When assembling or repairing an amplifier audio frequency, as well as other audio equipment, problems often arise with the source of interference - AC hum with a frequency of 50 Hz. It is very noticeable in loudspeakers or headphones and interferes with enjoying music.
If this happens, check...
- Is the microphone connected to the preamplifier (PU) correctly - the common wire of the device must be connected to the braided screen of the cord. There should be a good screening of the input circuits.
- Is the output of the PU and the input of the power amplifier (PA) connected correctly. The fact is that sometimes two amplifiers (preliminary and PA) are used in one device, having different polarities of the common wire. In amplifying circuitry, such inclusion is not a problem; the main thing for a high-quality amplifier is the compatibility of the input impedance and the amplifier's own noise level. However, incorrect (incorrect) connection of the amplifiers between themselves and the preamplifier to the sound source (for example, to a microphone) is often the cause of a hum with a frequency of 50 Hz.
- Wiring printed circuit board the amplifier must be wired so that the power paths converge to one point - on large capacitors (power filters).
- The power traces should be thick, and the chassis traces should also, if possible, cover the empty areas of the board.
Ways to eliminate background in bass amplifiers
To eliminate this problem, there is a simple way regarding the inclusion of sound sources to the preamplifier (it can be not only a microphone, but also another source with a low signal level up to 10 mV). Let's analyze this method based on the microphone connection example.
The center conductor in the braided microphone cord is connected to the PU input, as a rule, to an isolation capacitor, a limiting resistor or a voltage divider. The braid of the wire coming from the microphone (screen) is not connected directly to the common wire, but in series with the RC circuit (parallel connected 2kΩ resistor (± 20%) and an oxide capacitor with a capacity of about 10 μF with the same tolerance for possible deviation from the nominal) . Here, the resistance of the resistor and capacitor is calculated for devices with a power supply voltage in the range of 6-20 V.
The positive plate of the oxide capacitor in this case is switched on depending on the polarity of the power source so that if the common wire is connected to the “minus” of the power source, then the oxide capacitor is connected to the common wire with a negative plate, and vice versa.
This method eliminates hum in most amplifiers with varying common power supplies, including older tube amps where filtering of the rectified voltage leaves much to be desired.
In most cases, in this way it was possible to solve the problem of a background with a frequency of 50 Hz in dynamic heads, which occurs after replacing the standard microphone with another one (with similar electrical characteristics), as well as in the case of replacing a high-impedance microphone equipped with a matching transformer and having a resistance of 1600 Ohm with a low-impedance microphone with coil resistance of 200 ohms or similar in electrical characteristics.
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There are many reasons for PDU failures. Fall - in these cases, cracks form on the case, screws break out, the back cover of the batteries breaks, tracks on the board or electronic components break. There are those who like to sit on the consoles, in these cases the board or case may break. You can repair everything depending on the breakdown, another question is whether it is necessary if you can buy a new remote control.
It is possible, but there are exotic models for which the remote control cannot be found. Therefore, it is better to roll up your sleeves and spend an hour of your precious time on a creative impulse. And for one thing, to be proud of yourself for a small feat, maybe someone else will praise, is also nice.
One of the main problems that one has to deal with in the development and creation of high-quality tube ULFs is the background of the alternating current. In this case, the alternating current background is understood as the voltage existing at the output of the amplifier, in addition to the useful signal, which has a frequency equal to or a multiple of the frequency of the mains voltage. The presence of the considered AC background in any sound reproducing device is a very serious drawback, since such a background narrows the dynamic range of the amplifier and sharply worsens the subjective impression of the reproduced signal. of which are the main ones: ripple of supply voltages and induction of alternating current to various circuits in the amplifier. Therefore, the background should be eliminated in two directions, respectively, namely, by improving the filtering of supply voltages and reducing the effect of pickups. One of the main reasons for the appearance of a background in lamp ULFs is the ripple of the rectified voltage supplying the circuits of the anodes and lamp screen grids. In this case, the effect of ripples is the smaller, the higher the internal resistance of the lamp. As you know, the internal resistance of pentodes is greater than that of triodes, therefore, from this point of view, it is better to use pentodes in the first stages of a tube amplifier. In addition, it is possible to achieve a reduction in the background arising from voltage ripples by improving the circuit and improving the parameters of the rectifier.
When using a choke in a power supply filter, this element largely determines the hum level. The inductance of the inductor is usually on the order of 5-20 H and should depend little on the load current. To improve filtering, it is useful to shunt the inductor with a capacitor, the capacitance value of which is chosen so that a circuit is formed that is tuned to the ripple frequency (100 Hz with full-wave rectification). circuit diagram A filter with this type of circuit is shown in Fig. 1.
Fig.1. Schematic diagram of a filter with a circuit
The reasons for the occurrence of an alternating current background may also lie in the fact that either the screen grids of the lamps are powered by an insufficiently smoothed voltage, or the anode current overloads the elements of the smoothing filter unnecessarily. So, for example, in the final stages of amplifiers, the anode and screen circuits of the lamps are often powered by voltage with the same ripple. However, the allowable screen voltage ripple for most terminal pentodes and beam tetrodes is 20-30 times less than the anode voltage ripple. Therefore, screen grid circuits must be fed through an additional smoothing circuit.
In order to reduce the effect of leakage between the cathode and the filament, it is sometimes recommended for the first stages of the amplifier to use a separate rectifier with a filter instead of automatic bias circuits, with the help of which a constant bias voltage is applied to the lamp grid. Schematic diagrams of possible variants of such rectifiers are shown in fig. 2. As a source of input AC voltage, both the filament winding (Fig. 2, a) and the special winding (Fig. 2, b) of the power transformer can be used.
Fig.2. Schematic diagrams of rectifiers for generating DC bias voltage
In the process of designing, creating and establishing high-quality low-frequency tube amplifiers, the main attention should be paid to identifying and eliminating interference. The fact is that at present, amateur ULF designs usually use power supply circuits that practically do not differ from industrial designs that are described in detail in the literature and tested in operation. Therefore, with serviceable elements and no errors during the assembly of the rectifier, the effect of supply voltage ripple is significantly reduced, and the cause of the noise background at the amplifier output is usually AC interference.
To determine the cascade that is affected by pickup, it is enough to alternately close the control grids of all amplifier lamps to the case, starting with the first one. The cessation or sharp decrease in the background when the grid of one of the lamps is closed indicates that alternating current is being applied to the grid circuit of this particular lamp. If no interference is detected in the amplifier, but the background is heard during playback, this indicates that the background voltage is supplied to the amplifier from a device connected to its input.
Compared to AC static pickups, magnetic pickups generally have less of an effect, except when the pickup source is a power transformer field and the target is some winding amplifier element.
Quite often, the creators of amateur tube sound reproducing equipment have to deal with interference caused by the presence of common circuits for AC and signal, or the use of common circuits for AC and DC supply voltage. So, for example, it is not recommended to use a shielded wire braid as one of the wires leading the signal to the amplifier input. For signal routing, it is best to use two shielded wires or a double wire in a common shield, and connect the common braid to the amplifier chassis. If this rule is not observed, the background may have a significant value, since the voltage induced on the braid will be applied to the input along with the signal.
For the same reason, high-quality tube bass amplifiers should not use the common negative wire or chassis as one of the filament wires. On fig. 3. Examples of incorrect (a) and correct (b) installation of the first stage of the amplifier are given, in which the chassis serves as one of the filament wires.
Fig.3. Incorrect (a) and correct (6) installation of the first stage of the amplifier using the chassis as one of the filament wires
When used in the first stage of an amplifier, for example, a 6Zh1P pentode, improper installation of the filament circuit can lead to the fact that an increase in the contact resistance of the contact on the chassis to 0.05 Ohm will cause a significant background to appear at the output of the amplifier, equivalent to applying a voltage of 3 mV to its input.
One of the simplest and, at the same time, the most effective method of interference prevention is the use of screens. It should be noted that electrical and magnetic shields must be carefully grounded, otherwise their use may lead to the opposite result - to enhance rather than weaken the background. First of all, a special shield winding is wound between the primary and secondary windings of the power supply transformer. In addition, the input stage lamps should be placed on lamp panels with special screens. All branched grid and anode circuits of the first stages, for example, any corrective filters, should be carefully screened, placing all the details of this circuit with circuit boards in a common screen.
It is recommended to use shielded wires and coaxial connectors to connect a signal source to the input of the amplifier, since conventional pin sockets and plugs, with rather large unshielded surfaces, can cause strong hum.
All parts used in background-sensitive circuits should be kept as small as possible to reduce interference. At the same time, their metal cases should also be grounded. It is also necessary to reliably ground massive metal structural elements located near the input stages. Special attention should be given to the grounding of variable resistance cases, since most often they are not connected to the axis of the potentiometer.
One method often used to reduce AC hum is often referred to as compensation. Its essence lies in the fact that control grid one of the stages of the amplifier is supplied with an alternating voltage equal in magnitude to the background voltage acting on this grid. As a result, if the phases of the voltages of the background and the additional signal are exactly opposite, then the total voltage will be zero, and the background will be compensated. The main disadvantage of this method is that over time, due to aging, the parameters of lamps and other elements may change, which will lead to violation of compensation. Therefore, the use of such methods of background elimination in high-quality amplifiers is undesirable.
The compensation method can also be used to reduce AC ripple in power supplies. So, for example, with a large rectified current, the filter inductor core is significantly magnetized, which forces it to increase its cross section to maintain the same inductance. However, to reduce ripple, you can wind a compensation winding around the inductor. A schematic diagram of a filter with a compensation winding is shown in fig. 4. Unfortunately, full compensation cannot be obtained in this way, but the background level is noticeably reduced.
Fig.4. Schematic diagram of a filter with compensation winding
It should be noted that a sharp increase in the background level with a simultaneous decrease in the rectified voltage occurs in the event of any malfunctions of the rectifier elements, for example, when the leakage of the electrolytic filter capacitors increases, the kenotron emission is lost, or the filament of one of the kenotron diodes burns out. Therefore, before turning on the compensation winding, you should make sure that all elements of the rectifier are in good condition.
One of the options for applying the compensation method is to apply an antiphase signal to the cathode of the lamp of the last stage of the preamplifier. A schematic diagram of such a cascade is shown in fig. 5.
Fig.5. Schematic diagram of the compensation circuit with the supply of an antiphase signal to the lamp cathode
In this case, the control signal is removed from the engine of the tuning potentiometer R5, connected between the terminals of the filament winding of the power transformer according to the scheme with an artificial midpoint. This signal is fed through the R4C2 chain to the cathode of the lamp of the last stage of the preamplifier. In the process of working with the amplifier, by adjusting the potentiometer R5, you can set the minimum background level by ear.
One of the options for reducing AC hum compensation in the final stage of a low-frequency tube amplifier with a transformer output is to use an additional choke winding of the rectifier smoothing filter. This winding is connected in series with the voice coil and the secondary winding of the output transformer. As a result, the AC hum is compensated due to the fact that the voice coil of the woofer speaker system an alternating voltage is supplied, the phase of which is opposite to the phase of the background voltage induced in the secondary winding of the output transformer. A schematic diagram of the output stage with the connection of an additional inductor winding is shown in fig. 6.
Fig.6. Schematic diagram of the output stage with the connection of an additional winding of the inductor of the smoothing filter
The number of turns of the additional inductor winding depends on the resistance of the speaker's voice coil and usually ranges from 20 to 40 turns of varnished copper wire with a diameter of 0.8-1.0 mm. The phase of the voltage removed from this winding is selected empirically by changing the order of connecting the leads.
Naturally, this method of compensation can only be used if a smoothing inductor is used in the power supply circuit. In addition, with the help of the considered circuit, only that component of the background that is excited in the output stage is compensated. Therefore, this method of AC hum compensation is not widely used.
Audio frequency amplifiers, created and repaired by radio amateurs, often become a source of headache due to the subsequently occurring alternating current background with a frequency of 50 Hz, which is audible in loudspeakers or telephones (headphones).
If this happens, you should check whether the microphone is correctly connected to the PU (pre-amplifier) - the common wire of the device must be connected to the braided screen of the cord - and also - whether the output of the PU and the input of the power amplifier (PA) are connected correctly. The fact is that sometimes two amplifiers (preliminary and PA) are used in one device, having different polarities of the common wire. In amplifying circuitry, such inclusion is not a problem; the main thing for a high-quality amplifier is the compatibility of the input impedance and the amplifier's own noise level. However, incorrect (incorrect) connection of the amplifiers between themselves and the preamplifier to the sound source (for example, to a microphone) is often the cause of a hum with a frequency of 50 Hz.
Practical background elimination in amplifiers 34
To localize this problem, there is a simple way regarding the inclusion of sound sources to the preamplifier (it can be not only a microphone, but also another source with a low signal level up to 10 mV). Let's analyze this method based on an example with connecting a microphone.
The central conductor in the braid of the microphone cord is connected to the input of the PU, as a rule, to an isolation capacitor, a limiting resistor or a voltage divider. The braid (shield) is not connected directly to the common wire, but in series with the RC circuit (parallel connected resistor with a resistance of 2 kOhm (± 20%) and an oxide capacitor with a capacity of YumkF with the same tolerance for a possible deviation from the nominal value). Here, the resistance of the resistor and capacitor is calculated for devices with a power supply voltage in the range of 6-20 V.
The positive plate of the oxide capacitor in this case is switched on according to the polarity of the power source so that if the common wire is connected to the “minus” of the power source, then the oxide capacitor is connected to the common wire with a negative plate, and vice versa
This method eliminates hum in most amplifiers with varying common power supplies, including older tube amps where filtering of the rectified voltage leaves much to be desired. In most cases, in this way it was possible to solve the problem of background with a frequency of 50 Hz in dynamic heads, which occurs after replacing the standard microphone with another one (with similar electrical characteristics), as well as in the case of replacing a high-impedance microphone (for example, MD-47, equipped with a matching transformer and having a resistance of 1600 Ohm) to a low-impedance microphone of the MD-201 type with a coil resistance of 200 Ohm or similar in electrical characteristics.
So, for example, it is not recommended to use a shielded wire braid as one of the wires leading the signal to the amplifier input. For signal routing, it is best to use two shielded wires or a double wire in a common shield, and connect the common braid to the amplifier chassis. If this rule is not observed, the background may have a significant value, since the voltage induced on the braid will be applied to the input along with the signal.
For the same reason, high-quality tube bass amplifiers should not use the common negative wire or chassis as one of the filament wires.?
On fig. 3.36 shows examples of incorrect (a) and correct (b) installation of the first stage of the amplifier, in which the chassis serves as one of the filament wires.
When used in the first stage of an amplifier, for example, a 6Zh1P pentode, improper installation of the filament circuit can lead to the fact that an increase in the contact resistance of the contact on the chassis to 0.05 Ohm will cause a significant background to appear at the output of the amplifier, equivalent to applying a voltage of 3 mV to its input.
One of the simplest and, at the same time, the most effective method of interference prevention is the use of screens. It should be noted that electrical and magnetic shields must be carefully grounded, otherwise their use may have the opposite effect of amplifying rather than attenuating the background. First of all, a special shield winding is wound between the primary and secondary windings of the power supply transformer. In addition, the input stage lamps should be placed on lamp panels with special screens. All branched grid and anode circuits of the first stages, for example, any corrective filters, should be carefully screened, placing all the details of this circuit with circuit boards in a common screen.
To connect a signal source to the input of the amplifier, it is recommended to use shielded wires and
coaxial connectors, since conventional pin sockets and plugs, with rather large exposed surfaces, can cause strong hum.
All parts used in background-sensitive circuits should be kept as small as possible to reduce interference. At the same time, their metal cases should also be grounded. It is also necessary to reliably ground massive metal structural elements located near the input stages. Particular attention should be paid to the grounding of variable resistance cases, since most often they are not connected to the axis of the potentiometer.
One method often used to reduce AC hum is often referred to as compensation. Its essence lies in the fact that an alternating voltage is supplied to the control grid of one of the stages of the amplifier, equal in magnitude to the background voltage acting on this grid. As a result, if the phases of the voltages of the background and the additional signal are exactly opposite, then the total voltage will be zero, and the background will be compensated. The main disadvantage of this method is that over time, due to aging, the parameters of lamps and other elements may change, which will lead to violation of compensation. Therefore, the use of such methods of background elimination in high-quality amplifiers is undesirable.
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