CM6631 usb audio interface .... any good ?

[tolikas]

Hi,
i have this one:
24/192 , 24/96 CM6631 USB to COAX OPT SPDIF convertor for DAC - PCM270X killer | eBay

and it doesn't work with Win7, tried with 3 different drivers, LED flashes on,
but drivers won't work...

Seller suggested to flash firmware win a Win XP computer, but as do not have one, i still have a non working CM6631

[KlipschKid]

If the seller has suggested it needs re-flashing, then there must be a fault, so you should return it.

[KlipschKid]

Hi,

I thought I'd add some updates about the one I've bought.

I found the ASIO driver (downloaded through the Ebay ad) only works for I2S, not spdif, and again, no 88 or 176.

I can't find a datasheet to download but v0.8 (March 2010) can be viewed here :

CM6631_Datasheet_v0.8_°Ù¶ÈÎÄ¿â

The sheet says there is 3.3V output buffer on the spdif output (pin 38, 8ma output), and there is 1.63V DC on the pin, so it seems likely the logic levels need to be attenuated for spdif ? It also says its low level output is 0.4V and high level is 2.4V = 2v range ? Does anyone's pcb have any kind of attenuation on the spdif output circuit ?

My pcb has no spdif components :



So I added a 0.1uF COG smd cap in series, then a 300R smd resistor in series and then a pulse transformer (PE65612) and 100R res in parallel. This is about 75 ohms impedance and attenuates -12dB - so the assumed 2V output range drops to 0.5 for spdif - and the load should not exceed the pin's 8mA output - and it works well it seems. I'll need to get a look on a scope at some point to be sure how well...





So next up is to desolder the usb socket and give it clean power.

cheers

[syklab]

I gave my board to a friend for testing, may be can ask him to take a better picture on the s/pdif output section.

[KlipschKid]

Thanks for the bigger photo. It's hard to tell if that pin 38 goes into a via or C7 ?

Here's the full underside of mine. I was relieved to see no vias on the I2S lines and a contiguous ground plane on the other side. In fact, I'm very pleased with the design of this board.

[KlipschKid]

Hi,

I think I'm finished modifying this so here's a last update.

Here is the spdif circuit I'm using :



which I decided on after reading a bunch of threads, including this :

S/PDIF Digital output

I also made a "power cleaner" from a capacitance multiplier :



R1 = 100R, R2 = 910 (total 1K so that's an extra 5mA draw on 5V input), Q = BD139 (the board draws 132mA so a BC550 wouldn't quite handle the current), and C1 = 100uF Panasonic FM.

This gives 3.6V under load for a 5V input and the LP5900 is 3.3V with 80mV dropout, and corner frequency of a few Hz for the noise filter.

I also added a Nichicon PLE 820uF ultra low esr cap, after the cap multiplier to further reduce noise on the supply.

I can definitely recommend this board - it sounds better than any other usb receiver I've tried.

cheers

[luxury54]

Originally Posted by KlipschKid

I'm very pleased with the design of this board.

Hi, I saw on ebay this board that costs the same price like yours (29 $) but it is a more complex design having optical and coax outputs (with isolation transformer included) already mounted and also an extra power input switch for clean external power supply option; so what is that more preciselly on which you are so pleased with the design of your own board, what technical advantage does it have over this newest design?

CM6631 USB DAC Coaxial Fiber 192kHz 32bit I2S 192K 24bit SPDIF WLX 66 | eBay

[wizard1238]

Originally Posted by luxury54

Hi, I saw on ebay this board that costs the same price like yours (29 $) but it is a more complex design having optical and coax outputs (with isolation transformer included) already mounted and also an extra power input switch for clean external power supply option; so what is that more preciselly on which you are so pleased with the design of your own board, what technical advantage does it have over this newest design?

CM6631 USB DAC Coaxial Fiber 192kHz 32bit I2S 192K 24bit SPDIF WLX 66 | eBay

You can read this and find the different
TI USB 2.0 Board Design and Layout Guidelines
http://www.ti.com/lit/an/spraar7/spraar7.pdf

but if people only like eye catching big size capacitor, it's easy to choice

[KlipschKid]

Originally Posted by luxury54

Hi, I saw on ebay this board that costs the same price like yours (29 $) but it is a more complex design having optical and coax outputs (with isolation transformer included) already mounted and also an extra power input switch for clean external power supply option; so what is that more preciselly on which you are so pleased with the design of your own board, what technical advantage does it have over this newest design?

CM6631 USB DAC Coaxial Fiber 192kHz 32bit I2S 192K 24bit SPDIF WLX 66 | eBay

Hi,

I'd need a better photo than the one posted on Ebay to be certain, and definitely a photo of the underside, but it seems the component placement and routing on that board will not maintain signal integrity as well as the one I bought, however, it does seem to be pretty decent; perhaps the phase noise of the clocks and the noise of the regulators would be more critical.

Also - what reg does that one use ? The one I have uses an ultra low noise reg - LP5900. (EDIT Zoe Tsang text says AMS1117 - horribly noisy reg - definitely a strong disadvantage)

For example of a small improvement in layout, on the one I got, look at the signal lines from the clocks into the CM. That shows the designer knows how critical the signal on those lines is, so they are very short and unbroken with no vias, etc.

Unfortunately, they are both 2-layer boards but at least I know the one I got has a contigous ground plane on one side, so there is a ground return for all the lines, and none go through vias (holes to the underside), and none cross other lines of any kind. There are kinks in the lines but not a lot of them; so all in all, that's about as good as it gets for signal integrity. There's only one downside - the I2S output should really be available as GSGSGSGSG (g=ground s=signal) but as you can see on mine it is GSSSS. Also, I would expect to see some small smd resistors on the output (tx) to damp reflections from the receiver (rx)

There are lots of threads about this on this site. I'm no expert. I only finished one year of EE at uni, and I've forgotten most of that. But I'm a good learner, and a lot of it is common sense once you get the fundamentals about impedance at high frequency, ground returns, reflections, jitter, etc.

Most people want the best possible square wave signal because this produces the best possible sound, so noise levels and signal integrity are the most important things.

Hope this helps.

[diyinhk]

Hello everybody
GSGSGSGSG is already in one of the initial prototype
many prototype is tried before the pcb is available to the public,
hope everybody enjoy,
welcome any comment to improve