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Old 17th December 2012, 03:43 PM   #1
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Default Audio Alchemy DDE V 3.0 question

Hello

Inside my Audio Alchemy DDE 3.0 DAC is this sub-circuit as regulated psu for the analogue part:

Click the image to open in full size.

The opamps are supplied with heathsinks and they get very hot (guessed 70 to 80 degree celcius).

Why?
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Old 20th December 2012, 01:48 PM   #2
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Default PSU for the analogue part of the circuit

I would be happy for any advice.

My Audio Alchemy DDE 3.0 DAC does often not power up correctly. One or the other channel does not play.

Now, I recognized the fault within the shown psu part.

Sometimes, the positive or the negative voltage fails during startup. When the output voltage is O.K. I measure 7.5V over the zener diodes. When the output voltage is not O.K., some 0.7V over the zeners.

All LED's are always lighting.

My questions:

- What could be the reason for this fault?
- Why are the opamps supplied with heathsinks, why do they get hot by design in this circuit?

Franz
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Old 22nd December 2012, 10:00 AM   #3
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No answers here, so I looked in my theory books
This subcircuit isnt very good, same as LM7815/LM7915.
And I found this comment in Audio Alchemy (DDE V3.0)

Quote:
But i've looked into the schematic I found on the Internet and the problem is a very very very and VERY unstable +/-15V stabilizer circuit.
It looks like this section was designed by a nono because you have luck when this circuit is working at starting up the system.
I’ve made some fast modifications and now the +/-15V section starts up AND is stable.
I'm thinking about chaning tis section complete by inserting a shunt-regulator.
Any idea, what could be the fast modification? Dont pull the voltage for the bias zener (D32/D37) from the output but from the input?

Franz
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Old 22nd December 2012, 12:41 PM   #4
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Default I did it

My monologue
I repaired my Audio Alchemy DDE 3.0 DAC successfully, it is starting up now without problems!
As it seems to be a common problem with this devices, this posting could be helpful for other owners.
What I did:
Just removed the zener diodes and the 499R resistors on one side, joined it in the air and connected it with a 1k2 resistor to pin 4 or pin 8 of the opamp.
Before:
Click the image to open in full size.
After mod:
Click the image to open in full size.
Franz

/Edit
After posting this pictures, I remarked a loose piece of wire in the second picture, reopened the box and removed it. Good luck, isn't it?

Last edited by Franz Gysi; 22nd December 2012 at 12:47 PM.
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Old 14th January 2013, 04:55 PM   #5
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An illustration of my modification (one channel only):

Click the image to open in full size.
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Old 14th January 2013, 07:32 PM   #6
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Good move!

Now the +/- reference voltages are created from the unregulated input rather than the output. Clearly, there was a stability problem with this regulator and it had started to oscillate at a low frequency.

- Poetry2me
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