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.

[Enrico_I]

Thanks MasterT for replying.

Frequencies below 50Hz are "audible" with our diaphragm and bones and if you listen to music where is present a tuba, or a double bass, or a contrabass saxophone, a sarrusophone, congas, timbas, etc. the amplifier must be able to reproduce the low frequencies I spoke about above.

The high pass filter at the amplifier input, published on page 22 of the TDA7294 datasheet, has a 0.56uF capacitor and a 22k resistor. As you can see in the filter simulation graph, the response is flat after 100 Hz, which results in a loss of deep bass.

[MasterT]

22k & 0.56uF -> 12.9 Hz cut-off frequency -3dB.

Other question is acoustic system to reproduce anything below 20Hz.

[Peter BENSCH]

A switching regulator will have massive problems here because a positive and a negative voltage are required. Even at a signal frequency of 20Hz (period = 50ms), each of these voltages is fully loaded for 25ms, while the other voltage runs idle in the meantime. As a rule, such low frequencies also require the full power of the amp to drive the acoustic mass (speaker or similar), so that the switching regulators constantly alternate between full load and idle. This alternating, extreme load usually leads to massive problems with the stability of the switching regulators, which is why professional manufacturers then switch to the easier-to-handle Class D amps.

However, I agree with @MasterT that the capacitors are designed with plenty of reserve and for mains transformers with simple rectification. Nevertheless, such low frequencies are anything but easy to handle. Not to mention the difficulty of finding a speaker for it, which @MasterT also mentioned. The largest speakers of all time (80", 350kg) have a lower cut-off frequency of 20Hz, but also require 2500...5000W of drive power. I'm just imagining the diameter of a membrane of a speaker for 0.32Hz, which has to be pushed in one direction for 1.5625s...

With my comments, I actually wanted to direct the discussion to the fact that a cut-off frequency below 20Hz is nice for mind games, but doesn't really make sense. At the very least, this requires completely different power classes for the amplifier and power supply than TDAxxxx can deliver.

[Enrico_I]

Hello again,

it is clear that the switching power supply is not suitable for this amplifier. I have made recently a prototype of a linear power supply that instead of the passive diode bridge, has a LT4320 bridge controller and four power MOSfets. This eliminated thermal design problems, costly heat sinks and it reduced pcb area.

Dear Peter, I do not want to waste your time but back to my question from yesterday, with a 5uF capacitor at the input high pass filter, what are the capacitive values ​​for the bootstrap cap and for the two caps of the power supply?

Many thanks for your kind help.