Preamp difference : if it's not the frequency, not the slew rate, and not the harmonics, what is it ?

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
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.

Keep us posted, please.

One of the things that slowed down my ambisonic experiments is the HRTF function. A few have tried offering an online service but these seem to have vanished from the net. And the math involved is too complicated. It's not really I don't understand, it's more that I keep forgetting what I've learned.
 
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.
What is maybe missed by the average listener in evaluating speaker performance is what goes on when sound is recorded and how “attack” can be just as much from incursion as excursion from a speaker diaphragm. For example with a drum kit - the kick drum mic is put into a hole in the front skin (the side facing the audience not the drummer) behind and pointing at the back of the back skin where the beater strikes and the snare and toms and percussion mics are place above the skin surface, opposite to a kick. So the kick being initially positive pressure on strike it would appear that the other skinned instruments would be at negative pressure on strike - the fundamental mode 0,1 and the 0,2 mode contribute to the “thump” sound of a drumskin - the 0,2 at a point 0.436 of the distance to the edge from the centre has the opposite direction of travel, due to the nature of sound wave propagation on a circular membrane the subsequent contra rotating interacting waves at coincidence cause sequentially, compression and rarefaction of the air at the other vibrational modes of the skin and in different patterns according to the mode.
The mic in the kick drum is at a point where the initial impact of the beater provides a distinct positive “push”, for the other top miked drums the position of the mic can determine the polarity - aim it directly at and over the strike point in the centre you’ll get negative pressure, move out nearly 1/2 way to the side and further where a mic will normally reside and this reverses to the initial skin travel direction mode 0,1 negative pressure combined with resultant mode 0,2 positive pressure, which has a longer decay time but radiates less, plus of course the subsequent wave propagation around the skin - as the mic would get in the way of the drumstick it has to be placed towards the outer rim and so is also more subject to the mode 0,2 positive pressure on strike notwithstanding the initial fundamental negative pressure impulse. Looking at a top miked drum first impulse waveform there is usually an initial negative going travel followed immediately by a larger positive going waveform.
The tone of the drum comes from other modes of skin motion which contribute to the pitch and decay time of the vibration of the skin, modes 1,1 2,1, 3,1 4,1 and 5,1 in ever increasing complexity of reversals of skin direction.
Yamaha produces drivers using ceramic coating to rigidify speaker cones to negate the effect of ripple distortion and cone resonance.
Considering that no matter which mic you’re listening to that immediately after a positive pressure impulse there is a corresponding negative one (or vise versa) then positive and so on then it becomes apparent that not only is it important that the mic responds in kind but also the speakers and its enclosure must do exactly the same.
 
many people claim they can hear differences between preamps

I'm sure "many" people including myself could, and indeed would, hear the difference between a humble 741 v's an NE5534AN or OP27 used in gain stages but I'll also postulate far fewer could when the same op-amps were configured as unity gain buffers.

Part of the designer's skill is selecting what's appropriate rather than simply going with the best. Or as an approximation of something Brunel observed, "an engineer's skill is not in designing something strong enough but in designing something weak enough".

I'm guilty of this too - if I'm designing a preamp, I write 5532/4 even when I don't really need to because a TL07x would be perfectly adequate overkill for the kids' Karaoke machine or doorbell or ...

I truly wonder how many people could really hear the difference in a blind or double blind test. And if I were a betting man, I'd win money on this!
 
To try and stay on point, the item under evaluation depends on the rest of the signal path so what sounds "punchy" in one setup might not in another. Notwithstanding of course, that the perceived "punchiness" might actually be non-compliant performance (like the deviation from the RIAA Standard, mentioned elsewhere)
 
What is maybe missed by the average listener in evaluating speaker performance is what goes on when sound is recorded and how “attack” can be just as much from incursion as excursion from a speaker diaphragm.
isn't "attack" just another term for slew rate which is effectively limited by humans' 20kHz-ish upper frequency hearing range? I understand the "I just hit a snare drum" aspect but am I wrong to say that even if the rise time did exceed the 20kHz limit, we humans wouldn't be able to perceive it?
 
isn't "attack" just another term for slew rate which is effectively limited by humans' 20kHz-ish upper frequency hearing range? I understand the "I just hit a snare drum" aspect but am I wrong to say that even if the rise time did exceed the 20kHz limit, we humans wouldn't be able to perceive it?
There’s an interview with Rupert Neve worth reading - Geoff Emerick from Abbey Road would disagree as would Rupert. Neve design consoles were designed with 75KHz and above bandwidth for a very good reason - the effects of distortion in the octave above 20KHz (up to 40KHz) and even the next octave up can have detrimental downward effects due to harmonics on the perceivable audio spectrum. There are 3 parts to the interview - access parts 2 & 3 using the right arrow tabs on the interview page. Part 1 deals with what I mention above.
Interview with Rupert Neve:
https://www.audiotechnology.com/features/interview/rupert-neve-interview-part-1
 
Also worth noting that a lot of musical instruments have output ranging way above 20KHz - up to 40% of the energy in a crash cymbal is above 20KHZ. There is also much research done into our perception of much higher frequencies - even though we don’t “hear” them, apparently they can have an effect. Interesting test results here:
https://forum.audiob.us/discussion/...particular had,much weaker power than cymbals.
 
Some parameters are easily measured others are only detectable with the ear.

No. You are leaving an obvious gap between parameters that can be "easily measured" and those that are measureable but where it is not easy (however defined) to measure.
Nothing is unmeasurable given the right kit. The "ear" and subsequent processing in the brain is a very unreliable and unrepeatable 'measurement tool".
 
Also worth noting that a lot of musical instruments have output ranging way above 20KHz - up to 40% of the energy in a crash cymbal is above 20KHZ. There is also much research done into our perception of much higher frequencies - even though we don’t “hear” them, apparently they can have an effect. Interesting test results here:
https://forum.audiob.us/discussion/16234/ot-the-frequency-range-of-musical-instruments-above-20khz#:~:text=Crash cymbals in particular had,much weaker power than cymbals.

That article leans on an old conference paper by Oohashi which isn't open access. I found a much more recent meta-analysis about audibility of ultrasonics (from J. AES), which is:

https://qmro.qmul.ac.uk/xmlui/bitst...nalysis of High Resolution 2016 Published.pdf

There's also a later journal paper by Oohashi et al. downloadable for free from ResearchGate:

https://www.researchgate.net/public...ion_in_the_emergence_of_the_hypersonic_effect
 
isn't "attack" just another term for slew rate which is effectively limited by humans' 20kHz-ish upper frequency hearing range? I understand the "I just hit a snare drum" aspect but am I wrong to say that even if the rise time did exceed the 20kHz limit, we humans wouldn't be able to perceive it?
Did you ever mature intermodulation distorsion??
 
One simple, fun and not so scientific test would be to find a piece of music with say plenty of hard hitting drums and guitars - plenty of “punch”. Import that into your DAW. Route it out from the DAW and route it back direct in via just the audio interface I/O and record that, then using the same interface output, pass the original (not the re-recorded) music through the preamps under test and back in to the DAW via exactly the same input on the interface. If the preamps as I assume are mono, you’ll need to do a mono export of the track to start and use a mono signal routed through the preamps. Then since the same signal has been used with the same interface I/O the new recorded pieces should all line up timewise fairly closely (allowing that there will be slight delay differences with each pre) with each other and the original re-recorded DAW direct signal - you may need to slide them into position to line up exactly with the first re-recorded track by observing the expanded waveforms. Then you can phase invert each preamp’s recording in turn added to the non phase flipped original re-record and listen to the difference between what has passed through the preamps plus the interface I/O and what has only passed through the original interface I/O - whatever you hear if there is no absolute null is the characteristic difference of the preamp compared to no preamp - the “sound” of the pre. You will need to adjust level balance of each preamp’s recording to come as close to null as possible. What you hear will be the frequencies that deviate from the original signal.
Then you can play the recordings of two preamps at a time with one phase inverted and you will hear the difference between the preamps.
This is only an audible test with no other result than hearing how different each preamp is to another and also to the original I/O path which is also part of each preamp’s recording - this is then eliminated from the equation as they all share the same identical I/O sonic characteristics. You can also do this with a sweep, impulse or white noise signal. You can also record the sum differential of a pair of tracks and have a look at the waveform expanded. Or put the recordings of each plus the differential recordings through analyzer software and see where the differences lie.

Just a fun experiment. I have used this phase invert null method to line up compressors and EQs to be used on stereo mixes, channels on recording consoles etc.
At one stage I wanted to compare different DAW’s audio engines - in the test I compared Cubase, ProTools, Digital Performer and LogicPro using a commercial track straight from CD imported into each DAW and bounced out using the standard export function, then all imported into the same DAW to do the comparison and found that Cubase and ProTools import/exports had an absolute null (on phase inversion of one track), so they were identical - Logic and Digital Performer were way different to each other and to Cubase and ProTools.
 
am I wrong to say that even if the rise time did exceed the 20kHz limit, we humans wouldn't be able to perceive it?

That is not completely relevant in the context of this discussion. I think you are likely correct, to my understanding the most rigorous experiments have not shown any audibility of sound above 20kHz (which make sense if you consider the acoustic response from external ear to eardrum), but signals that have a very fast slew rate could potentially overload preamp circuitry (e.g. exceed feedback loop bandwidth) and cause distortion that is audible.
Even speakers can have intermodulation distortion. The "most rigorous" experiments I referred to above used multiple tweeters to play separate ultrasonic tones, because it was found that playing a complex ultrasonic signal through one tweeter resulted in distortion products being produced in the audible range.
 
That article leans on an old conference paper by Oohashi which isn't open access. I found a much more recent meta-analysis about audibility of ultrasonics (from J. AES), which is:

https://qmro.qmul.ac.uk/xmlui/bitstream/handle/123456789/13493/Reiss A Meta-Analysis of High Resolution 2016 Published.pdf

There's also a later journal paper by Oohashi et al. downloadable for free from ResearchGate:

https://www.researchgate.net/public...ion_in_the_emergence_of_the_hypersonic_effect
I am having flashbacks to an old thread about audition of high frequencies (above human hearing ?) on Gearslutz. IIRC that thread ran way too long with numerous claims about audibility.

I was glad when it finally ran out of steam. Any with interest and too much time on their hands try looking over there. I just wasted a few minutes doing a search and couldn't find it. There was one obscure musical instrument that was capable of output up to something like 50kHz that several embraced for justification.

I'd rather not revisit that fruitless search but do whatever floats your boat. ;)

JR
 
Back
Top