Optimised USB for Audio

Executive Summary:

  • Ultra low noise but more importantly, stable power is a critical aspect for driving audio device optimally

  • These LiFePO4 batteries (now supercapacitors) have such characteristics when employed correctly

  • Elimination of other sources of noise are also significant for optimal audio 

  • Noise on the USB signal lines is audibly obvious when removed

  • This noise is not fixed in frequency, it is dynamic & changing with the signal - see Noise page

  • This is noise on the USB signal wires, not on the USB ground or 5V lines

  • Depending on it's severity it can result in a lifeless, uninteresting sound at one end of the spectrum

  • At the other, more subtle end it is not noticed until removed when more realism & musicality is the audible result

ISO-HUB block diagram

A stable, ultra low noise power supply is the foundation for all devices to OPTIMALLY improve signal waveform or signal integrity. In these devices this low noise power is provided by very powerful but very safe, batteries which are user replaceable.


In the Ciunas devices, through various techniques, direct ultra low noise, clean power is provided from these sources without any intervening voltage regulators. I have found that using voltage regulator (even the best ones) on the battery output, negatively affects the quality of battery power output as evidence in the sound. It may be that regulators  do not produce the same power stability due to the interaction of their feedback output stages with the circuits they are driving.  A lot of digital audio components require currents in bursts & it’s important that the power system is capable of delivering these current demands without generating noise - in other words it's undisturbed, stable throughout its . For instance USB receivers handle a USB packet every 125mico seconds (USB high speed) & this involves handling the packets with bursts of internal processing in the receiver chips. A stable power delivery system is needed in order that these bursts of current delivery do not result in power system noise being generated on the ground plane.


Certain batteries are capable of this large dynamic current delivery - the batteries I use have 120 Amps instantaneous current capability. One result of achieving this are sonic benefits in this area of digital audio.




The Issues:

It seems that there are at least two factors at play - leakage current mitigation & stable, fast, ultra-low noise power (particularly low noise at very low frequencies).


Current leakage is an inherent phenomena in all mains powered equipment - the reason why medical grade electrical devices exist is a recognition of this fact & keep leakage currents within the limits of medical standards - usually < 100 uA for patient safety considerations.


Why are leakage current issues being focused on now when they have always existed in our playback systems? The rise in use of SMPS power is one of factors behind this. These tend to  have higher leakage currents than linear PSes. Another factor are the RFI filters needed in order to pass pass certification in devices handling high speed signals & these filters increase current leakage to ground/chassis. SMPSes which have a typical switching frequency of 40-80KHz tend to actively produce a strong leakage current at this frequency & its harmonics. Linear supplies are a passive window which pass 50-60Hz mains frequency & its harmonics.


Leakage currents are not the same as the 50/60Hz ground loop noise heard as hum or buzzes from speakers. This ground loop noise occurs when two or more grounds in the audio system are at different voltage levels & a current flows between these points. Leakage currents arise between the AC & DC of both linear & SMPS but are usually higher in SMPS. A small current is “leaked” onto the DC & often flow in signal cables in order to get back to ground or neutral.


Leakage currents is not noise identified as a separate audible entity but rather noise which is interwoven with the music signal. How this sounds is variable because it depends on the leakage characteristics of the power supplies through which the leakage currents are flowing. In a lot of cases it is perceived as a dull, flat uninteresting sound lacking dynamics, life & vitality.


Off-grid power supplies, such as batteries are one step towards addressing these leakage currents by simply eliminating one of the leakage loop pathways. USB isolators block another leakage current pathway - this time for common mode noise traveling on the actual signal lines, not just the ground or 5V power line. These two approaches, implemented in digital audio devices, prevent noise currents from entering the sensitive analogue areas of these devices (this includes the oscillators).


Another current finding in digital audio is that a well-formed digital waveform seems to result in improved sound quality. Why, is not currently known or revealed by current measurements. To achieve optimally formed waveforms requires a low noise, stable & fast power supply as the stable reference needed for generating such clean waveforms.  


In my experiments with USB isolators, USB signal cleaners on the outputs always improved the sound. Why? The reason is likely reducing the jitter which all isolators create on their outputs.


My experiments with reclockers/regenerators showed an obviously audible improvement when powered by battery or similar clean, off-grid PS even when these devices use highly regarded, low noise voltage regulators, LT3042 or TPSTA4700. An even greater improvement was heard when these internal voltage regulator were bypassed & direct battery power applied in certain critical areas such as clock & USB hub chip. I have posted instructions for performing this minor modification on an existing USB reclocker here & those who have tried it have reported the large audible improvements. The importance of this power supply approach cannot be underestimated. LiFePO4 batteries used in my devices have ultra-low noise levels all the way down to DC when supplying such currents. 

ISO-DAC block diagram

The result of optimising all of these approaches - off-grid clean, fast, ultra low-noise & ultra stable power supply; USB isolation; & USB reclocking, results in a very noticeable improvement & much more realistic sound - a deeper, more 3D soundstage where each element is more naturally portrayed - a portrayal which has an unforced flow & naturalness to it. Don't believe me - just look at the reviews