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Old 12th January 2013, 04:23 PM   #1
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Default TDA 1543 Distortion problems

I have recently removed my TDA 1543 from the original DAC board and fed it with a new 8.3V regulated supply. I have changed the output resistors to 2K7 and bias resistor to 1K5 as per;

Non Oversampling DAC-complementing CD-PRO
and
Pushing the limits of TDA1543 NOS DAC

I can not figure out whats up, I'm getting really bad distortion if I increase the volume over 5% from my digital source. The output is clear and listenable below this level, although at a relatively low level and not using anywhere near the range it should.

I have measured the dc voltage across the output resistors and it's at an expected 4.3V. I have also measured the ac output voltage after the output caps, with an audio source I get roughly 0.2V.

I originally thought that I may have blown the TDA, but I have replaced it with two others and am getting exactly the same results.

I'm stuck, I don't have a clue where to go from here?
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Old 12th January 2013, 04:30 PM   #2
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I'd not go beyond 8V which is the recommended maximum supply (although not the absolute max, which is 9V). Your volume control is a digital one? Try sending the DAC a 400Hz sinewave (generate the test waveform in Audacity) at full scale and look at the output on a scope (or at least measure with a multimeter if you have no scope).
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Old 12th January 2013, 05:26 PM   #3
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I use a computer as a digital source via coax cable to my DAC, so i can reduce the volume/output range using windows volume control. I do usually though, leave this as full scale and control the volume with my amplifier.

I downloaded a sine wave generator app, and set to it 400Hz. I measured the output at different output levels (from windows) with my rather inaccurate (at small ac voltages) multimeter;
2% - 0.0V
3% - 0.2V
4% - 0.5V
5% - 0.8V
6% - 1.3V
7% - 1.4V Distortion starts.

Looking at this, I figured that the output is not as low as I first thought, I checked my amp and realised that it also wasn't turned up as much as I first thought. Comparing with the 5.3V peak to peak, 1.85V rms available (at the 8.3V supply), its not surprising that distortion is setting in when it is.

So the question is, are the threads I was previously using as a guide on the output and bias resistors correct and if not, how do I calculate the values for my resistors?
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Old 12th January 2013, 05:36 PM   #4
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Since TDA1543's full scale current is 2.3mA I reckon your I/V resistor is a little on the high side, giving 6.2V peak to peak. But the real problem looks to be that you have too much digital gain in your system - as 5% corresponds to 0.8V then 100% would work out to be 16V. Perhaps there's a setting somewhere in Windows or on your S/PDIF output card's driver which is putting in excess digital gain.
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Old 13th January 2013, 05:47 AM   #5
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I believe to have traked down my problem to the DIR9001 reciver, spec here;
http://www.ti.com/lit/ds/symlink/dir9001.pdf

The output must have switched from 16 to 24 bit, which correlates with 2^8 / 2^12 = 6.25%

But what I can't figure out is that if you look on page 17 of the DIR9001 spec sheet, it only outputs I2S format in 24 bit anyway? This DAC used to work perfectly well, with full scale output before I lifted the TDA from the main board. I must be missing something, I just can't understand whats happening.
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Old 13th January 2013, 05:53 AM   #6
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I thought it only output I2S in 32 bit, 64X fs for the bit clock. I've never seen an S/PDIF receiver output 32X fs or 48X fs in hardware mode. It seems the WM8805 is the only one that can do 32X fs and that's only in software mode. In any case the rate of the bitclock is ignored - the TDA1543 accepts just the 16 relevant bits.

The only way the I2S interface can result in digital gain is if the phase of the WS gets shifted.
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Old 13th January 2013, 07:08 AM   #7
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I have read up on the I2S bus format, and now understand the truncation of the lower bits is not a problem. So this suggests the WS has somehow shifted four clock cycles?

I'm going to check the wiring I installed for the data bus to the new TDA board. Are there any faux pas that should be aware of, at the moment I'm just using three 1mm solid wires about 20mm long from the previous TDA socket straight up to the new board? Also, I wondered if the power supply to the new board may need a small bypass smoothing cap to help out with digital switching?
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Old 13th January 2013, 10:16 AM   #8
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Yes, assuming you've ruled out digital gain being provided by the PC, then 16X multiplication through a 4 bit shift of the WS is the next likely culprit. But I can't think of a simple way this error could be introduced so my own suspicions still lie with the computer.
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Old 13th January 2013, 11:22 AM   #9
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Its definitely not the computer, I have plugged in another DAC and it plays fine!

I added a 100nF across the TDA supply that made no difference.
I checked the new connections, all below 0.2ohms and over 2M insulation (to ground and each other)

Now I'm out of ideas, I'll take all and any suggestions.
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Old 13th January 2013, 11:34 AM   #10
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Have you implemented my original suggestion of reducing the supply to below the recommended maximum?
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