stereo amplifier problem

[epilot]

Hello guys,

Sorry but I have a question regarding a stereo amplifier which I made based on LM3785 chip.

I just used two separate LM3875 chips to make a stereo amplifier. To avoid crosstalk distortion the only thing which is common for both channels is the transformer. The supply chosen to be +-28V.
every channel is working just fine by using two separate mp3 Players, but unfortunately when I connect the Left and the right inputs of both amplifiers to a stereo input there is a hum and some kinds of distortion at the outputs(i.e at the L and the R speakers). I am not a master in the audio field so plaese help me and let me know what's wrong and how to handle the problem? I have used 2 coxial cables for L & F inputs and of course for L & R outputs. the ground trace of the whole system is common too.

Thanks in advance

[epilot]

ooops!!
I found that when I disconnect the ground wire of the center taped transformer which I used for the setereo amplifier the hum and distortion decreases significantly!!!
I am woundering why the circuit is working without the ground wire of the transformer and why the problem is decreased?

Any help please?

[firechief]

If you can post a picture that would help a lot. Are the grounds of both amplifiers connected in some way to the center tap of the transformer? When you disconnect the center tap of the transformer do things get pretty good or real good? Schematic is very important. If you dont have one, you would benifit a ton by sitting down a drawing one.
It will answer sooooo many questions.

[epilot]

Originally Posted by firechief

If you can post a picture that would help a lot. Are the grounds of both amplifiers connected in some way to the center tap of the transformer? When you disconnect the center tap of the transformer do things get pretty good or real good? Schematic is very important. If you dont have one, you would benifit a ton by sitting down a drawing one.
It will answer sooooo many questions.

Thanks a bunch for your help firechief,

I have no pic right now but maybe I put a pic...

Yea, actually the ground wire is the common thing for both amplifiers.
In any case the ground must be common becuase the L & R inputs need a common ground as well.

When I disconnect the center tap of the transformer the Hum is gone and both channels work fine without any hum. I have used four 6800uF filter cap for both channels (two for each) and yea I have used several 104 caps between the ground and the +- of power supply.

[epilot]

Ok, I just connected an inductor in series with the ground wire of the transformer and the problem is rather solved.
was that an common task to do??!

Are you thinking that a toronodial transformer will sovle the problem too? (I have used an EI transformer for the current circuit).

Anyway I don't know why I had the problem and of course why the problem was sovled by addin an inductor?

[ianpengelly]

Are you using two rectifier boards?

If so, then your problem is likely to be crosstalk caused by the transformer. I did a thread about it here:

Got Chipamp Hum? Possible Solution

[pacificblue]

Originally Posted by ianpengelly

Are you using two rectifier boards?

If so, then your problem is likely to be crosstalk caused by the transformer.

There is no crosstalk caused by the transformer. With two rectifier bridges you force the ground between each pair of rectifier bridges to a slightly different potential that depends on diode tolerances. When you connect those two grounds to each other, which you invariably do either through the source or through the PE, equalisation currents occur which produce hum.

Your solution is however correct. Use one transformer per pair of rectifier bridges.

Another solution is to use a center tapped transformer. Then you can use one rectifier bridge per amp, if one rectifier for all amps makes no sense to you.

[ianpengelly]

Pacific blue, you are right, it was the wrong term... bit late when I wrote that, but glad you agree with the solution. :-)