LM3886 Buzzing

[Bengali]

Hi,

I've finished the LM3886 chipamp/snubber dual mono
pcb. I'm using one transformer and two snubber boards
to each LM3886 pcb.

Upon power up, everthing is fine until I connect
the RCA cables from my preamp to the LM3886 inputs, it
buzzes like crazy. No connection at the input, no buzz.

This Buzzing noise is oscillation?

I'm using mogami 2534 for the interconnects. One
channel is about 3.5" long and the other is about 1.5"
from the pcb to the rca connector. I'm not using the shield,
just the twister pairs for + and GND.

Do I need to add a resistor between the + and GND
at the RCA connector?

I'm also using the Ci 47uF cap if that makes any difference.
It did lower the dc voltage to about 1mV for each channel.

Now only if I can get pass this buzzing problem...

Thanks.

[Bengali]

I have also found that if I disconnect the rca cable from one channel the buzzing goes away. If both Left and Right channels are connected, it will buzz.

The buzzing always comes from the same channel.

Now, I tried with just one channel, playing back a cd source, the volume level is low and distorted even if I turn it up.

I would appreciate your input. Thanks.

[dirtyk777]

From this "buzzing", assuming its coming from speakers, might be a grounding problem.

[dirtyk777]

Sorry..actually red your text. You need balanced audio. This means:
1. You must have a + , a -, and a Ground/Earth. You cannot have just a positive and ground to be considered balanced audio. This is more than likely the cause of your buzz due to interference.

Look it up. An Example.
One major difference with balance audio and unbalanced is the difference between a TRS(tip ring sleeve) and a TS jack(tip sleeve). TS quarter inch jacks are your common instrument cable jacks(bass, guitar) that plug into their specified amplifier cabinets. For amplifier audio that involves speaker output, you need a TRS jack. Tip positive, ring negative, sleeve ground.

[Bengali]

thanks. I will try standard coax rca cable instead and see what happens. The mogami 2534 has 2 twisted pairs and a shield.

I've tied the blue pair to positive and the white pair to (-) plus the shield as rca interconnects on my off the shelf equipment, no problems here.

I just did not use the shielding between the LM3886 pcb and the rca jack. I guess I could try using the shielding as well to the (-).

One of the channels is highly distorted and has no volume.
I guess the chip is defective. Will have to replace that one as well. not much luck so far.

[ianpengelly]

Hi Bengali,

I have a similar set up, are they the chipamp boards per chance? If so have you connected the CHG of each board to the other? I found that when I used a thick copper cable between them and kept its length as short as possible it massively reduces the buzz/hum.

Cheers,

Ian

[AndrewT]

Hi,
sounds like an earth loop is completed when the shields from the two RCAs connect.
The problem is that the chipamps each have their own central ground and these central grounds are connected, probably to safety earth.

You cannot expect ALL your source equipment to have isolated RCA grounds. So, you must isolate the grounds in the power amp.

The very short, thick wire between the chipamp central grounds attenuates the problem but does not eliminate it.

Can you provide an annotated pic of the internals or a layout drawing of the physical relationship between the various components inside the amp.

[ianpengelly]

Andrew,

What would be your proposal for isolating the grounds in the power amp? If I remove the ground wire connecting the two boards the problem gets worse, yet the grounds are effectively isolated?!

Ian

[markiemrboo]

Originally posted by ianpengelly
Andrew,

What would be your proposal for isolating the grounds in the power amp? If I remove the ground wire connecting the two boards the problem gets worse, yet the grounds are effectively isolated?!

Ian

Just to make sure, I think what Andrew means is that the RCA barrel should be insulated from touching the chassis, and thus connecting the input ground directly to safety earth. The RCA connectors should come with a sort of.. plastic washer to do this.

For what it's worth, attached is a picture of the grounding scheme that has worked for me, but I have totally separated the ground traces on my PCB (that is, on the amp PCB's there is no trace connecting power ground, speaker ground and input ground together).

My signal ground and speaker ground from the connectors actually go back to the PCB, then I take the trace they connect to back to the disconnecting network as shown in the picture. Andrew has advised me to just take these connector grounds from the connectors straight back to the disconnecting network separately instead of the PCB, but... it's working fine, so I haven't got around to doing this. You know how it is!

[AndrewT]

Hi Ian,
seems like a similar, but slightly different problem.

Put a multimeter set to low ohms between the input RCA barrels.
does it read less than 1ohm?
If so, you have an internal half loop.
Connect an external half loop and you will get hum and/or buzz.

All those wires running to the disconnecting network are not needed.
Your layout would suit a central star ground (audio ground) right in the middle of the group of four. Then run a single wire from audio ground to disconnecting network.

Beng,
have you connected the audio ground to the common wire/plate between the smoothing caps?
Then that is probably the source of the buzz rather than the hum.
Have you connected the mains safety earth to chassis?
Have you connected chassis to any part or parts of the audio ground system?