LM3886 buzzing on connection..

[diarav]

Hi,
Yes, I searched and I read most of the posts before asking this question. I am unable to conclude based on the posts hence I am asking this.

I constructed my LM3886 amp (ESP schematic - project 19 http://sound.westhost.com/project19.htm ) and powered up to test. Speakers are silent.

I connected the RCA of my adaptor powered CD player and the amp started buzzing. I connected my cable TV STB audio RCA and no buzzing - audio is clear. I connected my computer's audio out and no buzzing - audio is clear here too.

One of the remaining tasks is the disconnecting network between central star ground and mains earth. These are NOT connected now. Apart from this followed "almost" all grounding rules mentioned in most of the posts here.

Will the connection of the "disconnecting network" solve this buzzing from CD player?

I am also planning to add a simple pre-amp ckt between RCA and Poweramp, will this help?

[abid_rehan]

my only guess would be if everything else is working well and only CDP causes buzzing than probably your CDP uses positive ground.
If my guess is correct then whatever you add wont work, unless you change to other CDP. Then again there are lot of experts here may be they could enlighten.
Hope you have not connected RCA to "SIM" as mentioned in the schematic.

[diarav]

Hi Abid,

Thanks for your reply.

Hmm.. so it's time to change my CD player anyway.

No, the SIM is not a part of my circuit. Everything else is.

[abid_rehan]

hope u could borrow another CDP for testing.

[lineup]

Originally posted by diarav
Hi Abid,
Hmm.. so it's time to change my CD player anyway.

No, the SIM is not a part of my circuit. Everything else is.

connect a pair of headphones to CD RCA output
... and listen

Do not use very high volume!
You may have to use some adapter
... I would try to put together some custom home made adapter for this.

PS. If you have some older HeadPhones, use them.
I wouldn't use my new €200.00 phones

[abid_rehan]

Hmmmm!
righto even CDP may be faulty.
Sorry missed it.

[lineup]

Originally posted by abidr
Hmmmm!
righto even CDP may be faulty.
Sorry missed it.

Might be.
We have to exclude that possibility.

I think you are probably more close to the true reason, abidr.
It has got something with ground connection to do.
I wonder what kind of buzzing or sound it is. Is it hum (100hz)?

Strange he would get this, as he has tried to pay attention to good star grounding.

But would not need much, to create a ground loop.
That is, when Chassis is in touch with 0V Ground in more than 1 place.
This is why we use plastic insulated RCA chassis jacks in amplifiers.

[diarav]

Again, Thanks for your replies.. but..

Originally posted by lineup

connect a pair of headphones to CD RCA output
... and listen

Actually the CD player is an old compact portable type VideoCD player. It has a headphone jack splitting into an RCA connection. Would you please clarify what you mean by the above..I did not quite get you.

Originally posted by abidr
Hmmmm!
righto even CDP may be faulty.
Sorry missed it.

Hmm.. I dont think so. This plays without a hum on my TV (currently being used as an amp )

Originally posted by lineup


I wonder what kind of buzzing or sound it is. Is it hum (100hz)?

Not sure. Could be ..

Originally posted by lineup


But would not need much, to create a ground loop.
That is, when Chassis is in touch with 0V Ground in more than 1 place.
This is why we use plastic insulated RCA chassis jacks in amplifiers.

The RCA sockets are insulated from the chassis. Also the 0v is connected only at the central ground.

[lineup]

well
After this info, where everything seems pretty good, CDplayer + insulated RCA,
I am lost.
---------------------------------------------------------------


One thing you could do:
Try to reverse the signal and ground coming from CDplayer.
Connect signal to 0V-gnd in amplifier, and the cable shield to +input.

I have no more clue what could be trouble.
Hope you find the source to this strange issue.
If I were you, I should start checking my Chipamp.
Especially around the input pins.
Because there is where you attach your CDplayer.
And if it is dead quiet without CD connected,
then the trigger of this problem is via input rail .... somewhere there.
Check that you have connected, soldered all components correctly.

But frankly, if everything works nice with other sources ....
your amplifier should be perfectly alright.

lineup

Article and instructions, if anyone else wants to have a look:
http://sound.westhost.com/project19.htm

[CarlosT]

Does turning on the CD player make any difference?

Playing around my LM3875 iPod powered speaker, I found a similar phenomenon. When I hooked up an iPod though a 1/8" stereo jack to RCA adapter cable, the amp would buzz slightly. As soon as the iPod was turned on, it would go away never to rear its ugly head again until I turned off the iPod.

If you play with electric guitars, you'll find the same thing. If you connect an instrument cable into an amp, it will buzz until an instrument is plugged in at the other end.

This could be due to EMI coupling of the lead...maybe picking up stray EMI in the air or in your case maybe due to a ground loop? If the CD player is plugged in, its neutral AC main may be causing a ground loop with the amp's neutral AC main? Remember that the AC neutral is tied to ground at your house breaker panel...that could make for a very long ground loop.

If this sounds ridiculous, excuse my ignorance. I'm not an audio engineer but I play one on TV