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

[ubs28]

I believe Rob has said that both the Mojo, Hugo and Hugo TT have the same power.

 

[GRUMPYOLDGUY]

I measured both Mojo and Hugo and they have the same peak voltage output at around 5.3 Vrms. They have the same power output given the same phone. The difference in sound perceived as reported by other people could be coming from other variables. 

I have been comparing Hugo and Mojo and for most headphones I have tried they both sound identical. Except for a few iems (AKT8ie) where I perceive a very small difference. But then it's more likely the difference is just in my head and there is not much in reality. Except for the Stax headphones and HE-6 every headphones I have don't have any issues driven by Mojo or Hugo. 

A voice of reason... It's like finding a pot of gold at the end of a unicorn... Or something.

your head is reality.

Actually not true. What you measure is more reliable than what you hear... Many studies done on this.

 

[jmills8]

A voice of reason... It's like finding a pot of gold at the end of a unicorn... Or something.
Actually not true. What you measure is more reliable than what you hear... Many studies done on this.

so if you do not enjoy the sound but the measurements say you should then you will convince yourself to enjoy that sound ?

 

[ubs28]

so if you do not enjoy the sound but the measurements say you should then you will convince yourself to enjoy that sound ?

It means they measure the wrong thing.

 

[GRUMPYOLDGUY]

so if you do not enjoy the sound but the measurements say you should then you will convince yourself to enjoy that sound ?

Non sequitur.

 

[jmills8]

Non sequitur.

 

[GRUMPYOLDGUY]

Well, that escalated quickly.

 

[Jazzi]

1600 pages!!  Wow!  

 

[rkt31]

there are so many factors. all can't be measured. despite mojo and hugo both measure flat in frequency response, mojo definitely sounds warmer. once rob said that during development of dave , it started sounding too dark due to heavy noise suppression, then he had to improve the transient response by increasing tap count ( if I remember correctly) and other things to balance the dark sound. as far as i know with my limited knowledge if you increase tap count you can definitely listen the improvement in transients but it is difficult to measure . mojo imho has more noise suppression but fewer taps than hugo which may be the reason of mojo sounding warmer.

 

[x RELIC x]

there are so many factors. all can't be measured. despite mojo and hugo both measure flat in frequency response, mojo definitely sounds warmer. once rob said that during development of dave , it started sounding too dark due to heavy noise suppression, then he had to improve the transient response by increasing tap count ( if I remember correctly) and other things to balance the dark sound. as far as i know with my limited knowledge if you increase tap count you can definitely listen the improvement in transients but it is difficult to measure . mojo imho has more noise suppression but fewer taps than hugo which may be the reason of mojo sounding warmer.

Not quite, but close. It was down to the interpolation filters down to the nS. The timing brought the brightness back. Here is the full post from Rob:

http://www.head-fi.org/t/766517/chord-electronics-dave/735#post_12097191

Spoiler: Long post from Rob in spoiler

Listened to Dave? A little bit - just calculated I have spent 5,000 listening hours on it (first prototype to production).

And I lug it around on carry on for hotels because I miss it too much!

Sound quality has been an interesting story. Now my intention is to make things as transparent as possible, as the gap from listening to a live orchestra to hearing it in the home is vast. And if every link in the chain was perfectly transparent, it would not be a vast difference when listening to live unamplified sound.

Problem is nobody has heard a perfectly transparent DAC. Neutral really means average tonal balance and peoples idea of transparent is often etched and over emphasised details, giving a hyper sound quality but no musicality. And its musicality (defined as having emotional and involving music) that is the purpose of this - not something that sounds impressive for which only one recording actually works.

Half way through the development, I had made substantial improvements from Hugo. But it was getting smoother, and darker in tonal balance - to the point where I knew people would say its far too smooth. I felt it was too dark - I like the way Hugo's presentation can sound very fast and sharp but at the same time smooth. But Dave's extreme smoothness was not a colouration or error. Some designers add distortion (2nd harmonic) to the bass to make it sound phat, warm and soft. Or they use capacitors that create low frequency errors that fatten up the bass too. Dave's refinement was not this; it was based on reduction of distortion (most notably noise floor modulation - Dave's noise floor is at -180dB and is completely unchanged whether there is a signal or not - no other non Chord DAC has come even remotely close to this level of performance). So I knew for sure that Dave's smoothness was true transparency, not some tuning artefact or distortion. And I would have been prepared to leave it at that, as I mentioned earlier nobody has a clue as to what a perfect DAC would sound like.

But then another extremely important milestone was reached. One of the puzzles I needed to find out was the mystery of why Hugo had the timing performance it has. By timing I mean Hugo has this ability to enable one to perceive the starting and stopping of notes with remarkable ease, previous DAC's I designed sounded a bit soft, muddled and confused by comparison. Also, you could follow rhythm with ease, and perceive the interplay of different instruments - the way that musicians "talk" to one another. It also had the ability to paint a wide range of timbre and perception of timbre is down to timing too. All of these qualities is down to the perception of timing, so from an engineering POV where was this performance coming from as it was entirely unexpected. Now Hugo was the first project that had a major upgrade in all of the code that goes into the FPGA - this was a 6 year development. So there was something I had done that unexpectedly improved the perception of timing - and I know it was not the WTA filter, as I had previously designed 32,000 tap filters and they did not sound like Hugo.

What I had done was to improve noise floor modulation by using much better 2048 FS interpolation filters. And these proved to be the magic sauce of Hugo's ability to reproduce the starting and stopping of notes. In short it was the time domain accuracy that was important. Before Dave, my target was time domain accuracy of less than 1uS when reconstructing the missing bits from one sample to another. This number is based on the inter aural delay that we know is accurate to about 4uS, but its not based on how the brain processes ear data - science has no idea how the brain separates instruments out and calculates placement information. Things we completely take for granted. But the reality is that there is no real limit to how accurate the time domain needs to be - instead of targeting uS it needs to be nS. Once that was appreciated, I could radically redesign the interpolation filters to more accurately reconstruct the continuous time analogue signal that was present at the ADC.

Boy did this make a big difference. The perception of timing was much faster and brighter than before; instruments were more real with more sense of individual power. Now when it becomes easier to hear the starting and stopping of notes, things become much brighter and more immediate. Its like adjusting the focus on a lens; poor focus means the edges are blurred and soft and rounded; putting it back into focus gives edges much more immediacy and become more vivid. Now like the situation with Dave's refinement, the perception of speed and timing is not something artificial; a WTA filter running at 16 FS (1.4uS resolution) is fundamentally less time domain accurate than a filter running at 256 FS (88nS resolution).

What was nice was that the balance was restored (and I stress I am not trying to create a particular sound) in that Dave went from sounding really rich and dark and impossibly smooth to something much faster and immediate. Now Dave still sounds rich and smooth; but it is also capable (when the occasion demands) of sounding very fast and sharp.

The next aspect that had a lot of attention was depth perception. Now this is an area with audio I have had a considerable interest over; listen to a organ and a choir in a cathedral. If its 100 feet away, it sounds exactly 100 feet away. But play it back on your audio and its at best only a few feet back. Why is audio so bad at depth perception? The problem is down to being able to accurately reproduce very small signals. As soon as you get small errors in amplitude for small signals, the brain can no longer get a handle on depth and so does not give you an impression of depth -  you just get an organ sounding very ambient but with no sensation of depth - flat as a pancake. With DAC's there is a fundamental problem of small signal amplitude accuracy. R2R DAC's have enormous problems with this, as you simply can't get the resistors to match - and it shows with very large measurable errors. Delta sigma have better accuracy, but they still suffer. We can get a handle on a noise shapers performance by running simulations of the noise shaper. Simulations aren't a pretence, in case of Verilog simulation its actually the real performance of the noise shaper. The best noise shapers in high end silicon DAC's are at best 140dB devices - that is, if you run a Verilog simulation with say a 0dB 1kHz signal you will get -140dB THD and noise. This means the noise shapers noise floor is at best at -160 or -170dB. Now this means that a signal at -170 dB is completely lost - any signals below the noise shaper noise floor is eliminated. So small signals are attenuated or altered by the noise shaper - and this affects depth perception.

Now with Hugo, the noise shapers are 200 dB performance (that's about a thousand times more resolution than conventional high end DAC's), and this is one of the reasons why Hugo has the reputation for reproducing good depth. Now I thought that 200 dB performance was good enough - but you can never make assumptions as to whether something will make a difference unless you do rigorous carefully controlled listening tests. With Dave I had enough space to do more advanced noise shaping. So I increased it to 220 dB - and depth got better. Then 240 dB - again you could easily hear an improvement in depth. This process continued over several months with radical re-designs. At 330dB it was still sounding deeper - eventually I got 350 dB performance and you could still hear an improvement from 330dB. Now this is completely extraordinary; and if somebody told me that 350dB performance was necessary a few years ago I would have said they were nuts as it implies that there is no limit to how accurate small signals need to be - that the brain can detect any size of error no matter how small. Utterly amazing - but I can only report what I can easily and consistently hear.

At the end of this process, I could hear depth that is around four times deeper than with Hugo. Now that is only if depth is recorded, close miked sounds with no reverb still sound flat as it is supposed to.

So Dave has been a very exciting and rewarding project for me. I got to find discover more about DAC's in the last year than the previous decade. Most importantly I understand how digital degrades depth perception and upsets the perception of timing - and these things are absolutely key for trying to re-create the musicality of live unamplified music. In short I have vastly underestimated how sensitive the ear/brain is to extraordinarily small timing and amplitude errors.

So how does Dave sound? I am (quite rightly) not supposed to talk about the sound of my own projects but I guess I can hint at it. Its like a Hugo but with a lot more depth - that's the first thing one notices. For the first time ever, I am now starting to get depth to sound more like real life - some recordings can sound really spooky as to how deep they can go. And this aspect is (mostly) down to the noise shaper performance. But to get 350 dB on the noise shaper was pretty involved - I ended up with 17th order, with 46 integrators. Its so huge the noise shaper alone would not fit on Hugo's FPGA. But the improvements in noise floor modulation, and the timing accuracy, also have a very big influence, but the first thing you should notice is the uncanny depth.

Rob

There are so many things that are done that make audible differences that many just take for granted and presume 'good enough'. Assumptions abound. So no, power isn't the only factor that we hear as differences between Mojo, Hugo, TT, DAVE, as well as other gear.

 

[Soundizer]

May I ask a technical question. If you have additional USB devices such as cables to a phone and tablet could that add any noise or jitter into the computer- Apple iMac in my case.

The reason I ask is I would like to use the two USB slots to charge my other devices but not at any cost to audio quality data going into my Mojo.

 

[jarnopp]

there are so many factors. all can't be measured. despite mojo and hugo both measure flat in frequency response, mojo definitely sounds warmer. once rob said that during development of dave , it started sounding too dark due to heavy noise suppression, then he had to improve the transient response by increasing tap count ( if I remember correctly) and other things to balance the dark sound. as far as i know with my limited knowledge if you increase tap count you can definitely listen the improvement in transients but it is difficult to measure . mojo imho has more noise suppression but fewer taps than hugo which may be the reason of mojo sounding warmer.

I can't find the post right now, but Rob has said Mojo actually uses twice the taps that Hugo does, but at half the speed, for the same result. Something like that. And, more taps correlates to a warmer sound.

 

[GRUMPYOLDGUY]

I can't find the post right now, but Rob has said Mojo actually uses twice the taps that Hugo does, but at half the speed, for the same result. Something like that. And, more taps correlates to a warmer sound.

Wait, what information leads you to believe the filter coefficients changed?

And warm means more gain in the bass range to you, right? Why would filter length by itself affect gain? I can design a 100k tap filter and still have unity gain... You need the coefficients to know how frequency response is affected.

 

[Light - Man]

Wait, what information leads you to believe the filter coefficients changed?

And warm means more gain in the bass range to you, right? Why would filter length by itself affect gain? I can design a 100k tap filter and still have unity gain... You need the coefficients to know how frequency response is affected.

Why don't you do that and come back to us in 25 years?  

 

[Lord Raven]

Thank you Relic,
 
I have given up on the idea of an external amp, the convenience factor is huge. Talking about Focal Utopia, I would never jump that high in head-fi to own a Utopia headphones but I do have a Focal Utopia 3-way in my car as a reference, I can easily say that Mojo is very nice portable device and I might take it to my car audio and replace my head unit which is only CD and DVD player, I can play DSD and SACDs directly. The problem I face is that, while connected to a phone, it gives a lot of sound disconnections, pops, and crackles. Is my OTG cable faulty, or the Mojo is not behaving?
 
Moreover, I have been unable to fully charge Mojo via my PC. Do you connect two USB cables, one in each micro USB slot to achieve charging while listening to music? It takes about all night, while Mojo is off, to charge it fully and that too, using a Samsung 1A mobile phone charger. 
 
Is there a way to switch off the lights on Mojo to save battery? And, is it a good idea to connect both headphone ports to your external amplifier? One leg from the left and one from right. I heard someone say that it is better. Sounds like a myth.
 
Best Regards
LR
 
 
 
Quote:

For questions regarding the Mojo design, implementation and the views of the designer a good subsection in the third post is 'informative posts by Rob Watts'. Highly recommended that everyone read this.

Regarding adding an amp it all depends on what your goals are, your sonic preferences, and synergy with your headphones. The Mojo is powerful, as powerful as the Hugo and almost as powerful as the Hugo TT, so it can drive a wide variety of headphones. The Mojo is tuned a little smooth for it's intended purpose of a portable device, but honestly it isn't overly smooth at all compared to Chord's flagship DAC the DAVE, they share a similar tonality. Be aware that adding an amp can only add it's own sonic flavour and distortions, which is why Rob's designs do not use a separate headphone amp in his unique design. His goal is transparency to the original performance and the more components in the path will only reduce the transparency, which is against his design goals.

I've used separate amps such as the Oppo HA-1 (the amp portion), Cavalli Liquid Carbon, and Cavalli Liquid Crimson as well as a few portable amps with the Mojo. All of them added their own slight variation to the sound and some of them more pleasing than others, but none of them really added any drive capability with the headphones I've tested, namely the Focal Utopia, ETHER Flow, ETHER C(1.1), and LCD-2.2. Now none of these headphones are really difficult to drive and they all play at the lower end of the Mojo's volume to reach an average listening level of 80-85 dB, and they all sound great straight out of the Mojo.

I guess the short answer is that adding an extra amp can not increase measured performance (except power output depending on the amp and headphone requirements), but it may add a sonic flavour that's pleasing to the individual. The former point is unavoidable. The latter point is entirely a personal decision.