I wrote part 1 of this post a few days ago and was going to post in the USB Cables.... Again thread but decided against it. Then after sleeping on the thoughts below decided to try something else which may add weight to the ideas on the effect of USB power supply noise put forwards below. Part 2 of the post deals with observations today.
Part 1 I had a play about with isolating the 5v pin of my USB cable last night. Its something I had a go at quite a while ago with a diy platted silver usb cable and got "good" results at the time.
http://www.phasure.com/index.php?topic=1977.msg20341#msg20341I tried "cutting" the USB 5v supply (using PVC tape over the pin in the USB socket), and following this then went on to try replacing the PC 5v supply with regulated 5v and 3.3v supplied derived from inside the NOS1.
I though I would post some observations on sound changes and ideas about possible factors contributing to the changes.
First sound quality.1) With 5v PC USB connected.Boarder sound stage, fuller but possibly slightly less damped bass richer sound and tone.
2) Without 5v PC USB connection (USB power pin taped over)Sound stage shrinks slightly but is sharper in terms of pinpointing individual sounds. Sound is generally tighter but slightly thinner, tone is not so convincing. Bass is better damped but the room ambiance form low frequency is not there possibly giving the sense that the sound stage has reduced in size.
3) With 5v PC USB disconnected but NOS generated 5v or 3v supply provided to the USB power input.Sound is smooth but loses high frequency information which results in sound stage shrinking and a loss of perceived presence. This is very similar to the presentation of the Anadco fibre interface or the X-sound PCIe capacitance card.
My preference overall was easily listening with the PCs 5v USB connection in place. This is not what I expected.
Some thoughts on what may be happening.What does the 5v supply do within the NOS ? two things (and similar for other interfaces):-
- First it is used to pull up a bus enable signal into the USB chip (this does not appear to be needed because the NOS works fine without any 5v supply).
- Second and possibly important here is that the 5v usb supply connects within the NOS (and may other usb interfaces) into an anti surge protection chip that protects the d+ d- data lines of the USB interface connection from surge conditions on the USB connection.
The surge protection chip is a diode network that connects the d+ d- to the +5v USB supply and signal ground of the NOS. What all of this might mean is that conditions on the +5v supply wherever it comes from could link noise to the d+ d- signal lines ???
What is interesting though is that sound seemed better with the PC USB 5v line connected. This is what lead to the test replacing PC USB 5v supply with the NOS's own Internal supply voltages to see if a "low noise" 5v supply would have less influence on the d+ d- lines and signal ground. As reported above the sound was smooth and similar to having the 5v USB connection cut completely but still my preference was for using the PC USB 5v connection.
This is a stab here, but an idea that might to explain why the PC usb 5v supply sounded better than either no 5v connection or a "low noise" 5v connection, could be as follows. The noise level on the 5v line, wherever it is taken from is probably going to influence conditions on the d+ d- lines at the anti surge device. However it may actually be better from a jitter point of view to use this PC 5v USB. it might be that the d+ d- lines may be carrying similar noise spectrum to the PC 5v USB line which might present as common mode noise on all three USB lines and not impact the d+ d- data transmission so much as it reaches the usb decoder chip. Alternately when using low noise 5v supply (from the NOS internally) any noise carried on the regulated 5v supply will not have the same spectrum as noise on the data lines and as a result may have a greater impact on the d+ d- transmission to the USB decoder chip.
This is just an idea to provoke discussion, I'v probably has a bit too much to drink here last night when thinking about this but thought I would post the ideas. It would be good to get to the bottom of what causes the differences heard.
Part 2So today I wondered what would happen if the 5v PC USB supply were bypassed (smoothed) with capacitors at the NOS end of the USB cable. The answer turned out to be quite a lot happens and some potentially good stuff at that.
I added a range of capacitance values between the 5v PC USB supply and the NOS USB card signal ground. Observations on sound are as follows (note the NOS already has 100nf in this location so the values stated are in addition to this).
2200uf
Sound is smooth loses presence base is controlled and strong but sound is unbalanced with highs rolled off. Just like the regulated 5v and 3.2v supplies, a pattern forming perhaps.
470uf
Overall balance improving but sill nowhere near not having a capacitor for presence etc.
10uf
Still losing the harshness of the unfiltered 5v USB PC supply but still also not well enough balanced or detailed enough.
400pf smother sparkling top end mids and lows sounding good but transients are just a little slower than an unfiltered connection
220pf now we are really getting there. Hash gone, presence intact, beguiling sparkling highs, transients intact !
I'll try a few more values round 220pf and listen more but this is sounding very nice. It's a tuning point rather than extremely large change of the USB pcb tweaks but very nice all the same. Seems to give the sense of transparency and focus of lifting the PC 5v supply whilst keeping the life scale and tone of having the PC 5v supply connected.
I will post more thoughts after some more listening.
My guess is that the value of cap that gives good results will vary from system to system because it may be linked to the noise spectrum coming from a particular PC.
Nick
UPDATE 25.10.13
Looking at a few PCIe USB 3 cards most seem to use a DC to DC converter to take the PCIe 3.3v bus voltage and convert this to the 5v rail that is carried by the USB lead. So the cards i have looked have a SMPS running at 300 to 450 kHz straight onto the diode clamp mentioned above that terminates the USB d+ d- signal lines most DACs. For most of us the supply will just sit idle (supplying no current) 'humming' away on the USB card. The tests above showed that bypassing this 5v supply with a clean supply was not great. But what happens of the USB's 5v rail has some resistive rater than capacitive load applied to it ?
I put a 1k potentiometer between the USB 5v rail and the signal ground on the NOS1 interface. Having the load really help sound here. Best sound for me is around 200 ohms of load so around 25ma of current. If you try this be carful not to go below about 75 ohms as you will start approaching the 500ma rating of your USB card or mobo usb port (by 10 ohms load) and may get a hot potentiometer.
Sound quality changes with the resistance but is better in my system with a resistance in place. At 200 ohms there is subtle but important change throughout the frequency spectrum. Sounds like a slight reduction in jitter. Bass takes on a more tuneful deeper and percussive feel especially.
The resistor seems to work well for both pcie usb and mobo usb.
I was wondering of the effect might be about countering ground loop current between the PC and DAC. I know ATx supplies tend to elevate the pcs signal ground a few mv with respect to PE and in my case my amp signal ground is connected to PE which could be making a loop from PC to DAC to amp. Whatever the cause the resistor is defiantly staying in place for now
Regards,
Nick.