Background alternating current. In this case, the background of the alternating current is understood as the voltage existing at the output of the amplifier, in addition to the useful signal, which has

frequency equal to or a multiple of the frequency of the mains voltage. The presence of the considered alternating current 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 for the appearance of a background in low-frequency tube amplifiers can be conditionally divided into several groups, two of which are the main ones: supply voltage ripples and alternating current pickups on 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 tube ULFs is the ripple of the rectified voltage that feeds the circuits of the anodes and screen grids of the lamps. 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, in the first stages tube amplifier better to use pentodes. 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 520 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). A schematic diagram of a filter with this type of circuit is shown in fig. 3.34.

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.

often powered by voltage with the same ripple. However, the allowable screen voltage ripple for most terminal pentodes and beam tetrodes is 2030 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 options such rectifiers are shown in Fig. 3.35.

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...

1. Is the microphone properly connected to preamplifier(PU) - 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.
2. 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 own level amplifier noise. 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.
3. Wiring printed circuit board the amplifier must be wired so that the power paths converge to one point - on large capacitors (power filters).
4. 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 way 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.

AC background

Causes leading to the appearance of an alternating current background:

• Contact with AC power circuits into low frequency stages.
• Influence of electric and magnetic fields on low-frequency circuits, due to the unfortunate arrangement of individual wires and parts.
• Background overlay on high frequency circuits or a modulating hum that is only audible 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 ripples are sufficiently smoothed out. direct current rectifier filter. To do this, a verified high-voltage capacitor with a capacity of 40-100uF connected in parallel, first to the second, and then to the first capacitors of the smoothing filter of the repaired receiver or amplifier. If this gives the desired effect, then one or both of the filter capacitors should be replaced or the capacitors in the anode or grid decoupling filters should be increased. If such an event does not cause a noticeable weakening of the background, then most likely the second reason takes place.

In order to quickly find out in which low-frequency cascade the background is superimposed, all the lamps are taken out one by one, starting from the input and up to the terminal one, and watching which one of them stops the background when it is removed.

The final stage lamps cannot be removed when the power is on., since the sharp decrease in the load of the rectifier 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 due to pickup are breaks in the shielding shells, the appearance of a leak between the filament and the cathode at the input lamp of the bass amplifier. The cause of the modulating background can also be a bad ripple smoothing voltages supplying high-frequency lamps. The input stages of the receivers (HF amplifier and converter), as well as the local oscillator, are especially sensitive to this, and therefore an additional smoothing filter cell is sometimes arranged to power these stages.

The alternating current modulating background, heard 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 arms of the step-up winding of the transformer with capacitors with a capacitance 0.005-0.01uF; the operating voltage of these capacitors must be at least three times the voltage of the step-up winding arm of the power transformer ( 1000-1500 V).

Before eliminating the background that appears when receiving radio stations, you need to make sure that the hum modulation occurs in the receiver, and not at the transmitter. To do this, it is best to check the reception of the same radio station using another receiver.

Of particular note are the ways to eliminate the background in equipment with direct incandescent lamps when their filaments are fed with alternating current. Here it is necessary precise balancing of the heating circuit, which is not always provided by the device for removing the midpoint of the filament winding.

A more effective measure is to connect a low-resistance potentiometer between the leads of the filament, the slider of which should be considered as a lead from the cathode of the lamp. Precise balancing of the thread is carried out with the power on by ear by setting the potentiometer slider to the position at which the alternating current background is the least audible.

A similar measure can significantly reduce the background penetrating from the filament circuits in low-frequency amplifiers with a high gain (in tape recorders, microphone amplifiers). If the device is remounted, then the background interference may be caused by the unfortunate arrangement of individual circuits and transformers.

It is important to identify not only which circuit is affected by the undesirable influence, but also which circuit produces this influence. To do this, we apply the method of changing the reactivity of subsequent circuits, which consists in the fact that a capacitor of a larger or smaller capacity is connected in turn to the anode load resistances of the lamps, starting from the output of the receiver, and so they gradually approach the focus of self-excitation or its complete cessation.

Let us assume that the connection of a capacitor to the output transformer only reduced the volume, without changing the nature of the self-excitation. This means that the final stage is not covered by self-excitation and the circuit that creates an undesirable effect on the input of the amplifier must be sought before it. But, if, for example, when connecting a capacitor in parallel primary winding output transformer self-excitation is removed or its nature changes, then either this circuit or the subsequent one (the circuit of the secondary winding of the output transformer) affect the input circuit of the amplifier.

Having determined between which two circuits a harmful interaction occurs, it is easy to carefully examine their installation to find the place of the relationship and by shielding or partially changing the installation of these circuits, eliminate self-excitation.

RF self-excitation far from always manifesting itself in the form of constantly heard in the loudspeaker extraneous sound, more often it can be judged by the presence of loud whistles when tuning to a station or by characteristic distortions, a sharp decrease in volume and other specific features. You can detect such self-excitation using a lamp voltmeter or an electronic light indicator, which are connected in series to all oscillatory circuits investigated cascades ( pic.2 ).

Audio frequency amplifiers (AF), 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 and telephones.

If this happens, then you should check whether the microphone is correctly connected to the preamplifier - then 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 connected correctly. The fact is that sometimes two amplifiers (preliminary and PA) are used in one device, having different polarities of the common wire. As you know, in amplifying circuitry, such inclusion is not a problem - the main thing for a high-quality amplifier is the compatibility of the input resistance, the noise level. However, the incorrect (incorrect) connection of the amplifiers between themselves and the preamplifier to the sound source (including the microphone) is often the cause of the background with a frequency of 50 Hz.

In order to localize this problem, I propose a simple method 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 center conductor in the braid of the microphone cord is connected to the input of the amplifier (PU) according to the scheme, as a rule, to a separating capacitor, limiting resistor or voltage divider.

The braid (shield) is not connected directly to the common wire, but in series with an RC circuit, which is a 2 kΩ ± 20% resistor and a 10 μF oxide capacitor connected in parallel 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 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 (for example, MD-47, equipped with a matching transformer and having a resistance of 1600 Ohm) to low-resistance (MD-201 type).

Literature: Andrey Kashkarov - Electronic homemade products