00940
Headphoneus Supremus
- Joined
- Nov 6, 2002
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Here is what I'm planning to build this summer. If you have any suggestions or improvements, I'm willing to hear.
Digital section:
The digital section is nothing revolutionary and has been already seen more than once. It uses only proven TI ICs. The REG101s are a bit hard to get though but as I've a few left from another project.... The PCB is one fourth of an euroboard (50/80mm).
A PCM2707 is used as USB-to-I2S receiver. It is powered by its own REG101-33. A 12MHz crystal is used. A single AND gate takes care of the USB power's pull-up.
The ASRC chosen for jitter rejection is the SRC4192 (which is pin compatible with the AD1896). It shares a reg101-33 with the digital filter of the PCM1798. See why the src4192 rather than the AD1896 here : diyAudio Forums - Any feedback on new CS8421 high-res ASRC?
The DAC is the PCM1798. While its digital filter is not as good as the PCM1794's, it has the advantage of a weaker analog output. It thus makes it easier to use a discrete I/V. Its digital filter is set up to slow roll-off (is it the best option ? I'm still undecided). A REG101-5 powers the analog section.
A 24.576MHz clock feeds both the ASRC and the DAC. It can be either DIP8 (such as a Tent clock), DIP14 or SMD. A fourth and last REG101 powers it.
The PCM2707 outputs 44.1KHz/16bits. The ASRC turns that into 96KHz/24bits and the digital filter of the PCM1798 oversamples it 8 times. Aliases should thus appear at more than 700KHz, allowing for a gentle analog filtering.
schematic: http://membres.lycos.fr/sst00940/usbdac/dacsch.png
I/V section:
Each analog output will be connected to a discrete I/V stage. Based on a design by Cauthemoc, it should keep distortion quite low while allowing me to get rid of opamps. The idea behind this design (the current conveyor) is similar to what was found in Wadia products.
After each I/V, there is a simple emitter follower with a 2nd order Bessel Sallen Key filter. It provides 55dB of attenuation at 720KHz (while the digital filter already provides 80dB). At 20KHz, there is only 0.3dB of attenuation. It also buffers the output of the I/V.
They will be wired P2P.
Gilmore Dynalo:
The Dynalo sections are used as balanced to unbalanced converter. A 4-pole switch controls gain: either around 2.2VRMS (line out) or 7VRMS (headphones amplifier). A pot wired in between the hot and neutral allows to control volume.
Power supply:
The DAC will use two transformers. One is 18-0-18 @ 30VA for the I/V boards and the Dynalo. The other is 0-9 @10VA for the DAC board.
A pair of LM317-337 powers the Dynalo at around 18V. Before the regulators, there is 3* 4700µF per rail and a pi filter. To improve the performances of the LM*, zeners are used to set the voltage out rather than the typical resistors. (see Using 3-pin regulators off-piste: part 1 )
The 4 I/V sections will be powered from the regulated supply. They will be isolated from the Dynalo by zener-based open-loop regulators. The operating voltage will be around 15V. Those regulators will be wired P2P.
A stereo Dynalo draws around 180mA per rail. Each I/V section draws around 30-35mA per rail. The total dissipation will thus be around 15 to 16W for the analog section.
The DAC section has its own regulators onboard. However, each REG101 will be fed from a pre-regulator similar to those used for the I/V sections, set at 6VDC.
To compare it to well-known stuff, it’s a bit like TPA’s usb receiver + metronome + COD with a zapfilter on steroids.
Digital section:
The digital section is nothing revolutionary and has been already seen more than once. It uses only proven TI ICs. The REG101s are a bit hard to get though but as I've a few left from another project.... The PCB is one fourth of an euroboard (50/80mm).
A PCM2707 is used as USB-to-I2S receiver. It is powered by its own REG101-33. A 12MHz crystal is used. A single AND gate takes care of the USB power's pull-up.
The ASRC chosen for jitter rejection is the SRC4192 (which is pin compatible with the AD1896). It shares a reg101-33 with the digital filter of the PCM1798. See why the src4192 rather than the AD1896 here : diyAudio Forums - Any feedback on new CS8421 high-res ASRC?
The DAC is the PCM1798. While its digital filter is not as good as the PCM1794's, it has the advantage of a weaker analog output. It thus makes it easier to use a discrete I/V. Its digital filter is set up to slow roll-off (is it the best option ? I'm still undecided). A REG101-5 powers the analog section.
A 24.576MHz clock feeds both the ASRC and the DAC. It can be either DIP8 (such as a Tent clock), DIP14 or SMD. A fourth and last REG101 powers it.
The PCM2707 outputs 44.1KHz/16bits. The ASRC turns that into 96KHz/24bits and the digital filter of the PCM1798 oversamples it 8 times. Aliases should thus appear at more than 700KHz, allowing for a gentle analog filtering.
schematic: http://membres.lycos.fr/sst00940/usbdac/dacsch.png
I/V section:
Each analog output will be connected to a discrete I/V stage. Based on a design by Cauthemoc, it should keep distortion quite low while allowing me to get rid of opamps. The idea behind this design (the current conveyor) is similar to what was found in Wadia products.
After each I/V, there is a simple emitter follower with a 2nd order Bessel Sallen Key filter. It provides 55dB of attenuation at 720KHz (while the digital filter already provides 80dB). At 20KHz, there is only 0.3dB of attenuation. It also buffers the output of the I/V.
They will be wired P2P.
Gilmore Dynalo:
The Dynalo sections are used as balanced to unbalanced converter. A 4-pole switch controls gain: either around 2.2VRMS (line out) or 7VRMS (headphones amplifier). A pot wired in between the hot and neutral allows to control volume.
Power supply:
The DAC will use two transformers. One is 18-0-18 @ 30VA for the I/V boards and the Dynalo. The other is 0-9 @10VA for the DAC board.
A pair of LM317-337 powers the Dynalo at around 18V. Before the regulators, there is 3* 4700µF per rail and a pi filter. To improve the performances of the LM*, zeners are used to set the voltage out rather than the typical resistors. (see Using 3-pin regulators off-piste: part 1 )
The 4 I/V sections will be powered from the regulated supply. They will be isolated from the Dynalo by zener-based open-loop regulators. The operating voltage will be around 15V. Those regulators will be wired P2P.
A stereo Dynalo draws around 180mA per rail. Each I/V section draws around 30-35mA per rail. The total dissipation will thus be around 15 to 16W for the analog section.
The DAC section has its own regulators onboard. However, each REG101 will be fed from a pre-regulator similar to those used for the I/V sections, set at 6VDC.
To compare it to well-known stuff, it’s a bit like TPA’s usb receiver + metronome + COD with a zapfilter on steroids.