Experience with this DIY DAC ?

Hi guys, I've been reading this thread for long time, and heavily modded my DAC according to the suggestions here. Now I'm using UTC A-20 transformers. And one thing that is bothering me, is 50Hz hum. When DAC is off it is really audible, when it is on, hum is barely audible, at the same volume, but is still there. If I touch both DAC and amp cases it goes away. It is ground loop isn't? Trafos are not connected to ground, + and - terminals go to the RCA which are isolated from case. Maybe I need to isolate whole DAC board and power transfomer from the case?
Bill, if I remember correctly you wrote several posts about this, that we don't need to connect negative terminal to the DAC ground, but what will happen if we do?

Thanks,
Fedor
 
That's a real can of worms. Thinking logically it doesn't make sense, but I know strange things happen. Noise can be capacitively coupled through the trafos, but unless you have a connection to ground on the secondaries it cannot be a ground loop. Have you tried moving the dac away from all other equipment, the trafos could pick up stray magnetic fields from other sources. I would suspect your interconnect cables are at fault, too long maybe, or a problem with your preamp.
If you connect one side of the output to ground you will be creating a path for a possible ground loop, but it will work OK. Do not ground the output to the dac board, and don't ground the dac board to the chassis.
Let us know how you make out.
 
Thank you for your answer. Actually, I've tried few things, like taking power trafo out of the case, or holding output trafos in the air, so they won't touch the chassis, nothing helped. The interconnects are changed too, now they are 20cm long, so it can't be length. Interesting thing though, this problem appears with both of my DIY amps, but not with Yamaha RX-350 receiver. Maybe it's indeed problem with amplifier...
Anyway, thanks for the suggestions, I'll try them in a couple of days and post back.
 
I don't know anything about the UTC transformers, but in a general sense, nickle core transformers, and most signal level transformers are nickle core, are VERY susceptible to power transformer fields and really, any AC wires within about 2 feet of them. What you describe sounds like stray field coupling to the core of the buffer transformer. Start turning things at different angles, with respect to power and buffer. If it is an air borne field coupling between them, or between the buffer and any other source, a vector change will show up in the scope. You have to think in 3 dimensions for the transformer rotation with respect to outside fields. If a change does not show up on the scope, then you probably have a ground loop in front of the transformer primary.
 
Cans are your friends! Too bad that the only supplier left in the US requires a $1k purchase. You can use regular cold rolled steel, but you will want to have it heat treated to the Marstenite / Osteonite temperature to make it ductile again.
22 gauge buys you about 30 db of suppression and 16 gauge will get you 100 db.
 
I'm using those Tams I bought in my 2nd dac, I rotated them 90* and that got rid of the tiny hum I had originally.

In my old drag racing days we used to cook ring and pinion gears in self cleaning ovens to soften them up a little, stunk like hell, but it worked pretty good. A little OT.
 
That looks like a decent alternative to store bought opamps, very reasonable pricing. /QUOTE]

Contrary to popular belief Burson "borrowed" the design from GD and not the other way around.
I tried both Sun and Moon but did not notice any major improvement (soundwise) over LM4562.

It looked like a pretty simple circuit, and they usually benefit more from PS upgrades than the ICs. Did you try a shunt or anything better than the 317-337?
 
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