Computing stuff tied to the physical world

Audio DAC

In Hardware on Jan 11, 2013 at 00:01

Last year, I got one of these little low-cost audio Digital-to-Analog Converters on eBay:

DSC_4343

Takes digital SPDIF coax in, and produces analog left and right channels out. I got them before the bigger picture of the audio chain in the living room got fleshed out, and in the end I don’t really need this type of converter anymore.

Besides, there was some hiss and hum with these, so it’s not really high-end anyway.

Here’s what’s inside:

DSC_4344

That’s a Cirrus Logic CS8416 “192 kHz Digital Audio Interface Receiver”, which apparently has all the smarts to detect and decode a range of serial bit formats, plus a Cirrus Logic CS4344 “10-pin, 24-Bit, 192 kHz Stereo D/A Converter” to generate the L/R audio signals.

I’m a bit surprised by the hiss/hum issues, since the audio DAC is specified as -90 dB THD+N and 105 dB Dynamic Range, which to me sounds like it should be pretty good. Maybe not audiophile level, but hey… neither are my ears, anyway!

  1. These chips are good, the problem lays in the circuit design around them – properly handling digital and analog signal and ground in the same circuit. The designer probably skimped on parts – especially in the powersupply section.

  2. Or, what I stumbled upon: different ground loops between amp and the DAC. My router is downstairs and connecting the LAN to my Airport Express upstairs somehow pulls those two to the same ground level, presumably over the shield of the CAT cable. Now if I connect the (analog) audio out of the AE to my amplifier, I get a quite hefty hum on the speakers, unless I connect the shield of the CAT cable with the housing of my amp …

    Most likely nevertheless is that your DAC simply came with a poor wall wart.

    • “…presumably over the shield of the CAT cable.”

      Ethernet is usually sent over UTP – unshielded twisted pair.

      Even with STP I don’t think any of the conductors are connected to the shield – you need funny variants of the “RJ45” connectors to actually make a connection to the shield.

      I’m not so sure about this but I thought Ethernet is usually galvanically isolated from the cable by little pulse transformers at least in part to avoid these grounding problems. Those problems did apply to the coax ThinLAN cables which, IIRC, shouldn’t have been used between buildings with separate earths.

    • The ethernet port is nearly always isolated in your network equipment (the routers and switches for example) but it doesn’t have to be the case in de devices at the other end. I know that in at least the late 2006 macbooks the ethernet port wasn’t galvanically isolated. Also the jeelabs ehtercard that I have doesn’t seem to be isolated to me, but I didn’t dive into the datasheet to verify this.

      I was quite lucky that I could test and open up a few of these DAC’s before actually buying one. 2 of the 4 I had available had the same microchips in them One of those two was a cheap one and and another was around 50 euros. The expensive one had a huge number of components stacked away inside it and nearly no free space on the board left where the cheaper one looked quite simmilar to this one.

  3. Mixing digital and high quality audio is no trivial task. As Stephan said, the hum / hiss is probably due to poor circuit layout.

  4. Is there somewhere an easy introduction to designing analog audio circuits? I’m currently planning to build a DAB+ receiver. The sound comes out on a digital I2S port but at some place I’ll have to go analog. It will not be a high-end device but only a small radio for my workplace (~2x10W output).

  5. “Implementation is everything!” (nwavguy.com)

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