TDA7379 amp in Double Bridge config draws correct quiescent current but gives no sound
This would be my approach, but since I don't have that IC, I cannot guarantee the results.

Based on this diagram from the TDA7379 datasheet:




Circuit design


When you said "[...] after connecting STBY to VCC [...]", that is ambiguous since there is a point labelled ST&#8209;BY on the diagram (top left) but pin 7 of the IC is also called ST&#8209;BY.

Although I have an idea about the purpose of the \$10\;\textrm k\Omega\$ resistor R1, I don't see it explained in detail in this datasheet (the block diagram of the IC suggests pin 7 is the base of an internal BJT). In any case, both TDA7379 projects which I found do use that resistor. Therefore don't connect IC pin 7 (ST&#8209;BY) directly to the +ve power supply (you called it VCC, the diagram calls it \$V_S\$). Do include R1 and C5.
Make sure not to connect any speaker terminal to "ground". Both speaker terminals on both speakers, must be connected only to their respective pins on the IC.






Other projects using the TDA7379


For comparison, I found 2 website projects which use this IC. You might want to review their use of the IC, to see if you learn anything from them which is different from your usage of the IC:


This site (in Chinese) seems to be a simple circuit: http://gcbbs.digitw.com/phpbb2/viewtopic.php?f=3&amp;t=5397





This site (in Portugese) is more complex.
http://www.te1.com.br/2013/09/circuito-amplificador-2-1-tda7379-tda7377-tda7375-sta450-estereo-subwoofer/

The part with the TDA7379 (which uses the "Stereo/Bridge" output configuration) is copied below:







Diagnostic pin


You said:


  the diagnostic pin there are 0.1V (which I couldn't find any document on how to interpret).


As you said, there is no information about using the signal on the "Diagnostic Output" (pin 10) in the TDA7379 datasheet. However, an earlier audio amplifier, part number TDA1562Q (datasheet viewable via this webpage) had a similar-sounding feature.

The similar list of features might suggest that the implementation of the diagnostic output of the TDA7379 may be similar to that in the TDA1562Q:


TDA7379 datasheet says:





TDA1562Q datasheet says:







TDA7379 datasheet suggests Diagnostic Output pin 10 is probably an open&#8209;collector output, since on page 3 of the datasheet, the "clipping detector" output current is shown with the following note:


  (**) Pin 10 Pulled-up to 5V with 10KΩ; RL = 4Ω

TDA1562Q datasheet shows that its diagnostic output pin is an open&#8209;collector output:






So the question is: What can that diagnostic output tell us, and how does it do so?

The TDA1562Q datasheet (page 7) shows that its diagnostic output pin will be pulled low on an error condition (e.g. shorted output or over-temperature) or, briefly, when clipping occurs.

The main difference which I noticed, is that the TDA1562Q datasheet shows that its diagnostic pin is pulled-up via \$10\;\textrm k\Omega\$ to \$+V_P\$ i.e. the positive supply to the IC (see diagram above, which is an extract from figure 10 on page 14), whereas the TDA7379 datasheet mentions a \$10\;\textrm k\Omega\$ pull-up to +5V (as quoted earlier).

Therefore, to be safer for the TDA7379 in case it requires that pull-up to be to +5V and not a higher voltage, I would:


Use a lower supply voltage e.g. 12V instead of the 20V currently used (see the Absolute Maximum ratings on datasheet page 2).
Create a temporary +5V rail (common ground with the main power supply) and add appropriate input &amp; output capacitors as required by whatever regulator you use. The current drain will be minimal on this rail, so no heatsink should be required.
Connect a \$10\;\textrm k\Omega\$ pull-up resistor between that +5V rail and TDA7379 pin 10 (Diagnostic Output).
Disconnect any inputs from the TDA7379.
Then using your oscilloscope, view the output on TDA7379 pin 10, using figure 4 on page 10 of the TDA1562Q datasheet as a guide e.g.:


stuck low = output short to ground or power detected, or IC over&#8209;temperature;
mostly low with brief high pulses = short detected across a speaker;
stays high = no fault detected by the IC

It is a guess that the diagnostic output of the TDA7379 is similar to the (better documented) TDA1562Q. However we do know that it is an open collector output, and the TDA7379 datasheet mentions that \$10\;\textrm k\Omega\$ pull-up to +5V. Therefore I believe that temporary connection (to gather diagnostic information) should be safe. Of course you have to decide for yourself whether to take the risk, given that neither of us can find better information about the TDA7379!
You can also use your oscilloscope to carefully measure the voltages on the other TDA7379 pins, while the IC is powered and an audio signal is connected, to confirm that the signal is definitely reaching the IC and that the voltages on the other pins seem reasonable for their function. It's always possible that you might discover something :-)


Good luck - hope that helps.



Update: I found datasheets for two other similar audio amplifier ICs, which give more confidence in the interpretation of the diagnostic pin voltage mentioned above:


TDA7396 (mono audio amplifier)

In the April 2002 datasheet viewable via this webpage, the signal on CD-DIA (clipping detector &amp; diagnostic) pin 4, is described on page 8. On this IC, the pull-up resistor from that pin can be connected to a voltage between +5V and the supply voltage for the IC (\$V_S\$). That range of allowed voltage might also apply to your TDA7379, but gives additional confidence that a \$10\;\textrm k\Omega\$ pull-up resistor from the diagnostic pin to +5V is acceptable.
STA540

The STA540 seems very similar to your TDA7379. In this STA540 datasheet from 2008, again it confirms the interpretation of the output from the diagnostic pin (see sections 6.7 and 6.8 starting on page 17 of that datasheet).

Below is an extract from the above STA540 datasheet which also confirms my earlier recommendation that ST-BY pin 7 must not be directly connected to the supply voltage. Components R1 (series resistance) and C5 (capacitor between pin 7 and ground) on the original TDA7379 schematic are both required for correct behaviour.




I hope this additional information is helpful.