CM6631 usb audio interface .... any good ? - Page 18 - diyAudio
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Old 8th February 2013, 02:58 AM   #171
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Thanks for the links!

Alas, I have already purchased the two boards, with the original intent of simply using coax SPDIF and limiting my source material to <=96k (and/or living with soft resampling of the higher res stuff).

I already have a TE7022-based USB-SPDIF unit which sounds great but lacks 88.2k, which bugs me because I am getting more and more music at 88.2. I also already have a nice WM8524-based DAC. I also like it, but I suspect I will like the 8740 better, especially since I also have some nice opamps on the way as well.

For the prices, it'll be all fun to play with. I'll start with just using coax SPDIF until I figure out what all sounds good to me, then build it into an enclosure and either solder the SPIDF interfaces together or try to link I2S between the two. Even if I break a board (or both) it won't be the end of the world.
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Old 8th February 2013, 06:03 AM   #172
josha is offline josha  United Kingdom
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Those DAC boards usually have four resistors on the i2s lines, it would be easy to remove them and add a header instead, that would give you an easy 'tap' into the i2s feed on the DAC board. The other I'm not sure about - I've been lurking around on this thread trying to decide on the best option for me!
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Old 8th February 2013, 01:18 PM   #173
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Good idea. I wonder if a header is really necessary, though. Convenient, for sure - but perhaps it is feasible to tap directly onto the chip pins.

Can anyone tell me how flexible/pliable those chip pins might be? Could they be carefully released from the board and bent upwards and slightly away from each other to allow easier access to solder some fine wire on them? Or, are they so brittle/delicate that they will simply snap off the chip?
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Old 8th February 2013, 03:09 PM   #174
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I've been mapping things out and I think I have a plan.

On the DAC board, there are already connections between the DIR9001 I2S-out pins (4, 10, 11 and 12) and the WM8740 I2S-in pins (1, 2, 3 and 5). I am thinking I will disconnect the DIR9001 pins (4, 10, 11 and 12) and use those board contacts as a connection point to the WM8740. Perhaps I could even install makeshift headers there with some header pins harvested from an old sound card or video card.

On the CM6631 board, it appears that my only choice will be to solder directly onto pins 89, 93, 94 and 95. The good news is it appears (from the e-bay photos) that these pins are not currently connected to anything (electrically) on this board so there will be no real need to disconnect them from the board physically other than to maybe enable easier soldering (see my questions above).

If I am going to go to all this trouble (eventually, maybe) I should probably go ahead and terminate the far end of the tether wire with female header connectors so I can plug n' play it to the makeshift header pins on the DAC board (if I can successfully install them).

I am thinking CAT-5E might be a good choice for the wiring (?) and I will have to get a new soldering iron (I need a new/better one anyway).

Am I overthinking this? Underthinking it?
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Old 8th February 2013, 03:18 PM   #175
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It appears that this sort of thing certainly can be done: Click the image to open in full size.

I like a challenge, I am good with my hands, and I have lots of old computer parts to practice on...
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Old 9th February 2013, 12:32 AM   #176
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Hi,

When you get the DAC, please start a new thread about it, or perhaps you might find a thread already.

The CM6631 wont driver long (over 10cm) i2s lines very well because of the capacitance loading, and the signal quality will be degraded the longer the line - you'd really need to add a quad line driver (74vhc04) if you want this to be better than spdif. You also need to consider ground returns for the i2s signals, because if each signal wants to have current flow, it needs a return path immediately next to it for the return current. So use ribbon or cat cable with the grounds connected locally to the signals. If you don't do this you add inductance and again, degrade the signal. You din't mention the ground returns at all in your plan - no return = no flow = no sound.

It seems to me you've bought the wrong CM for what you want to do, and I'd suggest you might think first about optimising the DACs performance and the spdif circuit, and then, when the DAC is as good as it can be, think about whether i2s is even needed. The DAC might sound lousy due to poor routing, grounding, etc, and changing from spdif to i2s will yield precisely zero benefit.

Anyway, have fun - I think that's really why you've bought them right ?
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Old 9th February 2013, 04:13 AM   #177
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Here's photo of the current state of the CM6631 I'm hacking. I tried an adum3440 isolator but it seemed worse. So I pulled that off and now I'm going to use a Potato Chips 74G04 and a cat socket so I can use cat cable to connect into my DAC.

It isn't finished - trying to find some time to do that this week.....

Click the image to open in full size.
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Old 9th February 2013, 06:46 AM   #178
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Okay, so, I decided to get it finished. I used an adp151 to power the 74G04 because the LP5900 is already loaded near its limit. The LAN socket allows gsgsgsgsg connections. I used 62R as termination resistors.

I blacked out a section of the photo as a courtesy to the seller, who wants to keep the routing in that area a secret.

Click the image to open in full size.

The last thing to do is case it, and I'll fit a BNC socket for the spdif output. It'll fit very well in one of these :

Click the image to open in full size.

1455C801 - HAMMOND - ENCLOSURE, ALUMINIUM END PLATE | Farnell United Kingdom

Last edited by KlipschKid; 9th February 2013 at 07:05 AM.
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Old 9th February 2013, 11:08 AM   #179
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Hi, this is the "old" CM6631 pcb ? not the "A" version?
I see that you mounted an RJ45 for I2S output but you also consider BNC for the coax output, so have you find some way to have available both of the outputs simultaneously?

I don't understand why CM decided to have only 1 of the outputs available at a time, although I don't quite understand the ASUS Essence One construction either, where supposedly their firmware and driver which enables only SPDIF and mutes I2S would have to feed also the DAC present in that soundcard...so that means that the internal DAC isn't receiving I2S from the CM6631? they went with SPDIF all the way including internal DAC?
too bad there ain't any users around here with such a soundcard, it might be interesting to find out their solution by opening the box
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Old 9th February 2013, 01:11 PM   #180
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hi, yes, the A is being fitted to the ES9018. I've never tried simultaneous output - i have no idea if that works or not. it's not a feature i need. What i'd like is to set Foobar volume control to different bit depths

Last edited by KlipschKid; 9th February 2013 at 01:24 PM.
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