TDA7294 Bridge application

[Enrico_I]

Hello everyone.

Regarding an amplifier with two TDA7294 in bridge configuration, looking at the diagram, on IC 1 at pin 3, there is a high pass filter formed by the 0.56uF capacitor C1 and the 22K Ohm resistor. The cutoff frequency is 12,91 Hz.

I would like to get closer to 0 Hz by increasing the capacitance of C1 to 22uF (cutoff frequency is 0,32 Hz).

If you see IC 2, at pin 3 there is another filter formed by C2 (0.56uF) and the 22K resistor.

For symmetry with the first IC, should I also increase the capacitance of C2 to 22uF in your opinion?

Or leave it at 0.56uF?

I thank you very much in advance for your kind reply.

Cheers,

Enrico

[Peter BENSCH]

Welcome @Enrico_I, to the community!

Use for C2 the same capacitance value as for C1 at the first amp.

But: if you reduce the cut-off frequency of the input high-pass filter by a factor of 40, you must also increase the capacitors on pins 6 (BOOTSTRAP) by the same factor, because otherwise the internal stages will no longer be supplied with sufficient energy.

Hope that helps?

Regards
/Peter

[Enrico_I]

Thank you very much Peter for your prompt reply.

At page 7 of the TDA7294 datasheet (Application suggestions), the bootstrap cap has a value of 22uF. If I increase the input capacitance of the input high-pass filter, for example to 22uF or 47uF, what is the value of capcaitance to assign to the bootstrap capacitor? How is it calculated? Which formulas should I use?

Also, if I want to make a bridged amplifier using the TDA7293, the issue of increasing the bootstrap capacity is identical to that of the 7294, right?

I am looking forward to haring from you soon.

Best regards,

Enrico

[Peter BENSCH]

The bootstrap capacitor is charged while the output of the amp in question switches to -VS. During the other half-wave, this buffered voltage of the capacitor is shifted upwards and increases the supply voltage of the internal preamps.

The current bootstrap capacitor was dimensioned with some reserve for the lower cut-off frequency of the low-pass filter at the input. If you increase the capacitance of the low-pass capacitor to 22uF or 47uF, you must also increase the bootstrap capacitors to 1000uF or 2200uF for the same reserve.

Incidentally, the same also applies to the capacitors at +VS and -VS, which must therefore be increased from 2200uF to -~87000uF (87mF) and ~185000uF (185mF) respectively.

By now you should realise that your project will result in an immense increase in the size of the system - the power supply has not even been considered yet...

And yes, the same applies to the TDA7293.

[Enrico_I]

Many thanks Peter for your quick answer.

I would never have imagined that to extend the lower freqs under 10Hz, the 7294 would need 185000uF electrolytics for +Vs and -Vs, and 87000uF for the other. It's just not feasible...

Since a 22uF input capacitor cannot be used for the reasons you described, if a 5uF input cap were used instead, what would be the respective uF values ​​for the bootstrap and power capacitors?

I opted for the smaller value of 5uF because the response flattens out around 20Hz, so there is no big loss of deep bass frequencies, please see the simulated graph.

Thank you very much for your kind cooperation and I am looking forward to hearing from you again.
Enrico

[MasterT]

Those enormous capacitors likely suggested based on assumption of simplest power supply in use (transformer + diodes bridge rectifier) and also 4 Ohm load. For switching power supply big cap's are not required.

Since <20 Hz imply not audio, than more details about your project 'd possible bring more options/ suggestions.