Stuck as a Duck on the Doede Dac!

[Edwin Dorre]

Originally posted by rbroer
Pssssttt,

A clue, 128*Fs

That's is what I though 128 * 44Khz = about 5.6Mhz ! Which is my oscillation. And that's why I was starting to think it is not 'my' problem but a more generic design problem (or done on purpose because of the lack of filtering) of the DAC .

However why am I the only one seeing and mentioning this behavoir ???

Edwin

[finneybear]

Edwin, I am not sure whether I am about to say is correct since I have not designed a DAC chip before. My understanding is that many multi-bit DAC chips have an osc inside locked to the external reference clock through PLL. This osc can run 128X or even higher and is used to control the current source switching.

What you have seen is the switching noise. In a tradtional DAC design, the LPF will block it out yet this is not the case here. As far as your amp, and especially the speaker tweeter can handle it, nothing to worry about. Not sure whether it is a good idea to add a RC filter to output end. This seems to contradict the idea of a filter-less DAC design. Plus most amps will serve as the filter for you anyway.

[Lucas_G]

After swapping parts, fitting Blackgates BG-N 33u, and exchanging the lt1085 for a 317, I still had the same problem...

Then I had to do what I didn't want to do: desoldering the DacTower with the 8 pieces of TDA 1543...

Then I put in one TDA without heatsink. For the first time I could hear undistorted sound coming from the dac...

The sound I heard was indeed lively in the midrange. The bass was somewhat lacking in dynamics. The treble however was harsh and undetailed. Not smooth at all, and without any sophistication...

Compared to my already heavily modified Philips DVD 963 SACD this sound wasn't very promising...

I must add however that only ONE TDA was running at its nominal 5 Volt (because of lack of heatsinking), that there hasn't been any break in period, and that the output caps were not a Blackgate N in super-e configuration like it is in the DVD 963 SA.
So maybe it will all improve dramatically.

Now I must consider building a new DacTower, or start a new pcb with 8 pieces of TDA sitting in a rectangular shape with a heatsink on top of them. We'll see...

[Edwin Dorre]

Originally posted by Lucas_G
After swapping parts, fitting Blackgates BG-N 33u, and exchanging the lt1085 for a 317, I still had the same problem...

Then I put in one TDA without heatsink. For the first time I could hear undistorted sound coming from the dac...

I had 100% the same!

One dac is working, 8 not, even with a LM317 it did not work... You only get to 2.7V output on the Lm317 of LT and lots and lots of oscillation. The only way for me to get it working with 8, is to have an extra 220uf on the + and - power rail very close to the LT1085 it self. I soldered it on the + and - power rails pins parallel to the + and - 12V wires from the battery. I also raised the output C to about 100uf !

This way it works at my place!

Ok, two with the same problem, hardly a coincedence.

E.

[Lucas_G]

Originally posted by Edwin Dorre


I had 100% the same!

One dac is working, 8 not, even with a LM317 it did not work... You only get to 2.7V output on the Lm317 of LT and lots and lots of oscillation. The only way for me to get it working with 8, is to have an extra 220uf on the + and - power rail very close to the LT1085 it self. I soldered it on the + and - power rails pins parallel to the + and - 12V wires from the battery. I also raised the output C to about 100uf !

This way it works at my place!

Ok, two with the same problem, hardly a coincedence.

E.

Hi Edwin,

In my case I could easily reach 8.5 Volt though.
I was thinking whether these DacTowers were so badly prone to oscillations... Maybe it is because the distance from the upper TDA on to the capacitor is some 6 cm. In my case the tower is even a bit taller because I put 2 mm thick aluminium between each tda for better cooling. This makes the tower somewhat taller than that of Doede...

I think I will try it again with an extra cap soldered on the upper (ultimate) tda. See whether that helps.

Regards,

Lucas

[Edwin Dorre]

Originally posted by Lucas_G


I think I will try it again with an extra cap soldered on the upper (ultimate) tda. See whether that helps.

I did not solder an extra cap on the tower. I put it were you connect the 12v from the battery for the dac power. So the cap is in front of the LT1085...

E.

[Lucas_G]

I had ordered some new TDA 1543 chips, so I finally decided to build a new Dac-tower...

And indeed, it had been the old DAC-Tower that prevented this Doede Dac to work from the start. Maybe one of the TDAs has been faulty

For the first time I could hear this design with 8 paralell TDAs.
The sound is indeed much better than with one TDA.

I found the sound with 5 Volt supply quite relaxed, but also a bit less controlled in the treble.
Then I upped the voltage to 8 Volt. It does alter the sound quite heavily. There is more definition, but it becomes also a bit leaner and with less ambience in the midrange.
I do like its dynamics in the midrange, but the treble is still way more primitive than with my modded Philips 963

Still not a bad DAC. I will fix it into its new enclosure and use better cabling. I will also try various output capacitors and finetrim the output reference by listening to it. Let's see how much improvement can be made to it. I don't think it will ever beat the 963 DAC, but I will give it a chance.

Regards, and thanks to Doede!

Lucas

[Bas Horneman]

Hi Lucas,

Have you tried the Wildmonkeysects loop filter mod yet?

Tonight I am going to test my Doede dac...hope it works...

But I am allready starting to look at possible mods...the wildmonkeysects one looks promising..

as a preventive measure I have also added little 20R resistors as stoppers for pin 1, 2 and 3 of the 1543. And 100R after the clock signal of guido tent's oscillator.

Regards,
Bas