Problem building regards weird noise (TDA7240)

[guitar89]

I was building amplifier based on chip TDA 7240A, but encounter a weird noise.
It only happen when i connect to my music source (netbook), but doesn't happen that when i directly connect to my speaker unit (without amplifier).

It notice when i'm not using AC (use battery power) on my netbook, that noise doesn't happen. This also same for my previously build discrete amplifier.

But if i use any commercial brought amplifier, no such problem. So i guess need to incorporate something to make this noise disappear ?

[guitar89]

hm.... i decided to record the noise using my netbook's in-built mic, so its a bit not good. Attached in following link :

super distorted noise.mp3 - DivShare

Anyway, at the first 4 second, it was the ambient noise (the amplifier is turn off) and was raining (doesn't sounds like raining is it my mic sucks).
After the first 4 second, i turn on the mic amplifier, with AC powered netbook as input. (without any music on, just plug in), the noise is obviously heard, but the pitch is different in real world (damn my lousy mic) Is it oscillation ?

About at 12 second, I plug out the AC charger, thus on battery powered only. That makes the noise goes away, and speaker is nice with just a little hum (acceptable and can't heard by the mic).

At 22 second, I plug in the AC charger into netbook again, thus the noise came out again. About at 28 second, I remove the input TRS jack, thus no noise heard. At 30 second, I plug in the input jack to netbook, thus making noise again.

Any advise how to remove such noise from my amplifier (discrete amp noise is different)?

[guitar89]

also when the input' hot and ground are shorted, the weird noise change into low pitch similar noise

[firechief]

Its possible that the amp is oscillating. This is often because of inadequate bypassing of the power pins. In any case please post a schematic as built, or the link to the design.
Pictures would also be a help. Lots of folks willing to help.

[John Audio]

This is a very common problem when plugging computers into audio systems. The most effective cure is to use an isolating transformer (1:1) - in the audio output feed from the computer.

[guitar89]

a-netz.de - Audio Amplifier with TDA7240
I have build based on this link, although my 470uF in the picture is 2 x 220uF, others mostly the same. (I use point to point solder technique, which uses point by point proto board)
THIS is the manufacturer PDF : http://www.st.com/internet/com/TECHN...CD00010656.pdf
As for the power supply, i'm using a computer power supply (capable to supply to 14A of 12V)

Originally Posted by John Audio

This is a very common problem when plugging computers into audio systems. The most effective cure is to use an isolating transformer (1:1) - in the audio output feed from the computer.

I think this is the answer i'm searching for, but I don't know much about isolating transformer, mind explain slightly ? What is the pro and con using this method for input ? Does commercial Hi-Fi or good equipment also include this in the design ? (since i have no problem using those)

[firechief]

I think I got the right schematic. The isolation transformer couples AC (as in audio) but not DC. This is a way to isolate your equipment. Cost can be considerable, but it dosent have to be. Point to point ---not a problem, Lots of fine equipment is built that way. Mabe someone can suggest a source for the transformer.

[guitar89]

hm.. then what could my problem ? I'm guessing the AC charger adapter of my netbook connected tend to 'suck' current from my power supply, thus making 'motorboating' sound ? (don't have any other idea why it will cause power supply to go crazy when AC charger of netbook is connected...)

I have a look into purchasing of isolating transformer, it doesn't looks so good... and most of it having frequency limit of 400-4kHz, which different alot with audio (20Hz-20kHz)..... and those can allow those audio frequency and expensive and build for high powered (>50W). A bit too much and over, both in function, quality and COST.

Any technique to isolate the input ground and amplifier ground to prevent this matter, at the same time allow the music into amplifier ?

Edit : I suddenly think of an idea, can i attach a capacitor in series with input ground to amplifier ground ? this kinda isolate DC between them. Its may not be an solution, but will it danger to my system/component ? (wanted to test)
If so, what value should i use to permit at least 20Hz or audio is allow to pass ?

[guitar89]

tried with input ground disconnected, the 'motorboat' sound still persist if i'm using AC charger.

Also measure using my DMM, but seems nothing much except for stable voltage drop due to large load(happen when i tune up the music a louder)

Tried connect directly from input connection to speaker unit (bypass amplifier), the sound doesn't change even if i connect to AC charger or using battery.

Also experiment using either input's hot or cold only, the 'motorboating' sound still there, but different pitch.
What could possibly induce this noise ?

[guitar89]

hm... now i tried to connect a high capacitor as reservoir (2.2mF) is quite big for this amp i think, but overkill won't be problem.
The problem is still persist, but its ONLY a little bit help it, ALMOST no different from the 1st record. (but slightly better, just SLIGHTLY)

http://www.divshare.com/download/16047354-510

This time, i'm not making it standby, but playing music.
About after 8 second, I connect the AC charger. Thus the heavy ''''music'''' came out !
about at 22 second, I disconnect it, thus nice music again. After few seconds, I unplug the input, there is a little buzz (hard to listen from the recording), it was worst when i didn't use the reservoir capacitor. Any idea to solve these problem ?