I maintain the opinion that all sources (at least all sources I have tried) sound very good with the DAVE and so to this extent, the DAVE is pretty well immune to "bad" sources but there are exceptional sources that can and do sound better and I am convinced that this is not merely a bit-perfect issue. I don't believe it's a jitter issue as I have convinced myself the DAVE truly is immune to source jitter (i.e. my Oppo/Tidal example is proof of this for me) and so this leaves me to believe that source RF is the issue despite the DAVE's galvanic isolation, Rob has suggested RF is difficult to completely mitigate against and compares it to a "fungus" that can't be completely eradicated. The mR uses 10 regulators in its design from it's ethernet PHY to its USB hub, with each regulator providing some level of isolation and so as each of the components within the mR create their own RF that adds pollution to the signal, there are enough regulators in the chain to mitigate it as it is created. The other supporting evidence for RF being the culprit is that a really good power supply (defined by John Swenson as one with very low output impedance) is supposed to make a significant difference with SQ. I made an inquiry on the CA forum why this makes a difference but I got no response. Paul Hynes, who will be building my power supply explained it well, however. The lower the output impedance of a power supply (in the ideal world, it would be zero), the higher the PSRR (power supply rejection ratio) will be and in the ideal world, PSRR would be infinity. PSRR is a measure of a power supply's immunity to the powered circuit that feeds it and an infinite PSRR would result in complete isolation to that circuit regardless of how dirty that circuit may be. Because an infinite PSRR is not possible, no power supply can be completely isolated against the powered circuit but certainly, this is what you strive to achieve. Again, this speaks to RF in the supply line as being the problem.
Considering John Swenson's mystery power supply is supposed to cost less than $500, this will probably be the best value proposition although how will it stack up against the best solutions?Even John doesn't know since he doesn't yet have a working prototype.
As of this morning, the mR continues to improve.
Agreed that jitter is not an issue with Dave; nor with any of my other DAC's. We can see this on the measured performance with Dave, as there simply are not any artefacts:
The reason why I am so confident that jitter is a non issue is because of a number of things:
1. USB operation gets its timing from the local Dave oscillator, and incoming data gets re-locked to the local clock.
2. When I add 2 uS (that's 2,000,000 pS of jitter) to the data input from the AP using optical or coax I measure absolutely no change whatsoever. Now that on its own is not enough, as I have had situations before where unmeasurable effects are audible - but not concerning jitter. I have always been able to hear an effect then measure it.
3. One way that an incoming data can effect the SQ is down to ground plane noise, and in the past this used to be a big issue, both in measurement and SQ. And it's technically possible that ground plane and power supply noise can affect the SQ (I have seen this many times before). But in the case of my modern DAC's I have been able to eliminate this issue by a combination of local RF filtering on power supplies, double layer ground planing,use of efficient local SMPS, and power efficient FPGA's, plus careful layout. Now this issue used to be a nightmare, particularly with the FPGA, when my DSP cores used power hungry FPGA fabric. I would have to construct a DSP core by creating my own multipliers (today I use dedicated FPFA resources that are extremely power efficient), and every time a new place and route occurred, I would get different sound and different measurements. Today this situation never happens for lots of reasons - better design of the ground planes, better local RF filtering, better quality of RF filters, and dramatically lower signal induced noise (actually this is thousands of times lower than ten years ago) from the FPGA. Today, different place and routes show no SQ changes, or measured changes. What I am alluding to here is that the noise from a jittery source can't upset the sound quality through induced noise, as it is now (as far as I can tell) completely isolated - its also one of the benefits of the USB galvanic isolation in that the USB processor gnd and PSU noise is isolated from Dave.
4. Pulse Array DAC is innately jitter insensitive. What is not readily appreciated is that different DAC architectures have very different sensitivity to clock jitter. DSD is horribly sensitive to jitter, R2R DAC's are very sensitive, but pulse array is innately insensitive. The reason for this is that signal switching activity is completely signal independent - it switches in exactly same way whether its reproducing 0 of fully positive or negative. Because of this, when I get some clock jitter, it only creates a fixed noise. Now one of the really cool things that happens today is that PC resources and simulation tools are so good today, in that I can write a simulation, and add some jitter to the simulation, then measure the results using an FFT. From this, I can see exactly what jitter and only jitter does - and this technique has revealed a few surprises. But what it has done is proven that adding random jitter creates zero signal correlated effects to pulse array - no distortion, noise floor modulation at all - just an insignificant level of unvarying random noise. This does not happen with other DAC architectures, as you will then get significant noise floor modulation, distortion and noise shaper related noise. This is because with the other types of DACs, the switching activity is signal related. So DSD has maximum switching reproducing zero, and no switching at 100% modulation. R2R has no activity for zero, but considerable switching activity when the signal changes.
As to RF noise yes it is like a fungal infection. In the mid 80's, when I began to appreciate the importance of RF noise, I created a RF noise mains filter, in order to eliminate the SQ changes that mains cable was making. This ended up being a scary design - a cascade of inductors and capacitors, with filtering from 100 kHz to over 1GHz. I even had to make my own PTFE air cored inductors to get the performance I needed. But it worked - you could absolutely not hear the effects of different mains cable before the filter, but I had to use insane levels of filtering.
So I know how crazily sensitive things are to RF noise, which is why I can't say for absolute certainty that RF noise from Dave or from the sources may or may not affect your system - simply as most power amplifiers are very RF sensitive. So it really is a case of YMWV. But I would like to make a few suggestions:
1. PSU design - as far as impedance is concerned, forget it in relation to RF noise. Now impedance is an important issue, but it has no bearing on RF noise. It can be important in that signal currents will cause distortion due to the OP impedance. That's why the reference power supply for the pulse array is less than 0.0005 ohms for each and every flip flop as that is an important source of distortion. But it has no bearing on RF noise, that is another issue. But today, filtering within the DAC can be done to a very high level of performance with modern SMD capacitors, inductors and ferrite beads.
2. Source - my advice with Dave is to use a source that is convenient for you. So far, in my system - and again YMMV, I have failed to hear any significant difference between different sources.
3. RF noise in terms of SQ. When you reduce RF noise, things sound softer and warmer and smoother. Bass is rounder with less slam and impact. Now if it is a bit perfect input source you are making changes, the only thing that can make a difference to the SQ is RF noise, so go for the warmer and softer sound - as that is the most transparent - even if it sounds too smooth! If it is too soft, don't worry - it is not a problem - you just need to then improve transparency elsewhere in your system - such as better cables, changing where the loudspeakers are sited, different HP, EQ etc. One of the profound problems we have with audio is that making fundamental improvements may affect the balance of the system, so it is less optimum. The trick is to understand when you have indeed made a fundamental improvement but that gives you an unbalanced sound then make other changes in your system to restore the overall balance.
Happy listening, Rob
