circuit development, LM3886 Inverting connection, RRR U-7101 Hi-Fi tuning, Idol 35U-102S tuning .

Moreover, according to many who have heard Stonecold, its sound is confidently ahead of all imported industrial devices up to $800 and many after. Distinctive features– a very comprehensive, soulful presentation of vocal material, but without the intrusiveness and “squealing” characteristic of many amplifiers (the impression that the performer is singing just for you, and not into space). Detailed elaboration of fast group passages, collected, elastic bass. Not too deep, but more accurate than many. If you like bass that rolls in waves, powerful and enveloping, but less dynamic, put the complementary pair 2SA1943/2SC5200 at the amplifier output. High frequencies are silvery, without whistling or disruption. Detailed, thanks to a fairly low level of intermodulation. For example, in the song “Away from Me” by Evanescence, the sound of raindrops hitting metal is easily distinguishable from those falling on the ground. In the vast majority of the above amplifiers, the sound of rain simply merges into a general mess.

And yet, it’s just pleasant to listen to it for hours, it doesn’t tire you. He just gives you music... See you in a new consciousness.

Literature:

Gumelya E. Quality and circuitry of UMZCH – Radio No. 9 1985
Schmidt G. Current Dumping Amplifier

I thank everyone who took part in the fate of the project.Special thanks to Shabalin Leonid, who supported me and contributed to the creation of this material.

2005

Lincor_ nobox@ inbox. ru

The amplifier is built on ThermalTrak series transistors from famous manufacturer On Semiconductor. These transistors are new version top models MJL3281A and MJL1302A and have built-in diodes for organizing temperature-compensated bias circuits of the output stage.

As a result, regulation of the quiescent current of the output stage is eliminated and the need for a classic voltage multiplier for thermal stabilization of the quiescent current of the output stage is eliminated, and a number of design issues are resolved to reduce the thermal resistance of the radiator-transistor.

The amplifier is made on a double-sided printed circuit board, although for such a relatively simple design this would seem unnecessary. However, double-sided wiring of conductors allows you to optimize their location in order to minimize mutual interference and compensate for the magnetic fields created asymmetrical currents class B push-pull output stage (we wrote about this in the series of articles “”).

Features and Specifications

First, a small note: in the description of their amplifier, the authors often mention either the “AB” mode or the “B” mode. In fact, the amplifier belongs to class “AB”, that is, at low signal levels it operates in class “A”, and at high powers it goes into class “B”.

If in the first case (for small signals, class “A”) the fight against magnetic fields and ripples in power circuits do not present any great difficulties due to the small values and symmetry of the currents, then when the amplifier moves to class “B” the currents become asymmetrical and the magnetic field strength will be significant. Operating an amplifier with a maximum power of 200 W at levels of 3-5 W is somehow impractical. Therefore, the authors paid special attention to obtaining maximum performance (and, accordingly, eliminating or compensating for all negative factors) at powers close to peak, that is, in mode “B”.

Circuit design and design solutions used in the design made it possible to obtain:

Very low distortion
No quiescent current adjustment
Double-sided printed circuit board with simple conductor topology
Compensation for magnetic field interference when working in class “B”

Main technical characteristics of the amplifier:

Output power: 200 W into 4 ohm load; 135 W at 8 ohm load,
Frequency response (at 1 W power): 4 Hz at –3 dB level, 50 kHz at –1 dB level
Input voltage: 1.26 V at 135 W output power and 8 ohm load
Input impedance: ~12 kOhm
Harmonic distortion:< 0.008% в полосе 20 Гц-20 кГц (нагрузка 8 Ом); типовое значение < 0.001%
Signal-to-noise ratio: less than 122 dB at 135 W and 8 ohm load.
Damping coefficient:<170 при нагрузке 8 Ом на частоте 100 Гц; <50 на частоте 10 кГц

Description of the circuit

The figure shows a schematic diagram of a power amplifier:

Schematic diagram of the amplifier (click to enlarge)

The input signal through a capacitor with a capacity of 47 μF and a resistor with a resistance of 100 Ohms is supplied to the base of transistor Q1, a differential stage assembled on transistors Q1 and Q2. Low-noise transistors from Toshiba 2SA970 are used here, so it is this stage that makes the largest contribution to the final noise level of the entire amplifier.

The amplifier is covered by a general negative feedback loop, the values of the elements of which determine the gain. With the denominations indicated in the diagram, it is 24.5 times.

The negative feedback capacitor provides 100% DC coupling to maintain the amplifier output at zero potential without the need for additional integrators, etc. With a capacitance of 220 μF, it provides a lower cutoff frequency of 1.4 Hz at a level of -3 dB.

Feedback capacitors

The capacitance of the capacitors at the input and in the negative feedback circuit is slightly larger than is usually installed in these circuits. These values are chosen to minimize possible distortion in the audio frequency band.

For example, the output impedance of a CD player is typically several hundred ohms. If you install a capacitor with a capacity of 2.2 μF at the input (typical value for input circuits), then at a frequency of 50 Hz the input stage will “see” the resistance of the signal source of about one and a half kiloohms. A capacitor with a capacity of 47 uF at the same frequency will have an impedance of only 67 ohms. (Remember that the signal source is essentially a voltage generator, so it must have a low output impedance)

Here also Not(usually recommended) non-polar capacitors are used. They are several times larger in size than simple electrolytic capacitors, which is why they tend to pick up more noise and interference. Since the goal is to make an amplifier with a minimum level of noise and distortion, all measures have been taken for this: circuit solutions, choice of element base, design solutions.

The amplifier has a wide bandwidth, which also imposes its own requirements and restrictions on the choice of elements, installation, etc. in order to minimize picked up noise and interference.

Diodes D1 and D2 protect the relatively low voltage electrolytic capacitor in the negative feedback circuit if the amplifier fails. By the way, it is strongly recommended to equip the amplifier with some kind of speaker protection system. The authors migrated it from the previous design, so its description is not given here.

Using two diodes instead of one guarantees the absence of non-linear distortion due to the limitation of signal peaks in the feedback circuit (about 1 V, and two diodes will give a limitation at a level of about 1.4 V).

Driver cascade

The main voltage gain is provided by the cascade on transistor Q9. To reduce nonlinear distortions, the input stage is decoupled from the driver stage through an emitter follower on transistor Q8.

To obtain maximum linearity and maximum gain, the driver stage is loaded onto an active current source (made using transistor Q7). The base bias for both it and the input stage current source (Q5) is created by transistor Q6. The somewhat complex bias circuits of transistors Q5, Q6, Q7 provide maximum suppression of noise and ripple in the power supply circuits, which is important for a class “B” amplifier, where large (up to 9 A!) and, most importantly, asymmetrical pulse currents flow along the power buses.

If the ripples of the power circuits get into the input stage, they will be amplified by all stages and will end up in the load - the speaker system. We most likely will not like what we hear as a result. Therefore, the amplifier has taken all measures to prevent the penetration of noise and ripple from the power circuits into the amplification path.

The oscillogram in the center shows a 1 kHz oscillator signal. The upper (red) graph is the ripple modulation of the positive power supply bus by the input signal, the lower graph is the modulation of the negative power bus:

A 100 pF capacitor between the collector of Q9 and the base of Q8 limits the amplifier's bandwidth. Since it is subject to the full amplitude of the stage's output, it must be rated for 100 V or more.

Output stage

The output signal of the driver stage on transistor Q9 is fed to the output stage transistors through 100 Ohm resistors, which protect transistors Q7 and Q9 from a short circuit at the amplifier output, although, of course, the fuses should blow first. In addition, these resistors prevent possible excitation of the output stage.

The output stage is built on composite complementary Darlington transistors. Firstly, this made it possible to use highly linear transistors from ThermalTrak with built-in diodes, and secondly, to obtain the maximum full power at a 4 Ohm load (to minimize the voltage drop across the output stage).

Thermal offset compensation

When using four Thermaltrak transistors in the output stage, we have four built-in diodes to organize a temperature-compensated bias circuit.

As shown in the diagram, four diodes are connected in series between the collectors of transistors Q7 and Q9. This method of organizing the bias of the output stage was widespread in the 60-70s. Later it was replaced, which became a classic solution, by a voltage multiplier on a transistor.

Typically, the quiescent current of the output stage is set by a stage on a transistor, which is mounted on the same heatsink with the output transistors, thereby ensuring thermal coupling. This method has disadvantages: firstly, the bias circuit transistor must be selected to ensure optimal thermal compensation, and secondly, in any case, thermal inertia is present: the output transistor must heat the radiator, the radiator will heat the bias circuit transistor, and only then will thermal compensation of the output stage current occur.

Placing diodes for thermal stabilization in the same package with the transistor solves these problems: the diodes have characteristics that are maximally consistent with the transistors, so thermal stabilization occurs as accurately as possible, and secondly, they are located on the same substrate with the transistor crystals, which makes them heat up as quickly as possible, eliminating the intermediary radiator.

With Thermaltrak transistors, thanks to the built-in diodes, the amplifier's quiescent current quickly stabilizes after switching on and is maintained very accurately, regardless of changes in supply voltage or output signal level. The manufacturer also claims that the linearity of the cascade with such a bias is higher than when using a conventional transistor multiplier.

The figure explains how to set the output stage bias:

Four integrated diodes compensate the four base-emitter junctions and determine the output stage current. Taking into account the fact that the output transistors are connected in parallel and 0.1 Ohm resistors are installed in the emitter circuits, four series-connected diodes provide a quiescent current of the output stage at a level of 70-100 mA, which is slightly higher than usually set by the transistor bias unit.

Output filter

The output filter is an RLC circuit consisting of an inductance (without core) of 6.8 mH, a resistor with a resistance of 6.8 Ohms and a capacitor with a capacity of 150 nF. This filter has been used by the authors in many amplifier designs and has been shown to be highly effective in isolating the output stage from any reverse currents caused by a reactive load, thereby ensuring high amplifier stability. The filter also effectively suppresses RF signals picked up by long speaker wires, preventing them from entering the amplifier's input circuits.

Circuit breakers

The output stage is fed through 5 A fuses from the ±55 V rails. These provide the amplifier's only protection against output shorts or other faults that result in increased current draw.

Double sided PCB

To simplify and optimize wiring power circuits The amplifier's printed circuit board is double-sided. Firstly, this made it possible to organize the wiring of the common wire in the form of a “star”, when all conductors with zero potential converge to one point, which eliminates the formation of “ground” loops and the penetration of the output signal into the input circuits. We wrote about this in the series of articles “”

Secondly, and more importantly, the wiring and placement of parts on the board are designed to compensate for the magnetic fields created by large pulsed currents. We also wrote about this in the series of articles “,” where it was proposed to twist bifilar conductors with large and antiphase currents. You can’t connect conductors like that on a printed circuit board, but it’s still possible to compensate for fields.

For example, the positive power fuse is located side by side and parallel with output stage emitter resistors Q12 and Q13. The elements are connected so that current flows through them in different directions, due to which mutual compensation of magnetic fields occurs. Similarly, the parts are placed along the negative bus.

The power paths from the CON2 connector to the fuses run side by side parallel to each other, and in the middle of the board they diverge in different directions. Under the diverging conductors are the tracks of the emitter circuits of the output stage, and under the parallel tracks is the ground bus. Due to this layout of the printed circuit board, the magnetic fields created by these tracks are mutually compensated.

The applied methods of suppressing magnetic fields made it possible to significantly reduce amplifier distortion.

Results of measurements of amplifier parameters: