My "audiophile" LM3886 approach

[Groundloops]

Originally posted by Russ White
It looks like you have your shielded cable attached to your case which is attached to mains GND. As a test try leaving mains GND unconnected and see if the hum goes away.

Well, that (nearly) did it. All what´s left with open inputs is a little, but clearly aubible hum. With CDP connected I got to put my ear at the speaker to hear it. I´ll try conventionally shielded input cables and shielding the wires from trafo too. Tinker, tinker ...

[troystg]

SOP-

Is your trafo center tap connected to the chss or pwr grd?

[Groundloops]

Originally posted by rabstg
Is your trafo center tap connected to the chss or pwr grd?

It´s connected to power ground. trafo has 4 secondaries, I use 2 of those per mono-amp.

[troystg]

Beleive it or not, you can connect the pwr grnd from the AC to the chassis, and NOT connect the secondary cntr tap ground to it. Then there will not be a grnd loop between earth and pwr.

As long as the input AC grd is tied to the chassis the cahssis can't become "live", and any pwr issues in the chassis will be grnded out to the AC input grnd.

The cntr tap grnd should remain "floating" from the chassis and AC input grd. As should the signal ground from the RCA's.

If that is done, I would then tie the signal sheild you have to the chassis again. But only once the AC grd and the secondary grd are seperated.

[Groundloops]

Originally posted by rabstg
Beleive it or not, you can connect the pwr grnd from the AC to the chassis, and NOT connect the secondary cntr tap ground to it. Then there will not be a grnd loop between earth and pwr.

As long as the input AC grd is tied to the chassis the cahssis can't become "live", and any pwr issues in the chassis will be grnded out to the AC input grnd.

The cntr tap grnd should remain "floating" from the chassis and AC input grd. As should the signal ground from the RCA's.

If that is done, I would then tie the signal sheild you have to the chassis again. But only once the AC grd and the secondary grd are seperated.

By "floating" you mean the center tap ground isn´t connected neither to chassis nor to AC ground. That´s what I did before. RCA jacks are fully insulated. HUMMMM !!!

Please ave a look at the two last pics: http://www.diyaudio.com/forums/showt...874#post744874 If I understand you right I did anything as you described ...

[carlosfm]

Originally posted by SmellOfPoo
Please ave a look at the two last pics: http://www.diyaudio.com/forums/showt...874#post744874 If I understand you right I did anything as you described ...

Oh... what a download...
Smell, if you want help, you need to post smaller pics.
That page takes almost 3Mb to load, most people will quit. I almost did.

Regarding you amp, it looks like you are using the shield of the signal cable to connect to the chassis, and at two different points. (!)
Am I right?
Disconnect those wires from the chassis.

I mean these two wires I'm pointing out on your pic.
PS: with a technical miracle, your 1Mb pic was cropped and compressed to 98.5Kb.

[troystg]

SOP-

I see a dual secondary trafo... I can only ASSUME you are using 1 bridge rectifier and are paralleling the pwr leads from the supply to the two broards.

I ASSUME the first configuration correct?

EDIT:

I just assumed he had 2 wires inside the shield, and the shield was only connected to the chassis.

SOP- Can you say if there is 1 or 2 wires in the shield, and is the shield tied to the grnd signal wire under the red heat shrink?

By the way, I still think it is a great execution of an amp, I'm just trying to help make it quiet.

[troystg]

Well I left work early today to come home and tinker.

I pulled the relay / pot brd off the chassis and looked on the bottom.

What to my wondering eyes did appear, but I reversed the coil polarity to each relay. DC coils, 50-50 chance. Go figure. Once I swapped the polarity so + went to +12VDC and the switch controlled the grnds as planned, everything worked great.

I fired it up and listened to a cd (Sony walkman portable) on my test speakers. ( I measured voltage offset the other day, 0.03 for the left, 0.00 for the right. Those "be" Milli volts from my Fluke 87!)

After listening on the old test speakers with my wife we closed each others jaw and I went set it up in my living room.

I connected my Sony DVD player via copper interconnects with WBT midline RCA's to the int. amp then out via Canare quad spkr cable to my "refurbished" with Hi-Vi drivers B&W Matrix 805's.

I listened to Lyle Lovett’s "I love Everybody" CD and sat in amazement.

It was equal to my Sony DVD transport via Co-ax to Proceed AVP via balanced Kimber ? ( the shielded PBJ, KC-1 maybe?) with Nuetrik XLR's to my Jeff Rowland Model 2.

And I can say that after 2 cd's! The tone, pitch and pace were dead on. It was smooth and DETAILED!

I will be bringing it to a friend’s house shortly to listen on his Madisound Thor kit. I happen to be "between" speakers at the moment. My new ones are not here yet.

Thus far I am absolutely amazed at what is coming out of this inexpensive amp!

Fabuluos job Mauro, and thanks for doing the boards / kits Russ and Brian! Now anyone can aford top notch audio.

[wwwblue]

I begin to make my amp...

NE5532 GM Vol control + LM3886TF classic circuit....

AB R,,,RIFA,WIMA Mkp...MUSE,,,Nippon Chel...

[Groundloops]

Originally posted by carlosfm
Oh... what a download...

Sorry for that, I have a SDSL connection and didn´t mention that others might go to the www with less bandwith. Next time I´ll use smaller and compressed pics.

Originally posted by carlosfm
Regarding you amp, it looks like you are using the shield of the signal cable to connect to the chassis, and at two different points. (!) Am I right? Disconnect those wires from the chassis.

I tried that before. No cure. Btw it is not a coaxial cable; it is a twisted pair. So signal ground is not connected to the chassis.

Originally posted by carlosfm
PS: with a technical miracle, your 1Mb pic was cropped and compressed to 98.5Kb.

I know that kind of miracle, next time I´ll make use of it.

Originally posted by rabstg
I see a dual secondary trafo... I can only ASSUME you are using 1 bridge rectifier and are paralleling the pwr leads from the supply to the two broards. ASSUME the first configuration correct?

Nope. Any board has it´s own rectifier. Power grounds are not tied to the chassis.

Originally posted by rabstg
I just assumed he had 2 wires inside the shield, and the shield was only connected to the chassis. Can you say if there is 1 or 2 wires in the shield, and is the shield tied to the grnd signal wire under the red heat shrink?

I can say that there are 2 wires in the shield and signal ground is NOT tied to the shield or chassis.

Originally posted by rabstg
By the way, I still think it is a great execution of an amp, I'm just trying to help make it quiet.

Thank you !