Audio amp noise reduction - help

[whiz115]

Hi ppl!

I have a cheap 2.1 sound system which has a small problem and i wanted to repair it just for educations reasons the problem is that i can hear a steady noise comming from the sub-woofer no matter how high or low i have the volume.

The noise is like if no smothing capacitor exists! i changed the smothing capacitor 2200uF/16 with a 3300uF/16 and nothing happend, i also changed all the other small decoupling 100uF/16 with new ones better quality and again nothing happend. Now i was thinking to change the 100uF/16 with bigger ones 220uF/16 believing that they don't have sufficient capacity to filter out the power line noise, but before i do that i wanted to hear some opinions of what might be the problem.

 

[audioguru]

Try replacing the input cable. Maybe its shield is poor or its ground is nearly broken.

Oh, it is cheap? My fairly cheap sound systems don't make hiss nor hum noises. Maybe yours is too cheap.

 

[whiz115]

hmmm i played a little with the cable... and i also changed the diodes for the
rectification bridge there are some 1n159 and i was thinking that they might have leakage but nothing happend again! the noise exists even if i haven't connected it in a source...

not many things left to change... some ICs and transistors left, should i change them too?

 

[audioguru]

Some kid straight from school designed the circuit and its pcb. His first one.
Too bad.

 

[chemelec]

Its time to draw out the Schematic of this, so we have some idea as to what might cause the problem here.

Usually its Either Filtering or Shielding.

Power transformer causing hum??? Possibly??

 

[whiz115]

Its time to draw out the Schematic of this, so we have some idea as to what might cause the problem here.

Usually its Either Filtering or Shielding.

Power transformer causing hum??? Possibly??

as Audioguru said it's probably student's design! (if i could design too, it would be great! even at the same level!)

well....about the problem...i don't know what to say to you chemelec today i tryed external power supply unfortunately i don't have a regulated one... and the noise still exists! the board has a TDA2822m for the satellite speakers and the other one is also a TDA but i don't remember which one, it has heatsink and it is a bit diffucult to remove it. But i know that it is bridged so it can output about 7W RMS for the sub-woofer.

I repeat the problem is not so important, i just want to fix it just for education reasons...so i can learn repairing things.

 

[chemelec]

Welll usually those TDA's don't pose a Hum Problem.

But it could be shielding somewhere.
Or just poor design. (possibly too much gain)

as Audioguru said it's probably student's design! (if i could design too, it would be great! even at the same level!)

well....about the problem...i don't know what to say to you chemelec today i tryed external power supply unfortunately i don't have a regulated one... and the noise still exists! the board has a TDA2822m for the satellite speakers and the other one is also a TDA but i don't remember which one, it has heatsink and it is a bit diffucult to remove it. But i know that it is bridged so it can output about 7W RMS for the sub-woofer.

I repeat the problem is not so important, i just want to fix it just for education reasons...so i can learn repairing things.

 

[whiz115]

it's not gain...i used a 1K resistor on the input jack between signal and ground and nothing happend.

 

[chemelec]

That Doesn't Change the Gain.
That Just helps Eliminate the possibility of it coming in from the input.

Is this a Hum, Or Static type noise

it's not gain...i used a 1K resistor on the input jack between signal and ground and nothing happend.

 

[whiz115]

That Doesn't Change the Gain.
That Just helps Eliminate the possibility of it coming in from the input.

Is this a Hum, Or Static type noise

sorry right....must be in series so i can change the gain right?

what do you mean by saying "static" ? i can hear it all the time, it's kinda low frequency noise and you can only hear it if you put your ear close to the speaker... it doesn't matter if i play with the volume or if the input is somewhere connected.

 

[chemelec]

A Steady 60 or 120 hz is a Hum.

An Eratic Scratchy Noise is typically a static problem.

If the Volume control is at Zero than the Preamp has effectively No Gain. But the Power Amp Still has gain.

This can Pick up Hum from the power supply or outside sources.

But if it that low in Volume, Why worry about it. No Amplifier is Totally Quiet.

Gary

sorry right....must be in series so i can change the gain right?

what do you mean by saying "static" ? i can hear it all the time, it's kinda low frequency noise and you can only hear it if you put your ear close to the speaker... it doesn't matter if i play with the volume or if the input is somewhere connected.

 

[whiz115]

But if it that low in Volume, Why worry about it. No Amplifier is Totally Quiet.

Gary

...........

I repeat the problem is not so important, i just want to fix it just for education reasons...so i can learn repairing things.

 

[whiz115]

A Steady 60 or 120 hz is a Hum.Gary

that's the case and i guess it's not in the normal behaviour of an amplifier
i have built a Gainclone (lm3875) a TDA1553 and not even these that i made
behave like that.

thanks for your help anyway.

 

[audioguru]

Maybe the power supply rectifier bridge has a blown diode in it.

 

[whiz115]

Maybe the power supply rectifier bridge has a blown diode in it.

Audioguru i said that i changed the diodes for the bridge!

 

[aljamri]

I've been adviced to do the following by a great fellow in this forum:

bring a metal sheet, put your device on it and connect a peice of wire to the ground of your dc side. This will eleminate any noise exist due to poor grounding.

 

[whiz115]

I've been adviced to do the following by a great fellow in this forum:

bring a metal sheet, put your device on it and connect a peice of wire to the ground of your dc side. This will eleminate any noise exist due to poor grounding.

As Audioguru said...i think it's poor design and probably i can't do much but can you explain what you said in more detail and i probably try it as a last chance maybe.

 

[whiz115]

looks like i'll short circuit everything if i do that... can you explain it better?

 

[whiz115]

Mr Aljamri the way you said it, is going to eleminate noise once and for all...that's for sure! can you explain your thinking please?!

here's some photos, pointless maybe but anyway...

 

[audioguru]

There is a connector with 2 thick wires and another connector with 3 thick wires. One connector has AC from the transformer and its wires must be kept away from the volume control.

The rectifier diodes might be radiating a buzz into the nearby opamp wiring.