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Old 7th January 2004, 04:59 PM   #11
jcx is online now jcx  United States
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you should look at your current budget as well as considering supply voltage; 32 ohm headphones @ 100 mW rms require 5 Vpp and 2x 80 mA = 160 mA pk, if you multiply the usb voltage 3-4x to get 15 V you only have 120-160 mA available and the dac requires some of that 2x max1681 doubles the usb V and gives 250 mA current output (you can do 2x with inductive switchers as well but I worry that the inductor spews rf mag fields not that sw cap converters are noiseless)

another issue is the power ground to sink the current from the headphones, simplest is to AC couple the headphones but @ 32 Ohms this could require 500-5000 uF caps depending on your quality expectations and beliefs about capacitor distortion
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Old 8th January 2004, 03:08 AM   #12
jcx is online now jcx  United States
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a better arrangement might be to use the 2 x parallel max1681 to invert the usb power with respect to gnd, a diode V doubler can be pumped by the max1681 to provide a high enough V for linear 5 V regulation for the DAC analog - see applicatin notes for the sw-C converters

then the buffer in the loop works from raw usb and inverted usb Vin to provide high current into the 32 Ohm loads (up to ~ 200 mA), and a RC filtered version of the supplies can power the op amp, ac input coupling and dc ouput coupling similar to your 1st schematic is then practical - the AD823 is a rail-rail output, fet input op amp with very good reputaion at low voltage audio application, check the ouput swing of op amp and buffer with the low supplies, but acceptable performance sould be easy to find today with the emphasis on lower Vs op amps
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Old 9th January 2004, 02:02 AM   #13
Benjlv is offline Benjlv  United States
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first off i would like to thank you all for your input...expecially jcx.

I have gone with a relatively complete redesign. Because i wanted a SPDIF output as well, but not until recently did i realize that i could impliment one relatively easily.

it will work like this

PCM2903 SPDIF out to DIR1703 reciever to PCM1793 DAC to OPA134 to BUF634

power will be LDOs for 3.3v for the PCM2903 and DIR1703. the PCM1794 requires 3.3v and 5v. A LDO will provide the 3.3v and a Voltage doubler based off the max1681 and a LDO will provide the 5v.

The opamp stage will be powered by a max1681 in inverter mode. So there will be a -Vin...so maybe something as low as -4.5volts cause of the fluctuations of USB. The -4.5v will be followed by a regulator so the voltage will be more or less around -4volts. There will be another regulator off the USB bus for the positive rail that will provide the 4volts needed. volume control will be done in the digital domain. and there will be no coupling in the output stage becuase of the differential outputs of the PCM1793

so what do you all think?
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Old 9th January 2004, 03:04 AM   #14
Benjlv is offline Benjlv  United States
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oh i did look at those analog devices chips...they look real nice...particularly one of their DACs...but I am unable to obtain them for a reasonable sum.
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