USB-S/PDIF converter and DAC

[paulb]

I designed this to provide a USB input for the ezdac (see thread ezDAC v.1.5 Builders Thread ). It's based pretty closely to the datasheet app circuit.
I wanted selection of USB or S/PDIF input via a DIP relay, which is controlled from another board that does analog input selection.
Because the PCM2704 provides both S/PDIF and analog outputs, I decided to make the board more versatile (and easier to test) by bringing out the analog outputs. I have 300-ohm headphones which require some gain from the DAC output, so I added opamps at the output. I wanted a bit more headroom than the 5V USB supply provides, so there is a -5V supply generated by a switched-capacitor voltage converter.
I spotted a couple of needed changes already. The two opamps should be in the same package (my last re-annotate broke them up); and I'll reduce the values of the resistors at the output of the opamps to maybe 33 ohms or so.
Comments are welcome.

[paulb]

Thanks for all your comments and suggestions. I've decided to get rid of the opamps. My thinking is that I'll move them onto a separate companion board that will include a Li-ion battery charger and boost converter, and allow for an MP3 player input or the USB audio.
So, the new schematic is closer to the datasheet's Figure 33. I've beefed up the filtering a bit, and kept the relay to select USB or S/PDIF input. The connector JP2 is for the aforementioned companion board.
I've also added a an output transformer to allow isolation, if needed. For driving the ezdac board, I'll just bypass the transformer because the ezdac has its own input transformer.
I'll be working on the PCB layout for this over the next week or so.

[analog_sa]

I tried a similar USB/SPDIF circuit when i first built the EzDac and didn't enjoy the sound much. For the last couple of years EzDac has been getting I2S from a 2707 and sounds much better.

[theAnonymous1]

Originally Posted by analog_sa

I tried a similar USB/SPDIF circuit when i first built the EzDac and didn't enjoy the sound much. For the last couple of years EzDac has been getting I2S from a 2707 and sounds much better.

Is it galvanically connected to the PC?

If so, have you considered a USB isolator?

USB Isolator. « Circuits@Home

[paulb]

Originally Posted by analog_sa

I tried a similar USB/SPDIF circuit when i first built the EzDac and didn't enjoy the sound much. For the last couple of years EzDac has been getting I2S from a 2707 and sounds much better.

The only difference is the S/PDIF output, and I seriously doubt I'd be able to tell the difference. The HagUSB uses the same part:
Hagerman Technology LLC: HagUsb USB to SPDIF Audiophile Converter
and seems to be popular.
For those that are interested in an an I2S input DAC, there are other (probably more suitable) choices. Here's one:
The γ1 Modular Miniature DAC
USB input is available with this kit.

[paulb]

Here's an initial placement for this PCB. USB and S/PDIF inputs on the left (which will be mounted on the rear panel) along with their LEDs; S/PDIF output at the bottom right, headphone output on the right. The schematic has changed slightly, I now have two connectors JP2 and JP3 that will connect to the companion board I referred to. JP1 is 24VDC relay control input.

[mutter]

Why not use mini-USB? Does save much on the footprint, but the cables seem to be more readily available.

[paulb]

Originally Posted by mutter

Why not use mini-USB? Does save much on the footprint, but the cables seem to be more readily available.

I did consider it, but my current USB DAC uses the standard connector and I didn't see the point in designing something that requires me to buy a new cable.

[TICUL38]

Paul,
Just curious at this point... are you definitely set on routing the USB into a S-P/DIF conversion?
I am back and forth between several projects, but still planning for USB input to the DAC.
I am also introducing need for a "transparent" HP Amp., so reviewing DAC's output interface with a good OPAmp/volume control scheme driving a discrete transistor amp.
Any suggestions/ guidance re. output amps would be well taken.
Thanks,
Louis

[paulb]

Hi Louis,
I'm still working on this. I'm now planning it for a dual purpose: besides just USB to S/PDIF, it will also have a battery-powered headphone output amp, which could be used as a USB DAC headphone amp, or the headphone amp could be used by itself for an MP3 player. I had planned this as a second board but it ended up being much cheaper to put it all on one board. I'll go with more surface mount parts to fit it all onto a 75 x 120 mm board.
Are you asking about ideas for a power amplifier ( I like the P3A at Elliott Sound Products - The Audio Pages (Main Index)), or about a buffer stage after the ezDAC (I don't think it is required)?