The tube sound

The tube sound

Why tubes?

The theory

Some people say that tube amplifiers provide a warmer sound. It’s suggested that since tube amplifiers have their distortion primarily in the 2nd harmonic, it is more pleasing than some other electronics that might distort more in other, odd harmonics. Whether or not this is true is subject to some debate.

Modern electronics will have much lower distortion and are more accurate, so distortion from amplifiers is not generally a concern. While some might argue about difficult speaker loads, most speakers are probably not that difficult. So, why go from perfect sound to a more distorted and thus flawed sound? Only if the distorted sound has a more pleasant effect would it be preferable. This is why it becomes the subject of debate. However, I don’t see this as being any different from other enhancement devices, from 3D sound enhancers, to treble and bass controls.

Design Choices

While we have mostly moved on from using completely tube-based amplifiers, that is still an option today. However, there are a couple of choices in how one might capture some of the tube sound, while having many of the advantages of solid-state devices, such as more power.

  1. Hybrid amplifier
  2. Tube-based “buffer” pre-amp

Hybrid amps generally have tubes in the front-stage of the amp, while using solid-state for power. The tubes are there to color (a.k.a. distort) the sound. There is a lot of info. about this online. YouTube link

Instead of buying a new amplifier, you can get a taste of this with a “buffer” pre-amp. This device connects between your source device and your pre-amp, amplifier, or receiver, depending upon your setup.

FX-Audio Tube-01

There are a number of similar devices, but one the the least expensive models is made by FX-Audio. It comes with 2 vacuum tubes which can be replaced (upgraded!), as long as you choose an appropriate type. I wasn’t sure what to expect at such a low price. Tubes are part of the design, but the actual amplification is performed by a cheaper, more modern components than tubes. The first thing I did is swap out the tubes for higher-quality ones, as others had recommended. Sound output is pretty clean, without noticeable noise or hiss or other serious artifacts. How good is it really? And does it fulfill the promise of being tube-like in sound?

I decided to test using software, a sound card, and a test CD. I quickly realized that my soundcard has limitations, and can be overloaded, so I’m not able to test at full volume. This probably limits the ability to properly calculate the noise floor, so only use these graphs to get a general picture of the unit’s behavior with respect to the harmonic distortion. I don’t think the sound card can measure with enough accuracy to determine much else.

The first graphs are a 1khz tone from the CD, where the audio was recorded at -20db volume. The first graph shows the playback including the FX-Audio and the second shows the CD output without the FX-Audio. You can see that the FX-Audio version has a strong 2nd harmonic component to the signal, which we would expect to see from a tube. The CD version has a clean signal, with no noticeable harmonics.



Turning the volume/gain down on the FX-Audio, the signal is lower, of course, but so is the distortion. This means that as sounds (such as instruments) get louder, they’ll take on more of that 2nd harmonic distortion.



With a significant, but not overpowering, 2nd harmonic as the main distortion component, the FX-Audio successfully obtains at least part of the goal of a tube-like sound. Whether or not this actually sounds pleasant and is a good thing is up to you. Consider it to be a kind of effects-processor and enjoy it accordingly. Keep in mind that my results are not using the stock tubes, but replacement GE tubes.


The power supply died. I bought a replacement that can supply more amperage (but same voltage), just-in-case. The supplied power supply seemed awfully small. It’s not as great of a deal when you have to replace the PS and want to replace the tubes, but it’s still a really good value.