Reviews by Sword of Damocles
Technical specifications
Aune Audio, a Chinese company established in 2004, has gained a reputation for its interesting Class A headphone amplifiers and a few high-quality DACs. Recently, the manufacturer also released its first AR5000 over-ear headphones as a result of several years of development. For some time, I have been most interested in the S17 Pro headphone amplifier, priced at around 700 euros, whose JFET implementation differs from most other competitors under 1000 euros. Thus, I approached the manufacturer to inquire about the possibility of reviewing the S17 Pro. We quickly reached an agreement, so the device appeared at my front door earlier this year.
Before proceeding further, I will mention few words about me as reviewer. I usually write in Finnish on my headphone blog at www.kuulokenurkka.com, so Head-FI reviews are something new for me. However, I have already reviewed over 50 headphones as well as over 20 amplifiers and DACs, which makes me somewhat experienced, I guess. One could call my review style quite thorough and neutral, but I let you decide. This review is a faithful-ish translation from the original one found on my site, so I apologize any mistakes or silly language. Let’s get familiar with the S17 Pro now.
Externals
The S17 Pro is a large device, so it’s advisable to allocate sufficient space for it at your listening station. Its hefty weight of 3.5 kilograms instills confidence and signals that this is not just an ordinary headphone amplifier. The aluminum body feels extremely sturdy, and the overall impression of quality is high. It would not be easy to guess its sales price of around 700 euros, as based on its external appearance, the device could cost significantly more. I like the design of the S17 Pro, as the tilted sides and slightly curved top prevent the device from looking like a large rectangular box - on the other hand, it’s difficult to place any other device on top of it. Aune sells its amplifier in both black and silver. I chose the latter of these, as fingerprints are not visible on the silver surface, and dust is not particularly noticeable on it either. Many have certainly found the situation to be the opposite with black devices.
A large LCD display has been installed on the front of the amplifier. It indicates the selected volume, the input connector in use, the gain level, and the current mode. Interestingly, also the internal temperature can be found on the screen. All settings can be changed either by pressing the volume knob or with the included infrared remote control made from aluminum. The volume control is implemented with a 63-step electronic R2R chip, so the channel balance is always perfect. As a minor downside, however, the knob needs to be rotated quite a bit if you want to set the volume back to zero. Since the S17 Pro does not have a mute function, it might be a common action. Fortunately, adjusting is at least a bit faster with the remote control.
The amplifier features balanced 4-pin-XLR and 4.4 mm headphone outputs, in addition to a traditional 6.35 mm connector with lower power output. Otherwise, the only noteworthy thing about the outputs is the tightness of the 4.4 mm jack. Few of my cables have slippery shells, so it’s surprisingly difficult to unplug them from the S17 Pro. Based on this, the contact is likely very good. Overall, I like the 4.4 mm connection type, but there is still a lot of variation in the tolerances of female connectors. As a contrasting example, I have found the jacks on iFi’s Zen series devices to be looser than usual.
When we look behind the S17 Pro, in addition to the power input and switch, you’ll find XLR and RCA inputs as well as outputs. Therefore, the device can also be used as a preamplifier, which could very well be considered given the high-quality volume control and the excellent sound quality, which I will cover in a moment (apologies for the spoiler). However, it is quite unfortunate that Aune has not added a selector between the headphone outputs and the line output. This makes the line output always active.
Tiny holes have been made at the back of the amplifier, which the manufacturer says are sufficient for cooling, even though this is a high-power consuming and hot-running Class A implementation - we will find out more about this soon. I would have preferred the power switch to be located on the front of the device, rather than having to reach for it next to the power cord. If the S17 Pro is used as a preamplifier, for example, placed on a living room equipment rack, it may be difficult to reach the power switch. However, in desktop use, this has not bothered me much.
Internals
I was originally interested in the S17 Pro because it differs in its implementation from most modern solid-state amplifiers in its price range. There is nothing particularly surprising about a Class A structure assembled entirely from discrete components, but JFET transistors, which have tube like tone, are quite rarely used in headphone amplifiers. S17 Pro has two of these transistors, so Aune calls it Twin JFET. The amplifier has plenty of normal transistors too, and a hefty 50-watt toroidal transformer. The end result is an extremely powerful headphone amplifier that differs in sound from its competitors.
The S17 Pro offers a tremendous amount of output power, especially through its balanced headphone outputs and at its higher gain level. The device is, for example, made for low-impedance planar headphones with a hefty power budget of 7.5 watts at 32 ohms load. Even 600-ohm load has 0.45 watts on offer. The lower of the two gain levels is also quite sufficient for almost any headphones (2.77 W @ 32 ohm and 0.16 W @ 600 ohm). The unbalanced 6.35 mm output, on the other hand, is quite moderate in terms of power considering the the price of the amplifier, but it is also sufficient for conventional headphones. According to the manufacturer, the output impedance of the S17 Pro is 1 ohm, which should not affect the frequency response of the headphones.
One special feature of the amplifier is the option to select between two different current modes, which reminds me of Cayin iHA-6. In S17 Pro this affects how much current is fed to the transistors. By default, the amplifier operates in a 50 mA mode, but it can also be set to a 100 mA mode - at this point, the edges of the display change from blue to red. The internal thermometer I mentioned earlier comes in handy too, as increasing the amount of current causes the amplifier to heat up more. To prevent this from causing any problems, Aune has set a maximum temperature of 69 degrees, after which the amplifier automatically returns to the 50 mA mode. Despite its Class A operating principle, the distortion values of the S17 Pro are extremely low based on measurements, as is the background noise. The somewhat old-fashioned implementation does not seem to have any obvious disadvantage when objectively evaluated, except for higher than usual power consumption and heat production. This brings us to the cooling.
Experiences on heat build-up
The S17 Pro is the first solid-state amplifier I’ve tested that gets so hot that touching it is uncomfortable. In my opinion, the closed structure of the case has not been designed optimally, as Class A amplifiers usually have plenty of cooling holes through which warm air can escape. For example, the cover of the Rebel Amp is completely perforated, while the S17 Pro only has the minimal holes at the back. Aune has of course installed a large heat sink on both sides of the circuit board, but like the Burson Audio Soloist 3X Performance, heat could have been conducted directly to the metal body. The feet attached to the bottom of the S17 Pro are also low, so heat cannot evaporate properly through the bottom closest to the circuit board. Based on the Head-FI discussion, the amplifier also heats up differently for different users, so there may be slight variation in the installation of the heat sink.
In my case, the internal temperature never rises too high in the 50 mA mode, even though it exceeds 60 degrees in a couple of hours. However, this does not affect the operation, as the high temperature is a planned feature according to the manufacturer. On the other hand, in the 100 mA mode, the upper limit of 69 degrees is exceeded in less than 1.5 hours of use, after which the amplifier drops its current supply back to 50 mA. Unfortunately, after a certain point, the temperature does not drop much below 66 degrees, so 100 mA can only be returned temporarily for a couple of songs before the temperature rises again to 69 degrees. I often find the 100 mA mode to be slightly better in terms of sound quality, so it’s a shame that the cooling in my unit is not sufficient for the permanent use of it. This is the case also with headphones that require less power or when I am using a more moderate volume level.
For some owners of the S17 Pro, the internal temperature does not seem to ever rise above 60 degrees, so there is clearly unit variation. Many people have already developed various cooling solutions, to which I also ended up. Since I don’t want to drill holes in the case, I ended up installing the amplifier on a laptop stand, adding a 140 mm PC fan underneath it (900 RPM Noctua with 5-volt USB adapter). This setup solves the “overheating”, as the internal temperature stops at 55-60 degrees in the 100 mA mode depending on how close to the table I adjust the stand. Thus, I can use the amplifier permanently in the way I want and there is still plenty of leeway for summer use. Also, just lifting the amplifier off the table slows down the temperature rise so that I can stay in the 100 mA mode for over two hours.
I doubt that not many people are ready to build such a complex cooling solution, so as an easier means, I could recommend higher device feet. Also, ready-made laptop cooling platforms would probably work well and wouldn’t cost a fortune. If Aune ever releases a new version of the S17 Pro, I think its cooling could be improved.
Other experiences
Apart from the high operating temperature, I have nothing to complain about the amplifier. In the first production batches, the R2R volume control was apparently less linear, causing the volume to suddenly jump significantly upwards between the steps. As R2R chips have to be programmed, similar teething problems were also found in Burson Audio amplifiers, for example. Fortunately, Aune has been able to fix the issue this quickly, as I do not notice anything strange in the operation of my unit. The adjustment is smooth and there is plenty of volume range. On the other hand, with the high output power, the adjustment range may be somewhat limited when using the most sensitive in-ear-monitors, especially with the most attractive 4.4 mm output. However, with my affordable IEMs, such as the Moondrop Chu 2 and Tangzu Wan’er, the lower-powered 6.35 mm output offers surprisingly wide adjustment possibilities on the scale of 1–20 when using Tidal. I also don’t hear any background noise or hiss. Therefore, in my opinion, the S17 Pro is better suited for IEMs than, for example, the Cayin iHA-6.
I have noticed that it is worth letting the amplifier warm up for a moment before using it, as it initially sounds somewhat rough and congested. However, the sound clears up after about 20 minutes. This may be partly my imagination, but the manufacturer also mentions that the sound quality is optimal after the device has warmed up. In any case, there is nothing to lose in this matter, as the appropriate operating temperature is reached quickly.
Sound
I have compared the sound quality of the S17 Pro to the Cayin iHA-6 (only balanced headphone connector), which roughly costs the same and is also reasonably similar in operation. Another benchmark has been the Topping LA90 stereo amplifier, which represents as colorless a sound as possible, and costs about 800 €. The LA90 is designed for speakers, but due to its suitably low gain level and non-existent background noise, it can also be used with headphones. This requires an adapter cable costing at least 15 euros, which converts speaker outputs to a balanced 4-pin XLR female output. The LA90 sounds extremely neutral and controlled, so it has helped me to discover the characteristics of the Aune S17 Pro.
In my signal chain, I have used two different DACs: a neutral sounding S.M.S.L VMV D1SE and a warm sounding Schiit Audio Bifrost 2 (OG). I have listened to several different headphones (including various ZMF and Hifiman models as well as Sennheiser HD 600 and Modhouse T60RP Argon) and different music genres from the Tidal and from my computer’s FLAC library. I usually do not like to list every song or headphones one by one, as it is quite tedious. On the contrary, I test gear thoroughly until I am ready to tell something general, which does not depend on a brief moment in a song or one single headphone model. I hope at least some of you agree with me on this.
Impressively lifelike and warm experience
The S17 Pro sounds relatively neutral and even ordinary in the 50 mA power mode, so I focus more on the 100 mA mode, which I think is the absolute selling point of the device. I want to make it loud and clear, Aune has created a magically lifelike and pleasant sounding amplifier. An impressively holographic soundstage blends together with the feeling that there is a lot of warmth throughout the sound as if it were a separate element. I get goose bumps easily from realistic sounding vocals, and how present and layered the music is around me. The presentation is full-bodied and non-fatiguing, yet controlled and detailed. Actually, one of the best parts of the S17 Pro is that the warmth added to its sound does not significantly reduce the clarity and separation compared to more neutral amplifiers. Listening to music is extremely captivating, as long as the warm tone and somewhat dark treble suit your preferences.
In the 100 mA mode, the S17 Pro reproduces bass more robustly compared to the two other amplifiers I tested at the same time, but the low frequencies remain excellently controlled. The bass almost plays in its own layer in the soundstage. For example, if closed back headphones, like various ZMF models, have a somewhat boomy mid-bass, the S17 Pro puts it in check while adding even more punch to the sound. Powerful bass also brings more depth to the sound and is generally fun to listen to. The strength of the Cayin iHA-6, on the other hand, is the tightness and sharper attack of the hits, while the Topping LA90 is really controlled, so the low frequencies do not draw as much attention away from other frequencies. The S17 Pro itself becomes lighter and more neutral sounding in the 50 mA mode, but oddly enough, I don’t think the bass is as controlled as in the 100 mA mode. On the other hand, if some headphones are already thick and warm sounding, the more transparent 50 mA mode may suit them better.
The mid-frequencies are reproduced admirably rich and evocative. In this respect, the S17 Pro resembles quite a lot of warm-sounding tube amplifiers. The sound is extremely smooth and flowing in a liquid manner. The amplifier lacks most of the traditional solid-state dryness and sharpness, but clarity is still preserved. For the same reason, I also like the Garage1217 Project Polaris amplifier implemented with JFET, which, however, sounds quite unpolished and soft compared to the more expensive S17 Pro.
Like the bass frequencies, the mids also get lighter and more neutral in the 50 mA mode. The sound is more transparent, but it no longer evokes as strong feelings with its presence or sounds as spacious and layered. Compared to this, I prefer the sharper attack of the Cayin iHA-6. The Topping LA90, on the other hand, sounds airier and more distant, so its strength is rather uncolored presentation than evoking emotions. All in all, In the 100 mA mode I favor, the S17 Pro offers the most believable mids I've heard in its price class. The end result is pleasantly present, and each instrument is authentically reproduced in its own space. The soundstage also has great depth, which makes the experience even more lifelike.
Perhaps the most divisive feature in Aune's amplifier, however, is the way it reproduces treble. The high frequencies are clearly darker and calmer than usual, so it is possible to listen to the amplifier almost endlessly without any listening fatigue. Bright-sounding headphones, such as Hifimans, become easier to listen to, but on the other hand, they lose some of their energy and airiness. The subdued treble does not actually reduce the amount of detail, but the S17 Pro sounds a bit soft and rounded compared to, for example, the Cayin iHA-6. If the goal is a more energetic and thus more exciting sound, the iHA-6 might be a better option due to its tighter and more vibrant style. On the other hand, I think the S17 Pro works well with a DAC like the VMV D1SE, which adds a bit energy to the treble. For some headphones, I have found that the Bifrost 2 is an overly warm-sounding DAC, although it further enhances the pleasant characteristics of the S17 Pro.
The Topping LA90 is the most airy and spacious sounding of the three amplifiers I tried, but its soundstage remains more one-dimensional and less layered. Interestingly, the S17 Pro in the 50 mA mode does not sound much brighter than in the 100 mA mode, so I would call the amplifier warm and non-fatiguing regardless of the selected current mode. I actually feel that the transients are reproduced a bit sharper in the 100 mA mode, so the presentation feels clearer. For example, all kinds of effects and such in the background of music are more easily distinguishable than in the 50 mA mode. I would not have expected this considering how much warmth the 100 mA mode adds to the sound at the same time.
The full-bodied and authentic presentation of the S17 Pro ultimately suits my preferences well, as the amplifier gets close to its competitors in the accuracy. Sometimes using it feels almost like cheating, as it makes the headphones sound so much better than they normally do. On the other hand, the S17 Pro would not really be a reference amplifier, as it never hides its warm characteristics. All headphones sound more like the amplifier and slightly less like themselves. However, when enjoying the music, this does not bother me, as the S17 Pro is like a spell that adds emotion to the sound and makes you forget that you are listening to headphones. In a certain way, the experience resembles live music.
Preamplification
Out of curiosity, I also tried the S17 Pro as a preamplifier with the neutral Topping LA90, listening to both headphones and speakers. The neutral VMV D1SE as a DAC, I wanted to find out how much warmth the S17 Pro might add to the sound. My observations were clear.
The S17 Pro transfers most of its sound characteristics forward in the chain, as the Topping LA90 sounds noticeably fuller and more layered from its XLR outputs. The soundstage has more depth than before, and the warm tone makes everything sound pleasant. I would even say that the combination sounds more like the S17 Pro than the LA90, as the latter does not color the sound in any direction. Thus, the sound of the S17 Pro can be transferred quite directly to passive speakers with a neutral stereo amplifier and even more directly to active speakers. With headphones the combination I tried didn’t really make any sense, as the S17 Pro already includes a powerful headphone amplifier, so LA90 is not needed for that purpose.
Conclusion
I expected a lot from the S17 Pro in advance, but I was still surprised, how lifelike and pleasantly smooth it sounds, regardless of what kind of headphones I use with it. The same experience, similar to a warm hug, is also conveyed forward as a preamplifier. I don’t really understand how Aune has managed to add so much warmth to the sound without it being a result of high distortion or significantly impairing the clarity and detail of the music.
More expensive amplifiers, such as the Schiit Audio Mjolnir 3, might certainly take the listening experience even further, but at a price of 700 euros, the S17 Pro offers excellent sound, being a different kind of option to many of its competitors. The device also reacts quite well to different DACs in the signal chain, so the sound could be balanced to be a bit more neutral, if someone finds the fullness excessive. The S17 Pro is also well made, and it never runs out of power with any headphones.
The biggest weakness of the device, on the other hand, is its high heat build-up. This is not a problem for me in the winter, but summer could be something else. Unfortunately, this also means that the 100 mA mode, which I consider sounding better, may not be permanently usable without some kind of additional cooling. The amplifier could withstand a temperature of up to 80 degrees, but the maximum of 69 degrees set by Aune is too low for my unit, or the heat sink is poorly attached. Still, I was ready to solve the issue with a fan, as I consider the sound quality of the S17 Pro to be definitely worth the effort. With better luck, additional cooling might not be needed, however. In my opinion, the consumer should not even have to fix this kind of issue, so I hope Aune will find the reason why some units heat up more than the others.
Minor quirks, such as the power button located on the back or the lack of a mute function, are not things that I would not have already gotten used to with devices from other manufacturers. For beginners, I might not recommend the S17 Pro for now, but knowing the potential challenges related to high operating temperature, it would be an excellent choice for those tired of clinical sounding solid-state amplifiers.
Aune Audio, a Chinese company established in 2004, has gained a reputation for its interesting Class A headphone amplifiers and a few high-quality DACs. Recently, the manufacturer also released its first AR5000 over-ear headphones as a result of several years of development. For some time, I have been most interested in the S17 Pro headphone amplifier, priced at around 700 euros, whose JFET implementation differs from most other competitors under 1000 euros. Thus, I approached the manufacturer to inquire about the possibility of reviewing the S17 Pro. We quickly reached an agreement, so the device appeared at my front door earlier this year.
Before proceeding further, I will mention few words about me as reviewer. I usually write in Finnish on my headphone blog at www.kuulokenurkka.com, so Head-FI reviews are something new for me. However, I have already reviewed over 50 headphones as well as over 20 amplifiers and DACs, which makes me somewhat experienced, I guess. One could call my review style quite thorough and neutral, but I let you decide. This review is a faithful-ish translation from the original one found on my site, so I apologize any mistakes or silly language. Let’s get familiar with the S17 Pro now.
Externals
The S17 Pro is a large device, so it’s advisable to allocate sufficient space for it at your listening station. Its hefty weight of 3.5 kilograms instills confidence and signals that this is not just an ordinary headphone amplifier. The aluminum body feels extremely sturdy, and the overall impression of quality is high. It would not be easy to guess its sales price of around 700 euros, as based on its external appearance, the device could cost significantly more. I like the design of the S17 Pro, as the tilted sides and slightly curved top prevent the device from looking like a large rectangular box - on the other hand, it’s difficult to place any other device on top of it. Aune sells its amplifier in both black and silver. I chose the latter of these, as fingerprints are not visible on the silver surface, and dust is not particularly noticeable on it either. Many have certainly found the situation to be the opposite with black devices.
A large LCD display has been installed on the front of the amplifier. It indicates the selected volume, the input connector in use, the gain level, and the current mode. Interestingly, also the internal temperature can be found on the screen. All settings can be changed either by pressing the volume knob or with the included infrared remote control made from aluminum. The volume control is implemented with a 63-step electronic R2R chip, so the channel balance is always perfect. As a minor downside, however, the knob needs to be rotated quite a bit if you want to set the volume back to zero. Since the S17 Pro does not have a mute function, it might be a common action. Fortunately, adjusting is at least a bit faster with the remote control.
The amplifier features balanced 4-pin-XLR and 4.4 mm headphone outputs, in addition to a traditional 6.35 mm connector with lower power output. Otherwise, the only noteworthy thing about the outputs is the tightness of the 4.4 mm jack. Few of my cables have slippery shells, so it’s surprisingly difficult to unplug them from the S17 Pro. Based on this, the contact is likely very good. Overall, I like the 4.4 mm connection type, but there is still a lot of variation in the tolerances of female connectors. As a contrasting example, I have found the jacks on iFi’s Zen series devices to be looser than usual.
When we look behind the S17 Pro, in addition to the power input and switch, you’ll find XLR and RCA inputs as well as outputs. Therefore, the device can also be used as a preamplifier, which could very well be considered given the high-quality volume control and the excellent sound quality, which I will cover in a moment (apologies for the spoiler). However, it is quite unfortunate that Aune has not added a selector between the headphone outputs and the line output. This makes the line output always active.
Tiny holes have been made at the back of the amplifier, which the manufacturer says are sufficient for cooling, even though this is a high-power consuming and hot-running Class A implementation - we will find out more about this soon. I would have preferred the power switch to be located on the front of the device, rather than having to reach for it next to the power cord. If the S17 Pro is used as a preamplifier, for example, placed on a living room equipment rack, it may be difficult to reach the power switch. However, in desktop use, this has not bothered me much.
Internals
I was originally interested in the S17 Pro because it differs in its implementation from most modern solid-state amplifiers in its price range. There is nothing particularly surprising about a Class A structure assembled entirely from discrete components, but JFET transistors, which have tube like tone, are quite rarely used in headphone amplifiers. S17 Pro has two of these transistors, so Aune calls it Twin JFET. The amplifier has plenty of normal transistors too, and a hefty 50-watt toroidal transformer. The end result is an extremely powerful headphone amplifier that differs in sound from its competitors.
The S17 Pro offers a tremendous amount of output power, especially through its balanced headphone outputs and at its higher gain level. The device is, for example, made for low-impedance planar headphones with a hefty power budget of 7.5 watts at 32 ohms load. Even 600-ohm load has 0.45 watts on offer. The lower of the two gain levels is also quite sufficient for almost any headphones (2.77 W @ 32 ohm and 0.16 W @ 600 ohm). The unbalanced 6.35 mm output, on the other hand, is quite moderate in terms of power considering the the price of the amplifier, but it is also sufficient for conventional headphones. According to the manufacturer, the output impedance of the S17 Pro is 1 ohm, which should not affect the frequency response of the headphones.
One special feature of the amplifier is the option to select between two different current modes, which reminds me of Cayin iHA-6. In S17 Pro this affects how much current is fed to the transistors. By default, the amplifier operates in a 50 mA mode, but it can also be set to a 100 mA mode - at this point, the edges of the display change from blue to red. The internal thermometer I mentioned earlier comes in handy too, as increasing the amount of current causes the amplifier to heat up more. To prevent this from causing any problems, Aune has set a maximum temperature of 69 degrees, after which the amplifier automatically returns to the 50 mA mode. Despite its Class A operating principle, the distortion values of the S17 Pro are extremely low based on measurements, as is the background noise. The somewhat old-fashioned implementation does not seem to have any obvious disadvantage when objectively evaluated, except for higher than usual power consumption and heat production. This brings us to the cooling.
Experiences on heat build-up
The S17 Pro is the first solid-state amplifier I’ve tested that gets so hot that touching it is uncomfortable. In my opinion, the closed structure of the case has not been designed optimally, as Class A amplifiers usually have plenty of cooling holes through which warm air can escape. For example, the cover of the Rebel Amp is completely perforated, while the S17 Pro only has the minimal holes at the back. Aune has of course installed a large heat sink on both sides of the circuit board, but like the Burson Audio Soloist 3X Performance, heat could have been conducted directly to the metal body. The feet attached to the bottom of the S17 Pro are also low, so heat cannot evaporate properly through the bottom closest to the circuit board. Based on the Head-FI discussion, the amplifier also heats up differently for different users, so there may be slight variation in the installation of the heat sink.
In my case, the internal temperature never rises too high in the 50 mA mode, even though it exceeds 60 degrees in a couple of hours. However, this does not affect the operation, as the high temperature is a planned feature according to the manufacturer. On the other hand, in the 100 mA mode, the upper limit of 69 degrees is exceeded in less than 1.5 hours of use, after which the amplifier drops its current supply back to 50 mA. Unfortunately, after a certain point, the temperature does not drop much below 66 degrees, so 100 mA can only be returned temporarily for a couple of songs before the temperature rises again to 69 degrees. I often find the 100 mA mode to be slightly better in terms of sound quality, so it’s a shame that the cooling in my unit is not sufficient for the permanent use of it. This is the case also with headphones that require less power or when I am using a more moderate volume level.
For some owners of the S17 Pro, the internal temperature does not seem to ever rise above 60 degrees, so there is clearly unit variation. Many people have already developed various cooling solutions, to which I also ended up. Since I don’t want to drill holes in the case, I ended up installing the amplifier on a laptop stand, adding a 140 mm PC fan underneath it (900 RPM Noctua with 5-volt USB adapter). This setup solves the “overheating”, as the internal temperature stops at 55-60 degrees in the 100 mA mode depending on how close to the table I adjust the stand. Thus, I can use the amplifier permanently in the way I want and there is still plenty of leeway for summer use. Also, just lifting the amplifier off the table slows down the temperature rise so that I can stay in the 100 mA mode for over two hours.
I doubt that not many people are ready to build such a complex cooling solution, so as an easier means, I could recommend higher device feet. Also, ready-made laptop cooling platforms would probably work well and wouldn’t cost a fortune. If Aune ever releases a new version of the S17 Pro, I think its cooling could be improved.
Other experiences
Apart from the high operating temperature, I have nothing to complain about the amplifier. In the first production batches, the R2R volume control was apparently less linear, causing the volume to suddenly jump significantly upwards between the steps. As R2R chips have to be programmed, similar teething problems were also found in Burson Audio amplifiers, for example. Fortunately, Aune has been able to fix the issue this quickly, as I do not notice anything strange in the operation of my unit. The adjustment is smooth and there is plenty of volume range. On the other hand, with the high output power, the adjustment range may be somewhat limited when using the most sensitive in-ear-monitors, especially with the most attractive 4.4 mm output. However, with my affordable IEMs, such as the Moondrop Chu 2 and Tangzu Wan’er, the lower-powered 6.35 mm output offers surprisingly wide adjustment possibilities on the scale of 1–20 when using Tidal. I also don’t hear any background noise or hiss. Therefore, in my opinion, the S17 Pro is better suited for IEMs than, for example, the Cayin iHA-6.
I have noticed that it is worth letting the amplifier warm up for a moment before using it, as it initially sounds somewhat rough and congested. However, the sound clears up after about 20 minutes. This may be partly my imagination, but the manufacturer also mentions that the sound quality is optimal after the device has warmed up. In any case, there is nothing to lose in this matter, as the appropriate operating temperature is reached quickly.
Sound
I have compared the sound quality of the S17 Pro to the Cayin iHA-6 (only balanced headphone connector), which roughly costs the same and is also reasonably similar in operation. Another benchmark has been the Topping LA90 stereo amplifier, which represents as colorless a sound as possible, and costs about 800 €. The LA90 is designed for speakers, but due to its suitably low gain level and non-existent background noise, it can also be used with headphones. This requires an adapter cable costing at least 15 euros, which converts speaker outputs to a balanced 4-pin XLR female output. The LA90 sounds extremely neutral and controlled, so it has helped me to discover the characteristics of the Aune S17 Pro.
In my signal chain, I have used two different DACs: a neutral sounding S.M.S.L VMV D1SE and a warm sounding Schiit Audio Bifrost 2 (OG). I have listened to several different headphones (including various ZMF and Hifiman models as well as Sennheiser HD 600 and Modhouse T60RP Argon) and different music genres from the Tidal and from my computer’s FLAC library. I usually do not like to list every song or headphones one by one, as it is quite tedious. On the contrary, I test gear thoroughly until I am ready to tell something general, which does not depend on a brief moment in a song or one single headphone model. I hope at least some of you agree with me on this.
Impressively lifelike and warm experience
The S17 Pro sounds relatively neutral and even ordinary in the 50 mA power mode, so I focus more on the 100 mA mode, which I think is the absolute selling point of the device. I want to make it loud and clear, Aune has created a magically lifelike and pleasant sounding amplifier. An impressively holographic soundstage blends together with the feeling that there is a lot of warmth throughout the sound as if it were a separate element. I get goose bumps easily from realistic sounding vocals, and how present and layered the music is around me. The presentation is full-bodied and non-fatiguing, yet controlled and detailed. Actually, one of the best parts of the S17 Pro is that the warmth added to its sound does not significantly reduce the clarity and separation compared to more neutral amplifiers. Listening to music is extremely captivating, as long as the warm tone and somewhat dark treble suit your preferences.
In the 100 mA mode, the S17 Pro reproduces bass more robustly compared to the two other amplifiers I tested at the same time, but the low frequencies remain excellently controlled. The bass almost plays in its own layer in the soundstage. For example, if closed back headphones, like various ZMF models, have a somewhat boomy mid-bass, the S17 Pro puts it in check while adding even more punch to the sound. Powerful bass also brings more depth to the sound and is generally fun to listen to. The strength of the Cayin iHA-6, on the other hand, is the tightness and sharper attack of the hits, while the Topping LA90 is really controlled, so the low frequencies do not draw as much attention away from other frequencies. The S17 Pro itself becomes lighter and more neutral sounding in the 50 mA mode, but oddly enough, I don’t think the bass is as controlled as in the 100 mA mode. On the other hand, if some headphones are already thick and warm sounding, the more transparent 50 mA mode may suit them better.
The mid-frequencies are reproduced admirably rich and evocative. In this respect, the S17 Pro resembles quite a lot of warm-sounding tube amplifiers. The sound is extremely smooth and flowing in a liquid manner. The amplifier lacks most of the traditional solid-state dryness and sharpness, but clarity is still preserved. For the same reason, I also like the Garage1217 Project Polaris amplifier implemented with JFET, which, however, sounds quite unpolished and soft compared to the more expensive S17 Pro.
Like the bass frequencies, the mids also get lighter and more neutral in the 50 mA mode. The sound is more transparent, but it no longer evokes as strong feelings with its presence or sounds as spacious and layered. Compared to this, I prefer the sharper attack of the Cayin iHA-6. The Topping LA90, on the other hand, sounds airier and more distant, so its strength is rather uncolored presentation than evoking emotions. All in all, In the 100 mA mode I favor, the S17 Pro offers the most believable mids I've heard in its price class. The end result is pleasantly present, and each instrument is authentically reproduced in its own space. The soundstage also has great depth, which makes the experience even more lifelike.
Perhaps the most divisive feature in Aune's amplifier, however, is the way it reproduces treble. The high frequencies are clearly darker and calmer than usual, so it is possible to listen to the amplifier almost endlessly without any listening fatigue. Bright-sounding headphones, such as Hifimans, become easier to listen to, but on the other hand, they lose some of their energy and airiness. The subdued treble does not actually reduce the amount of detail, but the S17 Pro sounds a bit soft and rounded compared to, for example, the Cayin iHA-6. If the goal is a more energetic and thus more exciting sound, the iHA-6 might be a better option due to its tighter and more vibrant style. On the other hand, I think the S17 Pro works well with a DAC like the VMV D1SE, which adds a bit energy to the treble. For some headphones, I have found that the Bifrost 2 is an overly warm-sounding DAC, although it further enhances the pleasant characteristics of the S17 Pro.
The Topping LA90 is the most airy and spacious sounding of the three amplifiers I tried, but its soundstage remains more one-dimensional and less layered. Interestingly, the S17 Pro in the 50 mA mode does not sound much brighter than in the 100 mA mode, so I would call the amplifier warm and non-fatiguing regardless of the selected current mode. I actually feel that the transients are reproduced a bit sharper in the 100 mA mode, so the presentation feels clearer. For example, all kinds of effects and such in the background of music are more easily distinguishable than in the 50 mA mode. I would not have expected this considering how much warmth the 100 mA mode adds to the sound at the same time.
The full-bodied and authentic presentation of the S17 Pro ultimately suits my preferences well, as the amplifier gets close to its competitors in the accuracy. Sometimes using it feels almost like cheating, as it makes the headphones sound so much better than they normally do. On the other hand, the S17 Pro would not really be a reference amplifier, as it never hides its warm characteristics. All headphones sound more like the amplifier and slightly less like themselves. However, when enjoying the music, this does not bother me, as the S17 Pro is like a spell that adds emotion to the sound and makes you forget that you are listening to headphones. In a certain way, the experience resembles live music.
Preamplification
Out of curiosity, I also tried the S17 Pro as a preamplifier with the neutral Topping LA90, listening to both headphones and speakers. The neutral VMV D1SE as a DAC, I wanted to find out how much warmth the S17 Pro might add to the sound. My observations were clear.
The S17 Pro transfers most of its sound characteristics forward in the chain, as the Topping LA90 sounds noticeably fuller and more layered from its XLR outputs. The soundstage has more depth than before, and the warm tone makes everything sound pleasant. I would even say that the combination sounds more like the S17 Pro than the LA90, as the latter does not color the sound in any direction. Thus, the sound of the S17 Pro can be transferred quite directly to passive speakers with a neutral stereo amplifier and even more directly to active speakers. With headphones the combination I tried didn’t really make any sense, as the S17 Pro already includes a powerful headphone amplifier, so LA90 is not needed for that purpose.
Conclusion
I expected a lot from the S17 Pro in advance, but I was still surprised, how lifelike and pleasantly smooth it sounds, regardless of what kind of headphones I use with it. The same experience, similar to a warm hug, is also conveyed forward as a preamplifier. I don’t really understand how Aune has managed to add so much warmth to the sound without it being a result of high distortion or significantly impairing the clarity and detail of the music.
More expensive amplifiers, such as the Schiit Audio Mjolnir 3, might certainly take the listening experience even further, but at a price of 700 euros, the S17 Pro offers excellent sound, being a different kind of option to many of its competitors. The device also reacts quite well to different DACs in the signal chain, so the sound could be balanced to be a bit more neutral, if someone finds the fullness excessive. The S17 Pro is also well made, and it never runs out of power with any headphones.
The biggest weakness of the device, on the other hand, is its high heat build-up. This is not a problem for me in the winter, but summer could be something else. Unfortunately, this also means that the 100 mA mode, which I consider sounding better, may not be permanently usable without some kind of additional cooling. The amplifier could withstand a temperature of up to 80 degrees, but the maximum of 69 degrees set by Aune is too low for my unit, or the heat sink is poorly attached. Still, I was ready to solve the issue with a fan, as I consider the sound quality of the S17 Pro to be definitely worth the effort. With better luck, additional cooling might not be needed, however. In my opinion, the consumer should not even have to fix this kind of issue, so I hope Aune will find the reason why some units heat up more than the others.
Minor quirks, such as the power button located on the back or the lack of a mute function, are not things that I would not have already gotten used to with devices from other manufacturers. For beginners, I might not recommend the S17 Pro for now, but knowing the potential challenges related to high operating temperature, it would be an excellent choice for those tired of clinical sounding solid-state amplifiers.
Sword of Damocles
@RotundCatto As there isn't any clear measure for the amount of detail, I cannot really say anything for sure. However, the more energetic high frequencies of iHA-6 makes it sound somewhat more transparent and maybe more detailed at first. On the other hand, Cayin amp has a flatter and less holographic soundstage, so S17 Pro in the 100 mA mode reveals more spatial detail and might be a bit better after all. I would additionally call its soundstage better for my taste because of that, but let's not forget that the unbalanced headphone outputs of iHA-6 suit 300 ohm headphones very well making them wider sounding. iHA-6 has more options for different headphone types, but it never quite sounds as 3D-like and immersive as S17 Pro in the 100 mA. The 50 mA is more similar with iHA-6.
Sword of Damocles
@RotundCatto Dynamics are harder to compare, as S17 Pro sounds fuller that way more weighty when called for. iHA-6 on the other hand has faster and sharper attack, which I usually find more dynamic sounding. Overall, both styles have their own advantage, so I let you decide, which you might prefer. Thanks for the good questions, as many times it is easier to describe something when somebody asks something specific.
