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I have the Opus DAC configured as dual mono driven by 8804 and Volumite. I am getting distortion on the left channel. If I switch adresses of the DAC the distortion moves DAC. So could be data related from the 8804? Any ideas???
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Can you post a picture of your setup? It is very likely related to noise injection from the I2S lines.
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Could not work out how to attach a picture so I have sent it to you via email. I have shortened the I2S bus wires as short as they will go. I have also not connected the GND on the I2S as the digital boards all share the same PSU so they have a common ground there, possible ground loop issues? These measures have improved the situation but there is still some form of interference. Looking at the scope it is definitely digital noise. Not sure what else I can do?  steveww attached the following image(s):  opus.jpg (166kb) downloaded 179 time(s).You cannot view/download attachments. Try to login or register.
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You attach pictures by clicking the attach button on the top right of your post right after you have posted it.
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Rank: Advanced Member
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I see the attach button now. I was looking for it in the editor box for a new post. This forum works differently to others I have used.
Since my last post I have gone round all the modules and tied their regulated 0V together in a star earth scheme. This has again improved the situation but not entirely cured it. Adjusting the volume via the Volumite produces a crackle just like a rusty old pot (quite nostalgic really). My guess is that the 2 wire I2C bus is causing interference on the audio (I2S) bus??
I have used a twisted pair for the I2C bus, may be this should be a shielded cable? May be an extra twist with a ground cable would help??
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First, nice layout.
You can try routing the I2C bud wires away from the power supply wires for the digital modules. They would be safer near the balanced outputs, and any noise introduced there would be common-mode and cancelled.
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Another point that concerns me is the interface between the 8804 board and the Toslink module. The 8804 board is configured to receive a 75 ohm 500mV signal, the Toslink module is outputting a logic 3.3V signal. The Toslink module is rated for 2mA max output current, 3.3V in to 75 ohms is more than 2mA.
May be the Toslink module is not "strong" enough to drive the 75 ohm load and the resulting SPDIF signal is too poor and the 8804 is having trouble receiving it??? Amy be some form of buffer is required on the output of the Toslink module to drive 75 ohm sufficiently??
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Yes, you will probably want either a voltage divider there (374R/93R) or simply remove the 75R termination resistor on the receiver. Eother should work.
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Removing R1 from the 8804 board would give you a high-impedance target for the toslink module.
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That is what I thought and not what your website says!
I have used a BC549 into a divider of 220R & 110R via a 0.1uF to the 75 ohm coax. Thus the Toslink module is only driving the base current for the BC549. This still gives interference problems. Just about to try a re-route of the I2C bus.
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I have added a ground wire to the I2C bus twisted pair, there are now three wires plaited together. I have also re-routed the cable away from the power feed. This has reduced the volume adjustment noise to an acceptable level. However there is still the occasional snap, crackle and pop when playing back at a steady state, a bit like listening to vinyl. The noise comes from the DAC as I can see it on the DAC outputs with a scope. There are endless permutations and combination of available ground connections I could be re-wiring this thing until the end of time. Is there an official line on what should work? This is module hell.
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I've never had any problems with the OPUS in s/w configuration mode regardless of wiring configuration for spdif/toslink, I2S and I2C. However, I've always kept the power wires to a minimum by butting the PS to the modules.
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All apart from the Ballsie have short power lines.Not as short as I would like but I was limited by the case.
One extra clue has shown up. Occasionally when I touch the case the whole Opus resets. Obviously a static discharge from me through the case. This I believe highlights the system's susceptibility to noise.
On the positive side I have now put the DAC (with vinyl like crackles and pops) in to my main system. Apart from the background noise (main on the left channel) it sounds good. More forward in presentation than my other DAC, excellent detail and real slam in the base. If only I could loose the crackles and pops I would love this DAC.
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Have you tied any of the grounds to the chassis?
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As far as I know all the electronics are isolated from the chassis. I need to double check this with my multimeter.
The DAC consists of two aluminium enclosures; one for the mains and one for the electronics. One box has two transformers, bridge rectifiers and reservoir capacitors. It feeds raw DC via a multiple way cable to the other box where the regulated PSU and electronics live. The mains box is earthed for safety. The system should be double insulated such that signal ground is not connected to mains earth. I will double check this to make sure a sneaky little short has not found its way in.
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Rank: Advanced Member
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I finally have a dual mono Opus that works most of the time! Occasionally it does not boot up cleanly on power on with only one channel starting up. The power on reset could be held for a tad longer to improve this.
So here are some tips on wiring up the multiple modules.
Just because each module has multiple ground connections does not mean you should use them all! Sketch out what you have and try to avoid multiple paths to ground. If using the screw terminals to connect to the boards, test each connection thoroughly to make sure it is good. I had one that had not seated properly giving some resistance to ground.
Just the dead placid to sort out now!
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