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USB DAC - A Digital Voyager? |
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Graham Slee 
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Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16314 |
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 Posted: 05 Jun 2012 at 3:50pm |
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What we're trying to do is make a basic no fuss/no maintenance model, which at the same time, outperforms the computer's analogue outs...
(So far that is the case against the Realtek headphone jack on our HP Compac. Flat was an adjective to describe the PC headphone jack output, whereas 3D was the adjective used for headphones connected to the USB "Voyager". That wasn't me listening but John who used to play electric bass in a band and has younger ears than me.) We'd prefer this model to be bus powered, and the hub bus (inside the PC) has 120uF decoupling according to the USB specification. Obviously this 120uF is subject to the demands and glitches of other items plugged in, and it is the length of the USB cable away, but it counts in as part of the Kelvin load (headphone) current. The devices decoupling capacitor can be huge provided its charging current is limited to 100mA (one unit), but the USB spec contradicts itself by saying 10uF is OK, because at T = 0 a 10uF draws 500 Amps! What is lacking from the spec is the timing of the charging function. It doesn't make any reference to T (time) at all, which sounds a little ignorant to say the least. If you place a 3.3 Ohm resistor in series with any value capacitor it's T =0 current is 1.5 Amps, and if the capacitor is made 33uF it is in specification within 250 micro seconds. It says that whatever the instantaneous load is, that it must not cause a voltage drop of 330mV, but the approved 10uF will obviously cause a massive glitch at 500 Amps! (T = 0) So again, we have an ill thought through specification which would even Leave Einstein guessing! |
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That none should be able to park up and enjoy the view without a smartphone and the knowledge in how to use apps
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Graham Slee
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Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16314 |
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Posted: 06 Jun 2012 at 9:06am |
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Tried modeling the 10uF surge using an online capacitor charge modeler. The equation for capacitor charge is complex (calculus). Would need some serious electronics to charge anything more, and as this is supposed to be a small "gadget" I decided to try another approach, relying on the computer USB hub for low frequency energy, so out went the decoupling resistor, and in went a smaller cap to decouple at mid to high frequencies. The total capacitance seen by the hub including the DAC chip decoupling is now circa 10uF so it complies with USB there. How did it sound? Better, surprisingly. The computer was in use for other USB duties during tests and didn't seem to adversely affect the sound.
Next thing to comply with is the 2.5mA suspend (sleep mode). Could always claim it needs to be "left on". |
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That none should be able to park up and enjoy the view without a smartphone and the knowledge in how to use apps
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ServerBaboon
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Joined: 16 Jan 2008 Location: NW England Status: Offline Points: 970 |
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Posted: 06 Jun 2012 at 3:20pm |
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At this rate what will be next an Apple approved digital out headphone amp? Although knowing Apple they will make you pay until you bleed for the licence.
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Steve
------------- Various bits of GSP Kit ..well two so far, unless you count the cables that is. |
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Graham Slee
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Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16314 |
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Posted: 06 Jun 2012 at 4:10pm |
£2K - £4K for a USB licence, but seeing Texas Instruments paid the bill to allow communications between PC/Mac and DAC, we only have to ensure the rest of the circuit complies. Even so, we aren't allowed to use the USB logo unless we too pay £2K - £4K. We may be infringing copyright even calling it a USB DAC! |
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That none should be able to park up and enjoy the view without a smartphone and the knowledge in how to use apps
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Boris
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Joined: 30 May 2012 Status: Offline Points: 17 |
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Posted: 07 Jun 2012 at 12:14am |
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Life is bloody ridiculous sometimes
Hauliers would not travel up over jubilee weekend Got landed with £ 1200 bill for the pleasure of having latest usher shipment stored over the weekend God bless the queen New acronym gtw dac. That's gigabytes through wire Free and sounds better to me Who ever heard if a universal serial bus A killer bus?, travel for all ages? Somewhere to eat breakfast on the way to work? |
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