Buzzing Noise from USB

[ZGojira]

I'm not sure if this is the best place to post this question. Let me know if there is a better forum for this question

Long story short, I recent got a singxer SU-1 (along with a LPS), connected to a Holo Spring using I2s, and to a stax sr-001 system.

I plugged everything in and noticed a hum/buzzing noise coming through, the noise that you sometimes hear when you go near a speaker. After trying all the audio interconnects with no differences, it turned out to be the USB cable.

Initially I was using a 5m USB2.0 cable (a very nice looking and thick one, not a 50c hairline thin one), because the original cable was too short. After replacing it with the cable that came with it along with shorter (but also nice and thick) USB3.0 extension cable the noise disappeared.

I also tried using the Holo Audio Titanis (800ma version). with the 5m cable and in-between the extension cable and the original cable. In both instances the buzzing/humming only got much worse.

So here's the question... Why is this happening?

I am not a believer in digital cables, i.e. I believe that 1s and 0s are 1s and 0s, they either get there or they don't. As long as the cable is to spec, everything sound work as intended. (i.e. I don't see banks using "premium" USB/Ethernet cables) And I don't have enough experience with analog cables to make up my mind on the subject...

I understand that if the devices uses the power from USB, then noise from the power supplied from the USB might induce noise into the circuit.

Below are the possible reasons things I can come up with (in no particular order):

1. The SU-1 uses power from the USB (i.e. I wasted my money on an LPS)
2. The cable was too long and the humming was signal loss (I did not get any dropouts..only noise)
   
3. I'm crazy and digital cables matter
   
4. Something else I missed?

I do not intend so start an argument on cables, power, etc..., only to seek a possible explanation/theory of why this happens and how to prevent it in the future. i.e. I am not interested in how to make it "sound even better", but want to know "where this noise come from"?

 

[elan120]

The low humming noise typically signified a ground loop issue. How is the power connected to the LPS (does power core has earth ground)? If SU-1 can still be connected using its stock power supply without going through LPS, maybe you can try this route and see if the humming noise goes away. In any case, the first possible suspect based on your description sounds like grounding related problem. My $0.02...

 

[uncola]

definitely try replacing all cabling one by one.. also since you're using i2s I assume you set the jumpers on the su-1 correctly for your dac's i2s standard pinout. other than that.. yeah could be an lps issue, grounding etc

 

[ZGojira]

The LPS is connected to a monster power conditioner.

My other amp used to hum, that disappeared after I got the power conditioner.

The part that I don't understand is why this hum was gone once I replaced the USB cable with another (shorter) one...

 

[elan120]

First of all, SU-1 does require 5VDC input from USB as handshake, but the power supply output 5VDC is what supplies power for the mainboard.

USB cable supplies power to SU-1 and that power returns via the earthed shield of the USB cable. Now, that longer the cable, higher the amount of resistance.

The hum is coming from voltage difference across the USB cable. It's a very small voltage but it comes from what we call a 'low impedance' source and the moment we provide alternative routes for that source, via additional grounds, we let the hum into the systems.

When replacing the cable with shorter length unit, it reduces the impedance mentioned above, hence, caused the hum to go away.

 

[JSands82]

Grounding issue, I had the same problem I had to reconnect everything from the power supply to the motherboard. Problem fixed.

 

[ZGojira]

First of all, SU-1 does require 5VDC input from USB as handshake, but the power supply output 5VDC is what supplies power for the mainboard.

USB cable supplies power to SU-1 and that power returns via the earthed shield of the USB cable. Now, that longer the cable, higher the amount of resistance.

The hum is coming from voltage difference across the USB cable. It's a very small voltage but it comes from what we call a 'low impedance' source and the moment we provide alternative routes for that source, via additional grounds, we let the hum into the systems.

When replacing the cable with shorter length unit, it reduces the impedance mentioned above, hence, caused the hum to go away.

Thank you very much for your reply!!

You should be very proud of yourself as you are the only one that actually read my post before replying

 

[winders]

First of all, SU-1 does require 5VDC input from USB as handshake, but the power supply output 5VDC is what supplies power for the mainboard.

I am fairly confident that the 5v USB VBUS powers the "dirty side" of the SU-1 while the power supply powers the "clean side". The SU-1 uses isolation chips to separate the two sides. Look at this image:

In this photo, the left side of the board is the "dirty side". You can see the separator and the isolation chips. The USB input is there on the back left. The let side is powered by the USB VBUS. The right side is powered by that white plug with the silver wires on the right.

 

[ZGojira]

I am fairly confident that the 5v USB VBUS powers the "dirty side" of the SU-1 while the power supply powers the "clean side". The SU-1 uses isolation chips to separate the two sides. Look at this image:

In this photo, the left side of the board is the "dirty side". You can see the separator and the isolation chips. The USB input is there on the back left. The let side is powered by the USB VBUS. The right side is powered by that white plug with the silver wires on the right.

I see ....

Any idea what the "dirty" side does and what the "clean" does?

 

[winders]

I see ....

Any idea what the "dirty" side does and what the "clean" does?

It's fairly obvious. The "dirty side" is the input side and the "clean side" is the output side.

 

[ZGojira]

It's fairly obvious. The "dirty side" is the input side and the "clean side" is the output side.

I think that part was clear

Any idea to what extent is considered "input"?
My idea of what it does is USB input -> controller/decoder - > XMOS 208 -> various controllers for output -> output

Is the decoder part of the "dirty" side?

 

[pinnahertz]

I am fairly confident that the 5v USB VBUS powers the "dirty side" of the SU-1 while the power supply powers the "clean side". The SU-1 uses isolation chips to separate the two sides. Look at this image:



In this photo, the left side of the board is the "dirty side". You can see the separator and the isolation chips. The USB input is there on the back left. The let side is powered by the USB VBUS. The right side is powered by that white plug with the silver wires on the right.

Hmm...the whole board looks very clean to me. No dirt anywhere.