Very high order intermodulation - Myth or reality
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| Posted on May 18, 2014 at 08:36:56 | ||
 Some time ago I had a discussion (here in this asylum) with Ralph about the supposed mechanism of generation of very high order harmonics in case of use of NFB. At the time I was very skeptic. Now I can say that I have measured their reality. http://www.audioasylum.com/cgi/vt.mpl?f=tubediy&m=227895 The sonic signature in my listening room (measured @330Hz by the internal microphone of my ACER ONE netbook) CONFIRMS the reality of the re-growth of very high order harmonics around -60 dBc (allegedly generated by NF). They are clearly radiated in the room by the speakers and they must be real because they are absent from the acoustic noise floor when the test tone is removed. If I use in the same test set-up my only "zero feedback all triode amplifier" (with 12AT7/6AS7 instead of 6SN7/829B) I get a similar third harmonic level (-50 dBc) but a predominant second harmonic (-40 dBc). Now I am in the theoretical position of correlating my subjective listening impressions with some 'scientific measurement', even though the abyss of difference of damping factors between the two amplifiers could mask the phenomenon (the 'all triode zero NF one' exhibits some ohms of calculated output impedance whereas this one should have only some fraction of ohm). Well, to be honest I like both amplifiers and I am not able to express any clear personal preference. I hear relative differences but I can not decide which one is 'better'! By the way, I have also built an OTL and when I tested it some years ago in the same conditions it had similar third harmonic (-50 dBc), non predominant second harmonic (-55 dBc) but none of the currently measured very high order harmonics. The OTL is however my beloved and preferred one. Best regards Luca
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| Good work- have you read Norman Crowhurst? nt, posted on May 20, 2014 at 10:52:48 | |
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Posts: 4778
Location: Minnesota Joined: April 24, 2002 |
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| RE: Good work- have you read Norman Crowhurst? nt, posted on May 20, 2014 at 12:52:38 | |
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Of course yes! Best regards Luca
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