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In Reply to: Re: Harmonic Multiplication in Linear Devices posted by Scott Frankland on May 09, 2003 at 11:51:44:
I'm glad to note that we are discussing something useful to audio designers. In reality, we usually still have to use both local and loop feedback, but it is not as easy to decide which or how much, as some would think. ;-) For example, if you take mosfets, jfets and perhaps tubes, and then make a differential pair from them, you completely change the distortion. It goes from 2'nd with a little 3'rd (probably expansive) to almost pure compressive 3'rd. Is this good? Is this bad? Are the advantages of direct coupling, total distortion reduction, bias stability, etc, etc worth the change in the harmonic series? This is what real analog designers wrestle with every day, and why many tube designs stay single ended at the input stage.
WW is tough to get, but contact me first, if you need something. I might be able to help. The very BEST AES bargain is the CD rom collection of almost everything for about $400. I have it and love it, especially the stuff published 20 years ago and more.
Follow Ups:
Hi John,I can almost hear several violin or piano makers discussing this
subject 2 or 3 centuries ago. Not that they would be talking about
feedback, but they would have terms for 2nd, 3rds, overtones. They
would be discussing how sound changes from dull to bright, attacks
and decays, what produces rich tone and how to alter the tonal picture.Maybe your answer lies partly in looking way back, before electronics,
at how the designers back then plied their trade in making the best
sounding instruments they could.I can imagine today that these solutions would involve partly
electronic, partly mechanical (eliminate vibration???), and a lot
of subjective evaluations.I wonder about the first people who believed that a piano string
should be struck in a position giving the minimum amount of 7th
harmonic possible. If the makers of these fine instruments were
aware of this (without even knowing what it was), what other nuggets
lie waiting to be uncovered and applied to our latest technology.This reference shows a (perhaps) deeper look into the relationship
between the 7th and the fundamental and how it varies with frequency.
The way I read this is that more 7th can be tolerated at higher
frequencies.Also interesting is the reality that their is an initial pulse
down the string. How does an amp best capture these sorts of attacks,
since they are very important to the "pianoness" of the sound.You mention the 3rd harmonic as being compressive or expansive.
These aren't terms I'm familiar with. Care to elaborate?Thanks
Compressive third harmonic is when the 3 rd harmonic is phased to REDUCE the amplitude of the top of the sine wave. This is the normal third harmonic generated by analog tape compression, etc. Expansive third can actually cancel compressive 3'rd, so it is interesting for this reason. It is more difficult to generate. Thanks for the input on the piano, and good reading of that 2/3 of a century old book that I recommended. I think that you will like it.
Hi,I received the book from Amazon. That is one of the first "physics
of music" type books worth getting...and its less than $10 :)They even had subjectivist/objectivist debates back then. Page 181
discusses whether a tune played in the key of C major sounds any
brighter when played in the key of G. Great discussion! Lots of
them in this book.Thanks for the recommendation
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