TDA7294 DC offset problems

[SCKentrol]

Okay, I have built the amp circuit, having everything working and for some reason I haven't been able to figure out is why the output of the amplifier is having a 30V DC offset.
I am power the chip with a +/-35.5V supply, and more or less have the exact circuit built as the one in the datasheet, save for a different resistor value at the signal input (if i recall, 5K linear pot). any ideas?

http://www.datasheetcatalog.org/data...cs/mXqwvzw.pdf

[SCKentrol]

Come on 14 views ans nothing?
And for some reason the +voltage is dropping when the audio input is connected. Can the be because of my 7805 im for the standby and mute?

[singa]

Hi,
If you have 30V offset there is something terribly wrong.Recheck your
wiring or else the Lm3886 may be bad.
Edit: You are a little impatient relax!
When voltage drops as you described then something is shorting or
drawing current. regards.

[SCKentrol]

I understand that but what could be causing th short. I've looked for hours on end. And u can't use another chip

[SCKentrol]

I know but after hours of labour it's frustrating. Also I can't change the chip. Tho your comment has given me an idea...

[SCKentrol]

I just thought of this on my drive home; if the [+] voltage's filter cap is damaged could that cause the amp to act this way, as I am reading some oscillation also.

What's bothering me the most is that this problem was not att all present on my bread board.

[singa]

Hi,
Sorry I,ve mistaken your post with another one about LM3886,
I meant TDA7294.You're still impatient Calm down as it does not help matters.You need to be rational of course experience helps.

If you say you saw oscillations in your scope? are you sure? Anyway if it is then only the chip amp is doing it so it may be damaged.Assuming the connections are correct then replacing the ic is the way to go.I don't
think the cap is faulty.Although it's possible but I doubt it,of course
you are free to substitute if you have spares to try.

regards.
Ps. Since you mention it was not present on your breadboard all the more to recheck wiring and component values and position.
As I'm in a different timezone.I can only reply tomorrow after this.
Soooo.... stay calm and learn to meditate.

[singa]

Come to think of it,did you breadboard the tda 7294 with a heatsink?
If you did not you may have damaged the ic although it has thermal protection shutdown.

Regards.

[SCKentrol]

I put a brand new one in on the circuit board and still have the bread boarded one. I'll try a new one because I've tried just about everything else. Also I did replace one I fried by accidentally apply +70V to it. It's a brand new one.

[jackand08]

If you say you are getting some oscillation on the scope, this could be the reason for the large "DC" offset (just very high frequency AC which a meter can´t keep up with). Oscillation is usually due to inadequate bypassing of the power rails. As well as the main capacitors (large in size, around 100uF to 470uF for onboard caps and then over 4700uF for the main PSU caps), you should also have a 100nF polyester capacitor on each power leg of the chip, between the rail and 0V/GND. On a TDA7294 the power pins are 7 and 13 for +V along with 8 and 14 for -V.

These capacitors help to remove any high frequency noise that may be on the power rails and cause the output to oscillate.

Also, does the design you are using have a zobel network on the output? This helps to flatten the impedance variations from the speaker seen by the chip. Not having one can also make the output oscillate (although only usually with a load connected).