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Old 9th April 2008, 08:11 AM   #1
SM7UYJ is offline SM7UYJ  Sweden
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Default PCM1792 help needed!

I'm trying to build a DAC using a PCM1792, but it's behaving strange!

The DAC only works if I put my finger on the right side of the PCB - then it sounds realy nice! (no output without the hands on )

It is wired "as in the data sheet", accept for not using a controller och having a passive I/V-stage.

I'm using transformers on the output, and they seem to work as the sound is good.

Using a CS8412 to feed I2S to the PCM1792.

I have the mode select pin set to high for I2S.

From the data sheet, the default settings should allow this combo without a controler!?

Any ideas what the problem might be?

The test circuit:

Click the image to open in full size.
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Old 9th April 2008, 10:15 AM   #2
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PCM1792 will accept I2S by default with no micro connected. PCM1792 outputs 6 mA of steady current. Do you provide DC path for this current?
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Old 9th April 2008, 12:01 PM   #3
SM7UYJ is offline SM7UYJ  Sweden
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Quote:
Do you provide DC path for this current?
- Yes!

Transformer is a 1:1 with two windings, center tap om primary via resistor to ground, resistor paralell with secondary.

Can a output mis.config cause this problem?

/R
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Old 9th April 2008, 06:54 PM   #4
SM7UYJ is offline SM7UYJ  Sweden
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Problem solved!

Back engenered the protoboard schematic and found that the pins 10 -14 where groundes or pulled high.

3.3V and a 47k resistor and there was constant music!

Now I "only" have to get the transformers working optimaly - the output seems to be a bit high!

/R
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Old 16th June 2011, 08:29 PM   #5
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This DAC looks interesting. Is it a NOS version? What about the sound quality?
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Old 16th June 2011, 09:10 PM   #6
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Quote:
Originally Posted by SM7UYJ View Post

I have the mode select pin set to high for I2S.

From the data sheet, the default settings should allow this combo without a controler!?

As has been mentioned the PCM1792 defaults to accepting 24 bit I2S data with an OS rate of 64x and a stereo output. I am sure you misinterpreted the data-sheet, which says the mode select pin has to be set high for the DAC chip to accept I2C commands for the accessing and modifying of the control registers. The other option is to control these via SPI when the mode select pin would be held low.
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Old 16th June 2011, 09:29 PM   #7
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Quote:
Originally Posted by tiefbassuebertr View Post
This DAC looks interesting. Is it a NOS version? What about the sound quality?
No not NOS, but this family is very good sounding. I use PCM1794A in current sum connection, mono mode into THS4131 differential amps and then transformers. I suspect direct driving transformers may be possible as with the 1792, but have not tried it.

The 1792 is not recommended for new designs, and has some features that you probably don't need like DSD support.
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Old 20th June 2011, 07:59 AM   #8
SM7UYJ is offline SM7UYJ  Sweden
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This DAC now has a new owner, so I ca'nt verify how I ended up using the mode select pin - but probably I miss read the data sheet!

Here is a picture of my second proto-board (did'nt get past his version:

Click the image to open in full size.

It sounded very nice, but I ended up using my 18xTDA1543 DAC more, so I sold it!

/R
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