D100: Coaxial vs Toslink?

[NewBuyer]

Hello Wes, I was wondering if you might please share your thoughts on whether you recommend using the optical vs coaxial connection with the D100. For example: I believe the D100 coaxial input is transformer-isolated (correct?) - does this mean that ground isolation should be identical for its coax and optical inputs? Also, I've heard that optical is inherently more jitter-prone - is that your understanding as well, and have you found this (or jitter in general) to even be relevant to the D100? Any further info here from you on this topic would also be appreciated - thanks!

P.S. I'm really glad you are still offering the first edition of the D100, as 1V output is PLENTY for our system.

[NekoAudio]

Hi, NewBuyer. The short answer is either input is a good choice with the D100.

The coaxial input is indeed transformer-isolated, and it also shunts ground to the chassis to avoid potential issues. This provides a ground connection without allowing pollution of the D100's internal circuit. The optical circuit isn't "isolated" in the same sense because optical does not create an electrical connection between the transport and the D100. (For fun, hold your TOSLink cable a 1/2" out of the optical jack, and you'll still get music.)

I wouldn't say that optical is inherently going to contain more jitter. But I decided to not worry about jitter at all by choosing the Wolfson WM8804 S/PDIF receiver. Wolfson designed a chip that would not carry input jitter through to output jitter. So as long as the input signal is readable, you get a nice stable output signal. For more details, check out the white papers listed on the WM8804 product page.

Now, irrespective of all this you still want a relatively clean transport. After all, if your transport's digital output stage isn't reliable then there's not much a DAC can do. I've come across computer sound cards that were improperly grounded inside or included noise in the MHz bands on its coaxial output. The fidelity of such a signal would obviously be in question. In general I suggest using an optical connection from any transport that might be noisy like a computer.

[NewBuyer]

Thanks for the info. Does the D100 have its own onboard clock-crystal for timing, or does it strictly use PLL recovery on the transmitted S/PDIF clock?

[NekoAudio]

The WM8804 is driven by a 12MHz oscillator. The incoming S/PDIF signal is locked onto using a digital PLL. This data is then placed into a buffer. The 12MHz oscillator is used to generate a clock, and values are pulled out of the buffer as needed.

So now you have a data value synchronized with the internal clock, and the internal clock is clean and steady as it is derived from the 12MHz oscillator. There's a little more to it than that, which you can read in the white papers, but by rebuffering and using a good internal clock, you get a clean and steady audio stream. In fact, one use of the WM8804 is simply to reclock an incoming S/PDIF signal and output a "cleaned up" S/PDIF signal.

[NewBuyer]

Thanks again for the information. With its 1-volt peak output, I am thinking about using the D100 to directly drive an amplifier via the S/PDIF output of a Logitech Squeezebox Classic (which also allows digital volume control for fine-tuning the level). May I please ask you these follow-up questions:
1) Do you have any advice/recommendations regarding using a Squeezebox Classic's S/PDIF output to drive the D100 (i.e. quality of its output, etc)?
2) What is the output impedance of the D100?
3) Using a reasonably short interconnect cable: Do you feel the D100 is well-suited to directly driving an amplifier, and why (or why not)?
Thank you in advance...

[NekoAudio]

1) The D100 is very tolerant of transports so long as the signal is good because of the WM8804 operation I mentioned earlier. The Squeezebox Classic's S/PDIF signal will work great with the D100 and you should not have any issues.

2) The output impedance is 290 ohms. I recommend a pre-amp (or amp in your case) have an input impedance of at least 10 kohms.

3) If you use the volume control of the Squeezebox, then driving an amp directly works very well. I have done this myself and it definitely lets you avoid any coloration that may be introduced by a pre-amp. You may want to read about using the digital volume control of the Squeezebox though. Here's a thread that may interest you: SB3 volume control with 20-bit external DAC.

[NewBuyer]

Thanks again. I am currently using an amplifier that has a "signal-sense" automatic on/off feature, which works really well with a Squeezebox Classic (SBC) - i.e. the amp turns on/off automatically when I power on/off the SBC. The SBC has a software toggle-setting to either keep enabled, or to disable, the analog and S/PDIF outputs when the SBC is powered-off. Would this amp feature still work with a D100 between the SBC and the amplifier - does the D100 produce no voltage from its analog outputs when there is no digital signal present? Or, perhaps does the D100 power itself down completely after a certain period of inactivity?

[NekoAudio]

It depends. Usually amps that turn on in response to a signal do so because some voltage is detected. And then stay on for a short time after there is no longer any voltage.

When the D100 is on, but there is no S/PDIF signal or the signal is for no music, then there is no voltage on its analog outputs (any AC voltage would be heard, and any DC voltage would be bad). The D100 never powers down or enters an idle state, and is always attempting to detect an incoming S/PDIF signal. If you want it off, you have to do it manually. It draws very little power even when playing music (about 2W).