Background AC
Reasons leading to the appearance of AC background:
- Contact with AC power circuits into low frequency stages.
- Influence of electric and magnetic fields to low-frequency circuits due to poor placement of individual wires and parts.
- Background overlay on high frequency circuits or a modulating hum, audible only when the receiver is tuned to a radio station.
The presence of a constantly audible background indicates that it is superimposed in one way or another on the low-frequency circuit of the receiver. Therefore, first of all, you should check whether the pulsations are sufficiently smoothed out DC rectifier filter. For this purpose, a calibrated high-voltage capacitor with a capacity 40-100 µF connected in parallel first to the second and then to the first capacitors of the smoothing filter of the receiver or amplifier being repaired. If this gives the desired effect, then you need to replace one or both of the anti-aliasing filter capacitors or increase the capacitance of the capacitors in the anode or grid decoupling filters. If such an event does not cause a noticeable weakening of the background, then most likely there is a second reason.
To quickly detect in which low-frequency cascade the background is superimposed, remove all the lamps one by one, starting from the input and up to the pre-terminal one, and monitor which one of them stops the background when removed.
The final stage lamps cannot be removed when the power is on., since the sharp decrease in rectifier load caused by this leads to a significant increase in the anode voltage, which in turn can cause breakdown of the smoothing filter capacitors.
Common causes of background noise due to interference are breaks in the shielding shells and the appearance of a leak between the filament and the cathode at the input lamp of the low-frequency amplifier. The cause of the modulating background can also be poor pulsation smoothing voltages supplying high-frequency lamps. The input stages of receivers (RF amplifier and converter), as well as the local oscillator, are especially sensitive to this, and therefore an additional smoothing filter cell is sometimes installed to power these stages.
The modulating background of alternating current, audible only when receiving local stations, is easily eliminated by blocking the anode of the kenotron to its cathode or ground ( Fig.1 ), as well as blocking the shoulders of the step-up winding of the transformer with capacitors with a capacity 0.005-0.01 µF; the operating voltage of these capacitors must be no less than triple the voltage of the arm of the step-up winding of the power transformer ( 1000-1500 V).
Before eliminating the background that appears when receiving radio stations, you need to make sure that the background modulation occurs in the receiver and not in the transmitter. To do this, it is best to check the reception of the same radio station using another receiver.
Particular attention should be paid to methods for eliminating the background in equipment with direct incandescent lamps when their filaments are powered with alternating current. It is necessary here precise balancing of the filament circuit, which is not always ensured by the device for tapping the midpoint of the filament winding.
A more effective measure is to include a low-resistance potentiometer between the terminals of the filament, the slider of which should be considered as a terminal from the cathode of the lamp. Precise balancing of the thread is carried out when the power is turned on by ear by setting the potentiometer slider to a position in which the alternating current background is least audible.
A similar measure can significantly reduce the background penetrating from filament circuits in low-frequency amplifiers with high gain (in tape recorders, microphone amplifiers). If the device is installed again, the background noise may be caused by poor placement of individual circuits and transformers.
It is important to identify not only which circuit is affected by the unwanted influence, but also which circuit produces this influence. To do this, we apply a method for changing the reactivity of subsequent circuits, which consists in connecting a capacitor of larger or smaller capacity to the anode load resistances of the lamps, starting from the output of the receiver, and so gradually approaching the source of self-excitation or its complete cessation.
Let's assume that connecting a capacitor to the output transformer only reduced the volume without changing the nature of self-excitation. This means that the final stage is not covered by self-excitation and the circuit that creates an undesirable effect on the amplifier input must be sought before it. But, if, for example, when a capacitor is connected parallel to the primary winding of the output transformer, self-excitation is removed or its character changes, then either this circuit or the subsequent one (the circuit of the secondary winding of the output transformer) affects the input circuit of the amplifier.
Having determined between which two circuits a harmful interaction occurs, it is not difficult to carefully examine their installation to find the place of interconnection and to eliminate self-excitation by shielding or partially changing the installation of these circuits.
Self-excitation via HF does not always manifest itself in the form of an extraneous sound constantly heard in the loudspeaker; more often it can be judged by the presence of loud whistles when tuning into a station or by characteristic distortions, a sharp decrease in volume and other specific features. Such self-excitation can be detected using a lamp voltmeter or an electronic light indicator, which are connected in series to all oscillatory circuits cascades under study ( Fig.2 ).
Audio frequency amplifiers (AF), created and repaired by radio amateurs, often become a source of “headaches” due to the subsequent hum of alternating current with a frequency of 50 Hz, noticeable audibly in loudspeakers and telephones.
If this happens, you should check whether the microphone is correctly connected to the pre-amplifier - hereinafter referred to as the PU (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 correctly connected. The fact is that sometimes two amplifiers (preliminary and PA) are used in one device, having different polarities of the common wire. As is known, in amplifier circuitry such inclusion is not a problem—the main thing for quality amplifier compatibility of input impedance, noise level. However, incorrect (incorrect) connection of the amplifiers to each other and the pre-amplifier to the sound source (including a microphone) is often the cause of the hum with a frequency of 50 Hz.
In order to localize this problem, I propose a simple method involving the inclusion of sound sources to the pre-amplifier (this can be not only a microphone, but also another source with a low signal level of up to 10 mV). Let's sort it out this method based on the example of connecting a microphone.
The central conductor in the braided microphone cord is connected to the input of the amplifier (PA) according to the circuit, usually to a coupling capacitor, limiting resistor or voltage divider.
The braid (screen) is not connected directly to the common wire, but in series with an RC circuit, which represents a parallel connected resistor with a resistance of 2 kOhm ± 20% and an oxide capacitor with a capacity of 10 μF with the same tolerance for possible deviation from the nominal value.
Here, the resistance of the resistor and capacitor is calculated for devices with a power supply voltage of 6 to 20 V.
The positive plate of the oxide capacitor in this case is connected in accordance with the poles of the power source (PS) so that if the common wire is connected to the “minus” of the PS, then the oxide capacitor is connected to the common wire with the negative plate, and vice versa.
This method eliminates hum in most amplifiers with different power supply commons, including older tube amplifiers 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 arises after replacing a standard microphone with another (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 Ohms) to low-resistance (type MD-201).
Literature: Andrey Kashkarov - Electronic homemade products
Audio amplifiers built and repaired by radio amateurs often become a source of headaches due to the subsequent hum of alternating current with a frequency of 50 Hz, noticeable audibly 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 correctly connected. The fact is that sometimes two amplifiers (preliminary and PA) are used in one device, having different polarities of the common wire. In amplification 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 to each other and the pre-amplifier to the sound source (for example, to a microphone) is often the cause of the hum with a frequency of 50 Hz.
Practical hum elimination in amplifiers 34
To localize this problem, there is a simple way to connect sound sources to the pre-amplifier (this can be not only a microphone, but also another source with a low signal level of up to 10 mV). Let's analyze this method based on an example with connecting a microphone.
The central conductor in the braided microphone cord is connected to the input of the control unit, as a rule, to an isolation capacitor, limiting resistor or voltage divider. The braid (screen) is not connected directly to the common wire, but in series with the RC circuit (a parallel connected resistor with a resistance of 2 kOhm (±20%) and an oxide capacitor with a capacity of YumkF with the same tolerance for 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 turned on in accordance with 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 the negative plate, and vice versa
This method eliminates hum in most amplifiers with different power supply commons, including older tube amplifiers 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 arises after replacing a standard microphone with another (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 Ohms) to a low-impedance microphone type MD-201 with a coil resistance of 200 Ohms or similar in electrical characteristics.
One of the main problems that we have to contend with when designing and creating high-quality tube ULFs is the AC hum. In this case, the AC 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 power supply 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. The main reasons causing the appearance of background in tube bass amplifiers can be conditionally divided into several groups, two of which the main ones are: ripple of supply voltages and induction of alternating current into various circuits in the amplifier. Therefore, the elimination of background should be carried out in two directions, namely, improving the filtering of supply voltages and reducing the influence of interference. One of the main reasons for the appearance of background in lamp ULFs is the ripple of the rectified voltage supplying the circuits of the anodes and screen grids of the lamps. In this case, the influence of pulsations is less, the higher internal resistance lamps. As is known, the internal resistance of pentodes is greater than that of triodes, therefore, from this point of view, in the first stages tube amplifier It is better to use pentodes. In addition, the background noise arising from voltage ripple can be reduced 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 background 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 bypass the inductor with a capacitor, the capacitance value of which is selected so as to form a circuit tuned to the ripple frequency (100 Hz with full-wave rectification). Schematic diagram a filter with a circuit of this type 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 lie in the fact that either the screen grids of the lamps are powered by an insufficiently smoothed voltage, or the anode current unnecessarily loads the elements of the smoothing filter. For example, in the final stages of amplifiers, the anode and screen circuits of lamps are often supplied with voltage with the same ripple. However, the permissible screen voltage ripple for most terminal pentodes and beam tetrodes is 20-30 times less than the anode voltage ripple. Therefore, the screen grid circuits must be fed through an additional smoothing circuit.
In order to reduce the influence of leakage between the cathode and the filament, it is sometimes recommended for the first stages of the amplifier, instead of automatic bias circuits, to use a separate rectifier with a filter, with the help of which a constant bias voltage is generated, supplied to the lamp grid. Schematic diagrams possible options such rectifiers are shown in Fig. 2. Both the filament winding (Fig. 2, a) and the special winding (Fig. 2, b) of the power transformer can be used as a source of input alternating voltage.
Fig.2. Schematic diagrams of rectifiers for forming DC voltage offsets
In the process of developing, creating and setting up high-quality low-frequency tube amplifiers, the main attention should be paid to identifying and eliminating interference. The fact is that currently amateur designs ULF usually uses power supply circuits that are practically no different from industrial designs, described in detail in the literature and tested in operation. Therefore, if the elements are in good condition and there are no errors during rectifier assembly, the influence of supply voltage ripple is significantly reduced, and the cause of background noise at the amplifier output is usually AC interference.
To determine the stage that is affected by pickup, it is enough to alternately short-circuit the control grids of all amplifier lamps to the housing, starting with the first. The cessation or sharp decrease in background when the grid of one of the lamps is shorted indicates that alternating current is being induced into the grid circuit of this particular lamp. If no interference is detected in the amplifier, but hum is heard during playback, this indicates that hum voltage is supplied to the amplifier from a device connected to its input.
Compared to static AC pickups, magnetic pickups generally have less influence, except in cases where the source of the pickup is the field of a power transformer, and the object is some element of the amplifier that has a winding.
Quite often, creators of amateur tube sound-reproducing equipment have to deal with interference caused by the presence of common circuits for alternating current and signal, or the use of common circuits for alternating and direct supply voltage. For example, it is not recommended to use a shielded wire braid as one of the wires supplying the signal to the amplifier input. To supply the signal, 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 followed, the background can be significant, since the voltage induced on the braid will be supplied to the input along with the signal.
For the same reasons, high-quality tube bass amplifiers should not use the common negative wire or chassis as one of the filament wires. In Fig. 3. Examples of incorrect (a) and correct (b) installation of the first amplifier stage are given, in which the chassis serves as one of the filament wires.
Fig.3. Incorrect (a) and correct (6) installation of the first amplifier stage using the chassis as one of the filament wires
When using a 6Zh1P pentode in the first stage of an amplifier, for example, incorrect installation of the filament circuit can lead to the fact that an increase in the contact resistance of the chassis contact to 0.05 Ohm will cause a significant background to appear at the amplifier output, equivalent to applying a voltage of 3 mV to its input.
One of the simplest and, at the same time, the most effective method preventing interference 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 - enhancing rather than weakening the background. First of all, a special shielding winding is wound between the primary and secondary windings of the power transformer of the power source. In addition, the lamps of the input stages must be placed on lamp panels with special screens. All branched grid and anode circuits of the first stages, for example, any correction filters, should be carefully screened, placing all the parts of this circuit with circuit boards in a common screen.
To connect the signal source to the amplifier input, it is recommended to use shielded wires and coaxial connectors, since ordinary pin sockets and plugs, having rather large unprotected surfaces, can cause strong hum.
All parts used in background-sensitive circuits must be 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 grounding the variable resistance housings, since most often they are not connected to the potentiometer axis.
One method often used to reduce AC hum is often called compensation. Its essence is that control grid One of the amplifier stages 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 background and additional signal voltages 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 a violation of compensation. Therefore, the use of such hum elimination methods in high-quality amplifiers is undesirable.
The compensation method can also be used to reduce AC ripple in power supplies. For example, with a large rectified current, the filter choke core becomes significantly magnetized, which forces its cross-section to be increased to maintain the same inductance. However, to reduce ripple, you can wind a compensation winding around the inductor. The 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 a 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 malfunction of the rectifier elements, for example, an increase in the leakage of electrolytic filter capacitors, loss of kenotron emission, or burnout of the filament of one of the kenotron diodes. Therefore, before turning on the compensation winding, you should make sure that all elements of the rectifier are in good condition.
One option for using the compensation method is to supply an anti-phase signal to the cathode of the lamp of the last stage of the pre-amplifier. A schematic diagram of such a cascade is shown in Fig. 5.
Fig.5. Schematic diagram of a compensation circuit with an antiphase signal supplied 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 a circuit with an artificial midpoint. This signal is fed through the R4C2 chain to the cathode of the lamp of the last pre-amplifier stage. While working with the amplifier, by adjusting potentiometer R5, you can set the minimum background level by ear.
One of the options for reducing AC background compensation in the final stage of a low-frequency tube amplifier with a transformer output is to use an additional inductor 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. The 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 smoothing filter inductor
The number of turns of the additional choke winding depends on the resistance of the speaker 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 experimentally by changing the order of connecting the terminals.
Naturally, this compensation method can only be used if a smoothing choke is used in the power supply circuit. In addition, with the help of the considered circuit, only that background component that is excited in the output stage is compensated. Therefore, this method of compensating for alternating current background is not widely used.
AC background
Reasons leading to the appearance of AC background:
- Contact with AC power circuits into low frequency stages.
- Influence of electric and magnetic fields to low-frequency circuits due to poor placement of individual wires and parts.
- Background overlay on high frequency circuits or a modulating hum, audible only when the receiver is tuned to a radio station.
The presence of a constantly audible background indicates that it is superimposed in one way or another on the low-frequency circuit of the receiver. Therefore, first of all, you should check whether the DC ripple is sufficiently smoothed out by the rectifier filter. For this purpose, a calibrated high-voltage capacitor with a capacity 40-100 µF connected in parallel first to the second and then to the first capacitors of the smoothing filter of the receiver or amplifier being repaired.
If this gives the desired effect, then you need to replace one or both of the anti-aliasing filter capacitors or increase the capacitance of the capacitors in the anode or grid decoupling filters. If such an event does not cause a noticeable weakening of the background, then most likely there is a second reason.
To quickly detect in which low-frequency cascade the background is superimposed, remove all the lamps one by one, starting from the input and up to the pre-terminal one, and monitor which one of them stops the background when removed.
The final stage lamps cannot be removed when the power is on., since the sharp decrease in rectifier load caused by this leads to a significant increase in the anode voltage, which in turn can cause breakdown of the smoothing filter capacitors.
Common causes of background noise due to interference are breaks in the shielding shells and the appearance of a leak between the filament and the cathode at the input lamp of the low-frequency amplifier. The cause of the modulating background can also be poor pulsation smoothing voltages supplying high-frequency lamps. The input stages of receivers (RF amplifier and converter), as well as the local oscillator, are especially sensitive to this, and therefore an additional smoothing filter cell is sometimes installed to power these stages.
The modulating background of alternating current, audible only when receiving local stations, is easily eliminated by blocking the anode of the kenotron to its cathode or ground ( pic. 1 ), as well as blocking the shoulders of the step-up winding of the transformer with capacitors with a capacity 0.005-0.01 µF; the operating voltage of these capacitors must be no less than triple the voltage of the arm of the step-up winding of the power transformer ( 1000-1500 V).
Before eliminating the background that appears when receiving radio stations, you need to make sure that the background modulation occurs in the receiver and not in the transmitter. To do this, it is best to check the reception of the same radio station using another receiver.
Rice. 1. Elimination of modulating background
Particular attention should be paid to methods for eliminating the background in equipment with direct incandescent lamps when their filaments are powered with alternating current. It is necessary here precise balancing of the filament circuit, which is not always ensured by the device for tapping the midpoint of the filament winding. A more effective measure is to include a low-resistance potentiometer between the terminals of the filament, the slider of which should be considered as a terminal from the cathode of the lamp. Precise balancing of the thread is carried out when the power is turned on by ear by setting the potentiometer slider to a position in which the alternating current background is least audible.
A similar measure can significantly reduce the background coming from the filament circuits in low-frequency amplifiers with a high gain (in tape recorders, microphone amplifiers). If the device is installed again, the background noise may be caused by poor placement of individual circuits and transformers.
It is important to identify not only which circuit is affected by the unwanted influence, but also which circuit produces this influence. To do this, we apply a method for changing the reactivity of subsequent circuits, which consists in connecting a capacitor of larger or smaller capacity to the anode load resistances of the lamps, starting from the output of the receiver, and so gradually approaching the source of self-excitation or its complete cessation.
Let's assume that connecting a capacitor to the output transformer only reduced the volume without changing the nature of self-excitation. This means that the final stage is not covered by self-excitation and the circuit that creates an undesirable effect on the amplifier input must be sought before it. But, if, for example, when a capacitor is connected parallel to the primary winding of the output transformer, self-excitation is removed or its character changes, then either this circuit or the subsequent one (the circuit of the secondary winding of the output transformer) affects the input circuit of the amplifier.
Having determined between which two circuits a harmful interaction occurs, it is not difficult to carefully examine their installation to find the place of interconnection and to eliminate self-excitation by shielding or partially changing the installation of these circuits.
Rice. 2. Electronic light indicator
- Short probe
- Power hose
- No hesitation
- There are fluctuations.
Self-excitation via HF does not always manifest itself in the form of an extraneous sound constantly heard in the loudspeaker; more often it can be judged by the presence of loud whistles when tuning into a station or by characteristic distortions, a sharp decrease in volume and other specific features. Such self-excitation can be detected using a lamp voltmeter or an electronic light indicator, which are connected in series to all oscillatory circuits of the cascades under study ( pic. 2 ).
