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Old 6th March 2014, 04:32 AM   #51
lhquam is offline lhquam  United States
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I did another set of harmonic distortion measurements of 3 different front-ends for the F4. The front-ends are:
  • BA3-FE
  • Impasse
  • Curly-F4
Below are the spectrum plots. I am not sure why my BA3-FE implementation has such high H3. The MOSFET bias currents are 50ma as per spec. The MOSFETs are FQP3P20 and FQP3N30. My F4 power supply voltage is a bit low at 22.5V.

The Impasse and Curly-F4 sound much better than the BA3-FE. I currently give the Impasse the edge on sound quality.
Attached Images
File Type: jpg Impasse-swap-F4build1-20140304-sweeps2.jpg (235.4 KB, 451 views)
File Type: jpg CurlyCD-F4CD-Assembled-H2tweak-sweeps.jpg (228.8 KB, 435 views)
File Type: jpg BA3-F4build1-20140305-sweeps1.jpg (239.9 KB, 422 views)
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Old 7th March 2014, 06:07 AM   #52
rolandong is offline rolandong  Philippines
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Wonder if someone who has heard Impasse vs. F6 compare.
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Old 8th March 2014, 09:12 AM   #53
Sky63 is offline Sky63  Czech Republic
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I have a big request - can anyone compare the sound Aleph J and F4? I'm thinking about building and I cannot decide. Single-ended mode – yes, or not? I am only interested about sound description, not power or something else. Thank you.

Last edited by Sky63; 8th March 2014 at 09:15 AM.
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Old 8th March 2014, 09:22 AM   #54
Zen Mod is offline Zen Mod  Serbia
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Aleph J is amp with gain , so having some of own sound signature

F4 is power buffer , without voltage gain ....... and completely embracing sound signature of preceding voltage gain stage (preamp)

so - direct comparison isn't possible .......... while judging Aleph J on it's own , you need to judge preamp/F4 combo
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Old 12th March 2014, 11:04 PM   #55
gl is offline gl  United States
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Hi Ihquam,

Those graphs represent a serious amount of work. Thank you for posting them.

I am building a BA-3B + F4CR and I have to say that your results are somewhat discouraging.

I'm using the 313/2013 pair and I will place my faith in the hope that they will improve the sound over the Fairchild parts.

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Old 30th March 2014, 10:49 PM   #56
lhquam is offline lhquam  United States
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Default A/B listening tests

This post reports A/B testing of the Impasse+F4 vs. Curly+F4. The Curly gain stage is a variant of the LSK Pre without an output buffer, intended to drive an uncrippled F4.

I recently finished my A/B preamp+amp testing setup. It consists of two DPDT 24V DC relays to switch the speakers between two amplifier outputs, and a pair of trimmer pot attenuators for the inputs to one of the preamps (the Impasse in this case). The relays are controlled from the listening position with SPST switch at the end of a 16 foot telephone extension cable. The trimmers were carefully adjusted for identical amp outputs at 1kHz. With the gains balanced, flipping the switch is virtually undetectable.

I have listened for many hours to a variety of music and cannot repeatedly detect any differences between the Impasse+F4 and the Curly+F4.

I would like to do similar tests using my STAX Lambda Pro headphones without the F4 amps to better isolate differences between the two voltage gain stages. This could be done by using a stereo RCA switch box to select between the two front ends, and attenuating the output to the STAX by about 10:1.
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