TDA2005 Noise

[Roamin]

Hi everyone,

I have been trying to fix the noise emitted by this amp that we use here at work. I have not designed the layout, or the schematic (almost a carbon copy of the typical schematic). The chip amps are "TDA2005".

Here is the schematic :

The switch on the left side , is toggled to ground when we use a Projector instead of LCD monitor, because without grounding the input , noise was unbearable.

The tranformers are 10,000 : 10,000


Here is the pcb layout (pcb express format) :


Our problems is the noise. Under a lot of different conditions , the noise is more then annoying. First , we use this amp powered by a 12v Lead-acid battery. We use the same battery , to power a "car laptop charger" (step up 12v to 19v, to operate a laptop), when then plug the laptop's audio output , to our amp and thats when the unbearable noise begins. Now , the noise is mostly only present while the laptop is plugged to that dc-converter. The second we unplug the dc converter , noise stops. Now i have a few models or dc-converters , all make horrible noise , except one type, which we can't seem to find anymore. I don't really mind , because i know the amplifier is flawed designed, because a lot of equipment creates different levels of noise, and so i would like to fix the amplifiers once and for all. Its been so long since i studied audio analysis , that i have forgotten a lot. I know component placement is critical, that is why i am seeking for help.

Here are 2 different block diagrams of how we use our amps :

So the top block diagram gives us some noise while using the media player. I know the 5v supply is not the best , the 4ohm resistor is there to limit current, because there is a possibility of short circuit when someone powers the LCD monitor after the VGA cable is plugged, since the tip of the power connector can short with the lcd's case , and the ground returns via the VGA cable, busting our fuse. And since most of the time , the equipment is handled by people without electronics knowledge, its happen many times in the past.

The bottom block diagram is where our noise issues are at worst. If the dc converter is plugged in the laptop, horrible horrible noise comes out, making it unbearable to use like that.


I greatly appreciate any input from you audio Gurus.

Thanks in advance.

[mjf]

hello.
why are there no input caps (at pin 1 and 5)?
greetings........

[pacificblue]

Originally posted by mjf
hello.
why are there no input caps (at pin 1 and 5)?
greetings........

Because there are input transformers?

[Roamin]

Yes,

I was under the impression that the input capacitors weren't needed because the decoupling was done by the transformers. I'm pretty sure i have done this test before , but i will replace the transformers with coupling capacitors.

[pacificblue]

What kind of noise are you getting?

And could that laptop not work off his own batteries, so that you could skip the DC converter?

[Roamin]

Hi,

We do use the laptop on its battery , but since the this equipment is portable , and operates on batteries, we use the dc converter to keep the laptop charged at all times , so that it lasts longer and this way we only have to change the main battery, and not reboot the laptop. This is somewhat crucial , since our products are advertising mediums during events , the less downtime the better.

As i said in a previous post , we have 3 different dc converters , and 1 type has unbearable noise , the other is bearable, and the third , which we can't find anymore, is completely silent.

The exact item we use is this : http://www.dealextreme.com/details.dx/sku.5530

I don't have much time this week to fully check this out, but i did take a bit of time yesterday to scope the 19v dc coming out , and here is what i see when i scope the AC on the 19v dc out.





Timing was 10us , voltage division was 5mV. Now , this noise is going right through the laptop , and then onto our amp.

Output from the laptop (with nothing playing) :



Output from the laptop , with dc converter plugged in , still without any audio output :




Now , i know this is a lot of noise , i tried making a simple low pass filter (RC) , but it didn't change anything.. I would just like to completely remove any AC presence on the DC .. I just suspected our amp to be way to sensitive to noise, because whatever we plug in , seems to me there is always a bit a noise coming out from it. I also tried plugging the laptop's output to a manufactured built desktop amplifier (yamaha) , and there is no noise coming out at all, so it probably has a good filter at the input. Which is why , in the end , i blame the noise at our amp.

I really like any input you people give me , and i will post more findings as my tests go along , but i do have tons of other things to do at work at the same time.

For anyone interested in knowing a bit more about that equipment, www.pixman.com .

Thanks again.

[Roamin]

I just wanted to add that i tried the yamaha amp , powered by our batteries , and noise was still present , but seems a bit less then with the TDA amps.

Also, when we use the car adapters , with a car lighter socket , that is wired with a long "coiled" wire (like telephone cord , but bigger gauge) the noise is almost inexistant. We measured roughly 1 ohm on that wire, so we added a 1ohm resistor (5w) and the noise was considerably better. So again, i'm thinking it needs a proper RC filter.

Smile, its friday

[EWorkshop1708]

An inductor and about 4700uf or more on the 12V would help for sure. That kind of filter would clean it up. Clean DC is a must.

Usually in the car radios, those 12V amplifier IC chips have at least an inductor, and 2200uf on them, and that schematic posted shows only 470uf.