[diditmyself]

Why are there so many filtering caps? The simulated square wave looks really messed up. This is the the analog circuit of the 2009 Zero DAC. People in the FrankenZero thread worried about the 22 pF caps close to the output connectors and found it to seriously decrease treble

 

[rds]

Are you sure the part after U1 is right? It makes no sense to parallel R8 and R9 or C7 and C8...

R7 and C7 (C8 has basically no effect) are acting as a low pass filter. But if your numbers are correct it has a cut off frequency of approximately 1.6 MHz

 

[diditmyself]

Quote:

Originally Posted by rds /img/forum/go_quote.gif Are you sure the part after U1 is right? It makes no sense to parallel R8 and R9 or C7 and C8...

It's because the output relay isn't shown in the schematic. It's in series with the output "between" C7 and R9, so C8 and R9 make sense in the real circuit.

I'm concerned about the 1 nF caps C1, C5 and C6 to ground, and the 10 nF C7 from output to ground. I thought capacitive loads were something to be avoided. In LTSpice the cutoff frequency is about 300 kHz, and like I said the square wave looks awful.

If all caps are removed from the circuit, it simulates just perfect.

If I were to bypass the opamp stage à la Lampizator, there'll be no filtering at all. Is some kind of filtering necessary?

 

[hopeless]

It makes sense to add the 22 pF cap because it has a different dielectric material. Probably it was chosen for a certain effect on sound. BTW it's well known that the Zero is a copy of the Zhaolu D1.3, so maybe they just don't know what they're doing

BTW a 10 nF capacitive load for the opamp (since the isolating resistor is very small, 10 ohm) makes for heavy loading of the DAC opamp - which may explain why people prefer the discrete opamps for the Zero.

 

[cobaltmute]

C1 C5 C6 and C7 are all the same position (not value) as on the AD1852 datasheet.

 

[diditmyself]

Quote:

Originally Posted by hopeless /img/forum/go_quote.gif Probably it was chosen for a certain effect on sound.

LOL, they hear the sound of a 22 pF cap when there's one 1000 times bigger cap in parallel with it!

Quote:

BTW a 10 nF capacitive load for the opamp (since the isolating resistor is very small, 10 ohm) makes for heavy loading of the DAC opamp - which may explain why people prefer the discrete opamps for the Zero.

10nF is a tough game for any amp. Why is it there?

 

[diditmyself]

Quote:

Originally Posted by cobaltmute /img/forum/go_quote.gif C1 C5 C6 and C7 are all the same position (not value) as on the AD1852 datasheet.

Yes, but I don't understand why. It's not explained in the data sheet, and I don't know anything about DACs. I'm planning on bypassing the opamp so I want to know if I have to add some kind of filter then.

 

[cobaltmute]

From the data sheet:

Quote:

3RD ORDER LP BESSEL FILTER CORNER FREQUENCY: 75kHz GROUP DELAY: ~3.5ms

From what my basic brain can figure out:

R2/C5 and R4/C6 each are simple low pass filters. My guess is that they pre filter the signal into the DAC.

C4 is part of the low pass filter.

Not sure what C1 is there for. R1/C1 form a low pass filter. However on the datasheet C1=C4 - again I don't know the effect.

R7/C7 create another low pass filter, but the cut-off frequency with these values is really, really high.

C2 and C3 are DC blocking caps. If you look at the datasheet they don't need to be there.

From what I have read, most Sigma-Delta DACs need a following low pass filter as they create a lot of high frequency noise

 

[hopeless]

Quote:

Originally Posted by diditmyself /img/forum/go_quote.gif 10nF is a tough game for any amp. Why is it there?

But the discrete opamps tend to have more current, which helps.


Anyway, it's in the schematic in the AD1852 datasheet, but there it is preceeded by a 100 ohm isolating resistor.


I would guess that the AD1852 needs rather strong low pass filtering.

At least you know that changing it with something better than a ? brand polyester could improve the sound.

 

[hopeless]

Quote:

Originally Posted by diditmyself /img/forum/go_quote.gif LOL, they hear the sound of a 22 pF cap when there's one 1000 times bigger cap in parallel with it!

I would certainly not have thought of removing the last 22pF while there's another 10000 pF in parallel with the opamp's outputs.

 

[diditmyself]

Quote:

Originally Posted by cobaltmute /img/forum/go_quote.gif From what I have read, most Sigma-Delta DACs need a following low pass filter as they create a lot of high frequency noise

I also did some reading and came to the same understanding as you. So now I have two different approaches to take on, a new active or a passive. Both starts to cut at about 20 kHz, but the filtering is much steeper with the active. This makes me wonder about the lampizator mods, how sound they are. Are the "improvements" for real or ill behavior and wishful thinking?

The active is taken from one of the "CS4397 Ebay DAC" threads at diyaudio.com and the reports from using it are very favorable.

For the passive I'll be using Solen/SCR polypropylene caps.