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DAC Filtering - the "Rasmussen Effect"
DAC Filtering - the "Rasmussen Effect"
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Old 17th January 2014, 09:14 AM   #1
Coris is offline Coris  Norway
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DAC Filtering - the "Rasmussen Effect"
Default DAC Filtering - the "Rasmussen Effect"

What mean this kind of filtering?

In simple words (and even easier to do it in fact) is to place a right value cap/capacity over the output phases of a delta/sigma DAC (or maybe other DAC types?).

What does this filtering method?

Improve a lot the perceptual sound stage of a stereo audio system (DAC based). Increase the resolution of the sound stage, as the spatial distribution of the audio signal components. The low end of the audio spectre become very natural and much detailed. It make the speakers really transparent, and make very easy (natural) to identify the sound sources (instruments) in a sound stage.
Well, one may describe this, with even more details, but I think to let the descriptions for the further interventions her...

How it works this filtering?

Not so well known so far... Just speculations, and few attempts for answers... That because opening of this thread: to find out together more about.


A thanks and appreciation to Joe Rasmunssen who had the kindness to share with us the information about this kind of filter.


So, let`s try together to find out more about and improve (if possible) this filtering technique.

Last edited by Coris; 20th January 2014 at 10:24 PM.
 
Old 17th January 2014, 09:18 AM   #2
Julf is offline Julf  Europe
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Quote:
Originally Posted by Coris View Post
In simple words (and even more simple way to do it in fact) is to place a cap/capacity over the output phases of a delta/sigma DAC.
Doesn't that just form a first-order low-pass filter?
 
Old 17th January 2014, 09:46 AM   #3
Coris is offline Coris  Norway
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DAC Filtering - the "Rasmussen Effect"
Yes, it seems like a first-order low-pass filter... But it differ than a usual low pass filter placed on each phase of a DAC, and it looks like the result of this filtering is different...

Last edited by Coris; 17th January 2014 at 09:49 AM.
 
Old 17th January 2014, 10:04 AM   #4
Julf is offline Julf  Europe
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Originally Posted by Coris View Post
Yes, it seems like a first-order low-pass filter... But it differ than a usual low pass filter placed on each phase of a DAC, and it looks like the result of this filtering is different...
Not sure what is meant with "phase" here - are we talking about the inverting and non-inverting output of a differential stage, or something else? Is there a circuit diagram of the "Rasmussen Effect" circuit?
 
Old 17th January 2014, 10:06 AM   #5
Shinja is offline Shinja  Japan
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Simply putting cap from opamp's inverting input to GND is suggested in AD797's datasheet fig54.
Whether putting small resistor between Iout pin and inverting input or not, the cap and feedback resistor make a pole in beta circuit.
It must be tried with care ,I think.
 
Old 17th January 2014, 10:16 AM   #6
Julf is offline Julf  Europe
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It really sounds like it is just acting as a first-order low-pass filter that removes HF noise - but wouldn't the DAC already have a low-pass stage?
 
Old 17th January 2014, 10:34 AM   #7
FoMoCo is offline FoMoCo  United States
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Originally Posted by Coris View Post
Yes, it seems like a first-order low-pass filter... But it differ than a usual low pass filter placed on each phase of a DAC, and it looks like the result of this filtering is different...
No it isn't. It's a single pole lowpass. If by phases you mean the differential output, it's still a single pole, just differential instead of common mode.
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Old 17th January 2014, 10:39 AM   #8
Coris is offline Coris  Norway
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DAC Filtering - the "Rasmussen Effect"
This is the illustration of the concept.
Attached Images
File Type: jpg cap over DAC phases (one channel).jpg (31.1 KB, 2015 views)
 
Old 17th January 2014, 10:45 AM   #9
Julf is offline Julf  Europe
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Originally Posted by Coris View Post
This is the illustration of the concept.
As FoMoCo states, it is a standard single pole low pass filter.
 
Old 17th January 2014, 11:10 AM   #10
Joe Rasmussen is offline Joe Rasmussen  Australia
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Quote:
Originally Posted by Julf View Post
It really sounds like it is just acting as a first-order low-pass filter that removes HF noise - but wouldn't the DAC already have a low-pass stage?
Hi Julf

I understand, I have read your Paul Klipsch quote in your by-line. But please read on.

First of all, may I point out that it was Ken Newton that named this as the Rasmussen Effect.

Now that I have gotten that out of the way, then to answer your question - that a single-pole dominant filter (dominant as the very slow slope intrudes on what we normally think in a way unacceptable, it is not flat @20KHz and over-rides every other filter in the chain) has an unusual sweet spot and is not just about suppressing HF noise, although that may well be part of the answer too.

So far, this is expected to work only on delta-sigma DACs and may be related to noise shaping and dither functions of this type of DAC, which totally dominates the market place. I am yet to see it work with NOS DACs and Ladder DACs.

Find the implementation that suits your situation (DAC etc) best, then vary the value of a single cap to create the low-pass effect and find the sweet spot. I have successfully used -1.3dB @ 20KHz to work well with Sabre DAC (in "current" mode with very low few Ohm Z I/V) and Cirrus Logic DAC ("voltage", but with 1:1 transformer and in this instance adjusting the Zobel network on the Secondary). Coris and Ken have done it with Burr-Brown "current" DACs. Some of these suggestions/implementations will be posted.

We now have a significant number of people, some with initial skepticism, who has heard the 'effect' - and to use Ken's words, the 'effect' is not subtle.

Here is Ken's full quote, as he seems to have nailed it - taken from the Oppo 105 thread:

"The effect is not subtle. Not only does the sound become smoother, it simultaneously becomes more detailed, plus the soundstage becomes larger and more three dimensional. The effect is similar to what I hear via the 'slow' roll-off setting of the PCM1794A's digital filter, except it shows greater subjective focus, along with superior rejection of the undesired ultrasonic alias images than is obtained via the 'slow' filter setting. It's quite interesting to be able to make the sound go from 'digital edginess' to 'analog smoothness' via such a seemingly small change in a 1st order output filter."

http://www.diyaudio.com/forums/digit...ml#post3778273

Others have heard it use very similar descriptions as Ken does above. Did so without telegraphing what they were supposed to expect - indeed, what I simply did for a number of my clients, I didn't tell them what I did - just asked me to tell me what they heard. And it tallies perfectly with what Ken described.

So I hope that has allayed some of the skepticism (basically conservative instincts that are hard to shift or banish).

As far as I can tell, this is applicable to differential (two phases out) delta-sigma DACs, whether "current" or "voltage" types. It comes in the form of a single capacitor across those two [+] and [-], but generally, I would think, in most cases also the addition of resistors (2) are needed.

So it is an add-on filter, one that is "dominant" in the sense it will come in at a much lower frequency and thus dominate even other filtering higher up. The lowest frequency filter is the dominant pole. But the roll-off is very slow and likely only to be circa -3dB down at 40KHz.

I think I need to put together a few schematics illustrating various implementations, the difference between a "current" DAC that sees a Virtual Earth or uses passive I/V, and finally "voltage" DAC. My suggestions will of course be up for discussion and also what other methods that can be used.

So give me a day or so to post those.

But I hope to have made this a little clearer and hope that skepticism will not prevent actually getting the soldering out and try a few resistors and small caps.

The purpose of this thread is to figure out what exactly do we have here, as this unusual behaviour needs explanations for what we are hearing, something that is quite obvious to those that have experienced it.

Cheers, Joe

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Last edited by Joe Rasmussen; 17th January 2014 at 11:36 AM.
 

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