Humming noise from DAC?

I recently re-purchased a Direct Stream DAC and Scott has been working with me on this problem, but I wanted to throw it out there and see if anyone else has any bright ideas.

I got a new preamp and the DAC within a few weeks of each other.
Once I started using the DAC, I noticed a hum from the system that does not go away unless:
I pull the RCA cables from the DAC.
I switch off the preamp.
Float the ground on the DAC with a cheater plug. (Floating the ground on the MZ2 makes no difference)

The system is a Linear Tube Audio MZ2 preamp/headphone amp
PS Direct Stream DAC
Berning ZH-270 (with 2 inputs and it’s own volume control)
All plugged into a Synergistic Research power cell 8 power conditioner.

To experiment I borrowed my headphone system components. Chord Qutest DAC & ZMF Pendant headphone amp/preamp.
Everything is single ended RCA cabled.

Here’s how the different combos went.

DSD and LTA MZ2 (as a preamp) I get hum.
DSD and ZMF Pendant (as preamp) no hum.
Chord Qutest and LTA MZ2 no hum.
Chord Qutest and ZMF Pendant no hum.
DSD into the ZH-270 No Hum.

Anyone have any idea where this hum may be coming from? I put new 6SN7 tubes in the MZ2 today and the noise was still there. I don’t currently have an extra pair of 12AT7 tubes to check those.

Probably groundloop related, if your preamp supports it balanced cables might help. Otherwise perhaps plugging changing which outlets your equipment is plugged into might help. A little more extreme might be to tie the DS ground to the preamp’s ground with, 10 or 12 gauge stranded wire like you might between a turntable and it’s preamp.

I’m sure it’s a ground loop too.
Unfortunately no balanced connections available.

As for plugging into different outlets, all 8 outlets are the same and not isolated (i’m pretty sure but still check)

where on the DS do you suggest I connect the ground wire?

Just float the ground on the DS as since you are plugging it into the grounded Power Cell it won’t cause any issue and this is the simplest solution that works. Ted’s solution of ground the two components to each other might also work if you have the spare wire and access to chassis grounding points.

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The safest is to one of the I2S (HDMI) connectors. I don’t remember which screws have nuts that might fall off and which don’t.

[Edit: I also agree with dawkinsj.]

I have the ground floated now and is quiet.
However, I’ve not had to do this before, and when you look at the other gear combinations that I tried, and were quiet, this makes me think there is something wrong somewhere.
That bothers me.

Groundloops suck, but they are impossible to get rid of in a safe system without optical connections or careful galvanic isolation to break each loop. The amount of interference depends on the shape of the loop relative to any interference and, at times, particulars of each device in your system.

In my house my DVR satellite receivers were the worst since they had to have an power ground, a telephone ground and a satellite dish ground. I had to get quality signal isolation transformer to keep that from messing with the rest of the system. Still I had quiet noise until I used balanced connections everywhere. My system is overly complicated so none of this is unexpected and none of it bothers me.

For what it’s worth, I have not experienced this issue using a DS sr. with an LTA microzotl preamp (which is basically the mz3’s big brother).

With that said, there have been ground loop issues reported with the MZ2… the suggested solution, per LTA, is to open the power supply and disconnect the green wire.

Really? I talked to LTA and they did not say anything about this. I lifted the ground on the MZ2 and it made no difference anyway, so I wouldn’t bother disconnecting the ground internally.

I gave up and returned the DAC to the seller since I couldn’t get any answers.

Probably the right call, I don’t love the idea of disconnecting the ground wire inside of the power supply.

I’m planning on the LPS+ Upgrade. If there is any issues with future DACs I’m sure that will take care of it.