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Old 10th April 2013, 01:46 AM   #4091
lhquam is offline lhquam  United States
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I am trying to understand the evolution of this thread to talking about a Balanced F6. If we return to Papa's original F6 posting it discusses the highly symmetric nature of driving the output NFETs using transformer coupling. In principle (and reality), this can greatly reduce the 2nd harmonic. In practice, getting (now unobtainium) SemiSouth JFETs that are well matched and dealing with the transformer interwinding capacitances has resulted in underwhelming F6 performance.

Now some are pursuing balanced-F6 configurations. If we wish to go balanced, then everything changes regarding to 2nd harmonic cancellation. There is absolutely no need that the positive rail output fets and their drivers be matched to those on the negative rail. What is needed for 2nd harmonic cancellation is that positive rail fets and drivers be matched on each side and negative rail fets and drivers be matched on each side. But there is no need for matching between positive and negative rail fets and drivers, other than quiescent bias current.

I have some some drawings, math, and simulations to demonstrate this.
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Old 10th April 2013, 01:56 AM   #4092
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This is how mine are arranged. I concentrated on getting lower fets to match well on both sides. The lesser quality matches went on top. They turned out to match pretty well, numbers wise. I cannot justify the balanced version in any way. May be a stupid idea. But to me, it sounds good. Would you be willing to help me take phase measurements with visual analyzer. Also how do you the test for distortion over power band. Is a signal sweep used? I wouldike to provide measured data, but only know how to test for distortion and phase at specific power level. This weekend I hope to listen to balanced f6 vs f5tv3.
Here is measurements from two channels before bridging
Attached Files
File Type: pdf 192.pdf (18.8 KB, 80 views)
File Type: pdf 192 F6.pdf (20.1 KB, 41 views)
File Type: pdf 192 F6-2.pdf (20.2 KB, 56 views)
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Old 10th April 2013, 02:20 AM   #4093
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One last irritating wuestion. Does anyone know of a quick release type setup for mounting fets. I have thought of terminal blocks, but don't know how they would be accessible. I am redesigning the balanced boards and want to include flexibility for testing/tweaking.
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Old 10th April 2013, 02:27 AM   #4094
lhquam is offline lhquam  United States
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I do not understand how the first fft plot could have THD+N = .048%. Something weird is going on there. The 2nd and 3rd plots seem to show THD+N correctly, but the 3rd harmonic seems to be unusually low, visibly absent. That would be great for in balanced amp, which could cancel the 2nd harmonics.

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Originally Posted by buzzforb View Post
This is how mine are arranged. I concentrated on getting lower fets to match well on both sides. The lesser quality matches went on top. They turned out to match pretty well, numbers wise. I cannot justify the balanced version in any way. May be a stupid idea. But to me, it sounds good. Would you be willing to help me take phase measurements with visual analyzer. Also how do you the test for distortion over power band. Is a signal sweep used? I wouldike to provide measured data, but only know how to test for distortion and phase at specific power level. This weekend I hope to listen to balanced f6 vs f5tv3.
Here is measurements from two channels before bridging
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Old 10th April 2013, 02:31 AM   #4095
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First plot is soundcard only. I bought the MAudio192 to do my measuring.
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Old 10th April 2013, 02:37 AM   #4096
lhquam is offline lhquam  United States
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Quote:
Originally Posted by buzzforb View Post
First plot is soundcard only. I bought the MAudio192 to do my measuring.
How can the THD+N measurement in the first plot be plausible?
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Old 10th April 2013, 02:58 AM   #4097
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Might've wrong. What I did was increase output of soundcard until I had 1W across 8R dummy load I then disconnected the amp and looped backed that same signal level into the soundcard with nothing attached. I assumed this would show hat the native distortion mod the soundcard was at that output level. I then hooked up the amp again, double checked power through load, and measured distortion at 1W for each channel. It looked reasonable, with biggest concern being 7k blip on both. One showed higher 4k.
When I get my new boards, ill happily run all test again I you are willing to direct me in how to do so.
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Old 10th April 2013, 03:27 AM   #4098
lhquam is offline lhquam  United States
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I might have been wrong about the THD+N figure being implausible. You have 60 Hz harmonic problems, probably due to grounding issues. I see big components at 180 Hz and 240 Hz.

I have had very good luck with minimizing 60Hz ground loop problems using the techniques shown in this thread: Balanced Soundcard Interface for Power Amplifier Distortion Measurement.


Quote:
Originally Posted by buzzforb View Post
Might've wrong. What I did was increase output of soundcard until I had 1W across 8R dummy load I then disconnected the amp and looped backed that same signal level into the soundcard with nothing attached. I assumed this would show hat the native distortion mod the soundcard was at that output level. I then hooked up the amp again, double checked power through load, and measured distortion at 1W for each channel. It looked reasonable, with biggest concern being 7k blip on both. One showed higher 4k.
When I get my new boards, ill happily run all test again I you are willing to direct me in how to do so.
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Old 10th April 2013, 06:44 AM   #4099
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Yes. That was crappy measurement rig setup. The final iteration has zero GL noise
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Old 19th April 2013, 07:21 AM   #4100
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Complete, is working. G resistance to change 1K no Self-excited.
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