Preamp difference : if it's not the frequency, not the slew rate, and not the harmonics, what is it ?
- Thread starter AdrienPerinot
- Start date
Help Support GroupDIY Audio Forum:
A well known psychoacoustics researcher had what I thought was a good explanation, although it requires some knowledge of the human auditory system model:
If a phase change results in a change in critical band stimulus it will be audible.
That is from memory, so the wording may not be exact, but that was the summary.
Most natural electronic processes have a gradual change in phase (minimum phase behavior) that does not meet that criteria. Digital processes can, and physical non-minimum phase systems like multi-speaker arrangements.
One easily accessible demonstration is at Purifi, who work on low distortion speaker driver designs. This page show the difference in audibility between phase modulation and amplitude modulation which result in an identical power spectrum of distortion, but which sound quite different:
https://purifi-audio.com/blog/tech-notes-1/doppler-distortion-vs-imd-7
Square waves can reveal HF (rise time) and LF (tilt) response cut offs, not to mention stability (overshoot and/or ringing, not to be confused with Gibbs). An easy quick check if you know what you are looking at.
JR
Indeed. I've heard the effects of shifting phase when it results in a change of spectrum because it triggers non-linearities, whether in teh electronics or the transducers or teh air or the ears..
I'm not too convinced with this experiment.
PermO
Well-known member
The difference is very clear on my nearfields, I ignored the use headphones advice.
I found that I got inconsistent results with nearfields, because surprisingly tiny differences in head position matter. Earbuds did the trick.
OK, I had to turn the volume up significantly to hear the effect.
It shows that the difference in peak factor triggers non-linearities that are audible.
It shows that the difference in peak factor triggers non-linearities that are audible.
PermO
Well-known member
Last edited:
k brown
Well-known member
I'll prolly get royally flamed for this, but one of the most dramatic examples I heard of absolute phase in loudspeakers was when I had a phase switching box between my amp and speakers. Playing Peter Gabriel's "Red Rain", the prominence of different elements of the dense mix was quite audibly different between the two positions of the phase switch box.
I think confirmation bias not at play here, because I had no expectation of the effect, and the balances were simply different; one didn't "sound better" than the other.
On simpler, less dense mixes and simpler-mic'd recordings, the difference was mostly that subtleties of the sound were more readily audible in one position or the other; and different for different recordings.
Could be that these effects were more audible because the speakers I was using were two-ways, using very fast, high quality drivers (Dynaudio) and phase-linear (first order) crossovers. More complex speakers/crossovers may blur/scramble these effects.
I think confirmation bias not at play here, because I had no expectation of the effect, and the balances were simply different; one didn't "sound better" than the other.
On simpler, less dense mixes and simpler-mic'd recordings, the difference was mostly that subtleties of the sound were more readily audible in one position or the other; and different for different recordings.
Could be that these effects were more audible because the speakers I was using were two-ways, using very fast, high quality drivers (Dynaudio) and phase-linear (first order) crossovers. More complex speakers/crossovers may blur/scramble these effects.
Could the fact that most recordings don't show symmetrical waveforms have anything to do with that?
I have a hard time imagining how speakers reproduce non-symmetrical waveforms. Especially since speakers must have a different response going one way compared to the other.
It's hard to measure, I presume, since all measurements are done with symmetrical waves.
It could also explain why open baffle speakers are popular with audio enthusiasts...
I have a hard time imagining how speakers reproduce non-symmetrical waveforms. Especially since speakers must have a different response going one way compared to the other.
It's hard to measure, I presume, since all measurements are done with symmetrical waves.
It could also explain why open baffle speakers are popular with audio enthusiasts...
Bob Katz and I had an interesting email exchange recently regarding the audibility of absolute polarity and he summed it up quite nicely. I'm paraphrasing but it essentially boiled down to "the more asymmetric the transducer the greater the chance you can hear the audibility of absolute polarity."
By asymmetric he meant differences in distortion between positive-going and negative-going direction such as that found in moving coil sealed box speakers.
I did a test in my shop years ago with a small pair of metal Minimus 7s and a single-cycle symmetric burst around 100 Hz and polarity differences were obvious. These tiny speakers pressurize a room better than rarefacting it and positive acoustic polarity definitely had more punch. Bob's observations 30 years later made sense of it.
Am I right in assuming this box flipped polarity on both channels at a time?
I have noticed a number of times that phenomenon. It was always traced down to the fact that loudspeakers have asymmetrical response. It is particularly sensible on bass units, where the diaphragm excursion is important. The main reason for that is that the induction varies differently in a direction than in the other.
Most loudspeakers are subject to that. Loudspeaker designers have tried since ages to reduce this phenomenon. This is the base for JBL's SFG (Symmetrical Field Geometry) or Purifi's Constant Force Factor.
There are other factors that result in asymmetrical response, surround radiation, air compression vs. deflation.
@AdrienPerinot still hasn't answered my post #27 question about how he padded the line level output to feed the various mic pre DUTs.
Consider the following:
If a U-pad was used to lower the line level to mic level for testing and that pad was driven single-ended, i.e. with one leg driven and one grounded, the preamp input will see a common mode signal significantly higher than the differential. If the pad is 60 dB the stimulus' common mode signal seen by the input will only be attenuated by about 6 dB and the common mode signal presented to the input will be 54 dB hotter than the differential signal being amplified.
Depending on the preamp topology, how much it rejects common mode and how little or greatly it distorts common mode after rejecting it make measurable differences in distortion if driven in this manner. I've witnessed this effect testing an instrumentation amp front-end with a cross-coupled CM stage using a U-pad driven both single-ended and balanced. The high CM voltages in the former increased distortion by more than 20 dB.
Transformer-based inputs would seem to have an advantage if tested this way with transformerless preamps potentially varying a lot.
If the test was performed with a U-pad driven by a symmetrical balanced line output or transformer high CM voltage wouldn't exist and the differences could be attributed to something else. But until we're told how he ran the test we'll all just be left to guess.
Consider this as well:
In the "Cohen" (Demrow) transformerless topology I've found that front-end compensation capacitor tolerance, and the resulting increase in HF common mode rejection that matching provides, reduces twin tone 19:20 kHz 1:1 IM distortion significantly. Like 10's of dBs less distortion. https://proaudiodesignforum.com/forum/php/viewtopic.php?t=1286
Consider the following:
If a U-pad was used to lower the line level to mic level for testing and that pad was driven single-ended, i.e. with one leg driven and one grounded, the preamp input will see a common mode signal significantly higher than the differential. If the pad is 60 dB the stimulus' common mode signal seen by the input will only be attenuated by about 6 dB and the common mode signal presented to the input will be 54 dB hotter than the differential signal being amplified.
Depending on the preamp topology, how much it rejects common mode and how little or greatly it distorts common mode after rejecting it make measurable differences in distortion if driven in this manner. I've witnessed this effect testing an instrumentation amp front-end with a cross-coupled CM stage using a U-pad driven both single-ended and balanced. The high CM voltages in the former increased distortion by more than 20 dB.
Transformer-based inputs would seem to have an advantage if tested this way with transformerless preamps potentially varying a lot.
If the test was performed with a U-pad driven by a symmetrical balanced line output or transformer high CM voltage wouldn't exist and the differences could be attributed to something else. But until we're told how he ran the test we'll all just be left to guess.
Consider this as well:
In the "Cohen" (Demrow) transformerless topology I've found that front-end compensation capacitor tolerance, and the resulting increase in HF common mode rejection that matching provides, reduces twin tone 19:20 kHz 1:1 IM distortion significantly. Like 10's of dBs less distortion. https://proaudiodesignforum.com/forum/php/viewtopic.php?t=1286
Last edited:
You are talking about "absolute polarity". I recall first writing about this in the 80s. I even put a polarity switch in one of my phono preamp designs because recordings, and consoles at the time were not strict about maintaining absolute polarity. Since then the industry has become more disciplined about preserving absolute polarity.
This is at best a subtle audible phenomenon.
I am aware of one book on the subject "The Wood Effect" by R.C. Johnson c.1988. I know about the book because he quoted my "Audio Mythology" column on the subject in the book and sent me a copy. At the time I didn't argue that absolute polarity was very audible, only that signal paths should maintain absolute polarity for accuracy.
When in doubt do it right.
JR
Wow, boys. So many insights in just three posts. Thanks!
I only became aware that most audio is asymmetrical in nature a few years ago. I think some fellas on the Reaper forum pointed that out to me. It has been bothering me ever since, because it was a distortion of my reality field.
So it's nice to know someone like Bob Katz seems to agree with what I suspected. It strokes my ego...
I only became aware that most audio is asymmetrical in nature a few years ago. I think some fellas on the Reaper forum pointed that out to me. It has been bothering me ever since, because it was a distortion of my reality field.
So it's nice to know someone like Bob Katz seems to agree with what I suspected. It strokes my ego...
k brown
Well-known member
Yes, both L/R at the same time; I always understood that was the definition of 'absolute phase/polarity', as opposed to (L/R) relative polarity.
Last edited:
Absolute phase is pretty easy to hear if you a-b both + and - on LF instruments as the attack either comes at you or pulls away from you. A bass drum attack is heard live as coming at you when you're in front of the drum, the amount depending on the hardness of the beater, i.e attack transient. Slap bass may sound more aggressive one way or the other as the slapped string goes toward the pickup and if this results in a positive leading waveform. Depends on the polarity of the pickup. I think of it as "punch" which is a term you would apply to pop and jazz. Orchestral, not so much. . .
It is so music dependent.
It is so music dependent.
It's actually pretty common with many acoustic instruments.
The easiest to generate asymmetric waveform is your own voice.
It's been a few decades since I wrote about this but as I recall not only are vocals percussive and asymmetrical but brass and horns are notoriously asymmetrical.
I suspect there are also probably polarity related differences in human hearing (like different physical excursion limits of inner ear mechanisms).
JR
Lots of great insight in this thread reminding me my support of GroupDIY is money well spent.
On a sidebar and tangentially related I have been in a beta group with some state of the art "acoustic space" software...some of it is mind blowing but one of the deets that came up is that in order to appropriately simulate an acoustically treated space each participants HEAD needed a calibration method because of how our inner ear canals are different, so developing a test that each user could run prior to using the software so that the frequency spectrum was adjusted in the output of the software to accommodate for your own ears (AND making that test as simple and painless as possible because users are notoriously stubborn) has been in the works now for several months.
Frankly I was blown away at how realistically a set of "virtual" speakers at 3 meters sounded...once the test program ran my ears, switching to those speakers was like a vertigo event because I heard a space that I was physically not in...it physically felt like I moved while sitting still.
My point is each of us hears different things in different ways and some of that is physics...
Like my Dad used to say to me when I forgot to do a chore and said I didn't hear him ask me: "You only hear what you WANT to hear"
On a sidebar and tangentially related I have been in a beta group with some state of the art "acoustic space" software...some of it is mind blowing but one of the deets that came up is that in order to appropriately simulate an acoustically treated space each participants HEAD needed a calibration method because of how our inner ear canals are different, so developing a test that each user could run prior to using the software so that the frequency spectrum was adjusted in the output of the software to accommodate for your own ears (AND making that test as simple and painless as possible because users are notoriously stubborn) has been in the works now for several months.
Frankly I was blown away at how realistically a set of "virtual" speakers at 3 meters sounded...once the test program ran my ears, switching to those speakers was like a vertigo event because I heard a space that I was physically not in...it physically felt like I moved while sitting still.
My point is each of us hears different things in different ways and some of that is physics...
Like my Dad used to say to me when I forgot to do a chore and said I didn't hear him ask me: "You only hear what you WANT to hear"
