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Old 10th December 2011, 02:39 PM   #11
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Quote:
Originally Posted by SoNic_real_one View Post
The resistor series with the pot will just lower the singnal-noise ratio of the DAC. If the next stage has too much gain, reduce it there, don't reduce the input signal...
Plus, at 50k output impedance you are asking for EM interferences on the connecting cables.
A 10K pot would do it.
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Old 10th December 2011, 02:50 PM   #12
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Well, I could lower the gain of my headamp, no problem there, and then switch to a 10K pot instead, without any series resistance.

But then there would still be a varying output impedance, albeit with a smaller range.

WM8523 looks interesting, but requires more external circuitry for the digital volume control, which I'm not sure that I am willing to implement.

Last edited by Neutrality; 10th December 2011 at 02:53 PM.
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Old 10th December 2011, 02:52 PM   #13
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I am for buffering the outputs after the pot and keep the output impedance below 500 ohms. In this way the interconnect cables won't influence the top end of audio band.
Or just buffer the DAC and move the pot in the headend.
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Old 10th December 2011, 03:22 PM   #14
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So hows this, OPA2134 OP-Amp as output buffer, with a 10K pot after it and reduced gain in the headamp stage to make it work with 2Vrms.
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Old 10th December 2011, 03:39 PM   #15
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So basically, the DAC outputs doesnt like an output impedance that varies too much, but if you add an Op-amp buffer, that problem is solved, since the Op-amp doesnt mind the varying output impedance as much as the the DAC does?
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Old 10th December 2011, 05:43 PM   #16
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Quote:
Originally Posted by Neutrality View Post
Pin 12 on WM8524 is 1, see the 10K pull-up to VDD.
I read Z to be a resistance as I would not know otherwise what Z means. A logic 1 is I2S. My mistake ?


Quote:
Originally Posted by Neutrality View Post
Well, I could use an ES9023 DAC but the WM8524 is what I chose and I'm sticking with it. Maybe on some future V2 of my board I might try the ES9023 or something even better.

Im using TPS76133DVBT regulators, not the best there is but for my requirements it will work very well. This Input board will be mounted inside my head amp. My headamp will run on +15/-15 V regulated suppllies so the Input board regulators are fed with a +15V well regulated supply.

Regarding adding a fourth regulator, that is certainly possbible, did consider that myself.

You resistor choice for the primary and secondary sides of the SPDIF isolation transformer seems weird. Firstly, you shouldnt really need a resistor on the primary side in the first place and on the secondary side you usually terminate with 75 Ohm and not 200 Ohm. What is the reason for your resistor suggestions?

Also, why wouldnt the 50K resistor at the output work?
- Never go for better (and cheaper) DAC parts otherwise you will reach your goal of a good sounding DAC too fast

- TPS76133 don't work very well at all as they have a whopping 190 uV noise ( BW from 300 Hz to 50 kHz). This means they're totally unsuitable for this task as they even have more noise than for instance the 78XX series....

- the regs may become too hot as you will feed them +15 V. Do the math with the current the chips will use and calculate dissipating power. You should use preregs to drop the voltage or a dedicated transformer as suggested.

- Don't be surprised if feeding the DAC with the 15 V from the headamp will have its effect on that same 15 V if that is a voltage for analog purposes.

- the resistors present 75 Ohm but you can figure out why. Adding extra pads leave you the possibility for trying out.

- Reasons for not using 50 k in series have been explained well by others.

- try to make it good (as in: the best you ever made) and not mediocre as it will probably cost more than many Ebay DACs to build so it should be better... The difference between mediocre and very good might be just a few Euro.
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Last edited by jean-paul; 10th December 2011 at 06:04 PM.
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Old 10th December 2011, 05:59 PM   #17
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Quote:
Originally Posted by jean-paul View Post
I read Z to be a resistance as I would not know otherwise what Z means. A logic 1 is I2S. My mistake ?




- Never go for better (and cheaper) DAC parts otherwise you will reach your goal of a good sounding DAC too fast

- TPS76133 don't work very well at all as they have a whopping 190 uV noise ( BW from 300 Hz to 50 kHz). This means they're totally unsuitable for this task as they even have more noise than for instance the 78XX series....

- the regs may become hot as you will feed them +15 V. You should use preregs to drop the voltage or a dedicated transformer as suggested.

- Don't be surprised if feeding the DAC with the 15 V from the preamp will have its effect on that same 15 V.

- the resistors present 75 Ohm but you can figure out why. Adding extra pads leave you the possibility for trying out.

- Reasons for not using 50 k in series have been explained well by others.
Pin 12 can be tri-stated, that is 0, 1, and Z, with 0 being connetion to GND, 1 being pull-up to VDD and lastly Z being unconnected.

Hmm, I havent looked into the nosie figure of TPS76133, however, I might find a better regulator and then use a pre-regulator to drop the voltage.

EDIT : Also, one of the reasons for not using ES9023 is availability, buying from the distributor here in Scandinavia cost alot in shipping(80) so it is not readily available even though the price for the DAC itself is very reasonable.

Last edited by Neutrality; 10th December 2011 at 06:15 PM.
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Old 10th December 2011, 06:00 PM   #18
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OK that seems logic but I could not come up with this.
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Old 10th December 2011, 06:43 PM   #19
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Originally Posted by Neutrality View Post
So hows this, OPA2134 OP-Amp as output buffer, with a 10K pot after it and reduced gain in the headamp stage to make it work with 2Vrms.
An extra opamp is, well, an extra opamp. Extra (in the sense of more than required) stuff doesn't make things better usually. Keep it simple by applying a volume potentiometer where is should be (not at the DAC side).
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Last edited by jean-paul; 10th December 2011 at 07:00 PM.
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Old 10th December 2011, 07:40 PM   #20
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I think om more or less sold on ES9022/23. Simple supply, option of external XO, adjustment of output level via external resistor.

But nothing is decided yet.
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