Audio Alchemy DDE V 3.0 question

[Franz Gysi]

Hello

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

An externally hosted image should be here but it was not working when we last tested it.

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

Why?

 

[Franz Gysi]

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

 

[Franz Gysi]

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)

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

 

[Franz Gysi]

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:

An externally hosted image should be here but it was not working when we last tested it.

After mod:

An externally hosted image should be here but it was not working when we last tested it.

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?

 

[Franz Gysi]

An illustration of my modification (one channel only):

An externally hosted image should be here but it was not working when we last tested it.

 

[poetry2me]

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

 

[Franz Gysi]

As the images are lost and I also cannot find them on my harddisk anymore, here at least the schematics image of the mentionned modifications.

Hope this helps!
Franz

 

[Franz Gysi]

And I just reopened the device to shot new images. Here we go!

Carefully read this description:
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.