Stuck as a Duck on the Doede Dac!

[rwagter]

Damn, the lt1085 was the second most expensive part (€6,60) and a lm317t is very cheap....

[Leeuwarden]

Originally posted by dddac



My suggestion: Increase the Value of the output C to 47uF - 100uF when a LT1085 is used..........


Regards

doede [/B]

As far as I understand the value of the output C isn't critical. So why not put in a 100uF just to be sure... Or is this too simple?

Jan

[Lucas_G]

Originally posted by dddac
OK, I think I found the problem. It is 100% the LT1085....

When it starts cold, there is oscilation which can be heard as hard noise. In my DAC at home after a minute or so it is gone, I thought it was the DAC biasing and did not cared too much, as every thing worked fine after warm up. My tube amps have to warm up also, eh?

BUT, it might be, that in some other (critical) situations, the 1085 does NOT STOP making noise !!!

I have just measured it and just check the scope pictures....
Please do note the difference in scale !! When the warm up is done, the rest-noise is at an average of -90dB. This is ok enough, although I am sure some will diasagree,

The cold pictures show clear hard stuff !!!-15dB is not particulary a good noise floor level

ANYWAY: try a larger C at the output of the 1085 or take a LM317.....

Will make notes at the WEB Site as well....

Take care

doede

Hi Doede,

Thanks for your postings.
It is very kind of you to offer me to check the dac for me!

But I am sure that now that we have found the problem, I will manage to get the thing working.

Unfortunately (stressed by the lack of luck on that day), I had strifed plus and minus of the two 5 Volt supplies, last week, so I will almost certainly have to swap some parts now. I will start working on it after Christmas since work is keeping me extremely busy now (as is my 10 weeks old baby )

Anyway, thanks again for you kind offer, and I look forward to hearing the DAC somewhere in January!

Regards,

Lucas.

P.S. I am always amazed to see how much experience and expretise can come together through such a forum. Without internet people like me could never have undertaken such projects as these...

[Edwin Dorre]

I am spooked,

Perhaps I seeing things I should not looked for...

I still have an oscilation on the outputs! About 5Mhz, always. I have done:

- one dac (changed resistors to 2k2 and 1k5)
- 8 dacs
- changed voltage
- changed powersupply to 9.6V RC car battery pack
- changed C's up to 470uf
- changed all C's to a minimum of 100uf
- removed the clock circuit and changed to mode 2
- disconnected battery checker
- changed to a LM317
- removed and bypassed the complete 8.5V voltage source (LT1085) and put my variable power supply on 8V

HELP?

Edwin

Everyhing is working, both channels have normal output with this oscillation on top of it. I see the following image on the right and left output, which looks like an oscilation of about 5Mhz... Is this normal???



Snif

[Bricolo]

I'm sorry I can't help but,

Edwin: what's your scope? The screen looks similar to mine. It's a Tek 2232. I got it second hand, and I'm looking for infos, manuals...

Can you help me?

Alex

[Edwin Dorre]

Hello,

It is indeed an Tek 2232. I bought it secondhand also, it is in perfect condition and a great help with these kind of problems! I have a manual with the schematics, however I can not help you with a copy because it is almost impossible to copy with all these fold out pages. However I can help you if you have a problems with the scope, please ask me question if you need any help with it!

Edwin

[Bricolo]

Hi,


Does your manual explain how to connect it to a printer or a computer, to acquire data?
I'd like to know how to use the connectors on the left side.

Alex

[pukka]

Hi,

For those who have Doede PCBs, what are their sizes? is the PDF scale 1:1?

Looking for a suitable box on RS...

good luck with those oscillations.

Bricolo, have a look at http://www.slack.com/elec.html and http://www.slack.com/manavail.html#atek

[dddac]

I cannot see why you have the oscillations, but I assume from your info, you cannot hear them ?? I also doubt if it will do any thing bad to the music, but you might want to try to put a small Capacitor at the output of the dac (so parallel with Rload) Choose a corner freq of lets say 200kHz ?? 4,7nF with 270Ohm....

you can see if the oscillation goes away,

plan b is a RC at the output. lets say, 27 Ohm in series and then 47nF parallel.....

You can listen to music if you prefer one or the other or no filter at all.

good luck,

doede

[rbroer]

Pssssttt,

A clue, 128*Fs