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phrarod 27th April 2010 04:04 AM

Need ground advice on a DAC with tube output
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I had someone put my dac into two chassis and I still get a decent amount of hum. I went through the grounding scheme and made an illustration.

My questions are:

All the digital and the analog grounds are star grounded to one point in the Dac Box. I want to separate the digital grounds from the analog grounds.

I could float the B- for tube circuit including Fil supply in the Dac box. Since the DAC box is sharing chassis ground with the PS Box I'm not sure how to separate them.

I could separate them and float both star grounds of digital and analog above ground at separate points.

Also I want to get the filament supply as quiet as possible. In the past I would make pseudo center tap with 2 100 Ohm resistors and tap the middle to B-. Not sure the best way to do it when one is in another box.

I should also add that in addition to the dac board in the dac box it has its own separate power supplies - two (one bi-polar) for a total of three supplies being run by two torodial transformers coming off their own power cord. The torodials are shielded with mu-metal from the supplies which in turn are shielded by a grounded shield from the dac board and the analog stage.

samoloko 27th April 2010 05:58 AM

Is the hum only with chassis or every time
what uF you are using to filtrate B+
I had hum when I got 47 uF / 450 V as a first cap and 100 uF / 450 as a second
when I replace them with 330 uF / 450 V as a first and 220 uF / 450 as a second I got rid of hum

phrarod 27th April 2010 06:02 AM

Hum is consistant. The last caps are 33uF/400V. Before those are 47uF/400V and before those several chokes and before the chokes are EZ81 tubes.

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