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Old 17th July 2014, 11:29 AM   #1
Pengvim is offline Pengvim  Norway
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Default USB DAC noise issues

Hi.

I'm in the planning process of a DAC at the moment, to replace my Topping D20 DAC.

I've been experiencing problems when connecting my laptop to the stereo, IF the power supply for the laptop is connected. (The battery is old, so the power supply is neccessary)
The same noise also appared on the headphone outputs when connecting these to my amplifier by a minijack-RCA cable as soon as the ground ring touched connector on the laptop, leading me to the conclusion that some sort of ground loop is generated.

This is obviously irritating as ****, but I've been playing with a few ideas on how to combat this by DAC design.

A few details about the rest of the project is probably in order first.
The DAC shall have 3 digital inputs: 2xOptical and 1xUSB (PS3, TV decoder, PC).
I'm planning to use a classic combination of DIR9001 and PCM1794.
USB device has not been decided yet. I'm very open to suggestions.

To select between the three sources I'm considering to use a MUX, controller by switches. Or is this unneccessarily complex?
I really don't want to route the digital signals to a switch and back..

1.
Use a digital isolator circuit, like the ADuM4160 for instance, to isolate the connection to the laptop itself.
This will most likely introduce a bit of jitter, but I'm not really sure if this will be audible.

2.
Use a bus-powered USB interface, followed by a digital isolator on the digital signals to the DAC.

3.
Digital isolator after input selection, so everything is isolated. Would require isolated power supplies also, so I would like to avoid this..

What do you guys think?
Am I going in the wrong direction?
These are the best ideas I've had this far..

I hope this will not be a pure reiteration of an old post..

Edit:
The PMC2706 looks promising for the USB interface.

Last edited by Pengvim; 17th July 2014 at 11:39 AM.
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Old 17th July 2014, 11:51 AM   #2
Pengvim is offline Pengvim  Norway
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I realized that option 3 would mean risking the noise going into the other sources, depending on the architecture of the input selection circuitry.

I'm not so keen on simulating this, but I think that the first option might be better for jitter since the USB signal is jittery already. Anyone disagree with that assumption?

Last edited by Pengvim; 17th July 2014 at 11:56 AM.
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Old 17th July 2014, 12:27 PM   #3
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You might consider the ESS ES9018 K2M in stead
of the PCM1794.
It has it's own muliti input S/PDIF receiver
as well as I2S and DSD inputs.

The PCM2706 is pretty limited, only
capable of 48k at 16 bits even though
USB full speed is capable of 96k at 24 bits.
I'd look into one of the modern USB high speed
interfaces offered. Check this forum for some examples.
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Old 17th July 2014, 12:46 PM   #4
Pengvim is offline Pengvim  Norway
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Quote:
Originally Posted by Avro Arrow View Post
You might consider the ESS ES9018 K2M in stead
of the PCM1794.
It has it's own muliti input S/PDIF receiver
as well as I2S and DSD inputs.

The PCM2706 is pretty limited, only
capable of 48k at 16 bits even though
USB full speed is capable of 96k at 24 bits.
I'd look into one of the modern USB high speed
interfaces offered. Check this forum for some examples.
That sounds very good, but I don't find it in any of the stores. Who sells it?

Good point. I'll research that a bit further before I make a desicion.
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Old 17th July 2014, 04:45 PM   #5
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Location: Toronto, eh
In Europe, ESS can be found here
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