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Old 10th April 2011, 01:50 PM   #11
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I originally used 41 ohm resistors, yesterday I tried a pair of 30 ohm resistors and then a pair of 50 ohm resistors, - same result. Today, I plan to play the Dido CD and mark down the track and time location of the audible clipping. After that, I'll play the same CD on the player I have in the lab. The I have two Oppo players in my living room. The player in my workshop is an old Cal. Audio. Do you know if my analog scope can see the difference in digital outputs of CD players?
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Old 10th April 2011, 02:04 PM   #12
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With those resistor values I can't believe that the DAC itself is clipping - the TDA1541's output currents are 4mA. I'm more inclined to believe the clipping is occurring after the DAC - how much gain do you have in your following stage?

Can't answer in regard to your analogue scope as I'm not sure what kinds of differences you'd be looking for in digital outputs. I'd expect no difference in the data itself.
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Old 10th April 2011, 02:21 PM   #13
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More info.

I have tried both an Opamp (LM4562) and a tube output driver. Thinking at first it was the output driver, I lowered its gain.
So the problem is definitely in the DAC. I am using the Oppo as a transport. I have two Oppos , a 980 and a 970HD, in my liviing room. The player in the workshop is an old Cal Audio with only a COAX out.

If I play the Dido CD through the Airport Express using the optical input, I have no problems. If I switch to the digital output of either of the Oppos, I get a problem both trough the coax or optical inputs.

One possible problem is the digital output of the Oppo is too high for the DAC in some cases. And maybe some type of digital attenuation is needed.

I will mark the places where clipping occurs through the Oppo and play the same tracks on my CAL Audio player and see if I detect any clipping at the output with my scope.
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Old 10th April 2011, 02:28 PM   #14
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I have no experience of Airport Express but the only thing I can think of at this stage is that there's some digital attenuation being applied within it. Does it have software controlled volume? Use your scope not to look at the outputs of the CD players but rather across the 41/50/30 ohm resistors at the output of your DAC.
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Old 10th April 2011, 03:19 PM   #15
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I can adjust the attenuation of the digital signal through iTunes. So, I normally play music through the airport express at about 75 of the volume on the iTunes slider. I just tried pushing the itunes volume all the way to max. The clipping occurs at that volume setting no matter what volume set at my preamp. I didn't discover the problem until I tried to play this CD directly from the cd to the dac. I have two complete systems and this problem occurs in both systems.

If I take the analog signal from the Oppo on either system things are fine. If I use another dac, no problem.
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Old 10th April 2011, 03:32 PM   #16
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You haven't talked about the amount of gain before and after your volume control yet. Given that the output level from the raindrop DAC is going to be around 30dB lower than a standard CD output, you'll be listening with very different volume settings, no? Or do you have a gain stage you use with this DAC which you switch out when listening to your Oppo?
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Old 10th April 2011, 03:40 PM   #17
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I've lowered the output gain of the driver to around 10db. The clipping is still there.
The output from the OPPO is analog out to preamp or digital out through either coax or opitcal to raindrop to either opamp (around 10db gain), or tube driver (around 25 db gain).
So to answer you question more to the point, the volume directly from the cd player through the preamp is louder.
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Old 10th April 2011, 03:47 PM   #18
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I can't picture your setup with the details you've so far given. The only way I can think of for the DAC itself to be clipping would be if its got some digital gain built in. While I've not seen the schematic there's only one IC visible that could be responsible for digital gain and it doesn't look up to the job. Until you've scoped the output voltages (output of DAC, outputs of buffer/gain stages) I can't offer any further advice.
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Old 10th April 2011, 03:58 PM   #19
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I would suspect that the clipping is occurring on the digital side of the DAC chip. If there is not enough bandwidth in the digital receiver then clipping would occur. I will take the Dido CD out to the shop and look at the output across the 50 ohm i/v resistors. Even though, I pretty sure we'll see the clipping, we won't be able to tell if it is coming from within the DAC chip itself or somewhere beforehand. It would be nice to be able to measure the voltage of the digital signal to determine the bandwidth available at input.
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Old 10th April 2011, 04:39 PM   #20
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Update:
I played the Dido cd track 1 and set the scope probe across the 50 ohm output resistor. With the scope set on the 50 mv/div scale there are segments of the track where the peaks disappear. It seems the DAC can produce 200 mv peak to peak signals. Over that, clipping occurs. Do you know what the output voltage limits of the DAC?

I will try another CD to see if the output limitations are the same.
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