Chord Mojo(1) DAC-amp ☆★►FAQ in 3rd post!◄★☆

[x RELIC x]

Nice to see you back in the Mojo thread Rob.

 

[Arpiben]

There are two problems that USB has against toslink - and one benefit. The benefit is that timing comes from Mojo - but with toslink the incoming data has to be re-timed via the digital phase lock loop (DPLL) and this is not quite as good - but you will only hear the difference via a careful AB test, so it's in practice insignificant.

The downside with USB is the common ground connection. This will mean RF noise will get into Mojo, making noise floor modulation worse. Now I go to very careful lengths to remove this problem by using lots of RF filtering, and double ground planes on the PCB, but even minute amounts of RF is significant. The other problem is down to the way that digital code works - which is in twos complement. So zero is in 24 bits binary is 0000_0000_0000_0000_0000_0000. If the signal goes slightly positive then we get just one bit changing to: 0000_0000_0000_0000_0000_0001. But if it goes 1 bit negative all the bits change to:  1111_1111_1111_1111_1111_1111. Now the problem with this is that when a bit changes, more power is needed, and this injects current into the ground of the PC - and the ground will get noisier. Unfortunately the noise is worst for small signals. Now the problem with this is that it then couples through to Mojo's ground plane, and the distorted signal currents will add or subtract to small signals - thus changing the small signal linearity. This in turn degrades the ability of the brain to re-create depth information, and so we hear it in terms of depth being flattened. What is really weird about depth perception is that there seems to be no limit to how accurate it needs to be, so the smallest error is significant.

So with toslink we do not get these problems as there is no common ground - so no RF noise, no distorted signals on the ground, and it will sound smoother with better depth against a noisy PC. But the problem can be almost eliminated by using a power efficient USB source that is battery powered - such as a mobile phone. But with noisy PC's the only way of solving it is to use galvanic isolation on the USB - but this draws power from the source, and we can't do that with mobile devices. All of Chord's desktop DAC's have galvanic isolation on the USB, and then you can't hear whether its a noisy PC or a mobile phone. In this case, USB sounds slightly better than optical, because we have the (tiny) timing benefits of USB.

I hope that explains - its a complex subject.

Rob

Since Mojo has no galvanic isolation, I was thinking (maybe wrongly) using a DIY USB cable with Ground and +5VBus isolated, only D+ & D- wire directly connected.
At computer side a resistor would be inserted between Ground and +5VBus.
At Mojo side, a battery would be inserted for USB Audio port activation.

The aim is to remove RF ground noises without buying external devices (IMHO, too expensive).
I am also expecting some drawbacks by doing so.
Glad if you can comment.
Rgds.

 

[god-bluff]

Is it normal for the Mojo to emit a high pitched whistling sound whilst charging?

Its the sort of noise that drives me mad.

 

[audi0nick128]

Try different cable and or charger.

CHEERS

 

[Delayeed]

   
Anything I/we can help you with, or was it an ETA?

Oops, posted on the wrong thread. That's what I get for having over 10 Head-Fi tabs open in the morning hours... 

 

[Bricki]

  There are two problems that USB has against toslink - and one benefit. The benefit is that timing comes from Mojo - but with toslink the incoming data has to be re-timed via the digital phase lock loop (DPLL) and this is not quite as good - but you will only hear the difference via a careful AB test, so it's in practice insignificant.
 
The downside with USB is the common ground connection. This will mean RF noise will get into Mojo, making noise floor modulation worse. Now I go to very careful lengths to remove this problem by using lots of RF filtering, and double ground planes on the PCB, but even minute amounts of RF is significant. The other problem is down to the way that digital code works - which is in twos complement. So zero is in 24 bits binary is 0000_0000_0000_0000_0000_0000. If the signal goes slightly positive then we get just one bit changing to: 0000_0000_0000_0000_0000_0001. But if it goes 1 bit negative all the bits change to:  1111_1111_1111_1111_1111_1111. Now the problem with this is that when a bit changes, more power is needed, and this injects current into the ground of the PC - and the ground will get noisier. Unfortunately the noise is worst for small signals. Now the problem with this is that it then couples through to Mojo's ground plane, and the distorted signal currents will add or subtract to small signals - thus changing the small signal linearity. This in turn degrades the ability of the brain to re-create depth information, and so we hear it in terms of depth being flattened. What is really weird about depth perception is that there seems to be no limit to how accurate it needs to be, so the smallest error is significant.
 
So with toslink we do not get these problems as there is no common ground - so no RF noise, no distorted signals on the ground, and it will sound smoother with better depth against a noisy PC. But the problem can be almost eliminated by using a power efficient USB source that is battery powered - such as a mobile phone. But with noisy PC's the only way of solving it is to use galvanic isolation on the USB - but this draws power from the source, and we can't do that with mobile devices. All of Chord's desktop DAC's have galvanic isolation on the USB, and then you can't hear whether its a noisy PC or a mobile phone. In this case, USB sounds slightly better than optical, because we have the (tiny) timing benefits of USB.
 
I hope that explains - its a complex subject.
 
Rob

G'day Rob
 
Thanks so much for taking the time to share your wealth of information.
 
I think you have answered my question 

and very thoroughly too!! So if I understand correctly, asynchronous transmission over USB allows mojo to time the data, however via toslink the data is re-timed. I assume that the re-timing doesn't allow for as much wriggle room as timing it in the first place.....and so that explains why 2 optical sources can sound different because one had better timing to begin with than the other

 
So I need to get around my noisy PC issues with a jitterbug or something similar and ferite chokes or go down the spdif reclocker route.
 
I have had mojo for about 5 months now and during that time I have listened to so much more music than ever before, it has made me look back through my entire library and hear things I have never heard before. A truly amazing product that has taken me closer to all the music I listen to.
 
Seriously - thankyou Rob 

 
Cheers
Bricki

 

[audi0nick128]

Since Mojo has no galvanic isolation, I was thinking (maybe wrongly) using a DIY USB cable with Ground and +5VBus isolated, only D+ & D- wire directly connected.
At computer side a resistor would be inserted between Ground and +5VBus.
At Mojo side, a battery would be inserted for USB Audio port activation.

The aim is to remove RF ground noises without buying external devices (IMHO, too expensive).
I am also expecting some drawbacks by doing so.
Glad if you can comment.
Rgds.

Hey arpiben,
I also found an instruction and thought I share it here:

http://grizzlyaudio.blogspot.de/2015/08/diy-solid-core-audio-usb-cable-with.html?m=1

This instruction is describing an USB cable like you thought of, the only difference is this design uses a 5v linear power supply.
It should be easy to implement a 5v battery/power bank instead.

Cheers

 

[god-bluff]

Try different cable and or charger.

CHEERS

So it's the cable and charger that's at fault?

Tried various including Sony xperia and Amazon Kindle and the odd no name all same result. What charger is recommended?

 

[miketlse]

So it's the cable and charger that's at fault?

Tried various including Sony xperia and Amazon Kindle and the odd no name all same result. What charger is recommended?

A charger which generates noise, in the form of ripple current, will cause the inductors inside the mojo to buzz, which is what you are hearing. It is not dangerous to the mojo, but is annoying for some users. Replacing the charger with an Apple or Samsung phone charger usually works well.
 
Sometimes the cable is too long and/or the conductors contain a grade of copper with too high a resistance, so the 5V output by the charger is only 4.* V by the time the electricity reaches the mojo. So the advice is often to use a shorter cable as well, but in my experience I have no issues with the standard 1m cables that come with Samsung phones or OPPO dacs.

 

[tomwoo]

Could anyone post an ACTUAL picture of Audio Sanctuary's (formerly Custom Cable) 3.5mm Coaxial Cable for connecting FiiO X3II/X5II/X7 to Mojo? 



With all due respect, the picture on Custom Cable's website looks like it was photoshopped by a 3-year-old...  PhilW

 

[audi0nick128]

So it's the cable and charger that's at fault?

Tried various including Sony xperia and Amazon Kindle and the odd no name all same result. What charger is recommended?

There is also a dedicated chapter concerning 'Charging Noise' in post #3
It's under Battery/Charging

Cheers

 

[tkteo]

Could anyone post an ACTUAL picture of Audio Sanctuary's (formerly Custom Cable) 3.5mm Coaxial Cable for connecting FiiO X3II/X5II/X7 to Mojo? 

With all due respect, the picture on Custom Cable's website looks like it was photoshopped by a 3-year-old...  PhilW

 

[tkteo]

 

[tomwoo]

Thanks!
It looks much better than the website picture. (not unexpected tho )
Is it working well with your X7 and Mojo?

 

[Arpiben]

Hey arpiben,
I also found an instruction and thought I share it here:

http://grizzlyaudio.blogspot.de/2015/08/diy-solid-core-audio-usb-cable-with.html?m=1

This instruction is describing an USB cable like you thought of, the only difference is this design uses a 5v linear power supply.
It should be easy to implement a 5v battery/power bank instead.

Cheers

 
Hi audi0nick128,
 
  It is basically what I was having in mind. IMHO, I will prefer to keep the data cables shielded with one side grounded connection only (external power OV).
Please note that the USB modding may reduce RF Ground &+5VBus noises but it will not remove Data Bus noises(D+&D-) caught inside computer.
According to Rob Watts, the RF noises are sufficiently filtered by Mojo. Therefore, it is difficult to estimate any SQ improvement.
Please do note also that an unproperly done DIY cable may bring more issues than it is supposed to solve.
That said I am curious about it and eager to try, that was the aim of my previous post.
 
Cheers