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In Reply to: RE: I'll have to dig it up. I've read several articles on this topic posted by dave slagle on September 17, 2021 at 14:58:54
If you google "preamp variable 2nd harmonic added" you see that people are getting 'tube sound' with plug-ins and the like that add a variable 2nd harmonic.
Nelson made a harmonic generator that did that- he sold kits at the Burning Amp festival. Link below.
Follow Ups:
I am specifically looking for a citing in support of the following statement:The 2nd and 3rd are relatively innocuous so the ear can tolerate a lot of it.
this seems to suggest they are on equal footing and 1% 2nd is equivalent to 1% third. My experience has been that even order distortions are "friendlier" than odd and as the order increases the level of distortion needs to go down substantially to avoid objectionable sonics.
In respect to Nelson's 2d generator, I am having a hard time understanding what he actually means by the phase of the H2. As near as I can tell he is operating under the belief that there will already be some H2 in the signal and by adding additional H2 from an external source the phase relationships of the two are important.
I dove into spice and built a simple 300B stage that has 1.3% distortion with this spectra.
I then attenuated the signal to make it a unity gain stage and fed it to the grid of a second 300B operating in the same manner. I could then reverse the phase of the two "H2 generators" to illustrate what I believe to be the two conditions (H2+ and H2-) that nelson is referring to and came up with these two spectra.
if we then compare these harmonic profiles to nelsons genral tonal description things become much more (or less??) clear
from listening tests we learn that there is a tendency to
interpret negative phase 2nd as giving a deeper soundstage and improved localization than
otherwise. Positive phase seems to put the instruments and vocals closer and a little more
in-your-face with enhanced detail.It does seem to me that this is not a 100% direct correlation to exactly what nelson is discussing but I cannot envision how the phase of any given distortion spectra is relevant until it is combined with another spectra so you can get sum and difference effects. The biggest 2nd order generator in a system is typically the air speaker driver interface so none of this exists in a vacuum.
dave
Edits: 09/21/21
Take a look- see what you think.
the two things that strike me in the article (and the linked article by Lynn Olson) that jump out at me are this thought referenced to Norman Crowhurst:
Research by Norman Crowhurst points out that feedback mostly reduces the level of the 2nd and 3rd harmonics, leaving the upper harmonics more or less alone, or sometimes at higher level than before feedback. It doesn't take very much level of a 5th, 7th, or 9th harmonic to make the sound "bitter", "dry", "clinical", "cold" "small" "dynamically limited" or "harsh". This is all part of the psychoacoustics of distortion and masking.
This is all in reference to feedback reducing distortion and has nothing to do with masking even though both 2nd and 3rd are mentioned.
Later on in the article he does get into masking but at that point the "masking" generator is a tubed circuit that generates only 2nd order distortion and the "tube noise" raises the noise floor to bury all of the existing harmonics higher than 2nd in the increased noise floor.
both of the points that I extracted from a precursory glance do not seem to support the idea that the third harmonic is benign and helps mask higher orders. He does call 2nd order "sonic gold" but in all of the examples the third is way down and not seemingly a factor in the "sonic flavor" I do like that they give both the measured objective and the sonic subjective results for a number of different music selections which allows the reader to get a better feel for the relationships at hand.
dave
The pattern definitely matters, which is implied here by Crowhurst as the higher orders are largely left alone and the 2nd and 3rd are reduced having a negative impact on the sound.
Cheever also finds that SPL affects the sensitivity to high orders and the ear is more tolerant at higher SPL. This is why having no high order harmonics at lower SPL is critical because our hearing sensitivity to them is the highest.
-
I agree that anything odd other than the 3rd is not perceived well. They become more audible the further they are from the fundamental tone. That is why the 3rd is considered innocuous as the 2nd (and to a lessor degree, the 4th). I read this somewhere in the last 2 weeks but pounding google hasn't resulted in the page I viewed.
If you want to look at an example of where the 3rd shows up, look at any sort of analog tape recording. A properly working tape machine will have a primary distortion amount of the 3rd generated at 0VU which might be 3% at +3VU. Its also a product of output transformers although I've not spent a lot of time on the latter.
A lot of people say that a good reel to reel can't be touched by any other format; it would appear that the 3rd harmonic is relatively innocuous on that basis alone.
It seems that the 2nd and 3rd are somehow helpful to assist the ear/brain system in making out soundstage depth; I was interested to see that Nelson commented on that too. I've noticed using direct mic feeds in the studio if you use an amp that makes significant lower ordered harmonic distortion as opposed to one that does not, you seem to get a more accurate sense of depth out of the monitor system. I don't know why this is but Nelson's comment about this is not the only one I've seen. I'd love to see more research on that but I'm not holding my breath...
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