Optimal USB isolation

jkeny

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I've seen many USB DACs stating they incorporate "galvanic isolation" & although correct to some extent, it's a case of the "closing the barn door after the horse has bolted"

The reason being that this "galvanic isolation" is performed after the USB receiver which has some effect but is not the optimal solution

Optimal USB isolation is needed before the USB receiver, on the USB signal lines particularly (D+ & D- lines). It would appear that USB is a pseudo differential electrical protocol & needs particularly low noise, well formed signals in order that the downstream I2S signal output from the USB receiver stage is low in jitter. Isolating a jittery I2S signal does absolutely nothing to eliminate the sonic effects of this jitter, in fact isolation usually adds its own jitter.

Even some of the best DACs can be improved sonically with USB signal cleansing - see this report on the Mola Mola https://www.stereo.net.au/forums/topic/117214-mola-mola-dac/?page=2
"To me using the Uptone Audio regen eliminated what was otherwise an overly cool, sterile sound from the MolaMola DAC."
"the Mola Mola DAC with the regen in the chain sounds more fluid and musical to me."

Now this is without USB isolation as he couldn't get the Mola Mola to work with USB isolation only with USB signal cleansing via the Regen
 

jkeny

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If there was enough interest, I would be happy to start a tour of one of my USB isolation devices - hub (ISO-HUB), SPDIF converter (ISO-SPDIF) or DAC (ISO-DAC) - all powered by internal batteries as per all my devices.
It's the logical next step in USB isolation - build it into the audio device itself rather than as an external device & power it with an ultra low noise battery power source - I believe this is the only USB DAC or SPDIF converter having such USB isolation built-in
 
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paul79

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Hello, and thank you for the offer. I would love to try the ISO-HUB if you are open to US Clients participating in the tour. This device seems great to me, and I like that it is self powered. Hoping you get enough interest for this.

Paul
 

jkeny

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Hello, and thank you for the offer. I would love to try the ISO-HUB if you are open to US Clients participating in the tour. This device seems great to me, and I like that it is self powered. Hoping you get enough interest for this.

Paul

Thanks Paul
let's see what interest there might be in this tour?

Just to whet interest here's a recent review just published tnt-audio.com/sorgenti/ciunas_isodac_e.html
And there is a another review coming out shortly
 

jkeny

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Looks like there's no interest in touring a device?
312 views but no takers
 

jkeny

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Paul, I sent you a PM - if you are still interested, let me know - I have others interested
 

Elberoth

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I'm also surprised nobody signed up. There is great SQ improvement potential from USB isolation devices. If anyone is still using USB out of his Mac or a PC straight into his DAC, he should try this ASAP. The results may vary from noticable to trully spectacular.
 

jkeny

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I'm also surprised nobody signed up. There is great SQ improvement potential from USB isolation devices. If anyone is still using USB out of his Mac or a PC straight into his DAC, he should try this ASAP. The results may vary from noticable to trully spectacular.

Yea, maybe people haven't experienced just what an improvement TRUE USB isolation & reclocking can bring to sound quality. A number of products advertise they have isolation but it's not USB isolation done at the input - it's done after the USB signal processing which is not optimal.

Already reviewed in the press TNT Audio

Another press review being published on Sunday

Brucemck2 is receiving the ISO-HUB/ISO-SPDIF next week for a weeks auditioning & if no other takers the device is going to a good home in the US

The ISO-HUB/ISO-SPDIF acts like a 4 port USB hub except it is powered by internal battery & the USB signals are isolated & reclocked - this provides the best possible USB signal quality & makes a large difference in audio quality with all USB audio devices I have tried - Meitner, Lampizator, Soekris & many others

The ISO-HUB section sits between the computer (Mac or PC Linux or Win) & USB audio device isolating & reclocking the bi-directional USB hi-speed signals

The ISO-SPDIF section uses one of the isolated USB ports & converts the USB audio signals to SPDIF for connection to SPDIF input of DAC

A picture of the device might aid understanding
SPDIF rear CTM_2342.JPG SPDIF front CTM_2337.JPG
 
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Empirical Audio

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The optimum way to do integrated USB isolation in a DAC includes several pieces:

Design the isolation circuit so that it:
1) breaks the 5V line in the USB cable and inserts a high-quality separate DC supply with a separate power cord. This means two power cords for the DAC/power supply combo or two cords for the DAC.
2) isolates the USB interface, but allows the master clock to be generated on the DAC side of the isolation barrier, where it is powered by the DAC power subsystem
3) the master clock on the DAC side of the barrier drives back towards the computer side of the interface through the isolation devices, but this clock is not used for the final clocking of the I2S interface to the D/A. The local master clock on the DAC side is used to clock the I2S interface in order to keep the jitter as low as possible.
4) there is no connection of ground across the isolation barrier
5) There are no ground-loops, except between the computer and the DC supply on the computer side of the interface.

The devil is in the details here. Very few interfaces are designed like this. I have designed such an XMOS interface as an option for my DAC and it is working well. It requires an external power supply I call the "Power Block" that replaces the 5V power on the computer side of the isolation interface. Unfortunately, my Ethernet interface still beats it when comparing SQ. As a result, I have decided not to make it a stand-alone product, only integrate it in the DAC.

The other option is to regenerate the USB signaling on both sides of an isolation interface, basically a USB repeater with isolation. This requires a power supply for the DAC side and a second power supply for the computer side. This usually means 2 power cords. Not optimum if the computer side is powered from the cable 5V power IMO, because different USB cables and lengths will affect this, as will the quality of the supply in the computer or server. The DAC side must be powered from the same AC outlet as the DAC. This provides galvanic isolation, but it creates another ground-loop between the DAC and the output side of the USB regenerator/isolator. This ground-loop is not present in the 5 step design above. There is a second ground-loop between the computer and the computer side DC power supply. Two ground-loops.

Batteries eliminate these ground-loops, so that solves the ground-loop issue. I'm not aware of anyone that has just a USB repeater with isolation. Will this ISO-S/PDIF do that?

Steve N.
Empirical Audio
 
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jkeny

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The optimum way to do USB isolation includes several pieces:

Design the isolation circuit so that it:
1) breaks the 5V line in the USB cable and inserts a high-quality separate DC supply with a separate power cord. This means 2 power cords for the DAC/power supply combo or two cords for the DAC.
2) isolates the USB interface, but allows the master clock to be generated on the DAC side of the isolation barrier
3) the master clock on the DAC side of the barrier drives back towards the computer side of the interface, but this clock is not used for the final clocking of the I2S interface to the D/A. The local master clock on the DAC side is used to clock the I2S interface in order to keep the jitter as low as possible.
4) there is no connection of ground across the isolation barrier

The devil is in the details here. Not all interfaces are designed like this. I have designed such an XMOS interface as an option for my DAC and it is working well. Unfortunately, my Ethernet interface still beats it when comparing SQ.

Steve N.
Empirical Audio

Steve,
In my experience, the most important aspect of USB isolation is removing CM noise from the USB signals themselves - this is where the major improvement in sonics comes.
IMO, a 5V wire should not be part of a USB cable - 5V VBUS should not be needed as it is detrimental to sound
Funnily enough a lot of attention is given to USB ground but that's not where the noise is to be found - easy to test this on an ordinary USB connection - lift the ground after USB initial handshaking is done (about 2-3 seconds) - this is single ended USB communication needing the ground ref - after this all USB comms is differential & ground can be dispensed with. I notice no difference with ground connected or lifted.

One has to be careful about using the term "Master clock" - USB clock is 12/24 or 25MHz depending on USB2/USB3 chip. This is the clock which is timing the USB comms stream. The I2S audio signals output from USB receiver chip is timed by 2 audio master clocks which are located at the DAC. I reclock the I2S signals at the DAC chip using one of these audio clocks & send this master clock signal back to the I2S output of the USB receiver. Using this type of synchronous clocking is optimal as there are no clock crossing issues - I always found asynch clocking worse sounding than synch clocking

The USB recipe I follow is USB hi-speed signal isolation using Silanna isolator followed by USB reclocking using a USB hub chip - as isolation always adds signal jitter - all powered by ultra low noise battery power.

I reckon if you tried this approach you may find it the equal of ethernet transport!
 

Empirical Audio

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One has to be careful about using the term "Master clock" - USB clock is 12/24 or 25MHz depending on USB2/USB3 chip. This is the clock which is timing the USB comms stream. The I2S audio signals output from USB receiver chip is timed by 2 audio master clocks which are located at the DAC. I reclock the I2S signals at the DAC chip using one of these audio clocks & send this master clock signal back to the I2S output of the USB receiver. Using this type of synchronous clocking is optimal as there are no clock crossing issues - I always found asynch clocking worse sounding than synch clocking

Have you ever done a DAC design with an AK4114? Excellent jitter rejection and it's asynch. Almost indiscernible from driving direct I2S, which my device can do to my DAC as well as S/PDIF. Direct jitter measurements in the 16psec range (standard deviation of the period).

By Master Clock, I am talking about the I2S Master Clock. It is generated on the DAC side of the isolation interface and creates the I2S Master Clock directly.

From your product description, it sounds like I can get the benefits of isolation without using the S/PDIF output on it, just USB B input to USB A output. Is this correct?

BTW, I think its the computer interference in the data stream, including DSP etc., that makes USB have lesser SQ than Ethernet. Here is the study I performed:

http://www.audiocircle.com/index.php?topic=155232.0

Steve N.
Empirical Audio
 

jkeny

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Have you ever done a DAC design with an AK4114? Excellent jitter rejection and it's asynch. Almost indiscernible from driving direct I2S, which my device can do to my DAC as well as S/PDIF. Direct jitter measurements in the 16psec range (standard deviation of the period).
No I haven't tried it - I'll have a look at it, thanks but my devices to date have all been USB input only as I didn't need ASRC or PLLs to recover or use asynch clocking. My experience with Sabre DACs & their much vaunted ASRC demonstrated tome that bypassing their ASRC was always beneficial to the sound & very noticeably so

By Master Clock, I am talking about the I2S Master Clock. It is generated on the DAC side of the isolation interface and creates the I2S Master Clock directly.
Sure, I know what you were saying, I was just clarifying for readers

From your product description, it sounds like I can get the benefits of isolation without using the S/PDIF output on it, just USB B input to USB A output. Is this correct?
Yes, exactly - this is one product in a range of isolated USB devices - ISO-HUB, ISO-SPDIF, ISO-DAC - they all have the ISO-HUB functionality but SPDIF output is added in the ISO-SPDIF & Analogue output added in the ISO-DAC. If you want to try this touring device with one of your USB or SPDIF input DACs PM me & we can arrange it?

BTW, I think its the computer interference in the data stream, including DSP etc., that makes USB have lesser SQ than Ethernet. Here is the study I performed:

http://www.audiocircle.com/index.php?topic=155232.0
Yea, I read that at the time
The real weakness with USB is because of it's requirement for single-ended communication at handshaking & other times. If it had been designed as true differential it would have been better. I'm not sure what you mean by "computer interference in the data stream, including DSP etc."- I think it is a complex mixture of CM & differential noise that is partly created in the computer but also occurs in the cable itself through having a 4 wire USB cable with shield currents, etc - impossible to design such a cable with the tolerances needed for balanced differential rejection of noise. However the USB isolation devices do a very good job of handling this crud
 
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jkeny

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As promised - second Press review is online & titled "Digital done Right"

Excerpt from the conclusions
I believe the isolation and reclocking/regeneration techniques used with the ISO products are a real step forward for digital replay at a relatively affordable price. The signature of the ISO devices is one of precision, detail and life.

For those with an S/PDIF DAC I can heartily recommend the ISO-SPDIF for those with a PC (Windows, Apple or Linux) to connect to their DAC. For those eschewing PCs then the Raspberry Pi has fully come of age with the addition of the Allo DigiOne S/PDIF board, ideally with power supplied via the ISO-PS.

There are many options; maybe one of these options will form the basis for enhancing your digital playback.

I have to say it again – digital done right!
 

jkeny

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The technology & how it's put together is in this block diagram which might explain things?
View attachment 39589

What you see are essentially 4 elements before the DAC or SPDIF board:
- USB isolator board
- USB reclocking/reformatting HUB with 4 output ports
- one of these isolated/reclocked USB ports is feeding USB signals into the Amanero USB receiver which outputs I2S signals
- these I2S audio signals are reclocked in the I2S reclocker

All of this is powered by internal LiFePO4 batteries.
Each of these blocks is responsible for the final sound.
 

Empirical Audio

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I think it is a complex mixture of CM & differential noise that is partly created in the computer but also occurs in the cable itself through having a 4 wire USB cable with shield currents, etc - impossible to design such a cable with the tolerances needed for balanced differential rejection of noise. However the USB isolation devices do a very good job of handling this crud

If this were true then different USB player software would not sound so different. Even the same software from one release to the next sounds different. This points to deficiencies in the DSP software. When volume is set to max, it still impacts SQ.

this is one product in a range of isolated USB devices - ISO-HUB, ISO-SPDIF, ISO-DAC - they all have the ISO-HUB functionality but SPDIF output is added in the ISO-SPDIF & Analogue output added in the ISO-DAC. If you want to try this touring device with one of your USB or SPDIF input DACs PM me & we can arrange it?

That would be interesting. Do these devices eliminate the need for an expensive silver USB cable or expensive Ethernet cables for best results?

Would the ISO-Hub would replace the passive Ethernet isolator I use?

Steve N.
Empirical Audio
 

jkeny

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If this were true then different USB player software would not sound so different. Even the same software from one release to the next sounds different. This points to deficiencies in the DSP software.
I'm not so sure about your conclusion that it's deficiencies in DSP. I can see how different software/ different background tasks could cause ground noise fluctuations which ultimately can make their way into the USB cable ground or shield
When volume is set to max, it still impacts SQ.
I'm not sure what this means?



That would be interesting.
All I would ask is that you post your honest opinion of the sound
Do these devices eliminate the need for an expensive silver USB cable or expensive Ethernet cables for best results?
Eliminate is very definitive & I don't make definitive claims but the influence on SQ of USB cable quality is greatly diminished, by all accounts

Would the ISO-Hub would replace the passive Ethernet isolator I use?

Steve N.
Empirical Audio
I'm unfamiliar with your Ethernet isolator but you can see the ISO-HUB isolates USB signals & reformats them
 
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Empirical Audio

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I'm not so sure about your conclusion that it's deficiencies in DSP. I can see how different software/ different background tasks could cause ground noise fluctuations which ultimately can make their way into the USB cable ground or shield
I'm not sure what this means?

It means that DSP code is impacting SQ even though it is supposedly bypassed.

All I would ask is that you post your honest opinion of the sound Eliminate is very definitive & I don't make definitive claims but the influence on SQ of USB cable quality is greatly diminished, by all accounts

I can do that.

My isolator is just a transformer EMO EN-70e. Inserts in the cable between router and DAC.

What about the cables? Do I still need expensive cables or does this eliminate the need?

Steve N.
Empirical Audio
 

jkeny

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It means that DSP code is impacting SQ even though it is supposedly bypassed.
Yes, there should be no DSP processing changing the bits when digital volume is at max. Are you suggesting that DSP is changing bits then?

I can do that.
OK, PM me address, please

My isolator is just a transformer EMO EN-70e. Inserts in the cable between router and DAC.
OK but you know that a transformer allows certain frequencies pass from primary to secondary depending on capacitive leakage of the trafo?

What about the cables? Do I still need expensive cables or does this eliminate the need?

Steve N.
Empirical Audio
I don't believe different USB cables will have much effect on the sound but I haven't tested expensive USB cables - why not try some & report?
 

Empirical Audio

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Yes, there should be no DSP processing changing the bits when digital volume is at max. Are you suggesting that DSP is changing bits then?

Yes, that is my hypothesis.

OK but you know that a transformer allows certain frequencies pass from primary to secondary depending on capacitive leakage of the trafo?

Yes, I know that. Connecting earth ground to the DC common of the router minimizes the effect, but adding isolators also helps because it puts more than two capacitances in series.

I don't believe different USB cables will have much effect on the sound but I haven't tested expensive USB cables - why not try some & report?

I'll try what I have on hand.

Steve N.
Empirical Audio
 

jkeny

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Yes, that is my hypothesis.
This is easy to test & I've done it in the past - check the bit stream arriving at the DAC is the unchanged bit stream. It is unchanged so no DSP is happening.

Yes, I know that. Connecting earth ground to the DC common of the router minimizes the effect, but adding isolators also helps because it puts more than two capacitances in series.



I'll try what I have on hand.

Steve N.
Empirical Audio
OK, I look forward to your impressions/results
 

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