Mauro Penasa MyREF AMP hum problem.

[Amadeus00]

https://drive.google.com/open?id=0B3Ov7S3UKQSfdVVhc1l6ZTFpN0U

 

[bk856er]

Secondaries of the transformer are paired correctly to the correct terminals. Input is also connected correctly. RCA are isolated from the chassis. I was manipulating with the input cables from the RCA connector leading to the PCB, and it seems like the hum depends also on their location within the space.... strange ....
I am sure there is a DC in the mains, toroid used to have periodical noise, after I attached DC filter, periodical noise has disappeared.

Use a meter to verify the chassis RCA jacks really are isolated, even when they are moved around. Depending on the design and install method these can fool you.

Unless you use shorting plugs or something equivalent, you have floating inputs that can pick up noise and that's not an indication of a problem. With sources connected you can have ground loops that have origins in the source. Shorting plugs will help you to segment the issue.

Post some well-lit pics of the amp internals.

What are the specs on your toroid?

BK

 

[Amadeus00]

I verified with the meter that RCA connectors are really isolated from the chassis. I shorted the inputs and there were no hum cool. The toroid is 160 VA 2 X 24V from INDEL (Polish manufacturer).

 

[Amadeus00]

Some photos.

 

[AndrewT]

What type of cable (the four black insulated wires) have you used for the RCA to amp connections?
I can see some white inner insulation.

 

[vladimirb0b]

You MUST remove that connection from the transformer securing bolt.
It is acting as a shorted turn and injecting a large AC current into the Protective Earth.
Use Green/Yellow for the Protective Earth. That the standard colour in many countries.

I have wondered this before so will ask here:
How do you stop the bolt from touching the chassis? Heatsink tubing?

 

[Amadeus00]

I used stereo cable where each signal wire is shielded, but I did not used the shield. I was using only the signal wires. Maybe this one should be the right choice. In that case I would need only two of them instead of 4.

 

[AndrewT]

when you use coaxial, the core is the hot or flow part of the signal. The screen/shield carries the cold or return part of the signal.
You do not use two cores of two separate coax for the two halves of the signal.

 

[AndrewT]

I have wondered this before so will ask here:
How do you stop the bolt from touching the chassis? Heatsink tubing?

You can insulate the top plate and the nut and the bolt,
so that the lid can never touch the metal parts. A thick piece of tough (not brittle) plastic sheet glued over the top would do.

What you must not allow is a shorted turn around the transformer core.

 

[bk856er]

I used stereo cable where each signal wire is shielded, but I did not used the shield. I was using only the signal wires. Maybe this one should be the right choice. In that case I would need only two of them instead of 4.

Wire the RCA connector like your second pic, as AndrewT explained. Make sure none of the braided shield contacts the hot/flow. Molex KK are good.

No hum with shorted inputs means your amp is fine. Clean up the input wiring and you might be all set. Your other wiring could use some color coordination and shrinkwrap or sleeving for better protection ;-). Don't forget to connect the safety/earth wire at the filter entry.

If you still have hum (with source connected), seems you need to focus on your sources. Sometimes it's only a matter of plugging them into the same outlet as the amp to resolve pesky ground loops.

BK

 

[asuslover]

I have to ask... what kind of insulation you are using for the lm3886 chips?

 

[Amadeus00]

I am using ceramic insulation.

 

[Amadeus00]

There is no hum finally. I removed old wiring from RCA to PCB, and then used twisted pair. It is working very well.

 

[AndrewT]

Loop area !

The two separated wires for the flow and return signal had a big loop area and this operated as an aerial picking up any interference they could find.

Use two wire connections between modules and minimise loop area.

 

[Amadeus00]

What exactly do you mean by using two wire connections between modules?

 

[AndrewT]

What exactly do you mean by using two wire connections between modules?

All current that comes out of a Source MUST return to that Source.

That flow and return is mandatory for EVERY source to operate as a source of current flow.

Flow and Return equals two wires and they will pick up interference if not close coupled.

So we arrive at:
use two close coupled wires to connect every module to any other module.

Close coupled two wire connection can use:
A coaxial cable
A twisted pair cable
A figure of 8 cable pair
A screened version of any of the above.