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Old 5th April 2013, 12:24 PM   #291
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Great tips, ZM. Thank you.

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Old 1st May 2013, 04:09 PM   #292
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I finally got around to putting together a left and right channel of the F2J amp shown in the previous post quoted here. I even set them up in a decent room at home with ok speakers. I was eager to listen so I put my chair in what I judged to be a "sweet seat" position and listened without allowing a significant period of thermal settling nor adjusting the potentiometers for objective THD performance. It was pretty good, but I wasn't entirely pleased.

Without the sophisticated equipment I have in the lab, but based on objective testing I have been doing as part of my research to write articles, I made some adjustments to the drain source voltage on each of the SJEP120R125 parts in the two F2J's to approximate the bias location of the "sweet spot" found in other SemiSouth JFETs of similar size. Then my wife called me to help her with dinner and I abandoned the project for a couple of hours. I left the dc bias on to let the amps thermally stabilize.

When I got back, I sat back down in the sweet seat and proceeded to get my first mind-blowing audio experience. Now, I will admit that I have been listening to music primarily through ear buds and auto sound systems for so many years now that I have lost a sense of what good audio reproduction sounds like. And frankly before that I always had rather low end equipment for speaker reproduction. I simply was not prepared for what I heard.

The thing that struck me, besides the fantastic resolution of even subtle instruments and vocals, was the unbelievable directionality and spatial presence of the sound. It was almost disconcerting to close my eyes and hear lead vocals directly in front of me, and then open my eyes and see nothing there. My explanation is that the phase reproduction is much more precise than anything I had previously experienced, helped, obviously, by trying to get the room acoustics favorable. But I've had similar physical acoustic surroundings before, and yet this was far superior to anything I can recall hearing.

My first thought to explain it is that while we spend a lot of attention on reducing the harmonic distortion (i.e., distortion caused by non-linear processes) we don't always talk as much about the impact of linear distortion. Phase error is an example of linear filter distortion, but it appears to me that the phase distortion of these two F2J amps was so minimal that the spatial presence of the music and vocals were positively three dimensional. I put my college-age daughter in the sweet seat and she said exactly the same thing. (Motivation: She works in my lab at the university and is assembling additional amplifiers as a training experience.)

Well, I'm really pumped about my first real listening experience with SemiSouth JFETs in a Nelson clone. More to come soon.

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Originally Posted by Semisouthfan View Post
Well folks, I have been working on Part II of "What's the Buzz?" for so long now that I'm starting to think I'm married to it. That is probably the fate of your brides too: second fiddle to a DIY Project.

Anyway, I didn't think I could deliver anything credible on the weighty topic of load-line linearization with a SemiSouth JFET (or any transistor for that matter) without building an amplifier. Jim and I spent some time this morning checking out the single-channel F2J pictured in the attached images. We both were impressed with the listening this morning; but, that shows what lightweight audiophiles we are. We were in a noisy lab with a 60-Hz power transformer suspended from the ceiling directly behind us humming away. And we were using such a cheap speaker that I will make no further mention of the sacrilege.

But, anyway, we are taking some interesting distortion data, and for that you don't need ears of gold. Plus we're going to build the second channel and take both home for some real listening.

Hope to be posting the next installment of "What's the Buzz?" soon.
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Old 1st May 2013, 04:18 PM   #293
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Sounds like you're coming down with the audiophile bug!
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Old 1st May 2013, 04:56 PM   #294
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Old 1st May 2013, 05:09 PM   #295
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Run.
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Old 1st May 2013, 05:28 PM   #296
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......
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Old 1st May 2013, 05:40 PM   #297
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Unfortunately too late.
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Old 1st May 2013, 10:03 PM   #298
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Obviously you are delusional.
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Old 1st May 2013, 10:19 PM   #299
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ONly part of the time.

Mike, is the second article about the optimization process you used in selecting your source resistors for the J2. I have a pair waiting on such a bit of advice.
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Old 2nd May 2013, 12:36 PM   #300
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The graphs I made some time back showed promissing results for Rs values in the .25-.3 ohm range.
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