USB DAC 5V handshake

[semente]

Hi,

 

I have a linear 5V 2A L-PSU with two outputs which is currently powering a CuBox-i. Between the Cubox I have a Y split USB adapter.

Is it a good idea to use the other output for the 5V handshake?

Or would this be the same as feeding the DAC with Cubox's 5V, defeating the purpose of an external clean 5V feed?

 

Thanks,

Ricardo

[mansr]

Exactly what will happen depends on how the PSU is wired internally. Best case, the outputs are individually regulated. If they are simply connected to a single shared regulator, whatever fluctuations are caused by varying load on one output will be seen by the other as well. Even then, it may be better than the Vbus power supplied by the Cubox. Either way, it should be harmless to try and see.

[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.

[semente]

3 hours ago, mansr said:

Exactly what will happen depends on how the PSU is wired internally. Best case, the outputs are individually regulated. If they are simply connected to a single shared regulator, whatever fluctuations are caused by varying load on one output will be seen by the other as well. Even then, it may be better than the Vbus power supplied by the Cubox. Either way, it should be harmless to try and see.

 

Thanks, that's reassuring enough.

[audiobomber]

On 2/16/2019 at 5:55 AM, semente said:

I have a linear 5V 2A L-PSU with two outputs which is currently powering a CuBox-i. Between the Cubox I have a Y split USB adapter.

Is it a good idea to use the other output for the 5V handshake?

Or would this be the same as feeding the DAC with Cubox's 5V, defeating the purpose of an external clean 5V feed?

I'm not sure how you are connected. I believe the best way to configure this would be with the DC output of the LPS connected to the Cubox. The date leg of USB cable would be connected to a Cubox USB port. The power leg of the cable would be connected to the LPS USB port. Is that what you are doing?  

[semente]

1 hour ago, audiobomber said:

I'm not sure how you are connected. I believe the best way to configure this would be with the DC output of the LPS connected to the Cubox. The date leg of USB cable would be connected to a Cubox USB port. The power leg of the cable would be connected to the LPS USB port. Is that what you are doing?  

 

The LSPU has two outputs: one powers the Cubox, the other is doing handshake duties with the DAC.

[audiobomber]

On 2/28/2019 at 5:03 PM, semente said:

 

The LSPU has two outputs: one powers the Cubox, the other is doing handshake duties with the DAC.

I still don't understand how the Cubox, DAC and power supply (power supplies?) are interconnected. Can you explain in detail?

 

I have three dual head USB cables. I have two DAC's with separate power supplies, and a DAC that does not use USB power. Handshake is not something I've ever considered. All of my experience so far says it is advantageous to block USB 5V power from the source computer to the DAC.

[semente]

6 hours ago, audiobomber said:

I still don't understand how the Cubox, DAC and power supply (power supplies?) are interconnected. Can you explain in detail?

 

I have three dual head USB cables. I have two DAC's with separate power supplies, and a DAC that does not use USB power. Handshake is not something I've ever considered. All of my experience so far says it is advantageous to block USB 5V power from the source computer to the DAC.

 

My DAC doesn't work without the 5V handshake.

 

I have a single LPSU like this one with 3 outputs:

https://www.audiophonics.fr/en/hifi-power-supply/audiophonics-lpsu25-linear-regulated-low-noise-power-supply-usb-220v-to-5v-2a-25va-p-11364.html

 

One output of the PSU feeds the Cubox, the other handshakes the DAC using the USB type-A leg of this Y-cable:

https://www.audiophonics.fr/en/various-adapters/usb-b-adapter-cable-for-external-usb-a-power-supply-p-8389.html

 

The Cubox outputs to the DAC with a USB type-A male to type-B male adapter like this one into the type-B female socket of the Y-cable:

https://www.amazon.co.uk/gp/product/B01BV2VRK2/

 

 

These images found on the web may help you visualise the layout (yellow bit USB type-A male plug goes in the DAC):

 

 

[audiobomber]

I believe you have misunderstood the Audiphonics adapter cable's function. A normal USB 2.0 cable connector has four pins: 

The Audiophonics cable only has two pins, 1 & 4. The data pins, 2 & 3, are missing or blocked. The USB 2.0 cable that you connect to the Audiophonics adapter's USB A port has all five pins, but the USB cable's power pin will be blocked the Audiophonic adapter. In other words, 5V power is fed by the Audiophonics adapter only, data is fed by the second USB cable. The idea is to feed the DAC with a cleaner power supply than your Cubox USB port can supply. This has nothing to do with handshake.

 

You can prove this for yourself. Connect the Audiophonics adapter to the power supply and DAC, you will see that the DAC powers up. Connect the second USB cable to the DAC and Cubox, you will have data and power and can play music. Now disconnect the Audiophonics adapter from the PSU, the DAC will power down and cannot play. 

 

I don't know why you are using a USB gender changer. You should be using a normal USB A to USB B cable between the Cubox and Audiophonics adapter.

 

I don't know about the Cubox, but I get no improvement by using a better power supply to feed my Raspberry Pi. I do get better sound quality when I power my DAC from a separate power supply.

[semente]

As a non-native I may have expressed myself poorly but I am doing exactly what you describe: the Cubox into the female type-B socket provides data only, the L-PSU via leg with type-A plug provides power only.

 

My DAC has a proper inbuilt PSU (and no provision for using an external one) but needs the 5V to work. It is not powered by the USB 5V line.

 

I used to have two L-PSUs but sold one as a bundle with my R-Pi and HiFi Berry DAC.

This Audiophonics PSU is out of stock at the moment so I asked if it would be possible to use my only L-PSU to both power the Cubox and to perform the handshake. I've tried it and it appears to work normaly without any downsides.

 

Why should I use a cable? I could use a USB cable but the short adapter lets me fit the Cubox tidily behind the DAC. And cables should be as short as possible anyway.

[audiobomber]

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.

[asdf1000]

6 minutes ago, JohnSwenson said:

but in these cases there has ALWAYS been some other path where the computer ground gets connected to the USB receiver chip's ground.

 

 

Interesting John. Can you share these potential mechanisms?

[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.

 

 

[Jud]

9 hours ago, semente said:

As a non-native I may have expressed myself poorly but I am doing exactly what you describe: the Cubox into the female type-B socket provides data only, the L-PSU via leg with type-A plug provides power only.

 

My DAC has a proper inbuilt PSU (and no provision for using an external one) but needs the 5V to work. It is not powered by the USB 5V line.

 

I used to have two L-PSUs but sold one as a bundle with my R-Pi and HiFi Berry DAC.

This Audiophonics PSU is out of stock at the moment so I asked if it would be possible to use my only L-PSU to both power the Cubox and to perform the handshake. I've tried it and it appears to work normaly without any downsides.

 

Why should I use a cable? I could use a USB cable but the short adapter lets me fit the Cubox tidily behind the DAC. And cables should be as short as possible anyway.

 

I've also powered the CuBox with a battery - 24,000 mAh would go for days or weeks IIRC.

[asdf1000]

1 hour ago, JohnSwenson said:

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.

 

Ah yes. I remember having a discussion with you about leakage current loops and paths.

 

And after that discussion I wondered how critical isolating the USB source ground was - given all the ways leakage currents can get into and through DAC/pre/power amp etc... of course it depends on the overall system as you say. But trying to predict the paths these leakage currents can go, can't be easy. And the moment components in the system, the more complicated it can get.

 

I guess having more than one thing earthed can make things worse (more difficult) as you hinted there.