High Performance WM8741 Upsampling DAC New Version build thread (show 'n tell too) - Page 26 - diyAudio
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Old 10th April 2012, 07:21 PM   #251
TheGimp is offline TheGimp  United States
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I agree that the PCM2707 will take precedence if the USB cable is plugged in as the mux will select it as the source.

It matters not what the PCM2707 is doing.

One would have to unplug the USB port to enable the coax input.
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Old 11th April 2012, 02:35 AM   #252
TheGimp is offline TheGimp  United States
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Well, I got a bit of luck (possibly) as I found the WM8741 is not loading down the 3.3V supply. It measures 1.8V, but the regulator is bad and not putting out voltage. I believe the 1.8V is from I/O voltage back-feed through the protection diodes in the WM8741 and showing up on the power pin.

I wired up a LM317 to put out 3.3V and patched it in. I put a current meter in line to monitor it and it only pulls 38mA, which is close to nominal of 40mA. So possibly the WM8741 is not damaged.
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Old 11th April 2012, 12:48 PM   #253
TheGimp is offline TheGimp  United States
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On board regulator replaced. WM8741 now has 3.320V at the power pin.

When I turn on the headphone amp I get hum in the left ear. As I rotate the control CW, the hum goes down for the first 1/4 turn, then buzzing starts coming up in the right ear up to a little past half way, then both sides continue to increase with both hum and a buzzing (digital noise). This happens with or without any input.

No audio with USB input. No data from 2707.

No audio with S/PDIF although I can trace the signal to the SRC4192, and what looks like I2S from it to the 8741.

I've got 2 new 2707s on the way.
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Old 12th April 2012, 02:26 AM   #254
TheGimp is offline TheGimp  United States
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Turbon, do you still have chips running hot?

Mine are all slightly warm at best. However, after reading the regulator spec sheets I decided to run a pre-regulator.

I set up a LM317 for an output voltage of 6.15V which gives enough margin for the 5V regulator and minimizes the dissipation in the 3.3V regulators.

What is your input voltage?

The regulators will withstand the voltage up to 16V, but it is the dissipation that is a concern. Theta JC for the SOT-23 is 220C/W.

At 6.15V in the 3.3 V regulator will dissipate 0.114W for a temp rise of 25C. If you increase the supply to 8V the temp rise goes up to 41C.
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Old 12th April 2012, 04:09 AM   #255
Turbon is offline Turbon  Sweden
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Yes I have.

7.5V - the WM8741 being the hotest. I have tried to understand what its nominal temp should be from the datasheet but I don't understand yet how to calculate it.
Nominal current seems to be ~50mA for 5V and ~40mA for 3,3V which would give
a total of 0.382W or ~ 0.4W. And then what would I look after to get the temperature?

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Old 12th April 2012, 06:03 AM   #256
zinsula is offline zinsula  Switzerland
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Quote:
Originally Posted by Turbon View Post
[....]which would give
a total of 0.382W or ~ 0.4W. And then what would I look after to get the temperature?
Thermal resistance between junction and ambient:
NXP Semiconductors - ICs [Packaging - Thermal Resistance Rth(j-a) Data]

28 pin SSOP = 98C/W
0.4W * 98C/W = 39.2C above ambient, means that you can easily arrive at 65C or even more if in an enclosure.
This is already hot to touch.
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Old 12th April 2012, 08:15 AM   #257
Turbon is offline Turbon  Sweden
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Thanks a lot zinsula. I can keep my finger on it for a while but the sting forces me to remove the finger - max 20s. So no cooling needed?

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Old 12th April 2012, 10:46 AM   #258
TheGimp is offline TheGimp  United States
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Just remember you are calculating the die temperature which is the critical value for reliability. The package temperature will be significantly lower, although still quite warm.

I should probably check my analog current to the 8701 as mine is barely warm. I've measured the digital current at 38-39mA.

I'll measure the analog current today and take a temp reading with a FLIR IR camera if I can use it at lunchtime.

The purpose of Theta j-a and j-c is to be able to calculate the die temperature from the ambient temperature or case. Unfortunately it is not as accurate for estimating the package temperature from dissipation. This would require a different thermal coefficient.

Last edited by TheGimp; 12th April 2012 at 10:56 AM.
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Old 12th April 2012, 10:54 AM   #259
Turbon is offline Turbon  Sweden
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Thanks TheGimp. Both readings are interesting to me.
Typical for the analog is 55mA.

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Old 12th April 2012, 11:38 AM   #260
zinsula is offline zinsula  Switzerland
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Quote:
Originally Posted by Turbon View Post
[...]I can keep my finger on it for a while but the sting forces me to remove the finger - max 20s. So no cooling needed?
No, thats OK. If you can keep the finger for 20s, it may have 50-55C at the case.

New math:
Junction to case is 35C/W
Junction - Ambient is 98C/W

So, Case - Ambient is 98-35=63C/W
0.4W*63C/W=25.2C, if it's 25C in the "ambient" of the chip, the case has around 50C.

You do not have to worry, the chip being around 65-70C is ok.
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