Feliks Audio Envy Pure Class A 300B Tube Headphone Amplifier and HIFIMAN Susvara 1 Micron Membrane Headphones Review

Perusing the literature out there, I came upon a product that immediately got my attention: Feliks Audio Envy Headphone Amplifier. It is a single-ended output design, using 300B triode tubes, with CV-181 (6SN7) dual triodes for balanced input. I had not listened to 300B tubes before, and the Envy is so beautiful, I decided I must have this thing for review. So, I contacted Alex Brinkman, who is the USA distributor for the Envy, and asked him if I could get one. He said yes, and that he would include the HIFIMAN Susvara headphones with it. I had not listened to those either, so I was doubly excited.

The Feliks Audio Envy has a metal chassis that is covered by wood all the way around. You have a choice of oak or walnut, with other woods available (rosewood would be great, in my opinion). You just need to ask them. I obtained the oak version.

Here is the walnut version:

The input section is fully balanced, using one CV-181 (6SN7) dual-triode tube for each channel. The two triodes in each tube provide the two signal paths to produce a balanced circuit. One 300B is in the output stage of each channel.

There are two editions of the Envy.

Standard Edition (6,999 € – $7,631 USD) includes:

• Electro-Harmonix 300B power tubes
• Premium copper signal wiring

Performance Edition (7,799 € – $8,504 USD) includes:

• Full Music 300B power tubes
• UPOCC (single crystal copper) signal wiring

I received the Performance Edition. A pair of Full Music 300B’s and Electro-Harmonix 300B’s were supplied for the review. Both models have solid plates as shown in the photo below (click on the photo and zoom in). The Full Music 300B is on the left, and the Electro-Harmonix 300B is on the right. Mesh-plate 300B’s are available, but they tend to have lower output voltages. There are plenty of discussions out there about the differences in sound. Most of the mesh-plate 300B’s look like solid plates that have had small holes drilled in them, but at least one company uses an actual wire mesh.

Here is the Pin-Out for a 6SN7 which is the equivalent of the CV-181:

. . . and, the Pin-Out for a 300B:

Note that the CV-181 has an indirectly-heated cathode. The heater is pins 7 and 8, while the two cathodes themselves are  pins 3 and 6. In the 300B, which is a directly-heated cathode, the cathode is pins 1 and 4. Directly heating the cathode gives more efficiency, but great care has to be taken to keep hum out of the signal because the heating voltage is in the circuit with the signal voltage. The voltage on the Anode is created by Mosfet transistors. Indirectly heating a cathode is not without its problems. Care has to be taken to keep the voltage on the heater from attracting electrons from the cathode that would otherwise be going to the plate.

Here is a photo of the Envy without the tubes (the tubes are packed separately):

The sockets at the top are for the 300B’s. There are four pins. The CV-181 sockets are at the bottom. There are eight pins. Tube sockets on the Envy are gold-plated. Tube pins are also gold-plated.

Tube shields have to be mounted before the CV-181’s are inserted, using the enclosed hex bolts and hex wrench.

The Envy has output transformers because the output impedance of the 300B is too high to connect directly to the output. But, in this case, the output transformer is a very expensive one (Lundahl) that has low signal quality loss. The main power output transformer is custom-made for Feliks Audio by Edis Ogonowski (a renowned Polish transformer manufacturer for high-end audio applications). Feliks also uses two additional Lundahl symmetrical transformers to obtain the balanced signal path.

On the front, the original “Impedance Selector” label has been changed to “Gain”, as in reality, this is more accurate; it basically limits the maximum output voltage, so for more power-hungry, demanding headphones, the higher “Gain” setting should be selected. That’s why Susvara should be used on High, while such headphones as ZMF and Sennheiser will be fine at the Low setting.

The Envy’s volume control is also a very expensive model, the Alps RK-27.

The amplifier circuit has zero feedback and is self-biasing. It uses point-to-point wiring, and the Performance Model has single-crystal copper wiring.

Electro-Harmonix 300B tubes come with the Basic Model, while the Performance Model has Full Music 300B’s. I did hear a difference as noted in the In Use section. However, the sound difference in the Full Music tubes will not be preferred by everyone, necessarily.

The rear panel is shown below (click on it to see a larger version where you can see the details):

The rear on/off toggle is on the left (photo above). Once you turn on that toggle, you can use the capacitive touch switch on the front (photo below) to turn it on for use. The on/off touch switch is above the Feliks Audio logo in the center near the front, above the volume control. This is the oak version that I had for review.

Input jacks for balanced and unbalanced headphone cables are on the left. The knobs to select Gain and Input are on the right. Click on the photo to see the large version.

HIFIMAN Susvara Headphones are planar-magnetic in design. This means there is a thin polymer membrane (in the case of the Susvara, it is less than 1 micron (one-thousandth of a millimeter – 1/26,000th of an inch) in thickness. Because this approaches the wavelength of visible light, the membrane looks like it has colors, as seen below.

The membrane is covered by a metal mesh screen on both sides. You can see it in the photos below by clicking on them to see a large version. Outside of the mesh screens are the magnets. The mesh screen and the magnets have rounded edges which minimize diffraction that would otherwise occur when the sound passes through. The membrane is coated with a very thin layer of electrically conducting material which goes positive and negative with the music signal. This produces a magnetic field that is attracted or repelled by the magnets, and this produces the sound that is heard.

To my recollection, there are a couple of ways that the electrically conducting layer can be applied. One is by sputter coating under a vacuum, which is voltage-driven, and the other is heat-driven by evaporating a known mass of metal conductor, such as copper when it is heated to the melting point under a vacuum. Knowing the mass of the melted conductor and the distance to the target (the driver membrane), one can calculate the thickness of the coating.

Most planar-magnetic headphones use a copper voice coil that is imprinted onto the membrane rather than the electrically conducting powder in the Susvara. The reason for the powder in the Susvara is to keep the mass of the membrane as low as possible. This results in the lowest distortion. It is also the reason for the high price. This is a very difficult design to manufacture.

The Susvara is very comfortable with large soft ear cups. The headband is well designed, and the cable is marked left and right, but it does not matter which headphone you insert the left or right plug; you just place the one you connected to left over the left ear.

The Susvara is an open-back headphone, meaning that sound escapes through the rear to the outside. So, your ears seal the inside compartment with the front of the membrane delivering the music. If you put your hand up close to the outside of these headphones, you can hear a difference in the sound. The advantage of open-back headphones is that the soundstage is larger. It sounds like the music is coming from all around you. The disadvantage is that external sound can be heard, and those around you can hear the music you are listening to.  For me, this is not a problem because I listen to music in quiet surroundings, not at bus stops or in coffee shops. If I am on a jet headed for vacation, I listen to music using earbuds, not headphones. Noise cancellation is a part of those electronics.

The photo below is a good one to enlarge (click on it) to see the mesh.

I tested the Feliks Audio Envy and HIFIMAN Susvara by connecting the XLR analog outputs of the Lynx sound card in my computer to the XLR inputs on the Envy. I used the balanced output of the Susvara headphones. I played albums on Qobuz music streaming and music stored on my computer. Most of the Qobuz albums were released in the past couple of months.

I found that I preferred the high impedance setting on the Envy (Impedance knob turned all the way to the right). This resulted in a lower volume control setting.

The album below is titled, “Maisie Peters”.

I did not know that John Coltrane’s wife was also a published musician (singer).

What I found is that the 300B tubes added presence (fine detail texture) that is not very prominent in solid-state circuits. The Full Music 300B’s had more of this presence than the Electro-Harmonix 300B’s. There was also more mid-bass with the Electro-Harmonix tubes.

Male and female voices were superb. The twang of guitars sang perfectly. Choir was heavenly and not congested. Jazz saxophone, Piano? OMG.

The presence was most noticeable with high-frequency instruments like violins. However, the presence enhanced the overall sound. It was not something that stuck out all by itself. The presence, I believe, is a result of the 300B’s 2nd-ordered harmonics, which you will see in the bench tests below.

The combination of Envy and Susvara was amazing. Although my other headphones, which are not nearly that expensive, sounded good with this amplifier, it would be a shame to permanently use the Envy with lesser designs, and I only mean that relatively, not pejoratively. The Feliks Audio Envy is a product that demands ultra-high quality headphones to show off its licks. That does not mean you shouldn’t use “affordable” headphones with the Envy at all. Sure, there are situations where you just want to sit down and relax with the Envy, and a pair of your other headphones are handy. You just “need” to have that exceptional pair of headphones in your kit.

There are a lot of spectra to go through, so let’s get started.

I tested the Envy with the Full Music 300B tubes first. I used a low voltage input and the volume control set to 3:00 O’Clock.

At 50 Hz (Figure 1 below), with a low-level input and an output of 1.8 Volts, THD+N was 1.5%. Notice that the 2nd-ordered harmonic is quite a bit larger (1.46%) than the 3rd (0.35%). The noise floor is very low, less than 10 μVolts (micro-volts), which is -100 dBV. In general, the noise floor stayed between -100 dBV and -120 dBV. I was surprised by this, but the Envy is a very special, unique product.

At 1 kHz, distortion was 0.57%, and again, the 2nd-ordered harmonic is the most prominent.

At 5 kHz (Figure 3 below), the 2nd-ordered harmonic continues to be the largest one. Wait until you see the Impulse Response spectrum!).

At 10 kHz, the same thing. Notice how large a difference there is between the 2nd and 3rd-ordered harmonics. This is the “presence” I was talking about.

Although you cannot hear 30 kHz or 40 kHz, those are the 2nd and 3rd-ordered harmonics of 20 kHz respectively.

Now let’s take a look at Intermodulation Distortion IMD). First, 60 Hz and 7 kHz sine waves (Figure 6 below). IMD was 1.05%.

Expanding the region of interest on the X-axis, the individual IM peaks are shown. This is a bit more than I usually see, with solid-state amplifiers.

Now, on to 19 kHz – 20 kHz (Figure 8 below). This is also an IMD test. Distortion was 0.81%, which is less than the IM at 60 Hz – 7 kHz.

Enlarging the X-axis shows the details, Figure 9 below. This is actually a bit lower than I usually see in a solid-state amplifier.

In Figure 10 below, an Impulse Response-derived Frequency Response with corresponding harmonic distortion throughout the frequency range is shown (green line above the text). The response is within about 2 dB. The response between 10 kHz and 20 kHz slopes downward. Notice that the 2nd-ordered harmonic is the largest one through the entire frequency response, and it is a lot larger. It stays between 0.1% and 1% (the top yellow line below the text; the other two lines below the text are the 3rd and 4th-ordered harmonics respectively).

I have never seen a spectrum like this before. It is what makes the 300B tube shine!  If that were the 3rd-ordered harmonic, the sound would be terrible. The fact that it is the 2nd is what makes the sound glorious. This would make an excellent main preamplifier in an audiophile-grade stereo system.

Using the Impulse Response, I also calculated the Cumulative Spectral Display (CSD), in Figure 11 below. There is a bit of resonance in the 1 kHz – 3 kHz range. That is because tubes are microphonic.

Shown below is some information about tube resonance and how it is reduced by using a tetrode:

Next, I adjusted the input to 1 Volt and the volume control set to 11:00 O’Clock, which resulted in about 4 Volts of output. The noise floor was a bit lower. Distortion was 1.17%. Even so, the 2nd-ordered harmonic was the largest. Keep in mind that even-ordered harmonics are “euphonic”, meaning that they sound pleasing. Odd-ordered harmonics sound harsh. Because of the 2nd-ordered harmonic phenomenon with a triode tube amplifier like this one, higher distortion is acceptable.

The 60 Hz – 7 kHz IMD test yielded 2.95% IMD, so it would seem that setting the output to 4 Volts is at the limits.

Here is that same spectrum with the X-axis adjusted so the IM peaks can be seen more easily.

I should point out that it is very unlikely that you would ever need 4 Volts of output into your headphones unless you are trying to damage your ears. This test is just for seeing what happens when we push the amplifier.

I found similar results using 19 kHz and 20 kHz sine waves, although IMD was lower (1.66%). However, I have seen higher IMD of this nature in solid-state power amplifiers pushed to the limits.

For the next bench tests, I then switched out the Full Music 300B’s for the Electro-Harmonix 300B’s.

This first spectrum (Figure 17 below) is with the volume control at 3:00 O’Clock as with the Full Music 300B’s. Distortion was 0.67% which is lower than with the Full Music 300B’s, and the 2nd-ordered harmonic was the largest by far, just as it was with the Full Music 300B’s. The 2nd-ordered harmonic is lower than it was with the Full Music 300B’s at this setting. Perhaps that is why the Full Music 300B’s have more “presence” in the sound.

Here is the 60 Hz – 7 kHz IMD test with the Electro-Harmonix 300B tubes (Figures 18 and 19 below). IMD was lower (1.38%) than it was with the Full Music 300B tubes with the same settings, but the voltage was higher, and that can affect the distortion percentage.

Using 60 Hz – 7 kHz, and 19 kHz, 20 kHz IMD tests at 4 volts output with the Electro-Harmonix 300B tubes, IMD was a bit higher than with the Full Music 300B tubes (Figures 20 and 21 below), but I don’t consider it significant.

The Frequency Response and Harmonic Distortion spectrum derived from the Impulse Response, using the Electro-Harmonix 300B tubes, is shown below (Figure 22). Overall, it looks about the same as with the Full Music 300B’s. It is only with the individual sine wave tests that we can distinguish the differences.

Resonance was about the same with the Electro-Harmonix 300B tubes as with the Full Music 300B tubes (Figure 23 below). Perhaps a bit more with the Electro-Harmonix 300B’s, but not significantly so. Keep in mind that the bench tests are a measurement of the entire amplifier circuit, not just the 300B’s. The tests also reflect the CV-181 tubes too.

Crosstalk for the Envy with either type of 300B tubes was about -50 dB (top right corner in the two figures shown below). Solid-state equipment can have crosstalk levels at -120 dB or so. The -50 dB crosstalk level for the Feliks Audio Envy means that a low level of each channel can be heard in the opposite channel. This helps to reduce the “hole in the middle” effect that can plague headphone listening when some of the instruments are only in one channel, like the Art Pepper album that I use in so many of my listening tests. It was recorded at the beginning of the stereo movement in the 1950’s, and recording studios often placed instruments in the left or right channels only.

The HIFIMAN Susvara headphones are among the best I have ever listened to. I think part of the reason is that the driver membrane is so thin, less than 1 micron. Human hair is about 16 – 50 microns thick. The low mass delivers movement that has almost no momentum (the product of mass and velocity), so when the positive voltage in the waveform moves the driver membrane forward and negative voltage moves it backward, the lack of significant momentum means it does not stay at the end of its travel before moving backward. If it did stay there for a short time due to momentum, that would flatten the tips of the sine wave, which mathematically deconstructs to harmonic distortion. Since this occurs only a tiny amount with such a light driver, this translates to less harmonic distortion. Momentum and Inertia are variations on the same theme. While momentum is a measurement of mass and velocity, inertia is just the amount of mass sitting there (although it is common to use the terms interchangeably). Laws of physics state that it takes force to begin moving a mass and stopping the movement. The higher the mass, the more force it takes to start it moving, stopping it, and begin moving it in the opposite direction. An ultra-low mass membrane in the HIFIMAN Susvara headphones minimizes the problems that laws of physics create.

Using the Susvara with the Feliks Audio Envy Headphone Amplifier is an unusual experience. Its < 1%, > 0.1% 2nd-ordered harmonics add some detail that is otherwise silent. For example, I was listening to an album that had the sound of wind chimes. Listening to actual chimes, I can hear a tinge of brightness, sort of a pinging sound, that is not present with amplifiers that do not have the high 2nd-ordered distortion, but it is right there with the Envy. It does not sound artificial. It sounds like the real thing. For some reason, the recording does not pick that up. So, the distortion of the 300B brings back a reality that is lost in the recording.

The Feliks Audio’s Envy Headphone Amplifier is a masterpiece. For those of you who have not heard what 300B triodes can do to audiophile sound, you don’t know what you have been missing. It has to be paired with a fine set of headphones, and the HIFIMAN Susvara satisfies that requirement. It is an expensive investment, but that is the world of the audiophile.