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Just another lateral FET amp
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Old 22nd August 2019, 06:33 PM   #121
voltwide is offline voltwide  Ireland
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Today I brought in an Audio Precision Portable One Plus. Not the bleeding edge nowadays, it was built in the 90ties. Nonetheless its generator output delivers upto 30V output level with less noise than my sound card. So I fed the amp using the AP while recording the spectrum with my soundcard at 18Vrms output into 10R - just below clipping comes in.
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Old 22nd August 2019, 06:43 PM   #122
voltwide is offline voltwide  Ireland
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Noise floor dropped below 150dB, THD=0.00027%, THD+N=0.0034%.
Feeding the output into the AP input yielded this result:
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File Type: jpg AP_THD_IMG_20190822_191854267.jpg (321.6 KB, 222 views)
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Old 22nd August 2019, 06:48 PM   #123
synonymous is offline synonymous  United States
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You are having a blast I can see
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Old 22nd August 2019, 06:50 PM   #124
voltwide is offline voltwide  Ireland
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It is a pity the AP does not offer bare THD but only THD+N. The result is 0.001% being 3times below the readout of the soundcard. This confirms lower noise of the AP - anyway THD measurements are at their limit as well.
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Old 22nd August 2019, 06:53 PM   #125
voltwide is offline voltwide  Ireland
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Quote:
Originally Posted by synonymous View Post
You are having a blast I can see
Is it the AP display you are referring to? There is a problem with the backlight as well.
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Old 22nd August 2019, 10:33 PM   #126
Mark Tillotson is offline Mark Tillotson
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BTW You can't measure noise floor from a power spectrum plot, you need the power spectral density plot for that, because noise depends on bandwidth and you have to correct to unit bandwidth and correct for the window function noise-bandwidth.
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Old 22nd August 2019, 10:55 PM   #127
voltwide is offline voltwide  Ireland
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I am aware of that. What I did is comparing both noise floors with the same setup of FFT bins. And the observation that there is more noise with the soundcard than with the AP is backed up by the THD+N numbers imho.
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Old 22nd August 2019, 11:01 PM   #128
voltwide is offline voltwide  Ireland
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The display is faint, i.e. backlight is dim. So I looked for the backlight supply inside the AP. A small 60Hz xformer delivers approx 150Vac, so I think this is ok. Or have these LCD from the nineties all been that faint and I have simply forgotten the old ones?
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Old 26th August 2019, 06:05 PM   #129
voltwide is offline voltwide  Ireland
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Meanwhile I updated the change log and reworked the pcb layout accordingly. By minimizing the length of sensitive traces I hope to improve 50Hz hum screening.
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File Type: pdf LatFET_amp_2019_08_26.pdf (88.8 KB, 65 views)
File Type: pdf changelog.pdf (17.2 KB, 30 views)
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Old 26th August 2019, 06:10 PM   #130
voltwide is offline voltwide  Ireland
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This is the actual kiCAD preview:
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File Type: png tht.png (89.1 KB, 165 views)
File Type: png smd.png (92.1 KB, 156 views)
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