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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Amplifier on the chip TDA7294. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers Introduction Amplifier design has always been a challenge. Fortunately, in recent years, many integrated solutions have appeared that make life easier for amateur designers. I also did not complicate the task for myself and chose the most simple, high-quality, with a small number of parts, requiring no tuning and stable operation of the amplifier based on the TDA7294 chip from SGS-THOMSON MICROELECTRONICS. Recently, on the Internet, I have spread claims to this microcircuit, which were expressed approximately as follows: "spontaneously excited, with incorrect wiring; it burns, for any reason, etc." Nothing like this. You can burn it only by improper switching on or shorting, and cases of excitation have never been noticed, and not only with me. In addition, it has internal protection against short circuits in the load and protection against overheating. It also has a mute function (used to prevent clicks when turned on) and a standby function (when there is no signal). This IC is a ULF class AB. One of the main features of this microcircuit is the use of field-effect transistors in the preliminary and output amplification stages. Its advantages include high output power (up to 100 W at a load of 4 ohms), the ability to work in a wide range of supply voltages, high technical characteristics (low distortion, low noise level, wide operating frequency range, etc.), the minimum required external components and low cost. Main characteristics of TDA7294:
Company description and typical wiring diagrams from the manufacturer (PDF format). There are a lot of schemes for switching on this microcircuit, I will consider the simplest one. Typical switching circuit
Item List
The microcircuit must be installed on a radiator with an area of \u600e XNUMX cm2. Be careful, on the microcircuit case there is not a common, but a power minus! When installing a chip on a heatsink, it is better to use thermal paste. It is advisable to lay a dielectric between the microcircuit and the radiator (mica, for example). For the first time, I did not attach any importance to this, I thought, why would I be so scared to close the radiator to the case, but in the process of debugging the design, the tweezers that accidentally fell from the table shorted the radiator to the case. The explosion was great! Chips just smashed to pieces! In general, I got off with a slight fright and $ 10 :). On the board with an amplifier, it is also desirable to supply powerful electrolytes of 10000 microns x 50V, so that at power peaks the wires from the power supply do not give voltage drops. In general, the larger the capacitance of the capacitors on the power supply, the better, as they say, "you can't spoil the porridge with oil." Capacitor C3 can be removed (or not installed), I did just that. As it turned out, it was precisely because of him that when the volume control (a simple variable resistor) was turned on in front of the amplifier, an RC circuit was obtained, which mowed high frequencies when the volume was increased, but in general it is needed to prevent excitation of the amplifier when ultrasound is applied to the input. Instead of C6, C7, I put on the board 10000mk x 50v, C8, C9, you can put any close denomination - these are power filters, they can be in the power supply, or you can solder them by surface mounting, which I did. Plateau I personally do not really like to use ready-made boards, for one simple reason - it is difficult to find exactly the same size elements. But in an amplifier, the wiring can greatly affect the sound quality, so it's up to you which board to choose. Since I assembled the amplifier immediately for 5-6 channels, respectively, the board immediately for 3 channels:
In vector format (Corel Draw 12) Amplifier power supply, low-pass filter, etc. Power supply unit For some reason, the power supply of the amplifier raises many questions. In fact, just here, everything is quite simple. The transformer, diode bridge and capacitors are the main elements of the power supply. This is enough to assemble the simplest power supply.
To power the power amplifier, voltage stabilization is unimportant, but the capacitance of the capacitors for power is important, the more the better. The thickness of the wires from the power supply to the amplifier is also important. My power supply is implemented as follows:
The +-15V supply is designed to power the operational amplifiers in the preliminary stages of the amplifier. You can do without additional windings and diode bridges by powering the stabilization module from 40V, but the stabilizer will have to dampen a very large voltage drop, which will lead to significant heating of the stabilizer microcircuits. Stabilizer microcircuits 7805/7905 are imported analogues of our KREN. Variations of blocks A1 and A2 are possible:
Block A1 is a power supply noise suppression filter. Block A2 - block of stabilized voltages + -15V. The first alternative is easy to implement, for powering low-current sources, the second is a high-quality stabilizer, but it requires an accurate selection of components (resistors), otherwise you will get "+" and "-" arms skew, which will then give zero skew on operational amplifiers. Transformer The power supply transformer for a 100W stereo amplifier should be approximately 200W. Since I was making a 5-channel amplifier, I needed a more powerful transformer. But I didn’t have to pump out all 100W, and all channels cannot simultaneously take power. I came across a TESLA transformer on the market (below in the photo) watt commercial for 250 - 4 windings with 1,5 mm wire at 17V and 4 windings at 6,3V each. By connecting them in series, I got the necessary voltages, though I had to rewind two windings at 17V a little in order to get the total voltage of the two windings ~ 27-30V, since the windings were on top - it was not difficult. A great thing is a toroidal transformer, these are used to power halogens in lamps, there are a lot of them in markets and stores. If structurally two such transformers are placed one on top of the other, the radiation will be mutually compensated, which will reduce interference on the amplifier elements. The trouble is that they have one 12V winding. In our radio market, you can make such a transformer to order, but this pleasure will be well worth it. In principle, you can buy 2 transformers for 100-150W and rewind the secondary windings, the number of turns of the secondary winding will need to be increased by about 2-2,4 times. Diodes / diode bridges You can buy imported diode assemblies with a current of 8-12A, this greatly simplifies the design. I used KD 213 pulse diodes, and made a separate bridge for each arm to give a current margin for the diodes. When turned on, powerful capacitors are charged, the current surge is very significant, at a voltage of 40 V and a capacitance of 10000 μF, the charging current of such a capacitor is ~ 10 A, respectively, along two arms 20A. In this case, the transformer and rectifier diodes briefly operate in short circuit mode. The breakdown of diodes by current will give unpleasant consequences. The diodes were installed on the radiators, but I did not find any heating of the diodes themselves - the radiators were cold. To eliminate power supply interference, it is recommended to install a capacitor ~ 0,33 μF type K73-17 in parallel with each diode in the bridge. I really didn't do it. In the + -15V circuit, you can use bridges of the KTs405 type, for a current of 1-2A. Design
Finished design The most boring occupation is the body. As a case, I took an old slim case from a personal computer. I had to shorten it a little in depth, although it was not easy. I think that the case turned out to be successful - the power supply is located in a separate compartment and you can put 3 more channels of amplification into the case freely. General construction
After field tests, it turned out that it is not out of place to put the fans on the radiators, despite the fact that the radiators are very impressive in size. I had to make holes in the case from the bottom and top, for good ventilation. The fans are connected through a 100Ω 1W trimmer at the lowest speed (see the following figure). Amplifier block
Chips are on mica and thermal paste, the screws also need to be isolated. The heatsinks and the board are screwed to the case through dielectric racks.
Input circuits I really wanted not to do this, only with the hope that this is all temporary ....
After hanging these guts, a small hum appeared in the speakers, apparently something was wrong with the "ground". I dream of the day when I will throw it all out of the amplifier and use it only as a power amplifier. Adder board, low-pass filter, phase shifter
Regulation block
Experience the Power of Effective Results The back turned out to be more beautiful, even though you turn it booty forward ... :)
Construction cost
Yes, something came out cheap. Most likely, I didn’t take into account something, I just bought, as always, a lot more, because I still had to experiment, and I burned 2 microcircuits and blew up one powerful electrolyte (I didn’t take all this into account). This is a calculation of the amplifier for 5 channels. As you can see, the heatsinks turned out to be very expensive, I used inexpensive, but massive coolers for processors, at that time (a year and a half ago) they were very good for cooling processors. If you take into account that an entry-level receiver can be bought for $240, then you can think about whether you need it :), though there is a lower quality amplifier there. Amplifiers of this class cost about $500. Author: Novik P.E.; Publication: pavel.artmech.com
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Comments on the article: Victor Was it worth it to design all this when I bought speakers with such an amp built in for 3500 DNS. There, however, the “top” is made by a shameful company, so I changed a couple of HF conduits and added silver to the wires and everything was type-top. So I don’t think that designing is a good idea, you can just bring the purchased products to mind - it will be cheaper and less trouble, I think so.
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