Problem building regards weird noise (TDA7240)

[guitar89]

Originally Posted by danielwritesbac

Oh yes, of course, with TDA2009A
For Stereo amp to 2 speakers it wants ~23VDC power
For bridge amp to one 8 ohm speaker it wants 18VDC power
For bridge amp to one 4 ohm speaker it wants 16VDC power
Significant amperage is required.
The datasheet is an interesting read!
Its not difficult to read And it has interesting amplifier applications.
That should be good. Have fun!

Haha, I'm agree that it is an interesting chip, going to try build it within next week.
I personally will go for stereo amp, using 22-24V supply approx. It also has low idle current than tda7240 (double mono make it more !)

Originally Posted by gootee

Sounds like your power supply was far too weak. That was probably the whole problem.

Anyway, just so you know, the reason for suggesting that a resistor be the last thing before the input, and for it being close to the pin, was that I wasn't sure where that input went, inside the chip. If it happened to connect to a negative amplifier input that resembled a negative opamp input, then even stray capacitance from a length of wire or pcb trace could make the amp unstable. So the resistor was meant to isolate the input pin from any stray capacitance, which wouldn't hurt anything in the case of a positive input.

Then, adding a 220 pF capacitor to ground, downstream from the resistor, would create a low-pass RF filter. But in case the pin needed to be isolated from capacitance, per the above, I suggested putting it between two resistors.

I would shoot for a low-pass RF filter cutoff frequency of at least 220 kHz to 440 kHz. The cutoff frequency should be at 1 / (2 x Pi x R x C), except that when placed just before an input pin of an opamp with a feedback loop you have to double the C value that you would use if there was just one R, in order to keep the same cutoff frequency. Not sure, with this chip.

I suggest ALWAYS using a low-pass RF filter, for every amplifier stage's input.

Power supply far too weak ? I don't think computer power supply aren't that weak, maybe this amplifier consuming too much. (only car battery enough to feed it !)
About the low-pass RF filter, it is a RC network ? interested how to design one (for further use of other amplifier)
What if I didn't use this and let the RF to go through ? Will it cause oscillation, degradation of sound quality(frequency response), induce noises (hum/buzz), or overheat by giving other loads ?
If I were to make a low-pass and high-pass that only allow 20Hz to 25kHz sound roughly, what is the advantage or disadvantage of doing so ?

[danielwritesbac]

Yes. power supply far too weak.
Audio band wasn't defined by filtering, thus amplifier performed extraneous work.
So, low resources, extra work makes less stability and so you get noisy 4 watts.

[guitar89]

Originally Posted by danielwritesbac

Yes. power supply far too weak.
Audio band wasn't defined by filtering, thus amplifier performed extraneous work.
So, low resources, extra work makes less stability and so you get noisy 4 watts.

Then how should I make the input within audio bandwidth or at least nearly audio bandwidth (20Hz to 20kHz) ? at least this makes the it works less unnecessary works.

[gootee]

An R in series followed by C to ground makes a passive first-order low-pass filter with -3dB "cutoff" frequency at f = 1 / (2 x Pi x R x C), and 6 dB per decade slope after that. Put the cutoff freq at least 10x higher than any frequencies you don't want to be affected. Pi is approximately 3.14. R is in Ohms, f is in Hertz, C is in Farads.

Similarly, a series C followed by an R to ground makes a high-pass filter, with -3 dB frequency from same equation and with same roll-off slope.

You can cascade multiple stages to get steeper slope, but would probably want to put unity-gain (i.e. gain=1) opamp buffers after each of them, in which case you might as well download TI's free Filterpro software and use "active" opamp-based filters. But you can usually cascade one high-pass and one low-pass without needing a buffer.

Use polypropylene film capacitors (or teflon, or maybe polystyrene).

Typically you would put your high-pass f(-3dB) at 2 Hz or less and your low-pass at 220 kHz or more.

Google.com is your friend.

[guitar89]

So if I were to make a low-high-pass together at the input, should I make lowpass first or vice versa? ( I assume the low pass and high pass is in series ?)

Is there any way to make sharp cutoff ? instead of gradually decrease, I make a very steep or vertical cut off ?

[gootee]

Originally Posted by guitar89

So if I were to make a low-high-pass together at the input, should I make lowpass first or vice versa? ( I assume the low pass and high pass is in series ?)

Is there any way to make sharp cutoff ? instead of gradually decrease, I make a very steep or vertical cut off ?

Yes, they would be in series.

I would put the high-pass first, and then put the low-pass for RF right at the input pin or just before the input resistor, if there is one. Just remember that if it's a negative input pin for an opamp or chipamp, you need another resistor in series with the pin, to isolate the capacitor from the pin.

You could make steeper slopes but there is no need to do that, for this type of application, and it might even cause problems. And there are always trade-offs, to get the sharper cutoff.

However, just so you know, every time you cascade another simple first-order filter stage, the slope in the stop-band doubles (in dB per decade of frequency). BUT, you can't really just cascade those types of passive filter stages in series, UNLESS you use buffer amplifiers in between them. Basically, though, there are much better ways to make higher-order filters, with steeper slopes and with better control of their other characteristics.

See

ADI - Analog Dialogue | Op Amp Applications Handbook

which is a very nice free downloadable book from Analog Devices.

Also, as I already mentioned, Texas Instruments (ti.com) has a free download called filterpro. It designs nice opamp-based filters for you, up to 6th-order in the free version, I think.