Hi guys,
First post here despite reading many many threads over the past month or so. Bit of an electronics noob trying to build my first quality usb DAC and having some issues. Maybe you can help...
First of all I bought the aw-d3 kit from www.audioworkshop.com.hk.
I then decided I wanted to use a discrete 3.3v power supply to power the PCM2706 rather than use the dirty USB power. The board already has a ~6.8v regulated supply for the TDA1543 so I built myself a simple circuit to step this down to 3.3v using a voltage divider and BC109. The PCB then had to be modified for self powered operation. This involved removing the PCM2706, cutting traces and grounding the PSEL pin. This all went relatively smoothly and is now complete (despite lifting a few traces when desoldering the PCM2706 :S).
I now faced the issue that 3.3v must NOT be applied to the USB D+ pullup resistor or HOST pin when the USB cable is unplugged to prevent damage to the PCM2706. I borrowed part of the circuit from here and am using a 74HC08 AND gate in exactly the same setup to achieve this.
So in summary my power supply provides a steady 6.8v to the TDA1543 and 3.3v to the PCM2706. 3.3v is applied to the D+ pullup resistor and HOST pin when USB cable is inserted (this is 3.3v sourced from USB power as per dddac.de).
and IT WORKS! ....up to a point.
My problem is that after a few minutes of use (5, 10, 30 or even an hour) my computer reports that the device has been disabled due to possible EMI on the usb line. The drivers are then reloaded and it works again for a few minutes more. This will happen a few times and then my DAC will just 'die' and is no longer detected by the PC. If I then unplug the USB cable (I can leave the power supply plugged in) and leave the DAC sitting for an hour or two it will work again! but only after an hour or so has passed.
I do notice that the little red LED that is wired to the USB 5v (as part of the voltage divider for the AND gate) flickers slightly before the computer complains of EMI and disables it. It also sometimes flickers when the USB cable is first inserted. Of course if the USB 5v drops momentarily (and it flickers) then power to the D+ pullup and HOST pin might be removed which I presume would kill the USB connection. I have tried decoupling the USB 5v right before the AND gate with a ceramic 100n capacitor and also a 43u electrolytic but this doesnt seem to help and in fact the 43u cap stopped the PC detecting it (initial charging current to large?).
I believe these issues are related to instabilities in the USB 5v line however I may be wrong. Maybe the PC is removing USB power to my device when it detects interference? Maybe I have grounding issues. Currently all grounds are linked together.
It also puzzles me that I have to wait some period of time before the device works again as if current is being stored in some capacitor and causing problems. I cant think where though.
Maybe someone has some suggestions as to how I can help stabilise the USB link?
I have of course checked for dry joints/loose connections.
I apologise for rambling on and on!!
First post here despite reading many many threads over the past month or so. Bit of an electronics noob trying to build my first quality usb DAC and having some issues. Maybe you can help...
First of all I bought the aw-d3 kit from www.audioworkshop.com.hk.
I then decided I wanted to use a discrete 3.3v power supply to power the PCM2706 rather than use the dirty USB power. The board already has a ~6.8v regulated supply for the TDA1543 so I built myself a simple circuit to step this down to 3.3v using a voltage divider and BC109. The PCB then had to be modified for self powered operation. This involved removing the PCM2706, cutting traces and grounding the PSEL pin. This all went relatively smoothly and is now complete (despite lifting a few traces when desoldering the PCM2706 :S).
I now faced the issue that 3.3v must NOT be applied to the USB D+ pullup resistor or HOST pin when the USB cable is unplugged to prevent damage to the PCM2706. I borrowed part of the circuit from here and am using a 74HC08 AND gate in exactly the same setup to achieve this.
So in summary my power supply provides a steady 6.8v to the TDA1543 and 3.3v to the PCM2706. 3.3v is applied to the D+ pullup resistor and HOST pin when USB cable is inserted (this is 3.3v sourced from USB power as per dddac.de).
and IT WORKS! ....up to a point.
My problem is that after a few minutes of use (5, 10, 30 or even an hour) my computer reports that the device has been disabled due to possible EMI on the usb line. The drivers are then reloaded and it works again for a few minutes more. This will happen a few times and then my DAC will just 'die' and is no longer detected by the PC. If I then unplug the USB cable (I can leave the power supply plugged in) and leave the DAC sitting for an hour or two it will work again! but only after an hour or so has passed.
I do notice that the little red LED that is wired to the USB 5v (as part of the voltage divider for the AND gate) flickers slightly before the computer complains of EMI and disables it. It also sometimes flickers when the USB cable is first inserted. Of course if the USB 5v drops momentarily (and it flickers) then power to the D+ pullup and HOST pin might be removed which I presume would kill the USB connection. I have tried decoupling the USB 5v right before the AND gate with a ceramic 100n capacitor and also a 43u electrolytic but this doesnt seem to help and in fact the 43u cap stopped the PC detecting it (initial charging current to large?).
I believe these issues are related to instabilities in the USB 5v line however I may be wrong. Maybe the PC is removing USB power to my device when it detects interference? Maybe I have grounding issues. Currently all grounds are linked together.
It also puzzles me that I have to wait some period of time before the device works again as if current is being stored in some capacitor and causing problems. I cant think where though.
Maybe someone has some suggestions as to how I can help stabilise the USB link?
I have of course checked for dry joints/loose connections.
I apologise for rambling on and on!!