Help needed to sort noise issue with amp build

[ShinOBIWAN]

Hi folks,

I've recently completed a 4 channel amplifier based on Anthony' NXLSP400se modules. They work nicely but I've got troublesome noise issues that I'd like rid of. Unfortunately my knowledge only extent to soldering, drilling a few holes and wiring it all up in some fashion so hopefully you can help me out here.

Here's the problem:



This photo highlights the subjective amount of noise with no input and just the same loudspeaker connected in turn to each output. Number 1 is absolutely silent and I need to put my ear right up to the driver, number 2 is likewise nearly as quiet, number 3 is showing audible but quiet hum and number 4 is worst of all with a clearly audible hum from across the room.

With inputs connected the ranking is the same but on number 3 and 4, where hum was noted with no inputs, this disappears with inputs connected. However all channels have quite elevated noise levels in comparison to how very quiet number 1 was without input.

The circle in the photo shows where the chassis ground is.

These additional photo's show the wiring and general layout:





What I've tried so far:

Disconnecting the chassis ground from one of the PSU's and putting a ground link between it and the second PSU. This made no difference to the noise.

Thanks
Ant

[MJL21193]

Hi Ant,
I don't know how these amp modules are made, but in a multichannel unit like this, you need to ensure that the input grounds are isolated from the common ground. I use a 10 ohm resistor.
Also, you want to make sure none of the input leads are close to high power leads, AC especially.
I'll attach this drawing a made a while back to show how I do it. I'm not sure how helpful this will be for you, but I don't have any noise or hum with this.

[ShinOBIWAN]

Thanks John, I'm not sure about the input ground isolation either. These module have IP so no schematic and because I'm dumber than a bag of hammers I couldn't work it out from looking.

I'll ask the designer about the point you raised.

Good point about the main input, it is close to the input of one amp. I'll try jury rigging some wiring so I can take it well out of the case and away from modules. This'll let me check for any improvements that makes.

BTW I notice you've got 100ohm resistors inline with your chassis ground. Does this help with noise?

[MJL21193]

Originally posted by ShinOBIWAN


BTW I notice you've got 100ohm resistors inline with your chassis ground. Does this help with noise?

That is to isolate the chassis safety earth ground from the common star ground. In this scheme the chassis is connected directly to the safety earth on the IEC connector, and the common ground "floats" above it, separated by that 100 ohm resistor. The resistor I use is a 10 watt cement and for extra safety, it is paralleled with opposing 1N4007 diodes (just in case a large current develops across this resistor).
The 10 ohm resistor for the signal ground has the same thing - opposed diodes in parallel. A couple of these resistors have burned, so my lesson is learned.
Hope this helps.

[ShinOBIWAN]

I see, ta for the explanation. Your one of the few people that actually puts it in terms simple enough for me to understand

BTW The input gnd is isolated from power gnd on the PCB using 10ohm resistor.

[MJL21193]

Ok, the inputs at the rear of the case are RCA's? If so, did you isolate the ground part from the case. See here:



These do not touch the metal case, but go directly back, individually, to each amp module.

EDIT: looked at your pics, they are XLRs. Are you using balanced input?

[ShinOBIWAN]

I use XLR balanced inputs since my DAC's use these. All connectors are mounted on perspex and that's bolted to the case.

[MJL21193]

The shielding for the XLR is grounded? Each input is separate and goes to it's own module?

[ShinOBIWAN]

Originally posted by MJL21193
The shielding for the XLR is grounded? Each input is separate and goes to it's own module?

I'm not sure what you mean. There's 3 terminals and a shield.

Pins 1-3 are hot, cold, ground and finally there's the shield.

I've only connected the pins and ignored the shield as I wasn't sure if it was needed nor where it should go.

Each input is as you've said, isolated and goes to its own amp.

[ShinOBIWAN]

BTW I think this shield is used when you mount the inputs on metal ie. part of the chassis. I'm not sure though. I've only used RCA's in the past.