MayDus, PCM2706/CS4398 USB DAC w Transformer output. - Page 3 - diyAudio
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Old 3rd May 2012, 05:01 PM   #21
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I'll look into that.
However I imagine one less opamp(and alot of other parts) in the signalpath more or less has to be better.
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Old 4th May 2012, 05:25 AM   #22
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Quote:
Originally Posted by TheGimp View Post
The schematics I've seen on ebay for this design do not show pins M1,2,3 connected. However, the manual states they must be connected to gnd or Vcc. I found one PWB with a close up that shows them connected to ground, which is what I suspect for all of them. This puts the DAC in (M0-1) Left-Justified, up to 24-bit data and (M2-3) Single-Speed without de-Emphasis (32 to 50KHZ sample rates.

I believe that this will configure the device for Single-Speed (fast) Stopband Rejection per figure 20 on page 41.

Your dynamic range will be slightly degraded by not having the analog butterworth filter.

However, the output without a filter looks like it is better than 100dB. I suspect the filter gives another 20dB as they claim 120dB dynamic range.

You may lose some micro-detail and imaging without the filter.

It might be interesting to see if I can drive my sound card from the unbuffered output of the DAC to see how it looks on AudioTester.
A partial schematic for the DAC board:

Click the image to open in full size.
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Old 4th May 2012, 09:16 AM   #23
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This is a first draft. Pin nr on the transformers are not correct, I'm aware of that.

Click the image to open in full size.
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Old 4th May 2012, 06:20 PM   #24
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Changed resistor values to match Zin of the headphone amp the dac will be connected to.
Click the image to open in full size.
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Old 4th May 2012, 10:07 PM   #25
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Hi Mayday, Looks like it is coming along. You will have to keep me posted. Sure you don't want to add series (safety) resistors in the primary circuit? Well, 10K is pretty high I'm sure it will work. Dave
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Old 4th May 2012, 11:35 PM   #26
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I think the CS4398 is good down to 1Kohm if I remember correctly.
What might the issues be without series resistor on the primaries?
/Jonas
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Old 5th May 2012, 02:34 AM   #27
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Hi Jonas, If the secondary was accidentally shorted it would be reflected to the primary side. I look at the series resistor as a safety resistor above all else. 1K is so small into a 10K transformer that it has little impact beyond presenting just a little more DC resistance to the dac. This is a TSSOP dac so a pain to unsolder and replace. I think it would be fine without. Just voicing a concern. Dave
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Old 5th May 2012, 02:48 AM   #28
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Quote:
Originally Posted by Torchwood421 View Post
Hi Jonas, If the secondary was accidentally shorted it would be reflected to the primary side. I look at the series resistor as a safety resistor above all else. 1K is so small into a 10K transformer that it has little impact beyond presenting just a little more DC resistance to the dac. This is a TSSOP dac so a pain to unsolder and replace. I think it would be fine without. Just voicing a concern. Dave
Ok, I see
I'll try it without the resistors, I hope I don't have to regret that some day

/Jonas
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Old 5th May 2012, 02:50 AM   #29
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Another question, the resistors between the secondaries(and gnd)...do they have much impact on SQ?
Should I go for standard 1% metalfilm, Holco's etc?
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Old 5th May 2012, 06:52 PM   #30
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Never mind that question, the pot on the headphone amp is R1 and R2...duh.

Anyhow, not having heard the CS4398 before, how would you guys characterice the sound?
I'm thinking more in very loose terms, neutral, more on the bright side etc etc?

Got all the parts needed, just waiting for some tools to make the job with the enclosure a bit easier before I start the build.
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