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Old 10th March 2005, 05:22 PM   #1
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Default Why filter in tube DAC?

I have been researching DAC schematics on the web. Basically, all the tube DAC schematics include a filter stage, whether it be on the digital side or the analog side of the DAC chip. Yet plenty (all?) of the silicon DIY DAC designs have no such filtering. Why? Is it just a question of how refined the design is or it a question of the wide bandwidth of tubes possibly causing trouble with conversion artifacts? Can I build one without?
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Old 10th March 2005, 05:54 PM   #2
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A strange observation. Tube stages are not necessarily wider bandwidth than the ultra-fast opamps used in active I/V. More importantly, zero nfb tube stages are much more stable when presented with unfiltered square waves.
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Old 10th March 2005, 06:09 PM   #3
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Quote:
Originally posted by analog_sa
A strange observation.
The bandwidth was just a guess on my part. But the comment on filters is what I see:

http://members.home.nl/triode.dick/dacuitgang.html

http://www.fortunecity.com/rivendell...io/Adagio.html

http://members.brabant.chello.nl/~m....tml/dactop.htm

http://www.quadesl.com/dac.shtml

Filter stages on every one of them.
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Old 10th March 2005, 06:16 PM   #4
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Default Re: Why filter in tube DAC?

Konnichiwa,

Quote:
Originally posted by leadbelly
Yet plenty (all?) of the silicon DIY DAC designs have no such filtering. Why?
Actually, they don't have the filter because you don't see it. You either fail to identify the filter circuitry after the DAC because it is not a simple LC circuit but a complex Op-Amp or Discrete circuit with multiple RC sections for filtering or simply because the filter is actually placed on the DAC chip (many modern ones from Cirrus Logic, AKM, Burr Brown, AD).

Sayonara
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Old 10th March 2005, 07:48 PM   #5
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Interesting observation. I haven't checked the links. Instead I write to sort of join the "leadbelly camp." I'm not into tubes. But much of the deal about silicone non-oversampling DACs is to get away from filters. There's simply no need for it. (Only dogs and cats and perhaps some golden ear audiophiles can hear the 3rd harmonic distortion that takes place at 44.1 khz.) My thinking is that non-oversampling tube DACs would work the same. Different philosophies, perhaps?

The idea with a tube stage is to add warmth and timbre. That I can understand. But MY experience is that non-oversampling DACs have harder, more "digital" sound than oversamling DACs. If it has to do with the oversampling or DSP I don't know. But if it is the non-oversampling, which I think, why then don't the tube heads go all the way and make an oversampling DAC?

Now it's for the techies here to shed some real light on it.
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Old 10th March 2005, 08:01 PM   #6
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Quote:
But MY experience is that non-oversampling DACs have harder, more "digital" sound than oversamling DACs.
An even stranger observation. I thought i was the only one not entirely in love with NOS Dacs.
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Old 10th March 2005, 08:12 PM   #7
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Konnichia,

Quote:
Originally posted by phn
The idea with a tube stage is to add warmth and timbre.
It is? Since when?

I thought it was to subtract the sound of RF IMD folding back into the audio band due to overly slow Op_Amp's and the like....

Quote:
Originally posted by phn
But MY experience is that non-oversampling DACs have harder, more "digital" sound than oversamling DACs.
Hard to say. But non-os DAC's release a lot of supersonic output. If you do not limit the level and bandwidth of these Images (which is the technical term) gear after the DAC may end up being troubled (sonically).

Quote:
Originally posted by phn
If it has to do with the oversampling or DSP I don't know. But if it is the non-oversampling, which I think, why then don't the tube heads go all the way and make an oversampling DAC?
We "Tubeheads" have made oversampling DAC's for ages. Non-OS can sound better. But as with all things, much of the sound is not in the what but the how. More below in my note to analog_sa.

Quote:
Originally posted by analog_sa
An even stranger observation. I thought i was the only one not entirely in love with NOS Dacs.
As with anything else Sturgons law strikes: "90% of everything is rubbish!".

It is possible to get quite nice sound out of a TDA1543, but it takes a lot of skill and there is an ultimate limit build in. Other DAC's with a higher potential sonically tend to be much more difficult to implement and often implementations are too compromised. Many people really do not appreciate how to deal with the ultrasonic energy coming out of the Non-Os DAC and often make fatal mistakes implementing the analogue stages.

I remember well a certain kit maker attempting to take my Adagio DAC and to "improve" it. Having done so he found many problems which he had actually caused by his "improvements" and went on in great length how fundamentally flawed such a design is. In the end he brought Oversampling Back in through the backdoor, instead to simply strip out the "improvements"....

So, do not be surprsed if you don't like most Non-Os DAC's, i don't either....

Sayonara
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Old 10th March 2005, 08:19 PM   #8
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Well, I'm a fan of NOS DACs. I just didn't know what words to use. Maybe I should have said more detailed. I don't know. Not always easy to put musical experiences into words. But I do find "hard" a bit troublesome. (Belt-drive advocate Stereophile doesn't like the "hard sound of direct-drives.") Especially since I would never use "soft" to describe the characteristics of vinyl and tubes. Soft I like to reserve for cheapo CD and vinyl players alike.

Hopefully the core of my post makes more sense.
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Old 10th March 2005, 08:22 PM   #9
phn is offline phn  Sweden
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Quote:
I thought it was to subtract the sound of RF IMD folding back into the audio band due to overly slow Op_Amp's and the like....
My bad. My only excuse is that I'm not a tube head. I learn.
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Old 11th March 2005, 10:52 PM   #10
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Quote:
Originally posted by leadbelly


http://www.quadesl.com/dac.shtml

Filter stages on every one of them.
I use a digital filter to get the quantization noise up in the hundres of KHz range so I don't need an analog filter at all.

To my ears and measurements, Oversampling is a "good thing"

I've never seen a solid state DAC that doesn't use a filter. They are in there, you may have overlooked them.

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