TREEPER OVERLOAD PROTECTION
As a rule, if the speaker system is designed correctly and properly operated, then there will be no reliability problems. However, quite often some “lovers” of music turn on their acoustics, as they say, “to the fullest.” In this case, not only the immediate neighbors suffer, but also the entire house. Very often, in such a situation, the speakers cannot stand it and burn out, with high-frequency speakers burning most often. Why do tweeters burn most often? Well, firstly, it’s not always the high-frequency drivers that burn; sometimes the bass and midrange drivers burn. But still, (secondly) – the beepers light up quite often!
When the rated power is applied to the speaker, the voice coil heats up to a temperature of approximately 90-100 O With (sometimes more) it is quite clear that this heat(this reference data is taken from the book by I. Aldoshina “Electrodynamic Loudspeakers”). Moreover, the voice coils of low-frequency, mid-frequency and high-frequency speakers heat up at different rates, this is due to the so-called "thermal time constant" dynamics. For a woofer with a rated power of more than 30 W, the thermal constant is 15-20 seconds, i.e. When the rated power is applied to the speaker, the voice coil will heat up to the design temperature in 15-20 seconds. A mid-range speaker with a rated power of 15-25 watts has a thermal time constant of approximately 5-6 seconds. And finally, the tweeter has a thermal constant of about one and a half seconds! This means that if the tweeter is overloaded, the voice coil will burn out in almost one second. That’s why the tweeters “fly out” quite often.
Obviously, the thermal time constant depends on the frequency of the signal, but it also depends on the diameter of the wire with which the voice coil is wound. So, for low-frequency speakers the voice coil is usually wound with wire Ø( 0.25-0.35) mm, for mid-frequency – Ø (0.14-0.16) mm, for tweeters, wire diameter Ø 0.10 mm or slightly less. The thinner the wire, the lower the thermal constant and, accordingly, the less time it will take for the speaker to fail due to overload. Let's compare three high-frequency speakers of the same power with different impedances: 6GDV-4-8 (impedance 8 Ω), 6GDV-6-16 (16 Ω) and 6GDV-6-25 (25 Ω). A speaker with an 8 Ω impedance has a wire-wound voice coil Ø 0.10 mm, for a speaker with a resistance of 16 Ω, the voice coil is wound with wire Ø 0.08 mm, and the speaker with a resistance of 25 Ω uses an even thinner wire. In the context of the above, it is obvious that under the same overloads, the speaker with a resistance of 25 Ω will be the first to “burn out”, as the most unreliable of the three speakers mentioned here. And the most reliable of this trinity is a speaker with a resistance of 8 Ω (i.e. 6GDV-4-8).
Speakers burn not only from overload while listening to loud music. Sometimes this happens due to imperfect power amplifiers. When the power is turned on, so-called “transient processes” occur in the final power amplifier, due to which the voltage at the amplifier output can fluctuate for (1-2) seconds. Moreover, the amplitude of such an oscillation can approach the supply voltage of the final amplifier, and this amounts to ± (20-40) c. In this case, a loud clicking sound is heard from the speakers when the power is turned on. Similar transient processes occur when the power is turned off. So, very often one such “click” is enough to burn out a high-frequency speaker. Many old-style power amplifiers have this drawback, especially the 70s amplifier “Radiotekhnika UKU-020” from the Riga Radio Plant. In modern amplifiers, these shortcomings are eliminated by the fact that the speakers are connected to the output of the power amplifier through the contacts of a relay, which turns on with a delay of 3-4 seconds after the supply voltage is applied, and turns off immediately after turning off. As a result, transients in the power amplifier are not transmitted to the speakers.
In pop acoustics, high-frequency speakers, as a rule, are connected directly to a separate amplifier channel, i.e. without traditional separation filters. It is often not possible to control the power supplied to the high-frequency channel in such a situation, so the reliability (and overload protection) of high-frequency speakers in pop acoustics is a much more pressing problem.
IN general outline the problem is identified. Let's talk about one thing here in an interesting way protection of high-frequency speakers from overload.
Some modifications of S-30 type speaker systems use an overload indicator; when an overload occurs, the LED on the front panel of the speaker system lights up. However, this system is only an indicator; it only informs about overload, but does not protect the speakers from it.
IN speaker systems of the highest class of complexity “Cleaver 150AC-009” and “Corvette 150AC-001” the following speaker protection system is used. If an overload occurs, an additional resistance is connected in series to the speaker using a relay, and as a result, the power to the speaker is reduced. A similar system is applied separately to the high-frequency and mid-range speakers in the mentioned speakers. The woofer in these systems is connected via a fuse. The interested reader can find these diagrams in reference books or in data sheets for these speaker systems.
Some radio amateurs often use incandescent lamps to protect high-frequency speakers, which must be connected in series with the speaker (we are talking about miniature low-voltage incandescent lamps), in Fig. 1 shows such a diagram.
This protection system works as follows. At low powers, a small current flows through the load, because of this the filament of the lamp does not heat up, and therefore the lamp resistance is quite low. In such a situation, the lamp has almost no effect on the operation of the tweeter. If the power increases and the current through the load increases, this leads to the fact that the filament of the lamp becomes hot, the lamp begins to glow, and the resistance of the lamp increases sharply. From the diagram it can be seen that the lamp with the speaker is a divider, as it turns out, with a variable division coefficient. The greater the current through the load, the greater the lamp resistance, and the greater the voltage drop across the lamp U l, respectively, the voltage drop across the speaker U d– decreases relative to the total voltage U O, i.e. The power on the speaker is automatically limited, which means that the protection system is triggered. It's almost like a “compressor-limiter”!
The principle of operation of such a protection system is quite simple, however, how to calculate the parameters of the lamp? In other words, how to choose the right voltage of an incandescent lamp and its power? This is what is called the “essential” question, and this is what we will do next.
Rice. 1. Connection diagram for an incandescent lamp to protect the tweeter from overload. RF – high-frequency section filter, L – incandescent lamp (R l– lamp resistance), Gr. – Tweeter (R G– speaker impedance), U l(c) – lamp voltage, U d(c) – voltage on the speaker, U O(c) – total voltage across the load. Explanations in the text.
Here we will outline the “Simplified calculation” of the parameters of an incandescent lamp, which provides protection against 4-fold overload of the tweeter and the so-called “Verification calculation”. The verification calculation will be of interest to mathematics lovers. It represents a complete and general calculation, which allows one to calculate, for an arbitrarily given lamp, a kind of “overload characteristic” of the protection system, i.e. permissible overload value and degree of signal attenuation when various levels power.
SIMPLIFIED CALCULATION
We will demonstrate the calculation on a specific dynamics. Let's take, for example, the high-frequency speaker 6GDV-6-25; this 25-Ohm speaker from the Riga radio plant is used in some modifications of the S-90 and S-100 systems with a total speaker impedance of 8 Ω.
Let's assume that its rated power is 6 W, and its total resistance is 25 Ω. Let's imagine for a moment that the speaker is connected directly to the amplifier, and ask the question: “At what voltage will this speaker consume power equal to the rated power, i.e. 6 W"? Calculating this voltage is very simple:
where: N n(W) – rated speaker power, R G
It is quite clear that if a voltage of 12 volts is applied to this speaker, then the power consumed by it will be 6 watts. It is also obvious that if the voltage is applied to the speaker twice as much, i.e. 24 volts, then the power on the speaker will increase 4 times! This is because the power across a speaker (or any other load) is proportional to the square of the voltage:
where: N (W) – speaker power, U d(c) – voltage on the speaker, R G(Ω) is the impedance of the speaker.
Thus, in this particular case, the use of a lamp with an operating voltage of 12 volts and a power of 6 watts protects the 6GDV-6-25 speaker from 4-fold overload.
Let's voice the general formulation. To provide protection against 4x overload, the power of the incandescent lamp must be equal to the rated power of the tweeter, and the operating voltage of the lamp must be equal to the voltage at which the speaker consumes the rated power. So, the whole calculation comes down to just one formula, namely formula (1).
Obviously, the use of an incandescent lamp as protection will lead to some weakening of the sound pressure of the tweeter. The simplified calculation shown here does not allow us to determine the degree of sound pressure attenuation at different powers. For radio amateurs who want to know full description such a protection system, we recommend that you familiarize yourself with the “Verification calculation”.
VERIFICATION CALCULATION
The incandescent lamp is in in this case variable resistance and provides protection for the tweeter. In order to mathematically calculate a kind of “overload characteristic” of such a protection system, you need to know the characteristics of the lamp, namely, you need to know<Зависимость сопротивления лампы от напряжения на лампе>.
A few words need to be said about the designations of miniature incandescent lamps. The characteristics of a lamp are always indicated by two parameters. There are two ways to designate incandescent lamps: either voltage and power, or voltage and current. Let's give examples. So, a “12V x 4W” lamp has an operating voltage of 12 volts and a power of 4 watts. Another example, a “6.5V x 0.3A” lamp is designed for an operating voltage of 6.5 volts and an operating current of 0.3 amperes. Obviously, knowing the operating current and voltage of the lamp, you can always calculate the power of the lamp (we will show this using the example of a “6.5V x 0.3A” lamp):
where: N l(W) – incandescent lamp power, U RL(v) – lamp operating voltage, I RL(A) – operating current of the lamp.
Before proceeding with the calculation of the protection system, as already mentioned, let us determine experimentally the so-called<характеристику лампы>incandescent (i.e., the dependence of the lamp resistance on the voltage across the lamp). This is done as follows. The incandescent lamp should be connected to a power source, then you need to change the voltage on the lamp and at the same time measure the current flowing through the lamp (it makes no sense to show the diagram here due to simplicity). The voltage can vary from zero to a maximum value, which is equal to the operating voltage of the lamp. Thus, we get a dependence<тока лампы от напряжения на лампе>. Now it remains to calculate the lamp resistance using Ohm's law:
where: R l(Ω) – incandescent lamp resistance, U l(c) – lamp voltage, I l(A) is the current flowing through the lamp.
Using the described method, we obtain the characteristics for the following seven miniature incandescent lamps: 3.5V x 0.26A, 6.5V x 0.3A, 6V x 5W, 12V x 1.5W, 12V x 4W, 12V x 10W and 26V x 0.12A .
A small but very useful electronic fuse for expensive speaker tweeters can be assembled in just an hour using a dozen parts. This circuit in your tweeter is triggered when the voltage level applied to it is near the maximum permissible level. The first circuit uses a simple light bulb as a load. This lamp will light when the tweeter signal reaches a certain threshold level.
Scheme 1
Here the lamp functions as a resistor with a positive temperature coefficient(PTK) - its meaning is that the resistance increases in proportion to its temperature. The 2N3055 transistor will conduct the signal, thereby preventing the tweeter from overloading.
Scheme 2
The second scheme is an improved version. A fixed resistor replaces the light bulb and the switch is implemented using a composite Darlington transistor delayed by a capacitor. The scheme works just like the first one. However, due to the capacitor, the circuit will not respond to single peak overloads.
Printed circuit board
How to repair a speaker yourself? FAQ Part8
Here you will find a description of the process of restoring the high-frequency dynamic head with illustrations.
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But, if you do not have very good eyesight, you will have to use additional optics. The diameter of the wire used to wind the tweeter coils is usually less than 0.1mm.
In particular, the 4GDV-1 speaker coil is wound with wire with a diameter of only 0.08 mm. In such cases, I use binocular glasses with additional attachment lenses.
The paper tape securing the coil terminals turned out to be glued with 88-grade glue. To avoid damaging the sleeve, when removing the old coil, I soaked the adhesive joint only in those places where the coil leads were to be laid.
After laying the leads, I closed the ends of the tape and glued them with BF glue.
Assembling the speaker is done in the reverse order and is not difficult, since the alignment of the moving system is ensured by the speaker design itself.
Before final assembly You can check the phasing of the speaker, since, with such a small stroke of the moving system, it is more difficult to do this after assembly.
With correct phasing, the moving system should “jump” out of the housing.
You can remove varnish from the coil terminals using an Aspirin tablet. I already told how this can be done, but I showed it.