I have been subjectively evaluating the 'Rasmussen Effect' over the past few days. In my initial subjective description of the effect in the Oppo thread, I had noted that I hadn't yet decided whether the high treble was being unacceptably sacrificed in order to gain the effect. However, after further listening evaluation, I now feel that the effect produces a high treble that's much more like that produced by live instruments. To my ears, live music rarely sounds 'bright'. Instead, instruments such as bells, brass, and violin have a kind of great dynamic clarity which cuts through the sound of other instruments, yet they rarely sound 'bright' to me, at least, not in the HiFi sense of that word.
Prior to experimenting with the Rasmussen Effect, I had been ably to obtain what I felt was a very high level of replay quality via my long tweaked homebrew DAC, including in the treble, where I had got brass, cymbals, and bells to sound recognizably like the real thing. Except, for what has always sounded like an over emphasis of the upper midrange. The upper midrange always sounded too intense or bright. Just a little too brash and forward, giving vocals (especially evident on chorus) a somewhat 'shouty' quality. Now, in addition to the qualities which I've already reported, the Rasmussen Effect has removed this upper midrange brashness and glare. The audible result of this removal is the restoration of more the sort of natural treble balance produced by live instruments. In particular, brass, cymbals, and bells now have quite distinctive tone color and weight. In addition, and perhaps surprisingly, my DAC now produces a much more weighty sounding lower midrange and bass. I suspect that this seemingly newfound bass energy had always been there, but had been psycho-acoustically masked by the ever present upper midrange glare, or forwardness that normally characterizes 'digital sound'. The perceived beneficial affect on the lower frequency balance is quite strong.
About implementation. The circuit modification needed to produce the effect is so seemingly trivial that one's first inclination is to dismiss it out of hand as snake oil. What Joe has discovered is that if your either add a dominant 1st order output filter at your DAC chip output, or simply modify the response if it already has such a filter, so that the net frequency response at the DAC chip outputs is anywhere from -1.3dB to -1.5db @ 20KHz, then the perceived sound goes from 'digital edginess' to 'analog smoothness'. The effect is very obvious, and surprising. Such smoothness is not to be mistaken for some kind of 'smoothed over' character, as the clarity across the band increases.
I'm finding that a -1.5db @ 20KHz point better removes the upper midrange glare than does a -1.3dB point. With such seemingly minor frequency response differences in 1st order filter response producing such relatively significant changes in perceived tonal balance (including in the bass), I feel that there must be more responsible for the effect working as it does than could be due to simple frequency domain filtering. I suspect there may be some sort of time-domain related effect at work here. The effect itself 'turns on' quite abruptly as the output filter response is only slightly changed. As Joe has pointed out, there seems to be a sort of 'knee' point in the activation of the effect. My own empirical experiments indicate that an 1st order output filter frequency response change of less that 0.2dB @ 20KHz can separate whether the effect is obviously active or inaudible. Deeper technical investigation is in order.
I can confirm that the effect also works using only a single phase of the PCM1794A's differential outputs, making it function single-ended. In my homebrew experimental DAC, I use only the positively phased current output, via 75 ohm passive I\V resistor, while directly grounding the negatively phased output pin. So configured, I find that about 20nF of Wima FKP-2 (film & foil) across the I/V resistor well activates the effect.
Prior to experimenting with the Rasmussen Effect, I had been ably to obtain what I felt was a very high level of replay quality via my long tweaked homebrew DAC, including in the treble, where I had got brass, cymbals, and bells to sound recognizably like the real thing. Except, for what has always sounded like an over emphasis of the upper midrange. The upper midrange always sounded too intense or bright. Just a little too brash and forward, giving vocals (especially evident on chorus) a somewhat 'shouty' quality. Now, in addition to the qualities which I've already reported, the Rasmussen Effect has removed this upper midrange brashness and glare. The audible result of this removal is the restoration of more the sort of natural treble balance produced by live instruments. In particular, brass, cymbals, and bells now have quite distinctive tone color and weight. In addition, and perhaps surprisingly, my DAC now produces a much more weighty sounding lower midrange and bass. I suspect that this seemingly newfound bass energy had always been there, but had been psycho-acoustically masked by the ever present upper midrange glare, or forwardness that normally characterizes 'digital sound'. The perceived beneficial affect on the lower frequency balance is quite strong.
About implementation. The circuit modification needed to produce the effect is so seemingly trivial that one's first inclination is to dismiss it out of hand as snake oil. What Joe has discovered is that if your either add a dominant 1st order output filter at your DAC chip output, or simply modify the response if it already has such a filter, so that the net frequency response at the DAC chip outputs is anywhere from -1.3dB to -1.5db @ 20KHz, then the perceived sound goes from 'digital edginess' to 'analog smoothness'. The effect is very obvious, and surprising. Such smoothness is not to be mistaken for some kind of 'smoothed over' character, as the clarity across the band increases.
I'm finding that a -1.5db @ 20KHz point better removes the upper midrange glare than does a -1.3dB point. With such seemingly minor frequency response differences in 1st order filter response producing such relatively significant changes in perceived tonal balance (including in the bass), I feel that there must be more responsible for the effect working as it does than could be due to simple frequency domain filtering. I suspect there may be some sort of time-domain related effect at work here. The effect itself 'turns on' quite abruptly as the output filter response is only slightly changed. As Joe has pointed out, there seems to be a sort of 'knee' point in the activation of the effect. My own empirical experiments indicate that an 1st order output filter frequency response change of less that 0.2dB @ 20KHz can separate whether the effect is obviously active or inaudible. Deeper technical investigation is in order.
I can confirm that the effect also works using only a single phase of the PCM1794A's differential outputs, making it function single-ended. In my homebrew experimental DAC, I use only the positively phased current output, via 75 ohm passive I\V resistor, while directly grounding the negatively phased output pin. So configured, I find that about 20nF of Wima FKP-2 (film & foil) across the I/V resistor well activates the effect.
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I guess it depends on if they have forgotten to include the low-pass filter (that is an essential part of the conversion process).
Ken,
Interesting observations!
I have a couple of clarifying questions.
Did your homebrew design include a low-pass reconstruction filter?
Interesting observations!
I have a couple of clarifying questions.
Did your homebrew design include a low-pass reconstruction filter?
Have you tried a similar low-pass filter in any other place in the audio path?
If you modify a commercial product, you will void the warranty, but as this is diyaudio, I don't think we care...
Sorry for heaving just opposite opinion than you... As one who have tried this first. BTW, I`m very sceptic to filters in the audio signal path, and I have been very reserved about this trick. Until I have tried it...
The effect is very real, and the improvements are only obvious.
Before coming with such statements, I will suggest you to try this first. It is extremely easy and damage free (if one turn off the power first
) to solder some caps on the differential output of your DAC.This thread it were opened as a result of some experimenting, and some findings. At least to make this findings known to many others, give them the opportunity to try it first, and have eventually comments of those who have experienced this effect in real.
Many other members here, who eventually will want to try, have for sure some measurement devices, and can contribute with some more informations to all of us benefit.
Of course, some measurements it have to be made. I did actually, and it were surprisingly for me. I intend to repeat these in better conditions, when my I/V module (I`m working on now) it will be finished. In mean while, maybe somebody else will make his own measurements about this effect.
I`m pretty sure that after trying it , one may be enough intrigued by the results, that it will take some measurements (as I did).
No, it was without an analog output filter. However, it once had a 2-pole LC filter, but it sounded better to my ears without any analog filter - until now. It also once had a 1-pole filter, but the cut-off was much higher than is needed to activate the Rasmussen Effect.
No, but that is one of the many intriguing investigations which need to be performed. Does the filter need to be implemented right at the DAC chip outputs, or can the same transfer function be established anywhere in the replay chain (analog or digital) and still obtain the effect?
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The reconstruction (reconstitution) low pass filter is an essential part of the digital-to-analog conversion process. The mathematics underlying the sigma-delta conversion principle assume a low pass filter. Without it, you will generate high frequency noise that was not part of the original signal.
Ah. Any specific reason for omitting it?
It should have exactly the same function wherever it is placed in the analog signal chain after the sigma-delta converter.
Is exactly what I have experienced my self. You pointed this very accurate!
One really feel such sound.. The cymbals and bells deep in the ears (like in real), while the drums and bass instruments vibrations are to be felt with whole body. Just impressive I may say...
BTW, I`m use to use a quite low volume level, and low power for my amplifier...
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Yes, it sounds better without it.
In linear systems theory, yes. However, no linear theory I know would seem to explain the observed behavior this effect. So, it would be interesting to confirm.
Is not the question for me, but for discussion accuracy I may precise too that I do not use any of analogue filter after DAC chip in my set up. Now the only filtering is this cap across the phases. It looks to me that this effect it may more obvious in such (unfiltered by standard low pass filter) cases...
I have read at least your remarks (post) about modifying consumer devices, warranties and so on. BTW, I have modified also some professional devices too, with the same positive results...
Yes, is right, but who care? The main point is to bring improvements and get more out from a quite common designed product. But anyway, is not this the subject now.
This trick and the effect of it I have experimented on my own designed DAC using an PCM 1792 chip, and not yet on a modified consumer product.
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Isn't this thread an indication of the opposite?
How about sigma-delta theory that states that you need a low pass filter to properly reconstruct the original signal?
That cap adds the missing low pass filter that is an essential part of the conversion process.
But what I am curious about is if this effect is seen (or rather heard) on gear that already has the required low pass filter.
I don't need to try it, as I understand what is going on here. It's a simple low-pass filter that alters frequency response. Will a difference be heard? Sure. Is it any more magical than a single pole low-pass? Nope. Calling it anything other than a textbook filter is audiophoolery.The effect is very real, and the improvements are only obvious. Before coming with such statements, I will suggest you to try this first. It is extremely easy and damage free (if one turn off the power first
With no filter you get aliasing and the resulting extra high frequencies may cause intermod distortion, adding extra trash into the signal, in the following stages. Even ignoring this possibility, you've removed a filter that would remove high frequencies and changed the frequency response. Either way, you may hear a difference. The result is a less faithful version, but it happens to sound good to you.
Please re-read the last two words of the below excerpt taken from my post #26:
"However, it once had a 2-pole LC filter, but it sounded better to my ears without any analog filter - until now."
Sigma-delta DAC chips already feature an integrated low-pass reconstruction filter. It's called an oversampling digital interpolation filter, and has technical performance far superior to that of any practical analog reconstruction filter.
Julf, rather than first seeking to debate sigma-delta conversion theory with me, may I suggest you take the simple step of trying the technique for yourself?
Yes, i understand very well this. But this cap it filter in a different way than a normal/standard used low pass filter does. What and why it is this "different" is to be found it out...
Yes, again, I thought exactly at the same: how it may be the sound when using this cap on a system which have already all filtering in the right places ...
Maybe somebody with a "normal" gear will want to answer...
That's one theory. However, rather than blindly theorizing, perhaps, you would be better served by first trying such a simple to perform experiment.
This isn't blindly theorizing. This is 200 level electronics. You remove a filter, you alter frequency response. We've known this for longer than I've been alive. I suppose next you're going to suggest I use a yellow highlighter on my CDs, because I haven't tried it yet?
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