John Curl's Blowtorch preamplifier part II

[Wavebourn]

Originally Posted by kgrlee

When I did this in the early 80's, I couldn't find any sources for the maths either.

Sure, no Google then. Access to networks through doors guarded by guys with AK-47.

[bcarso]

Originally Posted by scott wurcer

... speaking from a fairly hermetic naivete

Nice expression.

[fas42]

Originally Posted by scott wurcer

EDIT - I read the whole thing. I like Rod but here he is speaking from a fairly hermetic naivete.

It appears that everyone that casually quotes the results of the typical 19, 20kHz IMD test for some ambitious audio gear are suffering the same problem. Which is, what his main point is ...

And, what stops people who do know from using a combo of 8 and 9kHz ... unless, it might be too revealing ...

Frank

[kgrlee]

Originally Posted by scott wurcer

You can derive all these results from a simple mathematical exercise, his comment about not being able to offer any references because there are none is baffling.

I might have done this in 1980 but this is beyond my brain capacity this millenium. Yus gurus, please point to something that dun nid Higher Maths for Dummies. I'm sure there's loadsa stuff if you have done HM4D.

[DF96]

Harmonics and IMD both come from trigonometry. Take two sines (or cosines or one of each). Multiply them together. There are trigonometric identities which tell you how to write the result in terms of other sines and cosines involving the sum and difference of the angles. That is how harmonics and IMD are formed: multiplying sines together in non-linearity.

Yes, when you slaved over triangles as a bored teenager in school you were learning the mathematical basis for low distortion amplifier design, but unfortunately the teacher never told you because he probably did not realise this himself.

[jneutron]

Originally Posted by gpapag

(this is a question from my part: In electromagnetics, with high energy, short time bursts, does vacuum properties (eo, μο) change values locally?)
George

The guys here seem to consider mu zero constant even in the megatesla range. I'll ask one of the carpool guys about the 7 TEV collisions. (chance are, I will not understand the answer. )

Originally Posted by gedlee

The nonlinearity of air is not an issue at reasonable SPL levels.

I've always pondered the spl's at the throat under high power. Prop velocity in air is pressure dependent, and putting a couple of acoustic watts into a few square inches is not exactly low level.

jn

[RNMarsh]

If you are playing at moderate-high levels in a large living room at listening distance, then up close to driver the spl is huge! As is distortion.
Measuring thd at 1W at 3 feet doesnt begin to tell you what you are really hearing/producing at the listening position under normal (for me) listening levels out in the room. I would imagine same for horn at throat entrance.... very high spl and distortion. Distortion is a Lot higher than typical measurement conditions/specs imply.

[Just use an SPL meter.... set up listening level out in the room's listening position and then move the spl meter to the driver up close (inch) and measure.... you get really high spl, then. What is the driver doing at such high spl?] I measured once very high thd under same real world conditions. -RNM

[scott wurcer]

Originally Posted by fas42

It appears that everyone that casually quotes the results of the typical 19, 20kHz IMD test for some ambitious audio gear are suffering the same problem. Which is, what his main point is ...

Frank

I was more interested in the "discovery" that odd functions can only make odd harmonics.

This is signal processing 101, even Wiki summarizes it nicely.

Even and odd functions - Wikipedia, the free encyclopedia

[scott wurcer]

Originally Posted by jneutron

I've always pondered the spl's at the throat under high power. Prop velocity in air is pressure dependent, and putting a couple of acoustic watts into a few square inches is not exactly low level.

jn

I was told a trumpet at full throtle was about 4W at the throat. A small silicon pressure guage could be placed at the throat of the waveguide/horn, an interesting experiment.

[bcarso]

Originally Posted by scott wurcer

I was told a trumpet at full throtle was about 4W at the throat. A small silicon pressure guage could be placed at the throat of the waveguide/horn, an interesting experiment.

I recall a paper by some of the Russians who Gene Cerwin hired which suggested a rearrangement of seating in big bands would produce lower acoustical IM distortion.

Of course, besides breaking up the intragroup camaraderies (and rivalries?), perhaps a bit of this IM is part of the particular arranger's sound.