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Old 4th May 2006, 03:55 PM   #1
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Default Cambridge Dacmagic problem.

My Dacmagic is not locking onto the coax input anymore...
When I use the optical connection, then it works.

What's wrong ??
SPDIF input buffer....??

Need some help on this one.
Thanks!

Toni
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Old 4th May 2006, 07:39 PM   #2
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If at all possible you should always use the optical connection. This provided electrical loop isolation and is rated by the designer to sound better than the coaxial by a long margin.
I tried the coaxial against the optical and it sounded much more refined with optical.

Shoog
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Old 4th May 2006, 08:05 PM   #3
tubenut is offline tubenut  South Africa
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Probably the buffer.
There is an 8 pin DIL chip near the CD co ax input. It pops easily on this unit especially if you connect or disconnect the co ax cable with the unit switched on.
Be careful, the tracks to this chip are thin!!!!
If I remember correctly this unit has another co ax input labelled DAT. It does not have the "buffer" chip on that input. If you can get your CDP to play/lock on this input I bet dollars to doughnuts it is the 8 pin DIL chip that flaked!!!

My friend had this DAC and he popped this chip several times, by round two we fitted a socket for the chip.
One of the units I had in my shop also popped this chip!!

Good luck
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Old 5th May 2006, 06:30 AM   #4
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Shoog: I know that in some cases optical sounds better, but in this case I need coax.
Because my main drive has only coax.
But thanks anyway.

Guillaume: Strange, because also when using the "DAT" coax input I still have no lock.

Fitting a socket for that chip is a good idea, I will do that.
Could that chip be a UA9637 ? That's a 8 pin DIL chip, which is located near the coax input.
The datasheet calls it a DUAL DIFFERENTIAL LINE RECEIVER

BTW: has anybody a schematic for this DAC ?
Would be much appreciated !!
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Old 5th May 2006, 07:40 AM   #5
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Despite repeated requests in various places on this forum, the schematic has never showed up, and probably never will.
The designer seems understandably a bit reticent about giving it out.

If you fail to sort it out, you could try a simple SPIDF to optical converter box.

Shoog
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Old 5th May 2006, 08:47 AM   #6
tubenut is offline tubenut  South Africa
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Differential Line driver sounds right.

I may be rusty on the DAT input but really thought that DAT would not work if this chip popped. ... It has been a few years.
The chip is cheap enough to just put in and hope for lock...

Another fault I have found on Audio Alchemy DACs and a QED Digit is that the 74 series chips they often use for input selection and or signal conditioning can die. On AA the coax 74HCU04 died but took out the 74HCU04 on optical as well. Me and other guy both suspected input receiver as all inputs were dead.

Turned out two separate chips had gone!

Best audio change we ever did to this Cambridge DAC btw was soft recovery diodes on the PSU.

Groetjes uit Zuid Afrika
In our winter I long for the terraces in Mestreech!!!
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Old 7th May 2006, 12:44 PM   #7
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Shoog: Thanks for the info!
I managed to obtain a schematic, but it's not a complete one.
It's still missing some parts, I think.


Guillaume: The 74 series chips in this DAC are (2x) SN74HC157N
So it's better to replace those as well ?

Do you know which type of soft recovery diodes on the PSU you have used ?

BTW: This weekend I was indeed enjoying the terraces in Mestreech!!!
Have had a long holiday weekend with my girl in Limburg.
Normally I live near Rotterdam.

Groetjes uit Zuid-Beijerland !
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Old 7th May 2006, 06:10 PM   #8
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Are the 74 series chips for signal conditioning or input selection from the diagram you have?
I would probably change the diff line driver first, if that does not sort it, I would go further.

I have always used BYT53 for DAC and CDPs PSUs. Never really tried anything else A-B.
They work, handle high enough voltage and current for the application and the lead thickness is thin enough to work in most existing holes.

There are whole threads dedicated to "the best diodes".




I live in Johannesburg now but grew up in Mestreech.....
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Old 11th May 2006, 12:34 PM   #9
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After changing the UA9637, everything works fine again.

Thanks to all who helped !!
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Old 11th May 2006, 08:18 PM   #10
tubenut is offline tubenut  South Africa
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Only a pleasure. Glad you got it sorted and thanks for letting us know. It is good to get feedback!!!
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