Stuck as a Duck on the Doede Dac!

[Ola]

Hi Doede!

I can't enter Your dac page, explorer crashes.
?

Best regards

Ola

[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

[dddac]

above was the cold start NF

below is the cold start HF

[dddac]

and now the warm situation,

note the different SCALE !!!!

just to make sure: These are FFT-spectrum images from the OUTPUT of the 1085.....

[dddac]

Originally posted by Ola
Hi Doede!

I can't enter Your dac page, explorer crashes.
?

Best regards

Ola

I have a cheap WEB-HOST

some times there is no site....

Should work ok in the meantime....

br
doede

[dddac]

OK, last image on this issue..........

This is a time recorder of the output of the 1085.
After switch-on the oscillations start at aprox 0,6 Volt (!)
As time goes by, the oscillations start to decreas and suddenly disapear after aprox 235 Seconds.....

OK, 4 minutes warm up, never realized it is that much

for easy to copy DIY-stuff I may be had better not mentioned the 1085 as an "alternative" for the LM317, which is clearly more stable!

doede

[Elso Kwak]

Hi Doede,
This thread might be helpfull:
Why 7805 is better than LM1085?

[dddac]

I could not stop

So close........., so I did some more checking. I tried a few capacitor values at the output and every thing above 40uF stopped the noises at cold start !!!

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

This can be done emperical of course as you can hear it, no need for scope

I am glad as well, as now I have no "warm-up" time any more of 4 minutes.... hahaha !!

Regards

doede

[finneybear]

Good finding, Doede. Now one less thing for me to worry about. Forgot to tell you that I got the board from you last week. Thank you. What a lovely board! The design is very good but now I am trying to see whether I can squeeze a bit more performance out it.

[Edwin Dorre]

Originally posted by dddac
I tried a few capacitor values at the output and every thing above 40uF stopped the noises at cold start !!!

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

I still have oscillations with my dac tower... I changed the C to 100uf (jamicon) and the output of the LT1085 ramped up to 5.5V instead of 1.87V and less oscilation (still a lot). Then I put a big 1000uf BG on the 12V voltage rails (for the record, I use a stabilized variable lab power supply for testing instead of an battery) and I get the 8.5V on the LT1085 but with an > 20Mhz oscilation with a top-top of 0.4V . Everything is working except for this oscililation.

This morning (had to test it before work! ) I still had some thoughts about my power supply and I changed it for a RC car 9.6V battery. Without the 1000uf BG on the voltage rails I still get a awfull lot of oscilcations. With the 1000uf BG I have the same picture as with my lab powersupply.

I just bought some 100uf, 220uf, 470uf C's and a lm317T to test more tonight...

So it seems that just ramping up the output C to 100uf is not the answer to everybodies problems

Nasty beast this LT1085...

Edwin