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cowana 1st August 2012 09:31 AM

Digital source, receiver and DAC for preamp
I'm designing a preamp with a range of inputs, and have decided I want to add digital capability to it.

This will obviously comprise of the following three parts. I wish to keep everything working in hardware mode, so with no I2C/SPI control required.

The sources:
The preamp will have three digital sources:
- Coaxial SPDIF
- Optical SPDIF

The USB obviously needs a decoder. I am currently looking at the PCM2707 as it seems to have positive reviews - but any advice is appreciated.

The receiver:
The receiver needs to switch between SPDIF sources and convert to I2S. I'm looking at the WM8805 for this (in hardware mode). Am I right in thinking that this requires an accurate 12.00MHz clock to run well?

*Edit: looking further into it, I'm not sure that the WM8805 can change between sources in hardware mode (which is very poorly documented). The CS8416 is MUCH better documented and has a more capable hardware mode - would there be much of a performance downgrade to using this?*

The DAC:
I've been looking at the WM8740 stereo DAC - this seems to have good reviews. However I don't need the differential output - the datasheet shows multistage external active filters and a lot of complexity. I'm trying to make a very compact design - the CS4344 DAC is a very compact solution requiring minimal external components. I don't mind a small performance decrease in return for simplicity and a design which is more likely to work first time - so I'll probably end up using the CS4344.

Any thoughts or opinions are valued!

Many thanks,

Autotat 1st August 2012 11:14 AM

This sounds rather similar to what I'm trying to achieve, only you seem to know what you are talking about where as I'm completely out of my depth! I'll watch this thread with interest.

korben69 1st August 2012 11:52 AM

Hi Andy

Have a look there :

Both analog and digital In, XOver, DSP, analog & digital Out and much more....

For a good quality USB/I2S-SPDIF another affordable source :

PCM2707 is limited to 48KHz/16bits, different range...

You can tap the USB/I2S-SPDIF lines directly to the mentionned DSP digital In ;)

For the DAC, as you mention WM8741, have a closer look to the Opus from Twisted Pear : Opus - A Balanced DAC


cowana 1st August 2012 12:21 PM

Thanks Phil - the product schematics at Twisted Pear are very helpful.

I've now decided to go with:
- PCM2707 USB to SPDIF (48khz max)
- CS8416 receiver and MUX (Very easy to use, no external clock required, very compact)
- PCM5102 DAC (very little configuration required, generates it's own master clock, excellent specs)

This will all be built into the rest of my preamp (I'm experienced with designing that part) - which will contain:
- An oLED display for user feedback
- A rotary encoder for source selection/settings/volume control
- A PGA2311 for analogue volume control
- Relays for source selection between 4 analogue inputs and the DAC output
- A very sleek (apple) remote control (IR)
- Mains switching to turn the amplifier on and off
- Either a USB or RS232 connection for PC control/automation

So in all, the preamp will have inputs for:
- Toslink
- Coaxial SPDIF
- RCA analogue audio x4

That should be pretty much everything I ever need!


cowana 1st August 2012 01:10 PM

I don't seem to be able to edit my previous post - but that should have read:
- A rotary encoder for source selection/settings/volume control

wintermute 1st August 2012 01:15 PM

fixed ;) the edit limit is 30 minutes except for the first post in a thread which has indefinite editing (intended mainly for build theads and group buy threads).


korben69 1st August 2012 04:03 PM

Nice to see that your project is evolving, I'll follow your built.
From what I've read WM8805 performs better than CS8416 in jitter's square.
More informations here :
Similar project here, less remote and PGA :
Another member has done an integration with The Wire & Odac using a PGA, LCD and remote... Can't find the thread...

cowana 5th August 2012 12:27 PM

1 Attachment(s)
Right! I feel the schematic for my preamp has got to the stage where the design is almost complete. I'd apprentice it if you could have a quick look over it - if there aren't any glaring errors, I'll move onto the PCB layout.

Although this has elements of digital source, line, and analogue source and line in it, I feel pairly happy with the analogue stuff. The digital stuff is all new to me, and it's the area I'm most likely to have made a mistake in, hence I'm posting here!

The circuit includes a large digital audio section - it now has an onboard USB audio decoder, a digital (SPDIF) source selector for changing between optical, digital and USB sound inputs, and a DAC for producing the audio output.

The analogue signal path is quite simple - the sources are summed through a buffer opamp, and passed through the PGA2311. There's a peak voltage detection on the input of the PGA2311, so the AVR can measure the volume level coming in.

Please ask any questions you might have!


cowana 8th August 2012 09:24 AM

I've started routing the board, and have come up with a few issues along the way (which have been fixed):
- The CS8416 config resistors were pulled to analogue ground, not digital ground
- Separate 3v3 regulators for the charge pump and digital aren't required
- Separate grounds for the charge pump and digital aren't required
- Capacitors added across the relay coils to further prevent (electrical) switching noise
- The USB cable connector was given an extra ground to connect a shielded cable
- Input power connector added

Issues still to fix:
- The PCM2707 power setup is wrong - it is currently wired as a mixture of self power and bus power. I will completely isolate it from the rest of the PCB (taking power from USB), and add another SPDIF transformer. This will stop any potential issues with USB noise or ground loops through the USB input and an analogue source (which are not isolated).


cowana 8th August 2012 01:07 PM

A couple more things that need fixing:
- The CS8416 outputs to the 3v3 DAC - so needs VL to be 3v3, not 5v
- That means the inputs need to be 3v3 max - so replace the TORX179 with a TORX147

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