Topping DX3 Pro+ - Quiet mains hum on RCA output

[Kaktus_Kontrafaktus]

I just bought a "used - as new" DX3 Pro+ via Amazon (Aoshida-Audio-DE), the price was low enough to take the risk (99.50€).

I resolved to thoroughly test it right away and I've found a possible defect (or maybe a design weakness?).

When I only have a toslink input connected (+ power & cinch on the back), there is a quiet hum on the cinch outputs while the headphone output is completely clean.
The hum goes away when I connect a USB cable (because that provides grounding?), it does not depend on which input is actually selected.

Somewhat annoyingly, the power amp I'm using for testing has a quiet mains hum (+ a buzz) of its own, I can't tell if the DX3 Pro+ hum fully goes away when USB is connected or if it just gets quiet enough to drown in the power amps hum. The hum does however clearly get louder (+~6-10db) when I disconnect the USB cable.
Grounding the DX3Pro+ through the metal body or the screws on the back (straight to a Schuko ground contact) doesn't change anything either.

Unfortunately, I only have access to a short ****** unshielded cinch cable right now (the power amp is connected to my Apogee Mini-DAC with an XLR/cinch cable), I'll have access to a Lake People G109 headphone amp, properly shielded cables and the Speakers the DX3Pro+ is actually intended to be used with (Adam T5V) next week, getting rid of my power amps own hum should make things clearer.

There is one thing I could try now, though. I have another external power supply, that I could try (it's working fine supplying my Apogee DAC):

Here are the specs compared to the Topping power supply (right):

This should work, right? Both center positive, the Voltcraft can be switched to 15VDC.

I once mixed up two barrel plugs behind my desk and accidentally connected AC to the 6-14VDC input of my Apogee DAC (result was fixable but not cheaply), so I'm careful about that sort of thing now...

 

[antcollinet]

What do you mean by "cinch"? Are you talking about the RCA output?

Im guessing your amp is not grounded (only two main pins connected to it)? If so you might cure both hums by connecting a ground wire to the amp or to the DAC. I wouldn't expect a different PSU to help, though properly shielded interconnect might.

 

[KaeliKoo]

I would be skeptical of a "Used / As New" DAC that is sold at a 50% discount from retail price...

 

[Kaktus_Kontrafaktus]

What do you mean by "cinch"? Are you talking about the RCA output?

Im guessing your amp is not grounded (only two main pins connected to it)? If so you might cure both hums by connecting a ground wire to the amp or to the DAC. I wouldn't expect a different PSU to help, though properly shielded interconnect might.

Yes, RCA.
The Amp is indeed not grounded.


Just to start with, there were some surge protectors used in the setup, removing those seems to have changed the behaviour.
I'm probably mostly testing the amps behaviour now, I'll be able to test the DX3Pro+ more cleanly next week with the Lake People G109.
(While this unit is/was intended for someone else, I also ordered one for myself)

I've spent the last hour A/B-testing, I have a number of noise sample to upload now...
I recorded very close to the left speaker, either between tweeter & mid driver or right in the base port.
I used my phone (LG V30) to record, very high, fixed gain setting.

For grounding, I connected one of the rear case screws of the Amp directly to the Schuko ground contact on a power strip. For A/B-testing, I plugged the plug of that power strip into the power strip that both Amp and DAC are connected to. The PC the DAC was connected to by USB is also plugged into that power strip.


Grounded vs ungrounded Amp (optical input only):
Amp ungrounded, DAC connected by RCA, only optical source connected (tweeter/mid):

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Amp ungrounded, DAC connected by RCA, only optical source connected (base):

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Amp grounded, DAC connected by RCA, only optical source connected (tweeter/mid):

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Amp grounded, DAC connected by RCA, only optical source connected (base):

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In this configuration, grounding the Amp seems to make things worse.


Connecting USB while ungrounded:
Amp ungrounded, DAC connected by RCA, plugging in USB at ~0:09 (tweeter/mid):

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Amp ungrounded, DAC connected by RCA, plugging in USB at ~0:09 (base):

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The hum gets louder when connecting USB, the higher frequency noise changes charakter but not volume.


Connecting USB while grounded:
Amp grounded, DAC connected by RCA, plugging in USB at ~0:05 (tweeter/mid):

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Amp grounded, DAC connected by RCA, plugging in USB at ~0:08 (base):

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With the Amp grounded, connecting USB makes things *much* worse.


Disconnecting the RCA connection (-->Amp off-->disconnect-->Amp on-->):
Amp ungrounded, USB connected (tweeter/mid):

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Amp grounded, USB connected (tweeter/mid):

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Ungrounded, disconnecting the source makes things worse.
Grounded, it makes things better.


Nothing connected to the Amp, going from ungrounded to grounded at ~0:05:

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I'm fully confused now.
Without a source connected, grounding the amp helps but with a source connected, it hurts.
Just a little with only the optical input connected on the DX3Pro+ but a lot with USB connected.

I can't seem to reproduce the original behaviour (with surge protectors in the mix), going from optical only to USB. USB makes things worse now.

The PC the USB cable is going to is on the same power strip as Amp and DAC, should I change that and see if that changes anything? (different outlet would be easy, different circuit hard but possible.)

 

[Kaktus_Kontrafaktus]

Quick update:
I switched back to my Apogee Mini-Dac. RCA to the Amp, toslink from the PC, also on the same power strip.
Should be equivalent to the Topping with only toslink connected. The Apogee has no USB, so no way to compare on that front.

With the Apogee, hum and other noise is like it was with the Topping and toslink only *but* it doesn't make any difference if the Amp is grounded or ungrounded.
This was the setup when I originally tried to diagnose the Amps hum problem (I decided that I was fine with not fixing it back then, didn't seem worth the effort).

The Topping is clearly making some difference here.

 

[antcollinet]

1 - Try grounding the DAC instead of the amp - ideally make sure ground is connected to any of the interconnect shields.

2 - Check interconnect routing. Make sure power cables are kept away from interonnect. Try to keep interconnect as far away from amp transformer as possible. Make sure no wires are coiled.

Are there any other connections to any device (including PC). If so disconnect everything not needed to get sound to the amp from one single source.

 

[Kaktus_Kontrafaktus]

1.
Grounding DAC (ungrounded->grounded->ungrounded->...) - Amp ungrounded, toslink only:
tweeter/mid:

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base:

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With USB connected to the PC, there is zero difference between the DAC being ungrounded & grounded.

2.
Already done as best I can.

There are no other connections, the Amp only has one input.

Power: PC, Amp, DAC - all on the same outlet through a power strip.
Signal: RCA DAC->Amp, Toslink PC->DAC, USB PC->DAC (when connected)


I'll the alternative power supply next.

 

[antcollinet]

On final thing I can think of to try. How about a good solid ground connection between DAC and Amp.

 

[Kaktus_Kontrafaktus]

On final thing I can think of to try. How about a good solid ground connection between DAC and Amp.

I'll try that after the power supply swap, brb.

 

[Kaktus_Kontrafaktus]

Amp & DAC ungrounded, toslink only (tweeter/mid):
Topping:

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Voltcraft:

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Amp & DAC ungrounded, toslink only (base):
Topping:

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Voltcraft:

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[Kaktus_Kontrafaktus]

On final thing I can think of to try. How about a good solid ground connection between DAC and Amp.

Amp ungrounded, toslink only (tweeter/mid) - Amp to DAC ground connection (using rear case screws, starting disconnected):

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[Kaktus_Kontrafaktus]

I think I'm done testing with this problematic Amp, I'll get back to it, when I have the Lake People G109 at hand.
That thing is dead quiet and it will we a simple setup.

There's nothing here that can be clearly attributed to the DX3Pro+ rather than whatever is actually wrong with the Sherwood AM-8500.