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Should DAC's USB inputs have galvanic isolation built-in?

B

busb

Mine's a pint of beer please
No point on headphone-only DACs or those that don’t have USB inputs. If I connect my Sony Vaio when powered by its external SM PSU to my M-DAC's USB port, I can hear mush regardless of music being played or not.
I can either run off the battery or buy an external isolator but it seems a lot more elegant to build this into the DAC unless it's a budget model.
 
Internal USB isolation does not offer effective RF isolation (the RF energy is already internal to the DAC chassis). For this reason its far better to isolate the 2 domains external to the DAC chassis. Sweeping with an RF spectrum analyser proves how ineffective internal USB isolation is WRT RF energy.

USB Isolation also requires power from the Host device - some products such as the Apple iPad CCK cannot supply the required power.
 
Thanks for your definative reply John. I therefore hope this topic is put to bed!
 
Not adding it in the box because it's better outside of the box, will tend to lead to a situation where some users have no filtering at all, as opposed to everyone with some filtering and those who give a rats ass and buying an external filter having too much. I know which i'd favour.
 
An optical connection would eliminate any electrical noise. I'm not talking about airbourne stuff like RFI and EMI, but noise transmitted from one unit to another by an electrical connection.

People bash toslink/optical. Toslink has a very good advantage over coax and USB under certain conditions.
 
That is the idea behind the pending MTRN which offers a Clock Locked optical link with a ASync USB 192KHz port for the MDAC - so both the CD and USB are clocked locked to the DAC and optical isolated for complete RFI and EMI isolation.
 
stu pitt said:
An optical connection would eliminate any electrical noise. I'm not talking about airbourne stuff like RFI and EMI, but noise transmitted from one unit to another by an electrical connection.

People bash toslink/optical. Toslink has a very good advantage over coax and USB under certain conditions.
Click to expand...

Actually, this only works if the toslink receiver part on the dac is really properly EMC shielded, otherwise it could be another entrance for RF or higher frequencies to enter the dac :eek:
all physical holes in the casing are important !
 
rbroer said:
Actually, this only works if the toslink receiver part on the dac is really properly EMC shielded, otherwise it could be another entrance for RF or higher frequencies to enter the dac :eek:
all physical holes in the casing are important !
Click to expand...
Like those venting holes on the top lid of the MDAC? :)
 
busb said:
No point on headphone-only DACs or those that don’t have USB inputs. If I connect my Sony Vaio when powered by its external SM PSU to my M-DAC's USB port, I can hear mush regardless of music being played or not.
I can either run off the battery or buy an external isolator but it seems a lot more elegant to build this into the DAC unless it's a budget model.
Click to expand...

. . . which is why it's always a good idea to minimise the problem at source.

If you seek the causes of that noise, you will find it's caused by a combination of the monitor's inverter, DC-DC switching on the mainboard and battery power management, the battery charger, the hard drive, bluetooth and wifi transmit/receivers, processor and bus activity, etc. For an audio-specific machine, much of this can be ameliorated with simple modification.

Once you allow the bull into the china shop, it's hard to stop it breaking something.
 
jirij said:
To be scientifically accurate here, quite the opposite. :)
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If it's on Youtube, I stand corrected! Possible, but still, I think difficult to stop a bull breaking china.

Sticking with the bovine, if you put a dead cow in a river, you could construct a water purifying plant downstream, but it's better just to remove the dead cow.

Obviously DAC designers continue to make every effort to reject jitter and noise, and transport builders should continue to minimise those problems at source. And amplifier, speaker and cable manufacturers should try to make their products good, too.
 
Is airborn RFI really the main issue? I'd have thought it mostly travels along the ground line. Even if it does enter the case, if you keep the track from socket to isolator very short it will cause significantly less problems than letting it run loose around the whole circuit. If it is still a concern then a small shield could be put around the socket and isolator.

Hmm.. or use toslink, or coax or AES/EBU with a simple transformer!
 
item said:
. . . which is why it's always a good idea to minimise the problem at source.

If you seek the causes of that noise, you will find it's caused by a combination of the monitor's inverter, DC-DC switching on the mainboard and battery power management, the battery charger, the hard drive, bluetooth and wifi transmit/receivers, processor and bus activity, etc. For an audio-specific machine, much of this can be ameliorated with simple modification.

Once you allow the bull into the china shop, it's hard to stop it breaking something.
Click to expand...

If I disconnect its external PSU, the noise disappears completely which points to the SM PSU & not other potential sources. Fortunately, I use an ATV2 & iTunes Match for most of my listening & if I wanted to use local files at higher than 256k, my PC is hard-wired to my router with only one leg over Wi Fi. If I disconnect the Sony's battery, I do get a very slight increase of SQ over USB than with the ATV's optical but not enough for me to worry about. The PC can be controlled remotely from my iPad as can iTunes Match itself.

Galvanic isolation could be built-in but John seems to be saying that it would not be worth the effort involved once the RFI is inside the main chassis. Although I've committed myself to iTunes I don't rule out experimenting with flac files at some point - an isolator from Farnell would probably be cheaper than a linear series mode PSU that may well be useless if I changed my laptop so eliminating the problem at source is not a serious option.
 
JohnW said:
That is the idea behind the pending MTRN which offers a Clock Locked optical link with a ASync USB 192KHz port for the MDAC - so both the CD and USB are clocked locked to the DAC and optical isolated for complete RFI and EMI isolation.
Click to expand...
This sounds very interesting but I'm not abolutely sure I understand where the optical isolation is, or to put it another way how the transport and the dac are connected. is there an optical clock link from dac to transport- but then how is the data sent from the transport to the dac. Is there still a 5v connection and ground to the dac? Is the Mtrans intended to be a sort of buffer between the dac and the computer?

Do you have a diagram that shows the connections. It sound like a different way of doing things.
 
busb, everyone assumes it is noise from the smps coming through the computer, maybe its coming straight through the mains from the smps?
 
sq225917 said:
busb, everyone assumes it is noise from the smps coming through the computer, maybe its coming straight through the mains from the smps?
Click to expand...

When I disconnect the USB lead (or PSU), the noise goes so I assume the RFI is conducted through the USB lead & not into anything else via mains. The level of RFI is fairly consistant & not sensitive to the time of day so it's not just crap on the mains.

At least Primare seem to have done a good job on the SMPS in my amp - no hint of conducted or radiated mush or from any PWM switching artefacts either.
 
busb said:
When I disconnect the USB lead (or PSU), the noise goes so I assume the RFI is conducted through the USB lead & not into anything else via mains. The level of RFI is fairly consistant & not sensitive to the time of day so it's not just crap on the mains.

At least Primare seem to have done a good job on the SMPS in my amp - no hint of conducted or radiated mush or from any PWM switching artefacts either.
Click to expand...

If you just look at the components in the PC you're plugging that USB lead into, it's truly scary. It's definitely the computer or its PSU that's causing the problem. That's what they do.
 
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