Problem DIY DAC / WM8740

[ah79]

Hello,

once again this forum is the last hope before my head explodes .

I'm building a DIY CD-player with a CD-drive from an old computer, D/A converter based on DAc Wolfson WM8740 + preamp with tubes. Basicly everything's ready and even the sound is nice but one little problem is making me crazy:

When the cable for digital input is not connected, the DAC gives out a continuous, loud noise. I have two Cd-drives and with the other the noise stops immediately when the digital input is connected in the player. Measured with oscilloscope, the player always gives out a regular signal even when it's not playing. The other drive I have is a lot older and does not give any signal when it's not playing and I get the same noise as with the cable unplugged. The older drive is much slower and more silent and I would absolutely like to use it instead of the newer one.

Having explored and measured things that far, which is already another story, I thought I could solve the thing by connecting the pin for detecting infinite zero signal to the mute-pin. It did'nt work out and I thought the pull-up-resistor in the mute pin caused this connection not to be stable and added a logical IF-circuit in between (or actually two NOT-circuits, but in practice it's the same). There wasn't any improvement and I did some more tests to find out that the logical levels are Ok and whenever the music's playing, grounding of the mute-pin silences the circuit as expected but when the cd-drive is stopped, not even direct grounding of the mute pin won't have any affect.

Now I'm running out of ideas: why does the WM8740 give out such a noise when it doesn't get any signal and how can I prevent it? Is this normal or is the circuit somehow defect? I cannot believe it because in general everything's perfect.

I hope someone is wiser than I and will make my day by sharing a bit of his wisdom . Otherwise I'll just have to use the newer Cd drive but that would be sad flaw in an otherwise completely succesful project.

[ah79]

And just for fun, here's a bad picture of the equipment in a test setup. As you can see, there's still some finishing works to do and the gadgets still won't be beautiful but I never intended them to be. As ugly as possible and sounding as good as possible, that's what I like . From right to left: CD-player, differential preamp and Leach Amp.

[ah79]

Having done some research on the internet I found out that the basic problem for noise is that it's a basic feature of all delta/sigma-DACs to create audible quantization noise if they get a infinite 0, or as well infinite 1-signal. The newer cd drive is probably constructed to work also with that kind of DACs connected on digital output, the older one not. The WM8740 seems to have even a function for infinite zero mute, but it's only available on software control mode. The normal mute available on hardware control mode doesn't seem to be working with infinite 0-signal.

Thus the reason for problems seems to be solved, the solution is still missing .

[PicMaster]

WM8740 does not output anything during "digital silence". If auto-mute is enabled (received 1024 zero samples) the DAC will mute it's analog output.

Dude, you mentioned "the cable for the digital input". Do you mean "a cable, connecting the cdrom transport and the DAC board", carrying for example I2S data? I'm not absolutely sure, but the I2S interface pins of WM8740 does not have any internall pull-up or pull-down resistors, so if you disconnect the cable, these pins of the DAC will be left floating in the air and you may hear some noise...

It's usually a good idea to attach resistors (10K for example) between these pins and ground, to provide guaranteed voltage levels when the input cable is not attached.

The much more interesting question is whether you hear some noise from the DAC output while the cdrom drive is idle and the DAC is connected to the drive...

Best regards.

[ah79]

Originally posted by PicMaster
WM8740 does not output anything during "digital silence". If auto-mute is enabled (received 1024 zero samples) the DAC will mute it's analog output.

That's how I supposed, but for some unknown reason that doesn't to function. When the music is on, setting the mute-pin down will silence the unit immediately. When the input to the DAC is continuous zero and the noise is on, the mute pin does not have any effect. I've tried this at least a million times because I was sure it should function but it still doesn't.

Originally posted by PicMaster Dude, you mentioned "the cable for the digital input". Do you mean "a cable, connecting the cdrom transport and the DAC board", carrying for example I2S data? I'm not absolutely sure, but the I2S interface pins of WM8740 does not have any internall pull-up or pull-down resistors, so if you disconnect the cable, these pins of the DAC will be left floating in the air and you may hear some noise...

I meant exactly this cable. If it's floating or not shouldn't have any effect because between the cdrom and interface receiver (TI DIR 9001) there's a connection with a logical circuit to keep the input either 1 or 0 at all circumstances. I've also tested grounding the input pin of WM8740 direct to make sure the problem is not on outside design but really WM8740 itself and the noise still continues even wit input pin grounded directly.

Originally posted by PicMaster
It's usually a good idea to attach resistors (10K for example) between these pins and ground, to provide guaranteed voltage levels when the input cable is not attached.

I haven't that kind of resistors but as mentioned I've tried to ground the input pin direct when the noise is on and it doesn't have any effect so the problem is in WM8740, not the outside components or design.

Originally posted by PicMaster
The much more interesting question is whether you hear some noise from the DAC output while the cdrom drive is idle and the DAC is connected to the drive...

Best regards.

With the older CD-drive (see first post) I hear the noise also when the cable is connected and the cdrom is on idle. With the other player, that sends a clear measurable signal even on idle, there is no noise.

Actually I was on last week able to bypass this problem by finding a cdrom that both playes the disc on low speed and sends a signal on idle, but I may later do some development on my player and would still be very happy to find what this problem is about.

Best regards,
-Aleksi

[PicMaster]

I've made at least 2 different DACs with WM8740 (single-DAC and dual-DAC configurations, as described in the datasheet) and both DAC boards are behaving perfectly, no noise/hiss/nothing when idle. The problem you're observing is very strange indeed...

Aleksi, can you post the schematic for your WM8740 board (if it's not secret )? It could help to understand the problem more easily.

Regards.

[SoNic_real_one]

On a side note - the "Digital Output" from the PC drives it is 5Vpp signal. Your DAC expects 05-1Vpp SPDIF signal probably?

[ah79]

Originally posted by PicMaster
I've made at least 2 different DACs with WM8740 (single-DAC and dual-DAC configurations, as described in the datasheet) and both DAC boards are behaving perfectly, no noise/hiss/nothing when idle. The problem you're observing is very strange indeed...

Aleksi, can you post the schematic for your WM8740 board (if it's not secret )? It could help to understand the problem more easily.

Regards.

Yes, I know the problem is more than strange . The schematics are far from a secret, but unfortunately I don't have any drawn schematics, just some obscure sketches and calculations. The final design is only on the board and in my head .

Basicly I have a CD-drive and then a logical circuit (74HC4 and some resistors) to keep the input stabile (1 or 0, no floating signal). The there's TI DIR9001 interface circuit with external components just as recommended on the datasheet, then WM8740 with external components just as recommended on the datasheet. The pins for different mode settings on both circuits are adjusted right, otherwise the player wouldn't be functioning. WM8740 is running on hardware mode, PIN24 connected to ground. What are the only "specialities" on my player are the very good quality voltage supply and differential preamp stage realized purely with tubes, but they should have nothing to do with this problem.

One big question I've been wondering: Why do some new CD-drives send out a regular, stable and measurable signal on idle that keeps the DAC quiet? For sure it must have some reason that some drives are designed that way instead of sending out just nothing when on idle. For second, on software mode the WM8740 has available also a separate function for infinite zero mute...

Originally posted by SoNic_real_one
On a side note - the "Digital Output" from the PC drives it is 5Vpp signal. Your DAC expects 05-1Vpp SPDIF signal probably?

Min. input HIGH level according to datasheet is 2.0V, max. rating for input signal 5,3V. Input level from receiver/decoder circuit DIR9001 is 3,3V so everythings goes just according to specs of the circuit.


I hope to have some more information soon because a friend decided to build a similar DAC after having listened at my player just once ( ), I'm waiting to see if he will have the same problem or if I've just done something wrong. Otherwise I wouldn't be surprised at all to have screwed up something when building the player and at the first tries I surely did some flaws, but I cannot imagine what kind of flaw could cause just this problem with everything else working absolutely perfect . But as mentioned, finding a proper CD-drive solved the acute probhlem, now it's just for interest to understand what's going on...

Regards,
-Aleksi

P.S Can you say if there's any audible difference on the single-DAc and dual-DAC configuration? I've been thinking if the dual-DAC version would be worth building next, but don't really know if it's in practice any better.