FAQ on the TDA7293/7294 chip. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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In this FAQ, we will try to consider all issues related to the recently popular VLF TDA7293 / 7294 chip. The information is taken from the Soldering Iron website forum topic of the same name, forum.cxem.net/index.php?showtopic=8669. I gathered all the information together and designed ~ D'Evil ~, for which many thanks to him. Microcircuit parameters, switching circuit, printed circuit board, all this is here.

1) Power supply
Oddly enough, but for many problems begin already here. The two most common mistakes are:
- Single supply
- Orientation to the voltage of the secondary winding of the transformer (effective value).

Here is the power supply schematic

(click to enlarge)

1.1 Transformer - should have two secondary windings. Or one secondary winding with a tap from the midpoint (very rare). So, if you have a transformer with two secondary windings, then they must be connected as shown in the diagram. Those. the beginning of one winding with the end of another (the beginning of the winding is indicated by a black dot, this is shown in the diagram). Mix it up, nothing will work. When both windings are connected, we check the voltage at points 1 and 2. If there is a voltage equal to the sum of the voltages of both windings, then you connected everything correctly. The connection point of the two windings will be "common" (ground, body, GND, call it what you want). This is the first common mistake, as we see: there should be two windings, not one.

Now the second error: The datasheet (technical description of the microcircuit) for the TDA7294 microcircuit indicates: +/-4 is recommended for a 27Ω load.

The mistake is that people often take a transformer with two windings 27V, this can't be done!!!

When you buy a transformer, they write on it effective value, and the voltmeter also shows you the effective value. After the voltage is rectified, it charges the capacitors. And they are already charging amplitude value which is 1.41 (root of 2) times the effective value. Therefore, in order for the microcircuit to have a voltage of 27V, then the transformer windings must be 20V (27 / 1,41 \u19,14d 20 Since transformers do not make such a voltage, we take the nearest one: XNUMXV). I think the point is clear.
Now about power: in order for TDA to give out its 70W, it needs a transformer with a power of at least 106W (the efficiency of the microcircuit is 66%), preferably more. For example, for a stereo amplifier on the TDA7294, a 250W transformer is very well suited

1.2 Rectifier bridge

There are usually no issues here, but still. I personally prefer to install rectifier bridges, because. no need to mess with 4 diodes, it's more convenient. The bridge must have the following characteristics: reverse voltage 100V, forward current 20A. We put such a bridge and do not worry that one "beautiful" day it will burn. Such a bridge is enough for two microcircuits and the capacitance of the capacitors in the PSU is 60'000uF (when the capacitors are charged, a very high current passes through the bridge)

1.3 Capacitors

As you can see, the power supply circuit uses 2 types of capacitors: polar (electrolytic) and non-polar (film). Non-polar (C2, C3) are necessary to suppress RF interference. According to the capacitance, set what will happen: from 0,33 microfarads to 4 microfarads. It is advisable to install our K73-17, pretty good capacitors. Polar (C4-C7) are necessary to suppress voltage ripple, and besides, they give up their energy at amplifier load peaks (when the transformer cannot provide the required current). In terms of capacity, people are still arguing how much is still needed. I realized from experience that for one microcircuit, 10000 microfarads per shoulder is enough. Capacitor voltage: choose yourself, depending on the power supply. If you have a 20V transformer, then the rectified voltage will be 28,2V (20 x 1,41 \u28,2d 35), the capacitors can be set to XNUMXV. Same thing with the non-polar ones. Looks like I didn't miss anything...

As a result, we got a power supply unit containing 3 terminals: "+", "-" and "common" With the power supply unit finished, let's move on to the microcircuit.

2) Chips TDA7294 and TDA7293

2.1.1 Description of the pins of the TDA7294 chip

1 - Signal ground
2 - Inverted input of the microcircuit (in the standard scheme, the OS is connected here)
3 - Non-inverse input of the microcircuit, we supply an audio signal here, through the isolation capacitor C1
4 - Also signal ground
5 - The output is not used, you can safely break it off (the main thing is not to confuse !!!)
6 - Boost (Bootstrap)
7 - "+" power
8 - "-" supply
9 - Output St-By. Designed to transfer the microcircuit to standby mode (that is, roughly speaking, the amplifying part of the microcircuit is turned off from the power supply)
10 - Mute output. Designed to attenuate the input signal (roughly speaking, the input of the microcircuit is turned off)
11 - Not used
12 - Not used
13 - "+" power
14 - Chip output
15 - "-" supply

2.1.2 Description of the pins of the TDA7293 chip

1 - Signal ground
2 - Inverted input of the microcircuit (in the standard scheme, the OS is connected here)
3 - Non-inverse input of the microcircuit, we supply an audio signal here, through the isolation capacitor C1
4 - Also signal ground
5 - Clipmeter, in principle, an absolutely unnecessary function
6 - Boost (Bootstrap)
7 - "+" power
8 - "-" supply
9 - Output St-By. Designed to transfer the microcircuit to standby mode (that is, roughly speaking, the amplifying part of the microcircuit is turned off from the power supply)
10 - Mute output. Designed to attenuate the input signal (roughly speaking, the input of the microcircuit is turned off)
11 - Input of the final amplification stage (used when cascading TDA7293 microcircuits)
12 - Capacitor POS (C5) is connected here when the supply voltage exceeds +/-40V
13 - "+" power
14 - Chip output
15 - "-" supply

2.2 Difference between TDA7293 and TDA7294 chips
Such questions come up all the time, so here are the main differences of the TDA7293:
- The possibility of parallel connection (complete garbage, you need a powerful amplifier - collect on transistors and you will be happy)
- Increased power (a couple of tens of watts)
- Increased supply voltage (otherwise the previous paragraph would not be relevant)
- They also seem to say that it is all made on field-effect transistors (what's the point?)
Here, it seems, are all the differences, I’ll just add from myself that all TDA7293s have increased buggy - they burn too often.

Another common question: Is it possible to replace TDA7294 with TDA7293?

Answer: Yes, but:
- With a supply voltage <40V, you can easily replace it (the POS capacitor between the 14th and 6th paws has been and remains)
- When the supply voltage is >40V, it is only necessary to change the location of the POS capacitor. It must be between the 12th and 6th paws of the microcircuit, otherwise glitches in the form of excitation, etc. are possible.

Here's how it looks in the datasheet for the TDA7293 chip:

As can be seen from the diagram, the capacitor is connected either between the 6th and 14th legs (supply voltage <40V) or between the 6th and 12th legs (supply voltage >40V)

2.3 Supply voltage

There are such extreme people, they feed the TDA7294 from 45V, then they are surprised: why is it burning? Lights up because the microcircuit is working at its limit. Now here they will tell me: “I have +/-50V and everything works, don’t drive !!!”, the answer is simple: “Turn it up to the maximum volume and mark the time with a stopwatch”

If you have a load of 4 ohms, then the optimal power supply will be +/- 27V (20V transformer windings)
If you have an 8 ohm load, then the optimal power supply will be +/- 35V (25V transformer windings)
With such a supply voltage, the microcircuit will work for a long time and without glitches (I withstood a short circuit of the output for a minute, and nothing burned out, I don’t know how things are with this among fellow extreme sportsmen, they are silent)
And one more thing: if you still decide to make the supply voltage higher than the norm, then do not forget: you still won’t get anywhere from distortion. it is impossible to listen to this rattle !!!

Here is a plot of distortion (THD) versus output power (Pout)

As we can see, with an output power of 70W, we have distortion in the region of 0,3-0,8% - this is quite acceptable and is not noticeable by ear. At a power of 85W, the distortion is already 10%, this is already wheezing and grinding, in general, it is impossible to listen to sound with such distortions. It turns out that by increasing the supply voltage, you increase the output power of the microcircuit, but what's the point? All the same, after 70W it is not possible to listen !!! So take note, there are no pluses here.

2.4.1 Switching schemes - original (usual)

Here is the schematic (taken from the datasheet)

C1 - It is better to put a K73-17 film capacitor, a capacitance of 0,33 μF and higher (the larger the capacitance, the less the low frequency is weakened, i.e. everyone's favorite bass).
С2- It is better to put 220uF 50V - again, the bass will get better
C3, C4 - 22uF 50V - determine the turn-on time of the microcircuit (the larger the capacitance, the longer the turn-on time)
С5 - here it is, the PIC capacitor (I wrote how to connect it in paragraph 2.1 (at the very end). It is also better to take 220uF 50V (guess it 3 times ... the bass will be better)
C7, C9 - Film, any rating: 0,33uF and higher for a voltage of 50V and higher
C6, C8 - You can not put it, we already have capacitors in the PSU

R2, R3 - Determine the gain. By default, it is 32 (R3 / R2), it is better not to change
R4, R5 - Essentially the same function as C3, C4

The diagram has incomprehensible terminals VM and VSTBY - they must be connected to the POSITIVE supply, otherwise nothing will work.

2.4.2. Switching schemes - bridge

The diagram is also taken from the datasheet.

In fact, this circuit consists of 2 simple amplifiers, with the only difference being that the column (load) is connected between the amplifier outputs. There are a couple more nuances, about them a little later. Such a scheme can be used when you have a load of 8 ohms (optimal power supply of chips +/-25V) or 16 ohms (optimal power supply +/-33V). For a load of 4 Ohm, it is pointless to make a bridge circuit, the microcircuits will not withstand the current - I think the result is known.

As I said above, the bridge circuit is assembled from 2 conventional amplifiers. In this case, the input of the second amplifier is connected to ground. I also ask you to pay attention to the resistor that is connected between the 14th "leg" of the first microcircuit (in the diagram: above) and the 2nd "leg" of the second microcircuit (in the diagram: below). This is a feedback resistor, if it is not connected, the amplifier will not work.

The Mute (10th "leg") and Stand-By (9th "leg") chains have also been changed here. It doesn't matter, do what you like. The main thing is that the voltage on the Mute and St-By paws is more than 5V, then the microcircuit will work.

2.4.3 Switching schemes - powering the microcircuit

My advice to you: do not suffer from garbage, you need more power - do it on transistors
Perhaps later I will write how the help is done.

2.5 A few words about the Mute and Stand-By functions

- Mute - At its core, this feature of the chip allows you to disable the input. When the voltage at the Mute pin (10th leg of the microcircuit) is from 0V to 2,3V, the input signal is attenuated by 80 dB. When the voltage on the 10th leg is more than 3,5V, there is no weakening
- Stand-By - Switching the amplifier to standby mode. This function turns off the power to the output stages of the microcircuit. When the voltage at the 9th output of the microcircuit is more than 3 volts, the output stages operate in their normal mode.

There are two ways to manage these functions:

Separate control Each function has its own control switch	Unified Management Both functions are controlled by one toggle switch

What is the difference? Essentially nothing, do as you please. I personally chose the first option (separate control).

The outputs of both circuits must be connected either to the "+" power supply (in this case, the microcircuit is on, there is sound), or to the "common" (the microcircuit is turned off, there is no sound).

3) PCB

Here is a printed circuit board for TDA7294 (TDA7293 can also be installed, provided that the supply voltage does not exceed 40V) in Sprint-Layout format: download.

The board is drawn from the side of the tracks, i.e. when printing, it is necessary to mirror (for the laser-ironing method of manufacturing printed circuit boards)

I made the printed circuit board universal, on it you can assemble both a simple circuit and a bridge circuit. Viewing requires Sprint Layout 4.0.

Let's go over the board and figure out what relates to what.

3.1 Main board (at the very top) - contains 4 simple circuits with the ability to combine them into bridges. Those. on this board, you can collect either 4 channels, or 2 bridge channels, or 2 simple channels and one bridge. Universal in one word.

Pay attention to the 22k resistor circled in red square, it must be soldered if you plan to make a bridge circuit, it is also necessary to solder the input capacitor as shown in the wiring (cross and arrow). The radiator can be bought at the Chip and Dip store, such a 10x30cm is sold there, the board was made just for it.

3.2 Mute/St-By board

It just so happened that for these functions I made a separate board. Connect everything according to the diagram. Mute (St-By) Switch is a switch (tumbler), the wiring shows which contacts to close in order for the microcircuit to work.

(Click to enlarge)

Connect the signal wires from the Mute/St-By board on the main board as follows:

Connect the power wires (+V and GND) to the power supply.

Capacitors can be supplied 22 uF 50V (not 5 pieces in a row, but one piece. The number of capacitors depends on the number of microcircuits controlled by this board).

3.3 PSU boards

Everything is simple here, we solder the bridge, electrolytic capacitors, connect the wires, DO NOT confuse the polarity !!!

I hope the assembly will not cause difficulties. The circuit board has been tested and everything works. With proper assembly, the amplifier starts immediately.

4) The amplifier did not work the first time

Well, it happens. We disconnect the amplifier from the network and start looking for an error in the installation, as a rule, in 80% of cases, the error is in the wrong installation.

If nothing is found, then turn on the amplifier again, take a voltmeter and check the voltage:

- Let's start with the supply voltage: on the 7th and 13th legs there should be a "+" supply; On the 8th and 15th paws there should be a "-" supply. The voltages must be the same value (at least the spread should be no more than 0,5V).
- On the 9th and 10th paws there should be a voltage of more than 5V. If the voltage is less, then you made a mistake in the Mute / St-By board (they mixed up the polarity, the toggle switch was set wrong)
- With the input shorted to ground, the output of the amplifier should be 0V. If the voltage there is more than 1V, then there is already something with the microcircuit (possibly a marriage or a left microcircuit)

If all the points are in order, then the microcircuit must work. Check the volume level of the sound source. When I just assembled this amplifier, I turn it on ... there is no sound ... after 2 seconds everything started to play, do you know why? The moment the amplifier was turned on fell on a pause between tracks, that's how it happens.

Other Tips:

Helping. TDA7293 / 94 is quite sharpened for connecting several cases in parallel, although there is one nuance - the outputs must be connected 3 ... 5 seconds after the supply voltage is applied, otherwise new m / s may be required.

Addition from Kolesnikov A.N.

In the process of reviving the amplifier on the TDA7294, I discovered that if the "zero" of the signal sits on the amplifier case, then it turns out to be a short circuit. between "minus" and "zero" power supply. It turned out that pin 8 is directly connected to the heat sink of the microcircuit and, according to the electrical circuit, to pin 15 and the “minus” of the power source.

Author: Mikhail aka ~D'Evil~ St. Petersburg; Publication: cxem.net

See other articles Section Transistor power amplifiers.

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