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

[headwhacker]

  @Shigzeo Thankfully it is easy to volume match inputs on the Mojo by simply not switching it off or changing the volume setting. 

 
There is a possibility given that optical doesn't carry noise and coax and USB potentially can that the different inputs could sound slightly different. They certainly did to me on the Hugo. This is unlikely to be something that can be shown clearly using RMAA through a sound card or common ADC. I'd love to own, or have access to a 'scope and suitable measurement gear and see if what we're hearing couldn't be figured out. I've seen the square waves showing reflections from non-75 Ohm cables being used posted by others, for example, but that hasn't been connected to anything anyone heard. 
 
Anyhow, the basic gist of things is that in the absence of hard data people are relegated to testing stuff out and using what they feel sounds best to them.

 
 
While USB and COAX are prone to noise  compared to Optical, the signal transmitted between a USB device and Mojo is still digital. A series of 1s and 0s when interpreted by the device before it is sent to Mojo must be the exact data when it is received by Mojo. Of course it's not 100% correct all the time that is why things like error correction exist.
 
With the technology inside Hugo I doubt these errors are relevant. I believe, the problem comparing optical and USB inputs of Mojo is that the devices are different. USB devices are usually Android/IOS device which may interrupt with the music player app and manifest as a series of clicks and pops.
 
However, difference reported in soundstage, details or bass would require IMO a shift in the interpreted analog signal which is highly unlikely because Mojo must interpret the same digital data to the exact analog signal all the time.
 
I don't know any device/transport that has all 3 digital outputs but sources being used to compare the 3 digital inputs of Mojo are also different. It's possible the difference is due to the transport used and not the inputs in Mojo.

 

[shigzeo]

In those reviews of the Mojo the reviewer mentions an audible hiss, is this a known issue with this DAC/Amp? Will I have issues with sensitive CIEMs?

I need to update the 'review'. The RMAA article was done with a hissy Mojo. The one I eventually purchased doesn't hiss much at all. 

 

[Currawong]

   
I don't know any device/transport that has all 3 digital outputs but sources being used to compare the 3 digital inputs of Mojo are also different. It's possible the difference is due to the transport used and not the inputs in Mojo.

I'd speculate that in a similar way to how people can mis-interpret brightness as detail, the same might be happening on the effects of the noise on the eventual analog output. I don't think it has anything to do with error correction, but you might be right about the transport.  Once the new year is in, maybe Rob can post his input on the matter. It was discussed somewhat in the Hugo threads. 

 
Ideally we should just plug in whatever and not care too much about this, but like I said in the Vali 2 thread about tube rolling it, it's just one of those things that are part of the hobby for us regulars and something we play around with and discuss for fun. 

 

[Mython]

Quote:

 

@naivesound What headphones are you using?

Shure se846, used with white filters and blue modded. Filters (trying to. Get some. Detailed treble) and while that is present, I'm looking for more engaging *fun* yet sparkling clear sound

 
 
'Sparkling' treble does not necessarily mean there is more detail being portrayed - it may actually relate to digital noise, so, when a DAC arrives on the market, that offers noise levels drastically lower than existing designs, there is a possibility that the treble may be perceived by some people as sounding a bit 'smooth'.
 
The following post, from the DAVE DAC thread, is interesting (see point # 1.):
 

  Depth perception is indeed mostly down to the reverberant signal to direct sound signal ratio. But another important cue is due to HF absorption of the air, so brighter sounding sounds are perceived as closer. Quite how the brain determines that the frequency response of signal is degraded I don't know; perhaps the brain subtracts the frequency response of  reverberant to direct to get a another depth cure, or perhaps its simply bright sounds is inferred as being closer. 
 
For sure, if the reverberant signal is attenuated or distorted with respect to the larger direct signal then depth perception suffers. Indeed, in all of my career of listening and then explaining the results of listening tests, small signal non-linearity is the only factor that I have correlated with depth perception. With noise shapers you get distortion of very small signals, in that signals below the noise shaper noise floor are lost and signals that approach the noise shaper noise floor have reduced amplitude. This is why fundamental linearity measurements are important, as they show the amplitude of small signals changing with level. Using FFT's with the result calibrated at -60.000 dB, then re run the test at -120 dB you check that the level is actually -120.000 dB (with results adjusted for noise). Using my Pulse Array DAC's you get pretty much perfect results (no consistent error). Conventional delta sigma have measurable errors, R2R DAC's have even more, as its impossible to match resistors accurately enough.
 
In the bad old days, when my designs had significant fundamental linearity errors, improving this problem did lead to better depth perception. Moreover, when I used to design cables in the 80's, one could always hear a correlation between conductor purity, metal surface oxides and depth perception. In this case, the oxides in metal to metal interfaces (crystal to crystal inside the wire and contact to contact) create small signal non linearities, with resistance being a tiny bit higher for small signals than larger signals. But although it was easy to hear depth being degraded, you could never measure this distortion.
 
Before the Dave project, my target for noise shapers was better than 200 dB distortion and noise performance. that's around 1000 times more capable than high end noise shapers. But with Dave I had ten times more gate capacity than ever before; also whilst listening to different noise shapers when developing Hugo I could hear large changes in depth perception. This indicated that my assumption that 200 dB was good enough was not correct.
 
At this stage I should indicate that there are actually two entirely different distortion mechanisms that are important to noise shaper designs (there are other things but this is the most important). These are:
 
1. Noise floor modulation. This is where the noise floor modulates with signal level - so large signals give greater noise than small signals. The brain is remarkably sensitive to this problem, and can easily hear unmeasurably small levels; it perceives it as smoothness or warmth to the sound when the problem gets smaller. With gross noise floor modulation you hear it as grain in the treble. Now all DAC's (except mine) have very large and measurable noise floor modulation, which explains why they sound harder (they normally add some 2nd harmonic to fatten up the sound to hid the innate lack of warmth). Additionally, reducing noise floor modulation improves instrument separation and focus, and you reduce the loudest instrument taking your attention problem.
 
2. Fundamental linearity - how accurately small signals are reproduced (small signal linearity). Now this only affects depth perception, it does not do anything else. And I have not come up with any change in depth that wasn't explained by small signal linearity.
 
Now a noise shaper has both problems - making a better noise shaper with say 220 dB performance than 200 dB will have lower noise floor modulation and better small signal linearity.
 
So when I designed a 220 dB noise shaper and listened to it, I could easily hear better depth - and maybe a bit smoother. So better than 200 dB is a good rule of thumb for noise floor modulation, but certainly not for depth. Indeed, over a 90 day period I constantly improved the noise shapers and came to the absolutely remarkable and frankly amazing conclusion that there was no limit to how good the noise shaper needs to be in order to resolve depth perception. I ended up with an very complex 17th order noise shaper that had 350 dB performance. 
 
With 90 days of work and getting an improvement every day in depth, I could hear now a cavernous depth perception. This is very exciting, as its long been a major problem of mine that one can hear depth so accurately in real life, but high end audio sounds flat as a pancake. Listen to a real organ and choir in a cathedral - if you are 100 feet away and shut your eyes, it sounds uncannily 100 feet away. I am now starting to get something like that depth perception with loudspeakers - but I guess the weak link now is the ADC, which is my next design challenge.
 
Now when I first got to 350 dB performance (that's the best I can do with available FPGA and flip flop speed) I was simply amazed that the brain was so sensitive. How can it detect such small errors? Perhaps its down to something else, and the noise shaper number is merely a proxy for something else going on in the analogue domain. But recently (a month ago) I upgraded internal digital (no analogue consequences at all) noise shapers within Dave from 220 dB to 350 dB - and lo and behold depth did get much better. So that suggests that the brain really is sensitive to absolutely no error for small signals. But it still perplexes me that minute effects can be so easy to hear.
 
But this brings me full circle. I have always been puzzled that cable small signal non linearity was never measurable and if its true that -350 dB performance is necessary then this proves why we can't measure cable effects - they are simply too small, and the brain is just too sensitive.
 
Funny that work in noise shapers can solve a puzzle with metal purity with wires...
 
Rob

 
[updated later contribution from Rob]:
 

  To eliminate the RF and signal correlated noise on USB you need galvanic isolation. The downside to galvanic isolation is that it draws power from the source - which is not something we can do with a mobile product. All Chord desktop DAC's have USB galvanic isolation now.
 
That said, mobile sources are much lower noise - they have very efficient processors, unlike a PC, and there is no ground, so circulating currents are much less, so it is a much smaller problem with mobile. If you can do it, use the optical, as this usually sounds the best and is completely isolated. Optical has a undeservedly poor reputation, as it sounds much smoother and darker than other inputs, and this is just a feature of lower noise floor modulation - its smoother with better instrument separation and focus - but lack of glare is often confused with a lack of detail resolution. Listening tests must be done with a lot of care, as it is easy to draw the wrong conclusions!
 
Rob

 
 
 
Quote:

 

Two questions related to Rob Watts' comments on optical output as a source to the Mojo;

1. What is audio "glare"? What does it sound like? And how does one distinguish it from detail?

2. Is the conclusion that one with an Apple computer should be using its optical/toslink output (that also serves as a headphone output) rather than its USB output, even if one uses an AudioQuest Jitterbug, Schiit Wyrd, Uptone Regen and an Akiko USB tuning stick, or similar devices ?

Glare is normally used for extreme form of hardness or grain in the treble. So I guess one could say going from bad to good glare, grainy, hard, bright, smooth, dark. 
 
Distinguishing it from detail is tricky as a brighter sound is easy to confuse it with more detail resolution. Indeed, truly more transparency, does sound brighter. So you have to be very careful, and I have been caught out in the past. One way of recognising it is with timbre - it the extra brightness is noise floor modulation for example, then all instruments will sound brighter - even those that are supposed to sound rich and dark. But if the brightness is better detail resolution, then smooth instruments will just sound clearer, not brighter. Also, if instrument separation and focus is worse, then it is not more transparency.
 
When somebody says it sounds better, but can't actually describe in details what the differences are, be warned! They may be preferring distortion. Fortunately, our lizard brain ignores all this - if its really better, it will be more emotional and involving, so you should use this as your goal. But assessing whether its more emotional or musical takes a lot of time, you can't do it on a quick AB test.
 
The USB filter devices help (hopefully) but do not solve the problem. It has to use galvanic isolation to do it properly.
 
Rob

 
 
 
And although mostly relevant to the DAVE DAC, some of the following holds true (to a lesser, but still significant, degree) for Mojo and Hugo:
 

   
Hugo and Dave don't use any kind of DAC chip, the analogue conversion is discrete using pulse array. The key benefit of pulse array - something I have not seen any other DAC technology achieve at all - is an analogue type distortion characteristic. By this I mean, as the signal gets smaller, the distortion gets smaller too. Indeed, I have posted before about Hugo's small signal performance - once you get to below -20 dBFS distortion disappears - no enharmonic, no harmonic distortion, and no noise floor modulation as the signal gets smaller. With Dave, it has even more remarkable performance - a noise floor that is measured at -180dB and is completely unchanged from 2.5v RMS output to no signal at all. And the benefit of an analogue character? Much smoother and more natural sound quality, with much better instrument separation and focus. Of course, some people like the sound of digital hardness - the aggression gets superficially confused with detail resolution - but it quickly tires with listening fatigue, and poor timbre variation, as all instruments sound hard, etched and up front. But if you like that sound, then fine, but its not for me.
 
On the digital filter front - original samples getting modified - actually the vast majority of FIR digital filters retain untouched the original samples, as they are known as half band filters. In this case, the coefficients are arranged so that one set is zero with one coefficient being 1, so the original sample is returned unchanged. The other set being used to create the new interpolated value. The key benefit of half band filters is that the computation is much easier, as nearly half the coefficients are zero, plus the filter can be folded so that the number of multiplications is a quarter of a non half band filter. When designing an audio DAC ASIC, the key part in terms of gate count is the multiplier, so reducing this gives a substantial improvement in die size, and hence cost. So traditional digital filters use a cascade of half band filters, each half band filter doubles up the oversampling - so a cascade of 3 half band filters will give you an 8 times over-sampled signal, with one sample being the unmodified original data. You can tell if the filter is like this as at FS/2 (22.05 kHz for CD) the attenuation is -6dB. The filters that are not like this are so called apodising filters, and my filter the WTA filter.
 
Going back eighteen years ago to the late 90's I was developing my own FIR filter using FPGA's. Initially, I was interested in increasing the FIR filter tap length as I knew from the mathematics of sampling theory that timing errors were reduced with increasing tap length. So the first test was to use half band Kaiser filters - going from 256 taps to 2048 taps gave an enormous sound quality improvement, so I had confirmed that tap length was indeed important subjectively. But at this point I was stuck; I knew that an infinite tap length filter with a sinc impulse response would return the original un-sampled signal perfectly - but the sinc function using only 16 bit accurate coefficients needs 1M tap FIR filter - and that would never happen, certainly not with 90's technology. So was it possible to improve the timing accuracy without using impossible tap lengths? After a lot of thinking and research, I thought there was a way - but it meant using a non half band filter, which would mean that the original sampled data would be modified. This was a big intellectual stumbling block - how can changing the original data be a good thing? But the trouble with audio is that neat simplistic ideas or preconceptions get in the way. Reality is always different, and reality can only be evaluated by a careful AB listening test. So I went ahead on this idea, and listened to the first WTA filter algorithm - and indeed it made a massive improvement in SQ - a 256 tap WTA sounded much better than 2048 tap half band Kaiser, even though the data is being modified. Why is this? The job of a DAC is NOT to reproduce the data it is given, but to reproduce the analogue signal before it is sampled. The WTA filter reconstructs the timing of the original transients much more accurately than using half band filters or filters that preserve the original data and it is timing of transients that is the most important SQ aspect.
 
So the moral of the tale? Don't let a simplistic technical story get in the way of enjoying music!                
  
 Rob

 

[Jawed]

Rob has a preference for optical on Hugo, but USB on Hugo TT. The difference between them is that TT has galvanic isolation.

Mojo and Hugo don't use galvanic isolation as this would suck power out of the mobile device that they are connected to. You would drain the battery in your phone more rapidly if it had galvanic isolation.

TT, being for home use, doesn't care if it drains power from the device that's connected to its "USB HD" input.

The "USB SD" input on TT doesn't have galvanic isolation and is there for phones. It also works on Windows PCs that do not have the Chord driver installed.

 

[joshk4]

 
I'd speculate that in a similar way to how people can mis-interpret brightness as detail, the same might be happening on the effects of the noise on the eventual analog output. I don't think it has anything to do with error correction, but you might be right about the transport.  Once the new year is in, maybe Rob can post his input on the matter. It was discussed somewhat in the Hugo threads. 

 
Ideally we should just plug in whatever and not care too much about this, but like I said in the Vali 2 thread about tube rolling it, it's just one of those things that are part of the hobby for us regulars and something we play around with and discuss for fun. 

 
I agree.
 
I do find it ironic, that people who dissect other people and then lay their own claims that a 1 db increase is a "big" difference somewhat puzzles me (I do find it a bit inconsistent - is this not an opinion/claim as well?).
 
I wonder to what length do these guys go through to make sure all variables are not factors (not sure if this is possible). i.e a/b testing, switch box, testing against pink noise (if they want to go to the extreme considering how important variables must be considered). In the end, I'm sure not all variables are considered, and so who has the right to make opinions?
 
After all, the Chord Mojo was a product to allow the general public to enjoy great sound. Should Chord put a note for each buyer, to tell them you must have volumn matching, must have a switch box before you can justify your own opinion on whether the sound has improve compared to your mobile/laptop.

 

[shigzeo]

   
I agree.
 
I do find it ironic, that people who dissect other people and then lay their own claims that a 1 db increase is a "big" difference somewhat puzzles me (I do find it a bit inconsistent).
 
I wonder to what length do these guys go through to make sure all variables are not factors (not sure if this is possible). i.e a/b testing, switch box, testing against pink noise (if they want to go to the extreme considering how important variables must be considered). In the end, I'm sure not all variables are considered, and so who has the right to make opinions?
 
After all, the Chord Mojo was a product to allow the general public to enjoy great sound. Should Chord put a note for each buyer, to tell them you must have volumn matching, must have a switch box before you can justify your own opinion on whether the sound has improve compared to your mobile/laptop.

Did I say 'big'? No, I said it was enough to elicit a repsonse. You are mis-representing what I said. Of course one cannot eliminate all 'variables', but before making bold claims such as the difference between cables or input methods, the least you can do is make sure the volumes are matched. And yes, because even small decibel differences can _elicit_ responses. As to the final argument: re: Chord taking responsibility for people's use of Mojo, of course not. That is silly. Of course you should decide what you like and what you dislike. But that never was the issue. The issue is making bold claims that have no practical method of falsifying. 
 
I can say headphone A has more bass than headphone B and that bassheads should buy it. According to the logic you used to refute me (again, which you attacked with a straw man), that's the end of story. There is no reason to make sure that a statement conforms to truth. When you can't factually verify a statement, it is best to put in a disclaimer. An 'I think', a 'perhaps', or an 'appears'. But that is what we do in most of life. We try to be as honest as possible, and where we are not, be ready to accept correction, or add provisos to our statements. 

Why is it that in Audio we can say whatever we want without the slightest attempt to conform to the rules etiquette or honesty? 

 

 

[joshk4]

 
Did I say 'big'? No, I said it was enough to elicit a repsonse. You are mis-representing what I said. Of course one cannot eliminate all 'variables', but before making bold claims such as the difference between cables or input methods, the least you can do is make sure the volumes are matched. And yes, because even small decibel differences can _elicit_ responses. As to the final argument: re: Chord taking responsibility for people's use of Mojo, of course not. That is silly. Of course you should decide what you like and what you dislike. But that never was the issue. The issue is making bold claims that have no practical method of falsifying. 
 
I can say headphone A has more bass than headphone B and that bassheads should buy it. According to the logic you used to refute me (again, which you attacked with a straw man), that's the end of story. There is no reason to make sure that a statement conforms to truth. When you can't factually verify a statement, it is best to put in a disclaimer. An 'I think', a 'perhaps', or an 'appears'. But that is what we do in most of life. We try to be as honest as possible, and where we are not, be ready to accept correction, or add provisos to our statements. 

Why is it that in Audio we can say whatever we want without the slightest attempt to conform to the rules etiquette or honesty? 

 

 
I never said you did

 

[Mimouille]

I completely concur with Shigzeo. When I was newer to this game, I took many comments at face value and was ill advised into certain purchases.

When I bought the Hugo, many claimed this source was clearly superior to that one, and when experiencing it myself, it made not a single difference to me. So for the Mojo, I will just trust my own ears.

 

[joshk4]

I completely concur with Shigzeo. When I was newer to this game, I took many comments at face value and was ill advised into certain purchases.

When I bought the Hugo, many claimed this source was clearly superior to that one, and when experiencing it myself, it made not a single difference to me. So for the Mojo, I will just trust my own ears.

 
I too agree with him about it being peoples opinions.
 
But for people to write that it is their opinion every comment, is superfluous.
 
I'm not sure if audio gears are an exception to believing everyone's comment as definitive truth. If you do, than I'm not sure what to say really.
 
Like you, trust your own ears is the best judgment. It is your ears after all.

 

[headwhacker]

 
I'd speculate that in a similar way to how people can mis-interpret brightness as detail, the same might be happening on the effects of the noise on the eventual analog output. I don't think it has anything to do with error correction, but you might be right about the transport.  Once the new year is in, maybe Rob can post his input on the matter. It was discussed somewhat in the Hugo threads. 

 
Ideally we should just plug in whatever and not care too much about this, but like I said in the Vali 2 thread about tube rolling it, it's just one of those things that are part of the hobby for us regulars and something we play around with and discuss for fun. 

 
I would not compare Mojo's input to Vali's tube rolling. First, they don't don't operate on the same domain. Second the input noise in Vali will have a larger impact on it's output as it is purely in analog domain. The noise in any of Mojo's input would have to be strong to flip a 1 to 0 or vice versa.

 

[headwhacker]

   
I agree.
 
I do find it ironic, that people who dissect other people and then lay their own claims that a 1 db increase is a "big" difference somewhat puzzles me (I do find it a bit inconsistent - is this not an opinion/claim as well?).
 
I wonder to what length do these guys go through to make sure all variables are not factors (not sure if this is possible). i.e a/b testing, switch box, testing against pink noise (if they want to go to the extreme considering how important variables must be considered). In the end, I'm sure not all variables are considered, and so who has the right to make opinions?
 
After all, the Chord Mojo was a product to allow the general public to enjoy great sound. Should Chord put a note for each buyer, to tell them you must have volumn matching, must have a switch box before you can justify your own opinion on whether the sound has improve compared to your mobile/laptop.

 
1dB difference is perceivable to almost everyone - claim.
 
Hugo's soundstage is bigger, wider than mojo - opinion
 
Got the difference?
 

   
I too agree with him about it being peoples opinions.
 
But for people to write that it is their opinion everyone comment, is superfluous.
 
I'm not sure if audio gears are an exception to believing everyone's comment as definitive truth. If you do, than I'm not sure what to say really.
 
Like you, trust your own ears is the best judgment. It is your ears after all.

 
Nobody is asking you not to trust your ears. But if you know beforehand that the louder sounding amp will give it an advantage. Will you still going to rely only with your ears?

 

[Currawong]

  I would not compare Mojo's input to Vali's tube rolling. First, they don't don't operate on the same domain. Second the input noise in Vali will have a larger impact on it's output as it is purely in analog domain. The noise in any of Mojo's input would have to be strong to flip a 1 to 0 or vice versa.

 
I'm sorry, I think you misunderstood what I was getting at. It was nothing to do with comparing tubes to inputs, but comparing how those of us much into the hobby fuss over small details to a degree that is arguably excessive. 

 
In the Vali thread some members were wondering if they purchased the Vali whether or not they'd have to purchase tubes costing half the value of the amp. Similarly we have had members asking whether or not they need to worry about special cables, software, DAPs and DSD files to get the most out of the Mojo, when none are necessary. In both cases it is just a bunch of us experimenting with things to see what effect they have -- most of them very minor. 

 

[headwhacker]

   
I'm sorry, I think you misunderstood what I was getting at. It was nothing to do with comparing tubes to inputs, but comparing how those of us much into the hobby fuss over small details to a degree that is arguably excessive. 

 
In the Vali thread some members were wondering if they purchased the Vali whether or not they'd have to purchase tubes costing half the value of the amp. Similarly we have had members asking whether or not they need to worry about special cables, software, DAPs and DSD files to get the most out of the Mojo, when none are necessary. In both cases it is just a bunch of us experimenting with things to see what effect they have -- most of them very minor. 

Ah, apologies. It's easy for me to jump the gun on this one and took what you said at face value as I am not a frequent visitor of the Vali thread.

 

 

[spook76]

1dB difference is perceivable to almost everyone - claim.

Hugo's soundstage is bigger, wider than mojo - opinion

Got the difference?


Nobody is asking you not to trust your ears. But if you know beforehand that the louder sounding amp will give it an advantage. Will you still going to rely only with your ears?

You need to look up the words "claim" and "opinion" they are almost synonyms. A claim is an assertion in the face of possible contradiction while an opinion implies a conclusion while thought out yet open to dispute.

Got the similarities?