John Curl's Blowtorch preamplifier part III

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#12,502
Peus from Neumann did a AES presentation on this topic including some informations regarding the difference measurement method (mainly on other microphone types though):

Peus, Stefan. Measurements on Studio Microphones, AES Preprint 4617

downloadable free of charge from:

File Finder

check "lectures" for file type on the left side; document should be on the first page (depends on browser window size though)

Other informations imo well worth a look too.
 
#12,504
The reason for my 3dB error brings up an interesting point one technique gave the amplitude deviation and the other gave the power in the two sidebands (3dB different). It's one of those things you rarely think about because THD is normally looked at as a single sided spectrum (it's also where the 2 in root(2qI) comes from for current noise).
 
#12,505
PMA said:
The only way how to get S/N into measurements. We are talking -80dB distortions here, at 1-2m you will have it buried in a room noise!
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maybe. However, it is not up to the task we put it to here. We need to measure distortion of a speaker when driven by an amount that would be used in home at listening distance. Depending on spkr sensitivity and room we need to drive it with up to 10W. Measuring that 10W level at < 1cm willl increase distortion of the mic significantly.. maybe even into over loading.


THx-RNMarsh

and, no, it is not the only way. Never-the-less, my room noise is very low and the range from 80-90db spl at listening location down to my room noise leaves enough dynamic range such that if the speaker distortion is significantly below my room noise (or an anechoic chamber) then that would be a super good speaker and We could be happy with it (distortion-wise)
 
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#12,507
- my issue, if it isnt clear by now... is to drive the speaker under test at realistic operating levels AND have a mic that wont over load or produce high distortion with that spkr drive. And, in my room, Scott is correct... I could measure spkr distortion at 10W and not over load the mic.

BUT, what is the distortion of a microphone at less than max spec spl? If it is 0.1% or less we are good to go.

The B&K paper I put up indicates it is much higher than I might have guessed at less than max rated spl... Too high in fact.

Back to... can we measure mics and find out ourselves or is there published thd specs at various spl levels for a mic/preamp somewhere which meets our spl/low thd needs? Which mic/pre would that be?



THx-RNMarsh
 
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#12,508
PMA said:
Let me have a look from another perspective.

We get some 0.02 - 0.03% IM distortion or HD distortion at 200Hz and even 10kHz from the whole chain, DAC - preamp - power amp - speaker - microphone - microphone preamp - ADC. 1 ppm at 10kHz frequency. This is not bad.

Now, this is an amplifier, better say a preamplifier thread. We may find many (even highly prized) power amplifiers, and even preamplifiers, that will certainly worsen the distortion found by acoustical path measurement (especially the 10kHz distortion) . So? Any thoughts? Famous "subtle sound differences"? 😉
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What mic and mic preamp are you using and at what mic SPL do you claim thsse distortion levels are good.

How do you measure the mic/pre distortion?

and which speaker?



THx-RNMarsh
 
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#12,509
RNMarsh said:
maybe. However, it is not up to the task we put it to here. We need to measure distortion of a speaker when driven by an amount that would be used in home at listening distance. Depending on spkr sensitivity and room we need to drive it with up to 10W. Measuring that 10W level at < 1cm willl increase distortion of the mic significantly.. maybe even into over loading.
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10W will give higher SPL an better S/N, so there would be a temptation to measure distortion in a far field.
There is one big problem then. Unless you have an anechoic chamber, the distortion measurement in-room would reflect rather room response (standing waves and reflections) than a real distortion. You know how uneven is a room freq. response. To get smooth response, a combination of near-field response and far-field response derived from gated impulse response is used. The impulse response is gated, windowed to cut the first reflection (from floor usually). This works for FR, but not for distortion. Distortion plot is still flawed by a room response because usually only frequency response plot is calculated from the gated impulse response.
In a near-field technique, useful frequencies are limited to c/(2pi*a), or 5475/a(cm), where a is the membrane effective radius. However, room effects are grossly eliminated and distortion plots for F <= 5475/a make some sense. For a 8" woofer, usable frequencies are up to 550Hz for a near field method.

The speakers used in my test were, as declared before, SEAS W18NX001 and T25CF001. The SPL at microphone placement was about 102dB.
 
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#12,511
PMA said:
10W will give higher SPL an better S/N, so there would be a temptation to measure distortion in a far field.
There is one big problem then. Unless you have an anechoic chamber, the distortion measurement in-room would reflect rather room response (standing waves and reflections) than a real distortion. You know how uneven is a room freq. response. To get smooth response, a combination of near-field response and far-field response derived from gated impulse response is used. The impulse response is gated, windowed to cut the first reflection (from floor usually). This works for FR, but not for distortion. Distortion plot is still flawed by a room response because usually only frequency response plot is calculated from the gated impulse response.
In a near-field technique, useful frequencies are limited to c/(2pi*a), or 5475/a(cm), where a is the membrane effective radius. However, room effects are grossly eliminated and distortion plots for F <= 5475/a make some sense. For a 8" woofer, usable frequencies are up to 550Hz for a near field method.

The speakers used in my test were, as declared before, SEAS W18NX001 and T25CF001. The SPL at microphone placement was about 102dB.
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I understand... in addition 2H and 3H may be attenuated/boosted at a particular placement in the room due to nulls and peaks in room response. If the room is reasonably quiet, you could window to allow 3H in and not higher.

My room is large and with fairly well controlled directivity might be useable. And I have some very large absorption panels to put in place for flat Fr for a least a couple octaves above 200Hz fund. I have measured the FR at my listening location and it is very flat... I would only have to check and locate flat response only at 2H and 3H level of one fundamental freq to be useful for speaker mods.

But, you didnt tell me the mic you used. How did you measure mic distortion? 102 near field could be 100 mW of power. This is the fundamental issue with near field... small signal distortion -- well below "normal" listening speaker drive power. Because speaker distortion increases with cone displacement and driving power... the distortion data from near field is very misleading at best for most people listening in far field. ... at least for audible issues.

On the other hand, if you listen in near field, as I suggest.... you can get some very good sound from your speakers; both distortion-wise and room interference-wise.


THx-RNMarsh
 
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#12,513
Hi George,


My Lynne Arriale Trio CD arrived today (which was your fault). If I like it will try and find the albums she did on DMP (back when CDs actually had dynamic range.


£19 for 10 CDs... good deal 🙂 (and I have an account with Presto).
 
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#12,516
Oh I have. The biggest danger with sets like these is I head off trying to find the original releases on vinyl!


(And don't tell Scott but I've been charged with locating more suitable Hindu spiritual music for SWMBO). Soundcloud is great for some of that

Durga Ashtotram - 108 Names of Durga by Davor Vdovic | Free Listening on SoundCloud


Croatian Man and German woman doing a pretty good job. I do wonder how much Indian music influenced 50s minimalism vs European church music of early times )
 
#12,519
Markw4 said:
Regarding the question of how do we teach people how to think, I believe we have gained a lot of insight into that question over the past several years. The answer is, you can't teach people how to think. Colleges don't do it. What they do is filter out people who can't think, and train those who can in professions.

What it means is that the best and the brightest have to want to go into science and engineering fields. All too often they go into investment banking to get rich. Our society doesn't really value scientists and engineers. We take them for granted. They are those nerdy people who make stuff for us.
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From Chris Langan, On intelligence and genius. Two quotes:

“[There is] no direct relationship between IQ and economic opportunity. In the supposed interests of fairness and “social justice”, the natural relationship has been all but obliterated.

Consider the first necessity of employment, filling out a job application. A generic job application does not ask for information on IQ. If such information is volunteered, this is likely to be interpreted as boastful exaggeration, narcissism, excessive entitlement, exceptionalism [...] and/or a lack of team spirit. None of these interpretations is likely to get you hired.

Instead, the application contains questions about job experience and educational background, neither of which necessarily has anything to do with IQ. Universities are in business for profit; they are run like companies, seek as many paying clients as they can get, and therefore routinely accept people with lukewarm IQ’s, especially if they fill a slot in some quota system (in which case they will often be allowed to stay despite substandard performance). Regarding the quotas themselves, these may in fact turn the tables, advantaging members of groups with lower mean IQ’s than other groups [...] sometimes, people with lower IQ’s are expressly advantaged in more ways than one.

These days, most decent jobs require a college education. Academia has worked relentlessly to bring this about, as it gains money and power by monopolizing the employment market across the spectrum. Because there is a glut of college-educated applicants for high-paying jobs, there is usually no need for an employer to deviate from general policy and hire an applicant with no degree. What about the civil service? While the civil service was once mostly open to people without college educations, this is no longer the case, and quotas make a very big difference in who gets hired. Back when I was in the New York job market, “minorities” (actually, worldwide majorities) were being spotted 30 (thirty) points on the civil service exam; for example, a Black person with a score as low as 70 was hired ahead of a White person with a score of 100. Obviously, any prior positive correlation between IQ and civil service employment has been reversed.

Add to this the fact that many people, including employers, resent or feel threatened by intelligent people [...] and the IQ-parameterized employment function is no longer what it was once cracked up to be. If you doubt it, just look at the people running things these days. They may run a little above average, but you’d better not be expecting to find any Aristotles or Newtons among them. Intelligence has been replaced in the job market with an increasingly poor substitute, possession of a college degree, and given that education has steadily given way to indoctrination and socialization as academic priorities, it would be naive to suppose that this is not dragging down the overall efficiency of society.

In short, there are presently many highly intelligent people working very “dumb” jobs, and conversely, many less intelligent people working jobs that would once have been filled by their intellectual superiors. Those sad stories about physics PhD’s flipping burgers at McDonald's are no longer so exceptional.

Sorry, folks, but this is not your grandfather’s meritocracy any more.”
― Christopher Langan
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“Owing to the shape of a bell curve, the education system is geared to the mean. Unfortunately, that kind of education is virtually calculated to bore and alienate gifted minds. But instead of making exceptions where it would do the most good, the educational bureaucracy often prefers not to be bothered.

In my case, for example, much of the schooling to which I was subjected was probably worse than nothing. It consisted not of real education, but of repetition and oppressive socialization (entirely superfluous given the dose of oppression I was getting away from school). Had I been left alone, preferably with access to a good library and a minimal amount of high-quality instruction, I would at least have been free to learn without useless distractions and gratuitous indoctrination. But alas, no such luck.

Let’s try to break the problem down a bit. The education system […] is committed to a warm and fuzzy but scientifically counterfactual form of egalitarianism which attributes all intellectual differences to environmental factors rather than biology, implying that the so-called 'gifted' are just pampered brats who, unless their parents can afford private schooling, should atone for their undeserved good fortune by staying behind and enriching the classroom environments of less privileged students.

This approach may appear admirable, but its effects on our educational and intellectual standards, and all that depends on them, have already proven to be overwhelmingly negative. This clearly betrays an ulterior motive, suggesting that it has more to do with social engineering than education. There is an obvious difference between saying that poor students have all of the human dignity and basic rights of better students, and saying that there are no inherent educationally and socially relevant differences among students. The first statement makes sense, while the second does not.

The gifted population accounts for a very large part of the world’s intellectual resources. As such, they can obviously be put to better use than smoothing the ruffled feathers of average or below-average students and their parents by decorating classroom environments which prevent the gifted from learning at their natural pace. The higher we go on the scale of intellectual brilliance – and we’re not necessarily talking just about IQ – the less support is offered by the education system, yet the more likely are conceptual syntheses and grand intellectual achievements of the kind seldom produced by any group of markedly less intelligent people. In some cases, the education system is discouraging or blocking such achievements, and thus cheating humanity of their benefits.”
― Christopher Langan
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#12,520
scott wurcer said:
Have some of those of course, the Brits were good at bringing the LP concept to India. I have several 50+ year old LP's of Hindu devotional music, you need the original no-fi versions of this.
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We also foisted our old car and motorbike factories on them when they were too outdated to sell anymore!


I have quite a lot of CD re-releases of the no-fi music. I find black and white film songs a bit of an aquired taste as the women, to my ear sound like they are being strangled but have been assured that this is the peak of art by my Father in Law.



With each passing year it sinks in more deeply that the more music I can appreciate the more I appreciate music. This does not extend to Miley Cyrus tho 😛
 
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