Let's measure tube-buffered preamps - FX-Audio Tube 03 MKII

[CuriousAli]

Hi,

I have seen many debates about the effectiveness of vacuum tube buffered preamps on frequency response linearity and harmonic distortion.

I have the FX-Audio Tube 03 MKII which is a tube-buffered preamp if I understand correctly.
I have used SSL 12 as the Audio interface to measure, at 192kHz (10Hz to 90kHz measured), Mixer mode disabled.
FX-Audio Tube 03 MKII comes with 6K4 vacuum tubes, but I also have JAN-5654W tubes (which came with my Fosi Audio Box X4 preamp) to compare.

Output: SSL 12 Line out 3 -> TRS to RCA cable -> FX-Audio Tube 03 MKII RCA Left in
Input: FX-Audio Tube 03 MKII RCA Left out -> RCA to TRS Cable -> SSL 12 Line in 1

Frequency Response Linearity:
So first let's see the frequency response linearity of SSL 12 without using the tube preamp:

Now let's add FX-Audio Tube 03 MKII with 6K4 tubes, bass and treble controls at center:

There is some bass rolloff right?

Now to do the same with FX-Audio Tube 03 MKII with JAN-5654W tubes, bass and treble controls at center:

I think they are pretty much the same in terms of frequency response linearity, right?

Distortions:
Well, the results are terrible (or are great, depending on what are your expectations, you may want distortion).
I didn't try to get the best SINAD out of them, just tried to get their distortion levels at a similar volume level and my method might be flawed, my cables might be at fault, but the same cables were used with both tubes, so we can compare the tubes:

Here is the FX-Audio Tube 03 MKII with 6K4 tubes:

And the same with JAN-5654W tubes:

Am I right to conclude the Jan-5654W adds much less distortion and is more transparent?

But aren't both horrible in terms of distortion? Maybe I'm measuring them wrong?

 

[truth_soldier]

don't waste your time measuring and looking at charts.
what does YOUR ears tell you? that's all that matters.

which one sounds better to you, or do they sound about the same?

after all, devices with tubes in the circuits are usually not designed to maximize measured spec's, they are used for subjectively pleasing sound quality - ie, that includes distortion that some people like.

 

[CuriousAli]

don't waste your time measuring and looking at charts.
what does YOUR ears tell you? that's all that matters.

which one sounds better to you, or do they sound about the same?

after all, devices with tubes in the circuits are usually not designed to maximize measured spec's, they are used for subjectively pleasing sound quality - ie, that includes distortion that some people like.

The reason for measuring it was to see if changing tubes in tube buffered preamps also changes frequency response linearity, which it doesn't.
As for listening, I had that preamp for more than a year, every once in a while I gave it another try, it sounded interesting with some old rock songs, but bad with others, so I always shelved it.
I think 6K4 was more colorful, but of course that was a sighted test and maybe biased.
And the preamp itself added noise which wasn't appreciated, so I stopped using it.

 

[truth_soldier]

The reason for measuring it was to see if changing tubes in tube buffered preamps also changes frequency response linearity, which it doesn't.
As for listening, I had that preamp for more than a year, every once in a while I gave it another try, it sounded interesting with some old rock songs, but bad with others, so I always shelved it.
I think 6K4 was more colorful, but of course that was a sighted test and maybe biased.
And the preamp itself added noise which wasn't appreciated, so I stopped using it.

Well it's very low cost. It and various other similar preamps were/are popular with some DIY/modders who upgrade the capacitors, tubes, opamps, etc. perhaps to lower the added noise/distortion and improve the sound, on a budget. It's difficult to find a very high quality tube preamp that's not expensive.

 

[LTig]

Hi,

I have seen many debates about the effectiveness of vacuum tube buffered preamps on frequency response linearity and harmonic distortion.

I have the FX-Audio Tube 03 MKII which is a tube-buffered preamp if I understand correctly.
I have used SSL 12 as the Audio interface to measure, at 192kHz (10Hz to 90kHz measured), Mixer mode disabled.
FX-Audio Tube 03 MKII comes with 6K4 vacuum tubes, but I also have JAN-5654W tubes (which came with my Fosi Audio Box X4 preamp) to compare.

View attachment 497775

Output: SSL 12 Line out 3 -> TRS to RCA cable -> FX-Audio Tube 03 MKII RCA Left in
Input: FX-Audio Tube 03 MKII RCA Left out -> RCA to TRS Cable -> SSL 12 Line in 1

Frequency Response Linearity:
So first let's see the frequency response linearity of SSL 12 without using the tube preamp:
View attachment 497767

Now let's add FX-Audio Tube 03 MKII with 6K4 tubes, bass and treble controls at center:
View attachment 497769
There is some bass rolloff right?

Now to do the same with FX-Audio Tube 03 MKII with JAN-5654W tubes, bass and treble controls at center:
View attachment 497770
I think they are pretty much the same in terms of frequency response linearity, right?

Distortions:
Well, the results are terrible (or are great, depending on what are your expectations, you may want distortion).
I didn't try to get the best SINAD out of them, just tried to get their distortion levels at a similar volume level and my method might be flawed, my cables might be at fault, but the same cables were used with both tubes, so we can compare the tubes:

Here is the FX-Audio Tube 03 MKII with 6K4 tubes:
View attachment 497772

And the same with JAN-5654W tubes:
View attachment 497773

Am I right to conclude the Jan-5654W adds much less distortion and is more transparent?

But aren't both horrible in terms of distortion? Maybe I'm measuring them wrong?

You should limit the range of the y scale. For frequency response use +10 to -10 dB, for THD use +10 to -130 dB. This makes it much easier to spot differences and see what is important.

 

[CuriousAli]

Well it's very low cost. It and various other similar preamps were/are popular with some DIY/modders who upgrade the capacitors, tubes, opamps, etc. perhaps to lower the added noise/distortion and improve the sound, on a budget. It's difficult to find a very high quality tube preamp that's not expensive.

Makes sense.

You should limit the range of the y scale. For frequency response use +10 to -10 dB, for THD use +10 to -130 dB. This makes it much easier to spot differences and see what is important.

I see, will try it next time, thanks!

 

[mhardy6647]

I feel oddly, perhaps perversely, compelled to note that PartsExpress, for better or worse and FWIW, has these for sale on clearance at a 'reasonable'* price.

CLEARANCE FX Audio TUBE-03 Tube Preamp with RCA Input Black

FX Audio TUBE-03 Tube Preamp with RCA Input Black The FX-Audio TUBE-03 tube preamplifier delivers tube sonic character for your desktop audio system. Complete with large front panel volume, bass, and treble controls the TUBE-03 provides ergonomic control of your audio. Featuring two 6K4 tubes...

www.parts-express.com

_______________
* 'Reasonable' in the sense of what the product is, how much (i.e., how little) these cost a few years back, and the realities of world economics on the threshold of 2026.

 

[DVDdoug]

I have seen many debates about the effectiveness of vacuum tube buffered preamps on frequency response linearity and harmonic distortion.

I'm not sure why there should be a debate. There are good and bad tube amps, and tube amps that intentionally "color" (distort) the sound. Back in the tube days McIntosh had a reputation for "perfect sound" and I assume their modern tube amps are just as good as any good solid state amp.

There is no "one tube sound" so if you like that kind of thing you do have to listen and choose an amp that you like. Guitar players tend to favor tube amps for the way they sound when overdriven so usually they have their favorite guitar and their favorite amp. (But, we don't want the whole band and the singer to sound distorted like that and we try not to overdrive our hi-fi amps into distortion.)

It is more expensive to make a good tube amp than to make a good solid state amp, especially with power amps where you need an output transformer. Good audio transformers are expensive and it's hard to get the low impedance that's "natural" from solid state amplifiers. It's not particularly hard to make a good tube preamp.

A GOOD tube will be stable and stay in-spec as long as the tubes are in-spec. "Tube rolling" shouldn't make any difference as long as we are using the correct tube. Tubes do age and eventually die, but they usually stay in-sped for many years.

Transistors and MOSFETs have tolerances and vary too, but if the design is solid it doesn't matter and the amp performs as-designed.

don't waste your time measuring and looking at charts.
what does YOUR ears tell you? that's all that matters.

It depends... Of course, what we hear is ONLY thing that really matters. But casual, uncontrolled listening tests can be VERY UNREILABLE. Amir's reviews here include uncontrolled "casual" listening tests but he backs-up everything he hears with measurements. And he knows that his ears/brain can be fooled.

What is a blind ABX test? (ABX tests only confirm if there is an audible difference. They don't tell you which one is better.)
Controlled Audio Blind Listening Tests (video)

Measurements can give you a "hint" that there is or isn't an audible difference and/or they can confirm what's causing the differences you are hearing.


...IMO - Tubes are dumb! This isn't 1955 and we have better (and more economical and more energy efficient) technology.

 

[truth_soldier]

note that PartsExpress, for better or worse and FWIW, has these for sale on clearance at a 'reasonable'* price.

it's half the price on AliExpress, that's crazy cheap.

https://pt.aliexpress.com/item/32761370949.html

 

[truth_soldier]

I'm not sure why there should be a debate. There are good and bad tube amps, and tube amps that intentionally "color" (distort) the sound. Back in the tube days McIntosh had a reputation for "perfect sound" and I assume their modern tube amps are just as good as any good solid state amp.

There is no "one tube sound" so if you like that kind of thing you do have to listen and choose an amp that you like. Guitar players tend to favor tube amps for the way they sound when overdriven so usually they have their favorite guitar and their favorite amp. (But, we don't want the whole band and the singer to sound distorted like that and we try not to overdrive our hi-fi amps into distortion.)

It is more expensive to make a good tube amp than to make a good solid state amp, especially with power amps where you need an output transformer. Good audio transformers are expensive and it's hard to get the low impedance that's "natural" from solid state amplifiers. It's not particularly hard to make a good tube preamp.

A GOOD tube will be stable and stay in-spec as long as the tubes are in-spec. "Tube rolling" shouldn't make any difference as long as we are using the correct tube. Tubes do age and eventually die, but they usually stay in-sped for many years.

Transistors and MOSFETs have tolerances and vary too, but if the design is solid it doesn't matter and the amp performs as-designed.


It depends... Of course, what we hear is ONLY thing that really matters. But casual, uncontrolled listening tests can be VERY UNREILABLE. Amir's reviews here include uncontrolled "casual" listening tests but he backs-up everything he hears with measurements. And he knows that his ears/brain can be fooled.

What is a blind ABX test? (ABX tests only confirm if there is an audible difference. They don't tell you which one is better.)
Controlled Audio Blind Listening Tests (video)

Measurements can give you a "hint" that there is or isn't an audible difference and/or they can confirm what's causing the differences you are hearing.


...IMO - Tubes are dumb! This isn't 1955 and we have better (and more economical and more energy efficient) technology.

I don't disagree with much of that, but I don't think it makes sense for a device that is so low cost to waste time measuring.
In the end only your ears will tell you if it's worth keeping, modding, or throwing in the trash.
If it cost 10X more, then yeah maybe worth the time to spend measuring it as well as auditioning.

 

[NotInYourHead]

One of the essential tweaks for this pre-amp is to swap out stock tubes for GE ones. The lower distortion levels nicely explained this choice.