Professional power amplifiers. Transistor low-frequency amplifier with low dynamic distortion (20 watts) Transformer low-frequency amplifier 20 watts
ULF for 20 watts (like a question for filling...) I needed a more powerful ULF for my EMR. I decided to assemble it myself. I reviewed a bunch of diagrams. I chose the one most suitable for me, with the necessary parameters, not very complicated, with protection for the output transistors and for my existing parts. Designed a signet. Well, along the way, the power supply with the stabilizer was also “bungled”. BP launched. I soldered the ULF (see photo). Turned it on and... the power transistors (KT819) immediately heated up and failed. It seems that I checked all the circuits on the short circuit beforehand, and the power supply also has protection (and the ULF itself also has one). Replaced the transistors. I checked all the circuits again for short circuit - everything is fine. I set the device to measure current consumption and turned it on for a second (the resistor that regulates the quiescent current was previously set to minimum). The current... is wild (it goes off scale... but there is no "short circuit"). In short, I've been fiddling with this amp for a long time, and I have experience - I've run similar ones more than once, but here - well, as a first-grader - I can't! By the way, I installed Soviet transistors (not soldered), but recommended ones. I won’t torment you for long - of course, I started this amp and it worked fine. But I decided not to reveal my setup progress yet. But I’ll give you a diagram and description of this amp (from the book). Let's organize a brainstorming session and try to collectively (even if in absentia) figure out this amplifier - why it didn't work immediately after assembly and what needs to be done to get it to work. Agree, radio components are now expensive - it’s a pity for power transistors... Well, then, if people can’t solve this riddle problem, I’ll tell you what’s wrong with this circuit... why it doesn’t start, what are the reasons for the failure of power transistors and, in general, how it should be launched. Sincerely! Rubtsov V.P. UN7BV. 04/28/2017 Astana, Kazakhstan. ULF debugging So, now about setting up this amplifier and what’s wrong with it... In this amplifier I used the following transistors: T9 and T11 - KT819VM, T8 - KT817, T10 - KT816, T1, T2, T7 - KT502, T4 , T5 – KT630, T3, T6, T7 – KT503. I checked the amplifier circuit for absence of a short circuit in the power supply circuits and applied power to it. Nothing was heard at the output (in the speaker). While I was trying to take measurements of current and voltage at control points, the output transistors burned out. Replaced with new ones. I checked for errors in the circuit diagram and the signet, as well as the design of the signet and with the real signet (checked) - I found no errors. I checked the circuit again for “short circuit” and turned on the ULF power supply for a few seconds through the tester’s ammeter (limit 1A) - it was off scale. I tried to set the quiescent current - nothing worked. But half the supply voltage at the connection point R17, R21 was set normally (+15 volts). I looked at the output with an oscilloscope - boom, two frequencies of enormous amplitude at once: 1...2 Hz and 15 kHz. And what’s interesting is that none of these frequencies can be heard at the output (previously I heard up to 17 kHz - in Moscow in a medical center in a room completely isolated from sounds, they checked it, but now, apparently, age has taken its toll...). The ULF excitation at a frequency of 15 kHz was removed by introducing a capacitor with a capacity of 36 pF between the base and collector T5 (negative feedback on alternating current). But with excitation at a frequency of 1...2 hertz, it’s more complicated. Assuming that this was the work of the protection circuit, I turned it off and removed T3, T6 and T7. Alas, the excitement remains. Then I reviewed (and compared) the circuit of this ULF with similar ones, and at the same time reread the theory of operation of this amplifier in a new way (refreshed my memory - maybe I’ve already forgotten something...). I came to the conclusion that it is necessary to install a resistor with a nominal value of 1.2 kOhm in the circuit between the nodes of the connection points C7, Gr and R4, R5, C5. I installed it and the excitation at a frequency of 1...2 hertz immediately stopped. I set the quiescent current of output transistors T9 and T11 to 50 mA. Gave a signal to the input. The signal at the ULF output is very weak, although up to 1 volt was supplied to the input. I looked where it disappeared - on the T1 collector its amplitude is much less than on the base. I removed the modes of this transistor relative to the positive power bus (!!!). At the base there are 10 volts, at the emitter 14 volts, that is, the transistor is “wildly” locked. And only after that it dawned on me that I was reading the constant modes with a tester, and its resistance was 20 kOhm (the base circuit is very high-resistance - the device bypasses this circuit) - this means that the problem here is not in the constant modes, but in something else. I measured it with an oscilloscope - the modes turned out to be normal (the oscilloscope input is high-impedance). But the cascade still doesn’t work. Okay, then I tried to eliminate the negative AC feedback arising from the presence of resistor R8 (12 kOhm) - I bypassed it with a 0.47 μF capacitor (!). The amplifier started working immediately (but similar amplifiers work fine without this...). Once again I carefully compared the circuit of this amplifier with similar others. I removed this additional capacitor, but at the same time replaced capacitor C4 with a capacity of 4.7 microfarads with 47 microfarads (50 volts). That's it... the amplifier worked as it should - the power is sufficient (both audible and visible from the current), and the output sine wave is symmetrical. And when I was putting away all the used literature, I came across an old Radio magazine (1973 No. 8), where I discovered the same circuit (I accidentally flipped through it...), but with my own “modifications” (except for the capacitor at 36 pF), which I reached... through “one place”. It was a shame - when I was looking through magazines to look for similar ULF circuits, I simply ignored the old magazines (you’re unlikely to find what you need in the old ones). I thought that he was smart and cunning - the diagram of this ULF was taken from a book published in 1982, and the magazine was published in 1973 (I am posting an article from it - see the scan) ... there is almost a ten year difference. And that’s how it turned out... It was in vain that he ignored the old times... oh, in vain! So I’m thinking, maybe someone will find my “games” of “starting up vintage amplifiers” useful? Or, maybe, for you, dear colleagues, everything starts immediately after assembly? Unfortunately, I… don’t! 04/29/2017 Rubtsov V.P. UN7BV. Astana, Kazakhstan.
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This automatic light switch circuit will automatically turn on the lights at night and turn them off in the morning. An LDR photoresistor is used as a light sensor. Any lamps (fluorescent, incandescent...) can be connected to the circuit. The basis of the circuit breaker is a Schmitt trigger on a 555 timer. The LDR and the 555 timer are used together for automatic switching. Light …
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This example shows the possibility of interaction between php and Arduino. The test is carried out on Ubuntu 14.04, Apache 2 web server, php 5.5 is installed. The test tested turning the digital output on and off, as well as polling the output status using php. test.php