USB DAC 5V handshake

[JohnSwenson]

7 hours ago, audiobomber said:

OK thanks, I see now. 

 

My Audiolab DAC is self-powered. I connect to my sms-200 with a normal USB 2.0 cable, but I found that blocking the 5V pin sounds better. Maybe it would be worthwhile trying that with your DAC? In other words, you would not use the Audiophonics cable adapter, just your other USB cable. I am under the impression that the handshake runs on ground, not 5V.

The term "VBUS handshake" is not an official term. There are two things that may fall under this name. According to the USB spec a device (such as a DAC) cannot connect to the D+,D- pins unless the VBUS pin is near 5V. Some DACs power some or parts of their circuitry from the VBUS pin, but even if they are self powered they are still supposed to look at the VBUS and not connect if VBUS is less than 4.75V or so. SOME DACs completely ignore VBUS and connect whether VBUS is powered or not, this is not spec compliant.

 

There is another use of VBUS, for OTG ports (which can be either host or device) under some circumstances VBUS is actually used as a communications line, the voltage is pulsed, this is used to resolve which type of functionality the port should take on. This is rarely if ever used with DACs, thus "VBUS handshake" usually refers to the above, the device should not connect if VBUS is not near 5V.

 

This all assumes that the ground line is always connected. Things get really wonky when people try and disconnect the ground line. Some aspects of the USB signaling will not work without the ground line, and the VBUS detect will also not work without the ground line. There are a few very specific instances where some people have gotten away with cutting the ground wire, but in these cases there has ALWAYS been some other path where the computer ground gets connected to the USB receiver chip's ground.

 

John S.

[JohnSwenson]

1 minute ago, Em2016 said:

 

Interesting John. Can you share these potential mechanisms?

Most of the time this happened the source had a connection to safety ground and something on the other side of the USB ground cut also had a connection to safety ground, such as DAC, preamp, power amp etc.

 

One system I heard about had several of these special USB cables with various injection schemes which wound up making a ground connection through one of the power supplies to something on the other side of the USB ground cut. That one took a long time to figure out what was going on since there were 4 different "special" USB cables involved. It took tests of all the cables to figure out what was actually connected to what to figure it out.

 

John S.