Modify an A/V receiver to provide multi-channel digital pre-out

[mhakman]

Would it be possible to modify a commercial A/V receiver to provide multi-channel digital pre-out in addition to existing analog? Background of this question is that currently available commercial all digital pre/pros are few and very expensive. Any decent modern and “cheap” A/V receiver with analog multi-channel pre-out must have that signal in digital form somewhere, before it gets converted to analog. Then, theoretically, it should be a “simple” matter to lead this signal out. Any thoughts?

[awpagan]

depends on which AV your looking at.
some will be easier than others.

I have a marantz sr5200 that has 4 cs4391's that go preamp out's.
thought about it a while ago, need to look at the sm again.

allan

[macboy]

I know for a fact that this would be possible on both of my (Sony) receivers. The DACs are off-the-shelf Burr-Brown units, and are fed with standard I2S signals. You could use the same signals to feed multiple digital transmitters (DIT4192 for example) to generate the S/PDIF digital signals. Then you need to either use analog circuits capable of driving a 75 ohm load at 2 Vpp (as per S/PDIF spec) for a coax output, or use TOSLINK transmitters for fibre optic output.

One issue you will face is that the volume control on your receiver is 99% certain to be analog, after the DACs. So the new digital outputs will be more like "line " outputs than "pre amp" outputs, i.e. fixed, full volume. You will need to implement some other form of volume control.

[mhakman]

I would be prepared to sell my current receiver and buy another if I got a “ready-to-implement” mod project for that. If I had to start from scratch and do all investigation and design then I would start with the one I own right now (a Denon). I was hoping that someone did that already (for any receiver) because it’s so obvious thing to do considering commercial prices for all-digital pre/pros – about 5 times that of an AV receiver.

Yes, it should be off-the-shelf DACs fed by I2S or alike – anything else wouldn’t be cost-effective for the vendors today. I was thinking about getting I2S out of the receiver and feeding it into small external box containing of-the-shelf S/PDIF circuits then terminated by S/PDIF coax and/or optical line drivers. Or why not AES/EBU balanced ones? Sounds pretty simple in theory but I don’t know whether I would be able to get all things right in practice – it was some time ago I did my last serious electronic project (and the world was different then).

Volume control (which is the only control I need) may be a problem if the receiver uses analog attenuation after DAC. I could adjust the level at some latter stage but that would require having a computer connected to the system at all times and perhaps couldn’t be done by an IR remote control. Using receiver’s volume control would be most “seamless”. Perhaps there is a simple way to implement digital volume control in connection with S/PDIF circuits but then IR remote may be difficult to implement.

It’s a tempting project and I personally know few other people that would appreciate this very much…

/Mikael