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PC noise with MiniDsp Flex RCA and valve amp

Rgv250

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Aug 3, 2024
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I'm having an issue with my MiniDsp Flex RCA and noise from the connected PC when using my Graaf integrated 25 valve amp. I also have a couple of Fosi V3 Mono's which I alternate with the Graaf and they are OK, although it seems (from what I've read) valve amps can be more susceptible to PC noise due to higher impedances? This has never been an issue with the Graaf at any time in the past. I mainly use the Graaf, but after a spell with the Fosi's returning to the Graaf was the first time I noticed the noise. The MiniDsp can even be off, but still connected in circuit and the noise remains.

I tried a few tests:-

  1. Tried different USB cables/ports between PC and MiniDsp.
  2. Use Different wall sockets.
  3. Moved things around, tidy cables etc.
  4. Removed all other cables (Sub and RCA) from the MiniDsp apart from the USB in and the RCA out.
None of the above made any difference. So, I removed the MiniDsp and substituted my SMSL SU-8 V2 DAC and that works perfectly, no noise. Same USB cable from the PC, same RCA cable from SMSL to the amp, just no PC noise.

Is there anything I could be doing wrong, anything else simple I have missed or any ideas why this is happening?
 
If the noise disappears once you remove the USB cable between PC and Flex, then you can fix the issue using a USB isolator (Topping HS01, Hifime isolator, JDS Synapse, Neutron isolator, etc)

Alternatively, you can use Optical to connect your PC to the Flex.
 
If the noise disappears once you remove the USB cable between PC and Flex, then you can fix the issue using a USB isolator (Topping HS01, Hifime isolator, JDS Synapse, Neutron isolator, etc)

Alternatively, you can use Optical to connect your PC to the Flex.
Great thanks for the info. It does disappear when removing the USB cable. I may have an optical cable I can try and will also look into USB isolators.

Out of curiosity, is this something that can start up out of the blue even though things were fine before? And be different depending on the amp in use i.e. one fine the other (valve amp) not?
 
Out of curiosity, is this something that can start up out of the blue even though things were fine before?

Yes - if you change anything about the wiring - eg where different devices are connected to power - or even just the position of wires relative to one another.

And be different depending on the amp in use i.e. one fine the other (valve amp) not?

Yes - Your tube amp will have an earth connection to it. The Fosi V3 don't (as far as I know). So that will significantly reduce any ground loop currents that can flow between DAC and AMP for the Fosi's


Similarly the Mini DSP has no mains ground, but the SMSL does. That will mean some of the ground current will flow to ground via the SMSL earth when that is in use, so less will flow between DAC and AMP along the analogue interconnect (which is where the noise is added to the signal)
 
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In case it helps someone reading this post with a similar problem, I purchased a Neutron Isolator V1 from the list suggested by @staticV3 and it worked perfectly. 100% elimination of USB PC noise when using my valve amp with a MiniDsp Flex
 
although it seems (from what I've read) valve amps can be more susceptible to PC noise due to higher impedances?
Partially true... Higher impedance inputs are more susceptible to noise pick-up BUT when you plug-in a low-impedance source, any radiated noise "sees" the parallel impedance which can be no higher than the lowest one.

And although tubes are "naturally" high impedance, sometimes there is a 10K (or so) resistor at the input.
 
In case it helps someone reading this post with a similar problem, I purchased a Neutron Isolator V1 from the list suggested by @staticV3 and it worked perfectly. 100% elimination of USB PC noise when using my valve amp with a MiniDsp Flex

Great that Isolator V1 managed to fix the issue! One more confirmed case of ground-loop noise elimination by USB isolator discussed actively here:
 
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