Fever FE-6J1-2 tube preamplifier kit review + listening test files

[pma]

Thank you, Daniel. Shall I make a follow-up? Some files recorded from this preamp output and you guys will try to hear a difference? A kind of fun on “audibility thresholds” . Something different from that DAC x DAC mania.

 

[DanielT]

Thank you, Daniel. Shall I make a follow-up? Some files recorded from this preamp output and you guys will try to hear a difference? A kind of fun on “audibility thresholds” . Something different from that DAC x DAC mania.

Yes, absolutely. It will be interesting to test.

 

[pma]

Yes, absolutely. It will be interesting to test.

OK. It has been prepared.

Test - Topping D10s/Cosmos ADC loop vs. Tube preamp (6J1) in the loop

link to the test files:

6J1_test.zip

drive.google.com

Please feel free to try the test and tell me if you hear the difference, and do not forget to post foobar abx test report.
I have also included a delta file (Deltawave difference) for you to hear how horrible is the "sound" of tube distortion in isolation. But, can you hear it when playing music?

I have also created an on-line version of the test:

6J1 tube preamp vs. Topping DAC/Cosmos ADC loop · FunkyABX

abx.funkybits.fr

 

[solderdude]

The one time the comparator did not 'crash' (volume clipping and one time playing different tracks together) Linux mint, firefox. I got 5 out of 10.
Thought the high distortion and hum would be easily audible.

There obviously is not a clear 'tubey sound'' sound for me.

 

[pma]

It only confirms that audiophiles debate about nothing. Those DAC sounds. And it is not much better here. But there must be a “topic”, right? Cables, parts, DACs, and all that stuff. Differences impossible to hear are discussed up and down. Relevant data are not posted. Audio Science? Really?

 

[KSTR]

There is a temptation to create a listening test with files recorded through this preamp, so here we go:

Test - Topping D10s/Cosmos ADC loop vs. Tube preamp (6J1) in the loop

link to the test files:

6J1_test.zip​

Pavel, did you correct for the polarity inversion of the tube preamp?

 

[DanielT]

OK. It has been prepared.

Test - Topping D10s/Cosmos ADC loop vs. Tube preamp (6J1) in the loop

link to the test files:

6J1_test.zip

drive.google.com

Please feel free to try the test and tell me if you hear the difference, and do not forget to post foobar abx test report.
I have also included a delta file (Deltawave difference) for you to hear how horrible is the "sound" of tube distortion in isolation. But, can you hear it when playing music?

I have also created an on-line version of the test:

6J1 tube preamp vs. Topping DAC/Cosmos ADC loop · FunkyABX

abx.funkybits.fr

View attachment 552034

Thanks for creating the test, pma. So far I don't hear any difference, with speakers I would say. I'll listen more intensively with headphones and see what happens.
However the delta file sounded awful as you mentioned.

But why 6J1-2 vacuum tubes? They are designed for VHF/UHF radio as far as I understand. Why not use the classic vacuum tube 12AX7 (ECC83)?
As far as I can see it was designed for audio:

12AX7 - Wikipedia

en.wikipedia.org

Is there that big of a price difference between 6J1-2 and 12AX7? I mean a little more money and maybe it could have been an ok tube pre amp? If the right type of vacuum tube was used that is. Perhaps a new design was needed in that case? Which probably mean a different pre amplifier, so it would be a case of comparing apples and oranges.
(Sorry for answering my own question )

 

[mhardy6647]

Thanks pma. I've seen worse, I guess. Seems odd they used pentodes triode-strapped but didn't use twin triode tubes and buffer the outputs with cathode followers.

Pretty sure the driver is cost. ThIs cadre of products (bargain priced "linestages") uses tubes available in large quantities and very inexpensively.

 

[fpitas]

Pretty sure the driver is cost. ThIs cadre of products (bargain priced "linestages") uses tubes available in large quantities and very inexpensively.

For $18 the profit margin can't be high.

 

[mhardy6647]

For $18 the profit margin can't be high.

At retail, in 2026, one would be hard pressed to buy a single 12AX7 for that sum!

 

[Ropeburn]

.But why 6J1-2 vacuum tubes? They are designed for VHF/UHF radio as far as I understand. Why not use the classic vacuum tube 12AX7 (ECC83)?
As far as I can see it was designed for audio:

12AX7 - Wikipedia

en.wikipedia.org

Is there that big of a price difference between 6J1-2 and 12AX7? I mean a little more money and maybe it could have been an ok tube pre amp? If the right type of vacuum tube was used that is. Perhaps a new design was needed in that case? Which probably mean a different pre amplifier, so it would be a case of comparing apples and oranges.
(Sorry for answering my own question )

Numbers and price really. Those HF types were made by the millions and cost virtually nothing. As opposed to 12AX7/ECC83: Those cost as much as the kit, each.

 

[pma]

For $18 the profit margin can't be high.

Yeah. Seems to me all these AliExpress and similar pizza lunch cost products must be funded. Fund and destroy competition, but cooperate at local level.

Some more images, including distortion in the tube path used for these test files. The hum that I have complained about is audible in fact only without music signal, or in extremely silent passages at high (unrealistic) volume setting. Otherwise we have ear masking. Ear masking works to mask non-linear distortion as well. Please note I am saying non-linear, not a mere "harmonic".
The woman voice used in the music sample is very sensitive to distortion - as we can hear from the delta file. Still - who can hear it when comparing the music files??

 

[Ropeburn]

So, yet another cheap distortion generator... interesting. You know I'm always looking for distortion, so thanks for the test.

Cheap enough to just get it for that. Shouldn't be hard to modify, set gain to ludicrous amounts, add attenuator at the output, done. Sounds like a fun project!

 

[Mike123]

Test results - 6J1 tube preamp vs. Topping DAC/Cosmos ADC loop · FunkyABX

abx.funkybits.fr

The link to the results doesn't work. It returns the home page.

https://cdn.abx.funkybits.fr/img/results/EwwXnuaWy7Q8sVZxxNLv7r/Fvo4QNgrHaJjib7RE6qnm2.png

 

[pma]

Test results - 6J1 tube preamp vs. Topping DAC/Cosmos ADC loop · FunkyABX

abx.funkybits.fr

https://cdn.abx.funkybits.fr/img/results/EwwXnuaWy7Q8sVZxxNLv7r/Fvo4QNgrHaJjib7RE6qnm2.png

Thank you! One more brave man.

 

[Mike123]

Thank you! One more brave man.

It's not a matter of brave, but laziness . I don't have golden ears, and I'm not afraid of being "deaf." Creating a website has made things a lot easier. That's why.

 

[DanielT]

At retail, in 2026, one would be hard pressed to buy a single 12AX7 for that sum!

Numbers and price really. Those HF types were made by the millions and cost virtually nothing. As opposed to 12AX7/ECC83: Those cost as much as the kit, each.

That explains it. On the other hand, it doesn't explain whether there would be any demand for a better, more expensive tube preamp kit? I suspect, however, that such kits do exist within more hardcore tube amp DIY circles. The demand might be there, even if it isn't huge?

OT: Michael Beeny has designed his own preamp that looks really good. It’s sold as a kit, by the way:

Spoiler: This one: The FCA-001A Preamp

 

[Janfi67]

Okay, my ears aren't exactly young anymore, 60+, even though my last audiogram suggested they look like a bit younger than that. They’re completely untrained, and I’m using Bluetooth/LDAC (32-bit/96KHz) to feed a DAC and headphones from an Android tablet, perhaps not the best combo.

But to my great surprise, I can't tell the difference between the two recordings
I didn't even try to answer, it would have been a total shot in the dark.

Anyway, thanks for the test and for the musical choice @pma

 

[fpitas]

We've gotten spoiled lately because the latest SS amps are so good, but it doesn't surprise me that smallish amounts of distortion aren't audible, except under special conditions.

 

[DonH56]

I tried and failed to find the test files I created a long time ago with added distortion. From memory, for a pure 1 kHz sine wave, virtually nobody detected 0.1% (-60 dB) distortion, most distinguished 1% (-40 dB), and essentially everyone 10% (-20 dB). Move to two/multi-tones or music, and most could not tell at 1%, and even 10% was harder.