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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Circuitry of tube amplifiers-correctors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers In recent years, the CD, of course, is the most popular type of music media. The processes of digital processing of audio signals are being continuously and very intensively improved, but despite this, the subjectively perceived quality of reproduction of modern CDs often hardly approaches the level achieved by mechanical sound recording 35-40 years ago! Also, oddly enough, along with the growing popularity of CDs came the "vinyl renaissance" of records made 40 years or more ago. It is the possibility of achieving a high level of subjective, emotional and aesthetic perception that explains the interest of serious music lovers in recording playback equipment. One of the most important components of this equipment is the corrective amplifier (UK). Readers are offered several options for such devices, where both radio tubes and semiconductor devices are used as active elements. Note that, unlike tube power amplifiers, where circuitry of the 20-50s of the last century is often used today, such an approach is inappropriate for preamplifiers. Classical ACs in the majority consist of two or four typical amplifying stages with capacitive coupling, covered by a fairly deep overall OOS. The elements of this OOS form the desired frequency response (Fig. 1). In UK, cathode and other repeaters are often used.
According to the author, a convincing reason for using bulky and vibration-sensitive tube amplifiers can only be an unconditional, subjectively assessed by the consumer advantage over a similar (in terms of accessibility) device based on semiconductor devices. In these ACs, it seems desirable to use the RIAA correction distributed over the amplification stages (including those with inductors). In addition, they try to achieve high linearity of amplifiers without the use of general and, if possible, local environmental protection. In the signal path, the number of transition capacitors is minimized, often refusing cathode followers. By the way, the author does not share the opinion about the inadmissibility of introducing semiconductor devices into the amplifying path, about the need to use exclusively triodes with a low value of "μ", about some special fundamental advantages of direct-heated cathodes and other "cardinal means". On the contrary, a reasonable combination in audio technology of the positive properties of semiconductor and electrovacuum devices is absolutely justified. It can be argued that the experience accumulated during the period of the "tube renaissance" made it possible to identify previously not obvious, but fundamental shortcomings of active and passive components, as well as the costs of the generally accepted ideology of building devices based on transistors. This helped to outline ways to significantly improve the sound reproduction of the recording. We remind readers that the magnetic transducers of pickup heads are conditionally divided into the following groups. Group 1 - heads with a rated output voltage of about 2 ... 4 mV, designed for connection to a preamplifier with an input resistance of 47 kOhm and a total input capacitance of 100 ... 250 pF (the internal resistance of such heads is 1 ... .2 kOhm). In this case, the required amplifier gain at a frequency of 1000 Hz is 50...60 dB. This group includes most MM (Moving Magnet) heads and MC (Moving Coil) heads with increased sensitivity. It is curious that some well-known companies (Shure, Grado, etc.) have recently begun to produce special inserts with needles for playing ordinary gramophone records ("at 78 revolutions") for well-known serial models of their MM heads. Group 2 - heads with a nominal output voltage of 0,2 ... 0,3 mV, for which the optimal load value is about 1 kOhm (the internal resistance of such heads is about 40 ... 50 Ohms), and the required gain reaches 70 ... 80 dB. This group includes common types of MS heads. Group 3 - heads with a rated output voltage of about 15 ... 20 μV, an internal resistance of about 3 ohms. They require a load with a resistance of about 100 ohms and a gain of up to 90 ... 100 dB (however, such heads are rare). As a rule, all heads of the 3rd group, and often the 2nd, work in a set with special matching transformers that allow the use of standard preamps designed for heads of the 1st group. In addition, the use of transformers makes it possible to increase the signal-to-noise ratio and make it easier to deal with the background of the alternating current in the network. However, the cost of such transformers is very high - up to 1000...3000 US dollars. The minimum achievable equivalent resistance of intra-tube noise is at best not lower than 100 ohms, which is why a purely tube preamplifier for heads of the 2nd group inevitably has an unimportant signal-to-noise ratio, and for heads of the 3rd group it is generally unacceptable. An alternative to a transformer in this aspect can only be cascades on low-noise field and bipolar transistors. If, nevertheless, it is a tube, and not, say, a hybrid preamplifier for an MC head, that the input triode must be made with low intrinsic noise (for example, 6N23P, 6N24P, 6S3P). For relatively high-resistance heads of the 1st group, it is advisable to make the input stage of the UK with a low-noise pentode, for example, 6Zh32P (analogous to EF-86), 6Zh9P, etc., since, unlike a triode, it has an insignificant dynamic input capacitance. I note in passing that the specialized "sound" pentode 6Zh32P, often ranked among the low-noise ones, is actually the least "phony" when its filament is powered by alternating current and is quite resistant to the microphone effect. Also, this lamp is distinguished by high linearity even at significant amplitudes of amplified signals and efficiency, in terms of noise properties it is inferior to pentodes 6Zh9P, 6ZhZP, 6Zh1 P. It is also possible to use rare octal tubes in the UK, which are distinguished by increased intrinsic noise and a noticeable microphone effect, but nevertheless loved by many audiophiles for their exceptional "musical" properties. For the input stage of the preamplifier, 6N9C and its numerous analogues are often recommended, less often - pentodes 6Zh7, 6Zh8, EF37, etc. Here it is appropriate to touch upon the issues of constructive implementation of the Criminal Code. Due to the specific rise in the frequency response according to the RIAA or RIAA-78 standard (Fig. 1), which has a maximum gain at frequencies of 50 Hz and below, and a low level of input signals (with high sensitivity of radio tubes to vibrations, and devices in general - to pickups) full electrical and magnetic shielding is mandatory. Measures should also be taken to mechanically isolate at least the parts of the input stage. For example, insulated lamp panels or a small sub-chassis with cascade details are fixed to the main chassis through an elastic (rubber) shock absorber, and electrical connections are made with soft wire segments (MGTF, LESHO, etc.). The lamp is covered with a massive steel cap, which can be pasted over with vibration-absorbing material. It is necessary to take the power transformers as far as possible from the preamplifier and the player (preferably with an annular magnetic circuit and reduced working induction). In a number of designs, the power supply is placed in a separate case. It is important to observe common for small-signal devices installation features of the common wire and ground. As a rule, a common wire or a copper strip of significant cross section (2 ... 5 mm2) is laid in isolation from the chassis and connected to it at one point near the input stage. A "star-shaped" connection (star ground) is also used, when all "grounded" conductors are connected to each other and to the chassis at one point. According to the author, there are no contraindications to the use of printed wiring, which is not accepted by do-it-yourselfers of lamp designs. Among other things, it is desirable to modify the player by providing symmetrical (without a common wire) signal outputs from the stereo head and, if possible, electrically "untying" the tonearm parts and screen braids of the output cables from the chassis and other "masses" of the player. Common conductors between all components of the audio system, as well as the "ground" wire of the mains cords, must not create closed loops. An example of a possible connection of parts of an electric player and a preamplifier-corrector is shown in fig. 2.
Although all the UC schemes proposed in the article look, one might say, alphabetically, the advantages of this simplicity can only be realized if careful design and painstaking adjustment are made. The prototypes of these circuits were borrowed by the author from such authoritative magazines as "Glass Audio" and "Sound Practices", as well as from the websites of foreign audiophiles, in particular, Jim de Kort and Ervin Wiesbauer [1,2]. The changes made during prototyping are explained by the use of a different element base and an increased output voltage level (1,5 ... 2 V - the limit value for CD players), convenient for matching with most tube UMZCH options, where a two-stage structure is considered preferable. It should be borne in mind that the AC described in the article requires high voltage sources that are life-threatening, as well as high-voltage capacitors with high charge energy (up to 100 ... 200 J!). Accidentally shorting a charged capacitor can result in metal melting and spattering, burns and injury. Therefore, proceed to the repetition of the described structures only with full confidence in the level of your qualifications. And now let's move on, finally, to the description of specific schemes of the Criminal Code. The first proposed option is a preamplifier on octal tubes for MM heads (Fig. 3 and following shows the diagrams of one of the channels).
The amplifier according to this scheme can also be built on finger lamps. Analogues for double triodes are as follows. Lamp 6H8C - a close analogue of 6SN7-GT, 5692, ECC32, ECC3Z (octal), ECC82, E82CC, ECC802S, 12AU7 (finger); with a slight adjustment in the values of the elements, domestic 6N1P, 6N6P, 6N14P and subminiature 6N16B, 6N18B are suitable. Lamp 6H9C - a close analogue of 6SL7-GT, 5691, ECC35 (octal), 5751 (finger), ECC83, E83CC, ECC803S, 12AX7; domestic finger 6N2P - an approximate analogue; from subminiature ones, 6N17B and 6S7B (single triode) are suitable. Often there are 6N2P with increased noise in the audio frequency range and poor insulation between the cathode and the heater. Analogues of the beam tetrode 6P6S - 6V6-GT (octal), EL90 and domestic 6P1P (finger type); exotic 6F6S and imported 6F6 are also close in parameters. In the output stage, it is also possible to connect two triodes of the 6N30P lamp in parallel, for which the voltage at the anode is reduced to 80 V and some resistor values \u13b\u12bare changed (R14 - 130 kOhm, RXNUMX - XNUMX Ohm). When designing UK with subminiature lamps, it should be taken into account that they have a slightly lower allowable power dissipated by the anode, the excess of which leads to their very rapid failure. By preliminary selection of parts, it is desirable to ensure the identity of the two amplification channels with a spread in the parameters of passive components of no more than 1%. This is especially true for the elements that form the frequency response (R4, R8, R11, C3, C4, C9). You can use resistors of types C2-23, C2-29, MLT, C1-4; and the R13 element is C5-16MV, C5-35V or PEV. Be careful about the advice sometimes found on using old carbon BC resistors, because due to aging their noise often increases, and the change in the actual value can reach 20 ... 25% relative to the nominal even for the E24 series. Capacitors C1, C7, C13 - types K50-24, K50-29 or imported (Rubicon, Weston, etc.), always foil. Capacitors of the K52-x, K53-x, ETO series are not recommended for signal circuits. Elements C2, C6, C8, C12, C14, C17 can be K73-4, K73-16, K73-17, MBGO or K42-x series, and C3, C4, C9, C10 - K78-2 or the like. In positions C5, C11, C15, C16, it is desirable to use capacitors of the K78-24 type, somewhat worse - MBGO, MBHC, in extreme cases, it is permissible to use oxide K50-27 (except C15). The types of components indicated here are also suitable for other CCs described in the article. Of course, with appropriate financial capabilities, you can give preference to the components of the so-called "audiophile" quality. Recommendations on this topic can be found on the pages of some Hi-Fi and Hi-End technology magazines, but they are often very subjective, sometimes going against the fundamental laws of physics. When establishing the UK, they adjust the DC amplifying stages to the recommended modes by selecting (if necessary) resistors R3, R9, R14 and eliminating deviations in the frequency response from the standard RIAA, as well as differences in the frequency response of the two channels by selecting elements C4, C9, R8. Capacitance Cm* should be chosen so that the capacitance of the connecting cable between the player and the preamplifier, the input capacitance of the first stage of the preamplifier (approximately - 40 ... The following scheme (Fig. 4) is a modification of the previous one, suitable for MS heads of the 2nd group. The author of the development is Arthur Loesch from the USA. Variants of this scheme, somewhat different in the types of lamps used, are invariably popular among foreign amateurs. The magazine "Sound Practices" its development is classified as Top End due to the peculiarities of the design and power organization of the cascades.
The output voltage of the basic version of the preamplifier when working with common heads (for example, DENON DL-103) is about 0,5 ... 0,7 V. A significant increase in this voltage is possible if lamps with high gain (μ > 30). This can lead to deterioration of the overload capacity if the supply voltages are not changed and the parameters of the components are adjusted accordingly. On the contrary, by reducing the gain of the second and third CC stages, it can be adapted to MM heads with excellent results. The use of galvanic cells as a bias voltage source simplifies the fight against pickups and eliminates cathode resistors and capacitors, which favorably affects the sound. The service life of the cells is practically determined by their self-discharge and can be two to three years. It is only necessary to ensure reliable, non-oxidizing contact with the terminals of the cells, and to avoid damage from heat and electrolyte leakage, protection from the heat generated by the lamps. By the way, in the studio equipment of the 40s and 50s, completely battery-powered input stages of microphone amplifiers were sometimes preferred. This direction is for extremist audiophiles; for the rest, I note that due to the fixed bias, all stages of the described preamplifier require well-stabilized sources of all supply voltages, including filament. In the original publication, Arthur Loesch states that the power supply contains a separate stabilized anode voltage source for each stage in each channel (i.e. 6 in total!). The original device is made on a sectioned chassis made of thick copper sheet. All capacitors - foil (copper foil and fluoroplastic dielectric), resistors - precision (tolerance no more than ± 0,5%) wire and metal film, oxide capacitors - from the top "Black Gate" series. The installation is made using branded silver wires and a special silver-containing solder. This example shows that high quality indicators of tube devices are achieved not due to the complication of the structure of the amplifying part, but due to careful execution; moreover, at least half of the success determines the quality of the power supply. With regard to the details of the Criminal Code, all of the above is true. Resistors R1-R4 must be metal film or wire. There is no direct replacement for the 6S45P (or 6S15P) lamp, the imported analogue 417 (Western Electric) or the triode 5842 close to them are practically inaccessible and expensive, therefore, in Table. 1 shows approximate replacements with approximate electrical modes.
In addition to those indicated, it is possible to use some low-noise high-frequency pentodes in the triode connection in the input stage, in particular, 6Zh11P, 6E5P, 6E6P, 6Zh52P, as well as a pentode from 6F12P. When using the 6F12P triode, it is recommended to connect a capacitor with a capacity of 3 uF at 1000 V in parallel with the resistor R6,3. As in the previous case, the selection of the resistor R1 and (if necessary) the capacitor connected in parallel to it should be carried out in accordance with the recommendations of the manufacturer of the used head. In the second stage, it is possible to use lamps 6N1P, 6N15P and 6NZP, in which both triodes must be connected in parallel. When using 6NZP lamps, it will be necessary to select the values of the resistors R6, R8. Another modification of the circuit in Fig. 4 shows its balanced version using differential cascades and partially - galvanic connection between the cascades (Fig. 5).
The author of the prototype bearing the name "Siren Song" is the American designer JC Morrison, well-known in audiophile circles. Of course, the input stage on an octal triode with a current source on a field-effect transistor looks very elegant (and works great), and in the original version - a current stabilizer chip 1N5309 or 1N5311. Due to the common mode suppression inherent in differential stages, as well as the compensation of the signal component of the current in their power circuit, the requirements for the anode power supply are significantly lower than in conventional stages. Nevertheless, the use of the input stage of a current source in the cathode circuit contributes to the stability of the mode. The original publication suggested a fully unregulated diet; despite this, I recommend that you stabilize the filament voltage when repeating. Of course, the implementation of this UK is also possible on finger lamps. For example, if you select 6N23P (ECC88, E88SS, 6922, 6DJ8) or 6N24P lamps for all stages and set the drain current value VT1 (Fig. 5) to 12 ... 15 mA (also reducing the resistance of resistors R4-R7, R15), then such a preamplifier is suitable for operation with an MC head. In the third stage, 6N15P (6J6) or vintage 6N7S (6N7, 6N7-GT) lamps are applicable with both triodes connected in parallel. If the preamplifier is supposed to work with a balanced symmetrical (relative to the common wire) load, you can remove the elements C7-C9, C11, and use a high-quality film or paper capacitor with a capacity of 10 ... 5 microfarads in position C10. If it is possible to reduce the total gain of the corrector by approximately 30%, it is advisable to install double triodes 3N4P or 6N6P in cascades with VL6, VL30; their exemplary modes are given in Table 2.
When setting up, the input stage is balanced with a resistor R8 until the voltages are equal on both anodes of the VL2 lamp, and the output stage is balanced with a resistor R22 until the voltages on the anodes VL3 and VL4 are equal. It is absolutely unacceptable to use variable resistors with unreliable contact as R8 and R22 (with a crack during regulation), since with further use this is fraught with failure of the power amplifier and speaker system! In the UK block, I advise you to use high-quality XLR type connectors. If for some reason a balanced input is not used, I recommend removing the elements R2, R3, and connecting the output of the left VL1 grid according to the grid scheme directly to the common wire at the point where the common wire contact of the input connector (RCA type) is also connected. The theme of "cooperation" of field-effect transistors and vacuum devices is continued by the circuit shown in fig. 6.
Unlike the diagram in Fig. 4, here the input stage uses a cascode connection of a low-noise field-effect transistor and a nuvistor lamp, used solely for size reasons. Part of the UK (according to the diagram in Fig. 6) to the left of section A-A was forced to be made in the form of a small block, placed directly to the base of the tonearm and connected to the rest of the cable with a length of about 0,3 m. No special advantages of the nuvistor over the 6N23P lamp not found. The advantages of the cascode structure are manifested in a low dynamic input capacitance and a significant amplification of the cascade, which makes it possible to recommend them for operation with heads of the MM and MS types (resistor R1 - 1 kOhm). Resistor R4 allows you to set the drain current of the transistor, which provides the necessary steepness of the drain-gate characteristic. In this case, the stabilizing effect of the transistor on the VL1 triode mode deteriorates somewhat, so the supply voltage of the input stage is stabilized by a chain of zener diodes VD.1-VD3. Sometimes the opinion is expressed that the use of semiconductor zener diodes in the power circuits of amplifying stages leads to a depletion of the "sound", and on this basis the use of gas-filled glow discharge zener diodes is recommended. The author's experience suggests that the followers of this "fruitful" idea run the risk of overly enriching the sound with a wide range of noise created by these devices, which sometimes even show a tendency to spurious generation (especially with a long operating time). Perhaps it is advisable to follow the advice from the magazine "Glass Audio" and limit the use of their valuable decorative properties - the mysterious and multi-colored glow in the twilight of these devices turned on idly, undoubtedly, successfully complements the intimate flickering of the filaments of rare straight-wire triodes and significantly enhances the emotional impact of the music being listened to. Those wishing to repeat the Criminal Code according to the scheme in Fig. 6 I recommend making a separate stabilizer to power the input stages, which can be supplemented with an LC filter to eliminate interference from the anode voltage source in the grid circuit of the VL2 lamp due to the increased output resistance of the cascode. By the way, this property of the cascode prompts some authors to recommend the direct connection of a correction capacitor (in this case C3) in parallel with the resistor in the anode circuit (R5). Such an inclusion leads to a significant frequency dependence of the load and, accordingly, to an increase in the dynamic component of the input capacitance, which is undesirable, at least for MM heads. Further, a purely tube version of the preamplifier-corrector is proposed using a cascode at the input (Fig. 7), which, like the previous one, is an attempt to improve the AC according to the circuit in Fig. 4.
The use of two triodes connected in parallel at the input is aimed at reducing the intrinsic noise of the corrector. With the ratings of the elements and supply voltage indicated in the figure, the overload capacity of the preamplifier when working with heads of the 1st group turns out to be about 20 dB. The gain of two stages of the UK at a frequency of 10OO Hz is approximately equal to 52 ... 56 dB, therefore, the output stage connected at points 1-2 must have a voltage gain of about 10 to obtain a nominal output voltage of 0,7 ... 1 V ( you can use the output stage according to the circuit in Fig. 3). If it is desirable to have an output level closer to 2 V, you can make an output stage similar to the circuit options in Fig. 4 and table. 2. We are talking, of course, about connecting a typical head of the 2nd group with a nominal output level of about 0,2 mV to the input of the UK. Obviously, triodes connected in parallel require careful selection for the identity of parameters in the operating mode, which may be difficult without a radio tube tester, but doable. Otherwise, the advantages of such circuitry are not realized. On fig. 8 shows a diagram of a simple corrective amplifier based on a 6Zh32P pentode, designed to work with heads of the 1st group. This kind of UK is popular among foreign amateurs in our country [3].
With careful execution, this CC, despite its simplicity, is able to show a tangible superiority over many "branded" products, including those whose price is more than $1000. In addition, the preamplifier according to the circuit in Fig. 8 makes it easier to achieve input matching with many types of heads; it is less critical to the output load due to the use of a cathode follower, which, according to the ideology of Hi-End Audio, is the most vulnerable to criticism. Formally, small non-linear distortions in the repeater at certain signal levels and load resistance values may be accompanied by a "dissonant" ratio of harmonic distortion components. But this cascade is easy to exclude, however, to the detriment of the sensitivity of the AC to the load, using the schemes of the previous options. If you intend to use an MC head with a step-up transformer, then I recommend just such a preamplifier, as it provides ample opportunities for optimizing the load for the head with a transformer. It is useful to stabilize the power of at least the input stage. With a lack of overall gain, the output stage should be performed by analogy with the already considered variants of the UK. A very original and elegant scheme of the UK for working with a MM head, using 6Zh32P and 6N6P lamps (with the introduction of a positive frequency-dependent feedback), was proposed by A. Likhnitsky [4]. For those who are interested in this circuit, I recommend supplementing the device with a buffer stage in order to avoid the influence of changes in the load of the corrector on its frequency response. On fig. 9a shows a variant of constructing a conventional cascade with an anode load, but improved power decoupling (optimal compensation is achieved by adjusting the resistor R4) The ratio between the resistances R1 and R2 (approximately equal) is chosen so that the components of the signal currents through them are equal. In addition to the considered options for cascades, we should also mention the SRPP (Series-Regulated Push Pull) cascade, when a dynamic load is installed in the anode circuit of the lamp. It is especially effective in the output amplification stage. Its varieties allow you to combine high gain and linearity with low output impedance (approximately 100 ... 300 ohms). The disadvantages include the need for a supply voltage of at least 300 V, an increased dynamic input capacitance (compared to a typical cascade), as well as increased requirements for the quality of insulation between the cathode and the heater if a double triode is used in the cascade. On fig. 9,6 shows a typical one, and in fig. 9c - the so-called "reinforced" SRPP cascade. There are more complex options using the pentode as a dynamic load; as a rule, their use is expedient as pre-terminal stages of power amplifiers. Nevertheless, it is fundamentally possible to build all stages of a corrective amplifier using SRPP circuitry. A galvanically coupled pair of stages on lamps with a common cathode and a common anode also has properties similar to the SRPP cascade. An example of such a cascade circuit is shown in fig. 9, city A very valuable property of this pair, with an appropriate choice of modes, is the almost complete absence of penetration of the signal component into the anode power circuit (as in a differential stage). Since in the cascades according to the diagrams in Fig. 9a and 9d, a significant reduction in the signal component released on the cathode resistor is achieved, it is possible to abandon the use of a large-capacity shunt capacitor (usually oxide).
The best option for building the output stage of the UK is, of course, a stage with an output transformer. Unfortunately, the proper implementation of the transformer is very laborious and only available to experienced radio amateurs. The final choice of any circuit design option is carried out mainly on the basis of subjective preferences based on the results of listening to carefully mock-up devices. In no case should a novice do-it-yourselfer trust "experienced" audiophiles in this matter, who utter phrases like: "The 6N6P lamp gives a fat and muddy sound ...", "Inexperienced listeners often confuse the acidic sound of the ECC88 lamp with excessive detail ... ", "Removing the top cover of the preamp resulted in a dramatic "airy" sound and a stunning "openness" to the sound...". Attempts to take into account the results of such "examinations" almost guarantee that the manufacture of the device being developed will not be completed, and the manufacturer will gradually develop a perception stereotype when, when listening to musical works, he will subconsciously focus on the detection of certain shortcomings, and not on the musical content of the work . Unfortunately, the volume of this article did not allow us to consider some important features of building power supplies for tube preamplifiers. These questions are worthy of a separate article. Literature
Author: N.Troshkin, Moscow
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Comments on the article: Sergei Thanks a good article.
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