Help needed to sort noise issue with amp build

[cbdb]

the shields wont work if there not grounded at 1 end but I dont think thats the problem? How much noise is there with the inputs shorted at the XLR (short +, to - )

 

[cbdb]

Dont move the transformer! Sounds like you figured it out. And nice job on the chassis and wiring!

 

[ShinOBIWAN]

Dont move the transformer! Sounds like you figured it out. And nice job on the chassis and wiring!

Thanks for the suggestions cbdb. I'll rewire the amp and Anthony suggests that the input caps aren't needed since the DAC has its own caps on the XLR output so they'll go too.

Once this is done I'll see what the noise situation is like and report back but at least I have some direction again now.

 

[ShinOBIWAN]

Hi Ant,

See in the pic the amps are shielded from the centre (where the toriod and power supplies are) with the solid sheet of aluminum. This joins with the shelf where the modules are attached to shield the bottom also. Notice the aluminum wire chase around the heatsink that carries the shielded input leads to each amp module. The power leads are twisted and neatly attached to the shield.

Hey again John, that's a mighty fine build you have there. The more closely I look the more impressed I am at the small details you've considered.

I fully believe you when you say its completely silent. Great work mate.

 

[ShinOBIWAN]

To bring this to a close. Removed input caps and rewired after that no noise issues.

Thanks for the suggestions.

 

[MJL21193]

Good news.
It looks suitably imposing

 

[Nico Ras]

Okay, this is it. I must throw in some magic now that you can try. First a brief explanation why this works.

Having any number of equipment with with centre tap power transformers connected together (in this case two amps, but it could also involve the other equipment connected to it) could introduce "mains hum" into the ground system. This is because all grounds are not necessarily at the same potential and causes ac current to flow.

Now this ground or centre tap of the transformer is carrying a 50 or 100 Hz signal and the amplifier will treat it as such and produce it faithfully.

For your amp to play it does not really need a centre tap on the transformer, the current passing through your speakers passes through you power supply caps and not the transformer at all. This "zero" potential is only needed as reference in the DC path of the amp.

Disconnect the center tap of both transformers and feed the centre tap via a smallish resistor, say 10 - 22 ohms (1/2 watt is fine) and the hum would be reduced significantly but more likely disappear entirely.

 

[ShinOBIWAN]

Nico

The amp were never subject to ground loop but thanks for the suggestion.

It was AC entering the amps through the large film caps that were in series with the input.

John, thanks buddy.

 

[jaycee]

Looks nice in that case! If I remember right these are cases made for building HTPC's in. They're not cheap but they look great!