Raspi 2 and HiFiberry DAC+ coupled to a LM3886T Gain Clone Amp

[HormusPeston]

Congratulations.

 

[sunilmani]

Congratulations.

Thank you!!

 

[sunilmani]

In the process of getting the raspi powered up. Built the circuit, testing it right now.

 

[sadik]

All the Best Bro....

 

[sunilmani]

All the Best Bro....

Thanks Sadik

 

[sunilmani]

The dreaded hummmmmmm.......

There is bit of a glitch i am seeing. When the amplifier is power up and no input is provided; the speakers are dead silent. When i connect the input cable to amplifier, its still silent; but when the input cable is connected to source, i can hear a hum in the speaker :-( I tried with mobile phone and laptop as a source, but its all the same.

Somehow feel the cable could be a culprit. Any thoughts ??

 

[jls001]

Please explain the grounding scheme of your 3886.

Looks like you've basically introduced a ground loop via the input signal cable.

 

[sunilmani]

You are correct. When i tried creating the overall circuit diagram, i could see where i am creating a loop. Not sure how to avoid it though.

At the input, when i connect a stereo cable, it invariably shorts the two input grounds and needless to say that should be causing the loop issue. The million dollar question is...how to resolve it?:sad:

 

[sunilmani]

Reconfirmed...when i remove one of the inputs the humm goes away....so its a ground loop getting completed at input. Any suggestions??

 

[jls001]

Please tell us how the ground lead of the incoming mains supply is grounded in the chassis.

Start from the rear of the IEC socket and describe how the ground wire is routed, etc.

 

[omishra]

Reconfirmed...when i remove one of the inputs the humm goes away....so its a ground loop getting completed at input. Any suggestions??

Remove ground connection from any one RCA metal shell. Other still has ground connection. Then connect both RCA grounds together. This will work as you have single power supply shared among two mono boards.

In case of separate power supplies case could have been complex.

 

[sadik]

In case of separate power supplies case could have been complex.

I have Separate Power Supplies for both Channels (Jean Hiraga 30 Watts) and i have faced this hum issue. When i run single channel there is no hum, but as soon as i connect the 2nd channel RCA the humming starts.

I tried various methods to eliminate the hum, I did brought down the hum within acceptable limits, but didn't succeeded to completely eliminate it. I also tried by disconnecting the ground from 1 channel but was in vein.

I am using Cap Multiplier (2 Nos) each for single channel.

Any suggestions Mishraji?

 

[sunilmani]

Hi Sunil. Very elegant cabinet. As a few forum members had mentioned earlier, the speaker connectors are too close to the AC mains. The cables from at least one of the channels will end up being close to either the AC socket or the transformer. Also, twist the wires to and from the transformer, and those from the bridge rectifier to the filter condensors.

An excellent guide to wiring was written by a member of diyaudio, called Bonsai: http://hifisonix.com/wordpress/wp-c.../How-to-wire-up-a-Power-Amplifier_Updated.pdf

Cheers,
~hp

With the humming problem coming up. I am referring this now...Thanks a lot for sharing.

 

[sunilmani]

Remove ground connection from any one RCA metal shell. Other still has ground connection. Then connect both RCA grounds together. This will work as you have single power supply shared among two mono boards.

In case of separate power supplies case could have been complex.

This works Mishraji, but then .... the other ground should have been there as well...i mean only one terminal would work in case of a mono connection...does not seem complete

 

[sunilmani]

Please tell us how the ground lead of the incoming mains supply is grounded in the chassis.

Start from the rear of the IEC socket and describe how the ground wire is routed, etc.

I have kept the mains ground open for now - . Wanted to put a bridge rectifier and close the loop. Referring the the doc

 

[omishra]

This works Mishraji, but then .... the other ground should have been there as well...i mean only one terminal would work in case of a mono connection...does not seem complete

It seems you did not read my post carefully. I told to remove RCA end of ground wire from one input but that RCA ground is now need to be connected from another RCA. So both will work individually.

 

[sunilmani]

This works Mishraji, but then .... the other ground should have been there as well...i mean only one terminal would work in case of a mono connection...does not seem complete

Edit - Did not think through. Yes Mishraji, it would work.

 

[sunilmani]

It seems you did not read my post carefully. I told to remove RCA end of ground wire from one input but that RCA ground is now need to be connected from another RCA. So both will work individually.

I agree. I was starting for office when i replied to your email. By the time i reached office, i realized my mistake . You are right.

 

[sunilmani]

Remove ground connection from any one RCA metal shell. Other still has ground connection. Then connect both RCA grounds together. This will work as you have single power supply shared among two mono boards.

In case of separate power supplies case could have been complex.

I tried this out and its not working. If anything, the humming turns worse . I have not grounded the body and the earthing pin is also open. I am thinking of trying that out over the weekend. Hope that resolves this issue.

Thank you for your suggestion. If anything else i need to be trying, do let me know.

 

[HormusPeston]

A few tips I've picked up: The RCA connector should not touch the metal case; the connector to your mobile phone should be a standard TRS (not a TRRS); if you're using a laptop, temporarily disconnect it from earth by using a two-pin adapter; if you're using a shielded pair cable from the input RCA connectors to the preamp, DO NOT connect the shield (the braided copper) to the case; speaker negative should go to the power supply T-ground, not the amplifier board.

Cheers,

~hp