HYBRID AMPLIFIER

Many have heard and probably made tube ULFs, some say their sound is the best, while others will say that transistors are in no way inferior to them and have much better parameters.

I did both and am ready to make the final conclusion: a cool sound amplifier has both tubes and transistors, to each his own:

The lamps work great at the entrance, and they look so stylish!and field-effect transistors at the output - and there is no need for huge output transformers.

Here are the circuits that I tested during the experiments and they all worked great!

And here is an example of the practical implementation of one of the hybrid ULFs according to the scheme given below:

For this amplifier I used an N-channel circuit field effect transistors from a radio hobby magazine, The lower part of the case, measuring 15x20 cm, is made of a centimeter aluminum sheet, and is used as a general radiator for transistors. The latter are powered through a conventional diode bridge and two 10,000 uF capacitors. Background alternating current Can not hear. 200 V for the anode is taken using a 12-volt small 10-watt trans, connected in reverse to the secondary of the main transformer. To indicate the position of the volume level, we place a blue LED through a piece of plexiglass. For beauty, the lamps from below are illuminated with red LEDs. The difference in hearing between 6N6P and 6N2P is practically unnoticeable. The setup consists of setting the required quiescent current (within 0.3 - 1 A). And lastly: don’t skimp on the radiator! Class "A" will require very decent cooling. For example, a radiator for a 100-watt ULF class “A” Mac weighs 8 kg! As a power source for such an amplifier you can use electronic transformer how in

For many years, power amplifiers used only vacuum tubes, but today modern amplifiers use transistors almost entirely. Tube amplifiers operate on the same principles as transistor amplifiers, but the internal design may be significantly different. In general, lamp devices operate at high voltage power supply and low current. Unlike transistors which operate at low voltage but with high currents. Additionally, tube amplifiers tend to dissipate a large amount of energy as heat and are generally not very efficient.

One of the most striking differences between tube and transistor amplifiers is the presence of an output transformer in a tube amplifier. Due to the high output impedance of the anode circuit, a transformer is usually required to properly transfer power to the loudspeaker. High-quality audio output transformers are not only difficult to make, but they tend to be large, heavy, and expensive. On the other hand, a transistor amplifier does not require an output transformer, and therefore tends to be more efficient. Many people find that the sound from tube amps can be excellent and have a unique character. What is certain is that there are sonic differences between tube and transistor amplifiers. I truly appreciate both worlds, and have had the opportunity to hear amazing systems using both technologies.

Figure 1: Simplified hybrid amplifier circuit

When developing this hybrid amplifier (Fig. 1), there was a desire to combine the best of tube and transistor technologies. The tubes offer full and faithful sound reproduction, with rich detail, brilliant clarity, and precision. They also reproduce deep better. Hybrid amplifier retains signature tube amplifier, complementing it low level distortion of the semiconductor output stage.

Figure 2: Hybrid amplifier circuit

The hybrid amplifier circuit (Fig. 2) is very simple, but includes interesting ideas: such as Erno Borbéli's low voltage tubes and Reinhard Hoffmann's bipolar-fed output stage. This hybrid is capable of delivering about 30W into an 8Ω load or 15W into a 4Ω load. You can easily increase the power by adding more output stages in parallel. This will increase the damping coefficient and reduce the dependence on load resistance. Amplifier with two outputs MOSFET transistors per channel will provide more than 50 +50 W of useful power of pure class A with a load of up to 6-8Ω. However, in such conditions the amplifier will dissipate more than 300 W, so you must use suitable heatsinks (at least 0.2 °C/W thermal resistance) in a suitable well-ventilated case.

Figure 3. Power supply circuit

The input stage is based on a double triode 6DJ8/ECC88 (analogous to 6N23P, you can also try 6N6P) and serves as a differential amplifier. I chose the 6DJ8 because of its linearity and Good work at 35-40V voltage at the anode. For 6DJ8/6922/ECC88/E88CC, MU is constant within 20% of 0.4mA, up to at least 6mA, and this trend continues up to 15mA. I chose an operating current of 3-5 mA for each half of the lamp, and a voltage of 35-40V to keep dissipation well below the nominal value of 1.8 W. Current is supplied to the cathode from the source direct current to Q3, while Q1 and Q2 represent the resistive load or current mirror. The active anode/cathode load of both triodes is almost equal, which reduces the second harmonic, promotes linearity and increases the slew rate of the output voltage. With potentiometer P3, you can adjust the bias current from 1 to about 7mA, P1 controls the output bias voltage, which needs to be adjusted close to 0.

OUTPUT CASCADE

Output stage consisting of one or more P-channel MOSFETs in single-ended, Class A mode, similar in configuration to the Nelson Pass Zen amplifier (for more detailed information see http://www.passlabs.com/

zenamp.htm). It is loaded onto current source Q4, which is set to 3A quiescent current using the specified values ​​of R14. You can experiment with different meanings current in rest mode by changing resistance R14 according to the formula Id = (Vz-Vgs)/R14 =0.9/R14.

It should be taken into account that the quiescent current should be 50% greater than the operating current. The overall gain of the amplifier is around 20 and this depends on the value of R8 and R9. Thus, 1V of the input signal will drive the amplifier to full power, so that the output level of a typical CD player is sufficient to drive the amplifier. You can calculate the required gain using the following formula: Av = 1 + (R9/R8). This amplifier's tested PCB is available in Ivex Win-Board format. To receive a free copy of the file, please send an email [email protected]. In this PCB, lamps and transistors are installed on the solder side.

Each channel of the hybrid amplifier requires ±35V DC/6A power supply for the main amplifier, and an adjustable 6.3V DC/0.5A for powering the filament lamps. The rectifiers of the amplifier's main power supply must withstand 20A.

RESULTS

This hybrid amplifier has a flat frequency response across the entire audio frequency range. Even with low-sensitivity speakers, you can appreciate its clarity and detail, especially when the CD player is directly connected to it. With a single output, the amplifier delivers up to 20W with less than 1% THD, but it will perform better with two in parallel. I've had the opportunity to evaluate some of the best Class A amplifiers on the market, and I believe this hybrid delivers the same flavor and fresh feeling when listening to high-end music.

1. “Low-Voltage Tube/MOSFET Line Amp,” GA 1/98.

2. “The Zen Cousins,” AE 4/98.

audioXpress 5/01

www.audioXpress.com

Corrected amplifier circuit.

1. Soft, detailed and clear sound
2. Excellent transmission of vocals, stage and volume
3. Simple design, no configuration required
4. A complete set of protections implemented on the chip chip
5. High concept - a vacuum double triode acts as a current buffer. The maximum linearity of the phase response and frequency response has been achieved, an inverting connection with T-OOS has been used.
6. The basis is the popular LM3886 MC manufactured by National Semiconductors
7. Average power – 68 W/4 Ohm. Peak – 135 W.

The LM series amplifier chips have the best sound among analogues. This also applies to flagship models of various levels, such as LM1875, LM3876 and its logical continuation - LM3886. The author's article continues the debate on the topic of circuit design and Thorsten's developments. An amplifier based on LM3875 is being considered. Its best sound, stability and linearity is achieved with inverting switching. However, this connection, when operating on the classic output impedance of the source, has a number of disadvantages. In short: with increasing frequency, the nonlinearity of the frequency response and phase increases. This is due to the fact that with an inverting connection, the signal must come from a current source, and CD players and sound cards have an output impedance of about 200 Ohms. The current source on field-effect transistors is also eliminated due to high losses, high input capacitance and pronounced nonlinearity. A current buffer on a triode successfully copes with this task.

In addition, this kind of buffer has a voltage gain of less than 1. Due to this, the OOS depth of the microcircuit itself is reduced, which also has an extremely beneficial effect on the sound quality. It is known that deep OOS, implemented by a classical divider, coarsens and deadens the sound. In the scheme proposed by Rasmussen ( Fig.1), a T-shaped OOS has been introduced, which increases the input resistance at the inverting input and makes it possible to reduce the grounding resistance at the direct input. The disadvantage of this approach is the increase in noise and interference, but this is the first impression. If the wiring and shielding of the amplifier unit are done properly, interference will be almost invisible.

Now let’s look at what I personally didn’t like about the original scheme.

The author has LM3875 installed as a PA. Its disadvantages are imperfect protection, operation only with an 8-Ohm load, and low power. Instead, the LM3886 MC with a full set of protections and a powerful output stage was chosen, allowing it to deliver long-term power of 68 W and short-term power of 135 W into a 4-Ohm load. In addition, the amplifier is equipped with a full set of protections and a built-in mute mode.

At the exit Fig.1 There is a current limiter - a wirewound SQP resistor. The SPiKe system implemented in the LM3886 allows you to abandon it.

For the convenience of mixing channel parameters and reducing the size of the amplifier, the popular vacuum double triode 6N23P-EV was used as a buffer. It is distinguished low voltage power supply, relevant in this circuit, and at the same time, good sound. Although we have to admit that in this case its application is far from classical.

For our own reasons, the following features were added to the board:

Taking into account all the above considerations, the scheme took the following form ( Fig.2):

Here are the elements C 1 , C 3 , C 4 as well as terminals CN 1.. CN 6 – common for both channels. Each channel also has half a double triode 6N23P-EV .

Here, let’s take a break from the circuit design of the PA for a few seconds and consider the power supply, so as not to return to this topic again.

To power the entire circuit, a four-polar power supply with a common ground and an independent heating winding is used, the circuit of which is shown in Fig.3:

Diode bridges are either ready-made or assembled from diodes of the types that appeal to you, everything from D213 to Schottky diodes. For ±36 V 0.2 A – D 1 for a voltage of at least 200 V and a current of at least 4 A. For ±27V 4 A – D 2 for a voltage of at least 100 V and a current of at least 8 A. For incandescent - D 3 for any voltage and current of at least 4 A. This seemingly overestimation of parameters is not accidental. The fact is that, despite the peak reserve of the diodes, the current during charging of the containers exceeds the nominal one several times. But the price of diodes or ready-made bridges does not differ much, so for your own peace of mind I do not advise saving.

Capacities C 1, C 2 (for voltage not less than 50 V), C 5, C 6 (for voltage not less than 35 V), C 9 (for a voltage of at least 16 V) – imported electrolytic type K50-35. C 3, C 4, C 7, C 8, C 10 – type K73-17 at 63 V.

Any power transformer with an overall power of at least 200 W that satisfies the parameters of currents and voltages in the secondary windings indicated in the diagram (incandescent current of at least 0.8 A per lamp) can be used as a transformer.

In addition, it is possible to use two separate transformers. One is powerful for powering the PA, and the other is for powering the lamp. The second can be selected from a number of standardized lamp " T transformers A butno- N Akalny". I use TAN1.

So, we managed to fit both channels onto one printed circuit board measuring 130x80 mm. Assembled module (without additional blocking containers) C8, C9 ) looked like this ( Fig.4).

Cute, isn't it?

The original layout of the elements is shown in Fig.5:

Now a few words about the details and the intricacies of assembly.

Resistors

Most resistors require pairing across channels with an accuracy of at least 1%. These conditions are fully satisfied by resistors of the C2-23 series. So, selection is required R 1 , R 3.. R 9 . Moreover R 1 , R 3 And R 4 It is better to use metal film type MLT, OMLT or imported analogues.

Resistors R 2 And R 10 no selection required. Can be of the MLT-0.25, S1-4 or S2-23 type at 0.125/0.25 W. R 11 And R 12 – imported at 2 W. The output inductance winds over R 11 , dressed in an insulating cambric, with a wire in enamel or epoxy insulation with a diameter of 0.6-0.8 mm until filled and soldered to the legs of the resistor. Although in this case I am a resistor R 11 didn't install. Instead, a coil was soldered, wound on the handle of a file and containing 15 turns of wire with a diameter of 0.8 mm.

VR 1 , VR2 – double variable resistor. In my case, Taiwan for 44 clicks, selected with an accuracy of 0.5% from 5 pieces.

Capacitors

C 1 , C 3 , C 8 , C 9 , C 10 – polar electrolytic type K50-35, preferably imported famous brands. However, the circuit does not contain electrolytes in the audio circuit, which significantly improves the sound, reduces the criticality of the elemental base and increases the reliability of the system as a whole.

C1 – 16 V, C3 – 100 V, S8-S10 – 50 V.

C 4 , C 5 , C 7 , C 11 – metal film type K73-17. C 4 - at 250 V, the rest - at 63 V.

C2 – metal film or metal paper of the highest available quality, preferably no worse than polypropylene. The permissible voltage is also not lower than 63 V. Although this circuit sounds great with a K73-17 capacitor.

C6 – ceramics, preferably without piezo effect. KM or disk type. In extreme cases, of course, the K10-17B will do, but it’s hard to imagine a worse option.

Active components

The LM3886 amplification IC can be replaced with similar pinouts, taking into account the features of each. Purely theoretically, the circuit works with any MS built on the principle of a powerful op-amp. Attention! On the MC body there is a minus power supply!

Lamp R.O. 1 6N23P-EV is changed to 6N23P or an imported analogue ECC88. It is installed in a ceramic or any other socket designed for mounting on a printed circuit board or on a UMZCH chassis and is connected to the board with copper conductors.

In addition, taking into account modern trends in design, separate amplifier blocks have been developed for L.M. 3886 , which are installed on the radiator inside the UMZCH housing, and the lamp is installed in a special socket located on the housing cover. In this version, the entire llama harness ( R 1 , R 2 , 2x R 3 , C 3 , C 4 ) is carried out by mounted mounting directly on the socket terminals. And then it is connected to the power amplification units using a shielded signal cable. Don't forget to ground the lamp shield.

The printed circuit board of one PA channel is given on Figure 6:

Since it takes about 5 s to warm up the lamp, all these 5 s the amplifier input “hangs in the air”. At this time, all imaginable interference and a very noticeable rumble are present at the output. This can be avoided in two ways - by using a mute circuit or a relay to delay the turn-on. In both cases, the control signal will be a bipolar transistor with an RC divider in the base. If the delay is not enough, simply increase the value R 1 .

A diagram of such a delay is given in Figure 7:

In addition, at the time of modeling I had relays lying around TR 81 companies TTI . A printed circuit board was laid out for them. Its drawing can also be used as a guide for wiring for any relay you like with a normally open contact group. The board layout is given on Fig.8.

Details:

VR 1 – to the supply voltage of the relay winding. You can take it a little higher (about 2 V - drop across the transistor). In my case 12 V, i.e. stabilizer 7812..7815 .

C2 – on the voltage of the PA supply arm.

C1 – higher than stabilization voltage VR 1

This protection is connected to the positive side of the PA power supply (powerful transformer). The negative power terminal and the mute circuits of both amplifier channels (or all, if there are more channels) connected together are connected to the relay.

So finally SOUND

Fans of “tube sound” will really like this amplifier. What immediately catches your eye is the excellent vocals, the stage design and its incredible depth for transistor amplifiers. Unlike the typical sound of the LM3886, the HF is not washed out in this inclusion. They sound very subtle and precise. Silver and crystal do not smudge, as in a non-inverting inclusion. It is also impossible not to note the presence of a dense, collected and powerful, but extremely well-developed bass, which has always been so difficult to achieve from LM. Jazz and Blues sound so soulful that when listening, I often found myself getting goosebumps running down my spine.

The sound of this amplifier cannot be called absolutely accurate with a multi-frequency signal, but this sound is much more pleasant to the ear than various “super-linear” designs with distortion coefficients of thousandths of a percent.

To summarize: This amplifier is intended for music, not for measurement systems. Its objective properties are questionable, but its sound and dynamic range are so mesmerizing that hearing the word “vector nonlinear distortion meter” makes you want to spit.

Moscow 2006 ( Lincor_ nobox@ inbox. ru)

Hybrid amplifiers are different high quality sound and ease of execution. We offer you enough simple diagram manufacturing that uses simple components. Using such a hybrid amplifier will allow you to get enhanced, clear and detailed sound at the output.

Zarathustra amplifier circuit

More detailed and well-presented information can be found from the author of this amplifier on the forum:

Hybrid amplifier Zarathustra

One of the features of hybrid amplifiers is output current limitations. At the same time, the amplifiers are characterized by stable operation and minimal heating. There is no need for additional cooling systems. The output current is equal to the output current of the cascade, and can reach 15A.

It is possible to operate in supply voltage mode. Improved cascade symmetry at high frequencies allows for significantly improved sound quality when operating at maximum volume and when reproducing high frequencies. Minimal distortion has a positive effect on sound quality.

To manufacture a hybrid amplifier, an SRPP output stage based on bipolar transistors was used; two 6E5P lamps were installed at the input. The use of a tetrode cascade ensures voltage stability and excellent output voltage performance. The output stage uses a virtual average using capacitors instead of bipolar power supply.

This eliminates the appearance of DC load in the circuit and avoids overcharging the power supply capacity. This eliminates impulse distortion that can occur at peak power levels. The output signal is connected to the midpoint, and the capacitors used are eliminated from the audio circuit. This makes it possible to eliminate the influence of capacitors on sound quality.

Capacitors are connected to the windings, which makes it possible to suppress the electrical background in the incandescent circuits. This improves the sound quality. The use of capacitors makes it possible to make the filament voltage on the output filaments completely symmetrical. At the same time, the implemented amplifier is simple and can be easily implemented by every radio amateur.

Note also affordable price components used. If in the manufacture of other amplifier circuits it is necessary to use high-quality foreign capacitors, then in this case it is possible to use inexpensive capacitors domestic production. The influence of their quality on the generated sound is minimal, which allows us to slightly reduce the cost of manufacturing the amplifier without losing sound quality.

At numerous requests from radio amateurs, I present an improved and more full diagram hybrid ULF with detailed description, parts list and power supply diagram. The lamp at the input of the 6N6P hybrid ULF circuit was replaced with a 6N2P. You can also install the 6N23P, which is more common in old lamps, in this unit. Field-effect transistors are replaceable with other similar ones - with an insulated gate and a drain current of 5A and higher. Variable R1 - 50 kOhm is a high-quality variable resistor for the volume control. You can set it up to 300 kOhm, nothing will worsen. Be sure to check the regulator for the absence of rustles and unpleasant friction during rotation. Ideally, you should use ALPS RG - this is a Japanese company producing high-quality regulators. Don't forget about the balance regulator.

Trimmer resistor R5 - 33 kOhm inserts voltage zero on the speaker in ULF silent mode. In other words, by applying power to the transistors and instead of a speaker (!), connecting a powerful 4-8 Ohm 15 watt resistor, we achieve zero voltage on it. We measure with a sensitive voltmeter, since it should be absolute zero. The diagram of one hybrid ULF channel is shown below.

The remaining resistors are 0.125 or 0.25 watts. In short, any small ones. A 10,000 µF capacitor can be safely reduced to 100 µF, but it is drawn according to the old designation. We set all capacitors for anad supply to 350V. If it’s difficult to get 6.8 μF, set it to at least 1 μF (that’s what I did). The quiescent current control transistor will be replaced with KT815 or KT817. This will not affect the sound, it simply corrects the current there. Naturally, we need another copy of the hybrid ULF for the second channel.

To power the transistors, you need a bipolar source of +-20 (35) V with a current of 4A. You can use a regular transformer. Since more power was not required, I installed a 60-watt trans from a VCR with a corresponding reduction in output power. Filtration is simple - a diode bridge and a capacitor. With a quiescent current of 0.5A, a capacity of 10,000 microfarads per channel is sufficient. Capacitors C3, C4, C5 are 160V each, no less. Or more just in case. R8 small trim resistor- turns with a screwdriver. It sets the quiescent current of the output transistors (in the absence of a signal). You need to set the current from 0.3A - mode AB to 2A - mode A. In the second case, the sound quality is much better, but it will not heat up much. It can be used for power supply with an additional ring and 12-turn windings - it receives 12V from the transformer, and two 20V each - this is the secondary. In this case, the bridge diodes must be high-frequency; simple KD202 will burn out instantly.

We feed the filament with 12 volts by connecting the filaments of both lamps in series. I took the anode voltage of 300V using a small transformer (5 watts) from a Chinese multi-voltage adapter. You can't power anything from that parody except an LED, but in this hybrid it comes in handy. We supply 12V to its 15-volt secondary from an electronic (or conventional) transformer, and remove the voltage from the 220-volt network. The current is certainly not that great, but both 6N2P lamps pull only 5mA across the anode, so they don’t need more.

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