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

[MasterT]

>> "A switching regulator will have massive problems here because a positive and a negative voltage are required. "

There are switching power supplies on a market that provide both: positive and negative voltages .

With all due respect, all post is just a speculation. 

Seems written by AI with very low IQ.

[Enrico_I]

2nd copy

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.

[Andrew Neil]

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@Peter BENSCH wrote:

 such low frequencies are anything but easy to handle. Not to mention the difficulty of finding a speaker for it.

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[AScha.3]

Hi,

- its for audio, right ?  (or for test equipment ?)

if audio : What are your speakers ? lower frequ. limit ( - 3 dB ) ?     frequ. response ? avg. SPL at 1 W ?

THEN we talk about sense or nonsense to lower the input filter frequ. .

[Enrico_I]

The speakers are La Scala II made by Klipsch.

Are you a musician? Do you know what pedal tones are?

THEN we talk about the need to lower the input filter freqs.

[Andrew Neil]

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@Enrico_I wrote:

Do you know what pedal tones are?

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Yes, but look at the sizes of those pipes!

They are "not feasible" for all but a very few, very specialist situations.

If you want extra-low tones, then you're inherently going to need extra-large components!

[Enrico_I]

Hi Andrew, I simply asked the ST technicians the question: if I raise the input filter capacitor from 0.56uF to 4.7uF for the reasons above, what should be the new value of C bootstrap?

I still don't have an answer regarding what formulas should be used etc.

[Andrew Neil]

That question was answered, and you dismissed the answer saying, "It's just not feasible"

[Enrico_I]

The mention of "not feasible" was about the 22uF input capacitor and the related 185000uF and 87000uF capacitors. I then asked if instead of 22uF a 4.7uF was used.

I am still waiting for an answer...

[MasterT]

There are a few issues with those bootstrap capacitors:

1. Setting big value proportional to (20 Hz / F-new) is likely to damage IC during power up /down, so you can't just increase x20 times w/o designing OCP OVP protection on pin 6.

2. Main purpose bootstrap capacitor is to lower THD, to specified value 0.005% or so. This objective is meaningless for low freq. end, since whatever acoustic system you have it's likely produce 1-5% THD. 

Vibrations <20 Hz is also travelling through surfaces, floor / walls, and distortion these "mediums" is also not measurable.