 My reference set of one has been upgraded with the Air Acoustics AX520 amplifier. What is this amp? Why was it chosen and why did the set of one need an update? Confused? You won't be after the next episode of the Hans Beekhuyzen channel. Let me start with the amp it replaces. The Audio Note Soro SE Phono has been with me for 20 years but has in the meantime been upgraded three times by AudioMagic Peter van Wilenswaart. I had the pleasure of working with Peter for five years in the mid 80s after which he went through the local Audio Note distributor. He then founded AudioMagic and was one of the founders of Grim Audio. Over the years he has improved my Audio Note by replacing the potential meters, replacing the lint cable between the input board and the preamp by AudioMagic's silver disabling the tape and pre outputs, replacing all electrolytic capacitors at critical places by Jensen audio grade types and since audio equipment makes long hours at my place, tubes are replaced every four to five years. Last year new Dongsol ECC82 new old stock preamp tubes and Softek 5881 power tubes are installed. On my request Peter also created a power amp input that is selected by a 12v trigger signal from the external source. I have this to connect the left and right analog outputs of my AV receiver to. So when watching TV or movies the left and right channel are played over what normally is my stereo setup. The true qualities of my Audio Note are transparency, especially in the mid range and a wide and deep stereo image with proper placement. That makes reviewing equipment debris, especially when I had limited myself to equipment up to 2000 Euros. Encouraged by you, my viewers, that limit was abandoned. Even higher quality DACs visited the test bench and although still quality differences were audible, for efficient working I needed an even better amplifier. So I decided to replace it and I am sure I will find a good home where its qualities will be appreciated. So exit Audio Note, welcome air. This is of course kicking in an open door as the Dutch expression goes. But this chapter is widely appreciated so let's not skip it. A set of loudspeakers need to be connected. In my case these are the well respected audio physics Scorpios. Then only sources with analog outputs can be connected, like a CD player, a streamer or tuner. Volume, input selection and power can be controlled from the included sturdy infrared remote. In my set of one the MiTech Brooklyn DAC is connected over SPDIF to the Mini DSB SHD Studio Dirac processor that in turn is connected digitally to the Auralic RSG2 network player. For TV and film viewing the NAD T758 Version 3 surround receiver has its left and right pre-outs connected to one set of single ended inputs on the air over RCA cables. This way the left and right channels are reproduced by the stereo setup. The center and surround speakers are connected to the corresponding speaker terminals on the surround receiver. Dirac, at full bandwidth setting, not only solves room modes but also sound character differences between the speakers. Works like a charm. I must admit that I am far less critical when sound for video is concerned. On shows air normally shows its silver models but I have chosen the black version. Both me and my wife don't like shiny metal apparatus in our living room and that is where my set of one is placed. The AX520 measures 440 x 480 x 120mm and weighs a hefty 21 kilos. The front has only four controls, a volume control, a standby button, in the middle the dimmable display, then the tape output button and the input selector. All buttons have secondary functions like naming inputs, setting processor pass-through and more. To do this you really need to have the manual at hand since it involves irrational button sequences. On the rear in the middle the IEC mains input with above it the power switch. Below it two RJ45 connectors to integrate control with other air products. On both sides four balanced analog inputs on XLR and two single ended inputs on RCA. There even is a set of tape outputs which shows the age of the initial design a bit. The speaker cables are clamped in these binding posts that are optimized for forks. There are no digital inputs nor is there a phono input. Those sources require additional gear like a dyke and a phono pre pre. With the lid taken off we see the centrally placed large mercury magnetics transformer. Apart from a part number there was no further specification. It feeds the mirrored layout power supply for left and right channel on the board in front of it. As far as I can see the circuit board mounted against the front panel holds the controller electronics. Then two large heat sinks hide the amplifier boards. Against the back panel the input boards are mounted with in between it the motorized volume control on which later on more. This controlled over a flat cable coming from the controller board. Time for the real $10,000 question, why this amp? Of course the first argument was sound quality. I have been following the brand for decades now. In my country Michael Halver represents air, already at the previous distributor and now at reference sounds a company he leads. His demos at shows and in his showroom are always at 99.9% of what is achievable. I had also lent an earlier air amplifier from him years ago. What I like especially is the time integrity and transparency over the entire frequency range. A good upright bass recording sounds so open and natural and the same goes for a glockenspiel. Strings are not velvety or harsh, they just are as they should be. It doesn't impress in a cheap way but with a lot of class. I did of course do extensive research on what my respected colleagues think of the AX520. I don't review amplifiers so for me it is hard to say if this is the best choice. So don't see this as a review, it's rather an explanation on why I decided for this amp. Eracoustics is somewhat a quirky company. All their products are balanced, used no feedback and used so called diamond circuits where possible, even in their power amps. And that's just the beginning, but let's take it step by step and start with a simplified explanation of the benefit of balanced circuits. Analog audio is nothing more than a varying voltage over time. In single ended designs this voltage varies between 0 volts and for instance 2 volts as with Redbook spec CD players. There is the ground, the tulip shaped contact on the RCA plug and the positive, the tip of the RCA and the voltage thus varies between 0 and 2 volts like this. The amplitude of the signal is the voltage between the ground plane and the top of the signal which at the blue arrow is 2 volts and at the red arrow 0 volts. When there is only a slight electrical pollution on the ground plane it will superimpose on the audio signal and cause distortion. If the ground plane is offset by a ground loop it will affect the audio signal as well. When we look at a symmetrical signal we see the ground level halfway the plus 2 volts and minus 2 volts. So the voltage is double that of a single ended signal according to Redbook specifications. Which is already nice. But when we look what happens when the ground plane is polluted we see that there is no direct influence on the audio signal since there is reference between plus 2 and minus 2 volts and not to the ground plane. That doesn't mean other things might not get wrong but the starting point at least is better. Air acoustics also adheres to zero feedback design. Negative feedback is used in most electronics to keep distortion low in the circuit stable. To explain negative feedback the layman's way you could compare it to riding a bike. Ideally you would keep your steers straight ahead but since you always cause some imbalance you automatically make small steering corrections to restore balance. Amplifiers that use feedback, and again most of them do, correct small errors this way. Let's see how that looks in a block diagram. There is an input signal that enters the amplifier where it is amplified and sent to the output. There is a tap in the output that sends the output signal to a feedback filter called beta that inverts polarity, attenuates it and sometimes also contains a filter. That signal is added to the input signal and since it has an inverted polarity it is effectively subtracted. Also like the steering movement it corrects the output of the amp. And just like with riding a bike it's always too late for a perfect straight course. You can consider that to be wrong or say that it's life's reality. And for a long time it seemed that zero feedback amplifiers was no reality. But already a few decades ago manufacturers, including Air, issued zero feedback amps. Yet another quirky approach to a simple circuit. Variable gain transconductance. To understand this, let's first look at how the average amplifier takes care of volume control. When a good network player is connected to the input of an amp, the signal potentially has a signal to noise ratio of about 120 dBs and a maximum output of 2 volts, or 4 volts if it's a balanced output. There are still old analog sources around, like tuners and cassette decks that only output 0.3 volts or thereabouts, which is a 16 dB lower level. An amplifier needs to be able to amplify these lower signals to normal playing levels too, so it has to have at least 16 dBs more gain than needed for digital sources. Above that you don't normally play music at full level so our volume control is needed. Let's say that the level is reduced by 18 dBs. Since an amplifier will only have a few dBs more signal to noise than the 120 dBs a digital player offers, the signal to noise of the audio signal is reduced by the amplifier noise. Let's say 105 dBs signal to noise remains. That signal is sent to the power amplifier to be amplified, resulting in the amplifier noise also being amplified. The signal to the speaker remains to have 105 dBs signal to noise at best. If the gain is further reduced, even less signal to noise remains. You could argument that at lower levels the environmental noise in the room will be louder and thus mask the amplifier noise. But then again Louis D. Fielder in the 70s proved that we are able to hear signals 20 dBs deep into noise. So what's there to say? Again most amplifiers work this way, but not the air amps. When a signal enters an air amp it is not attenuated. This is not needed since the amp has a variable gain, something that is only possible with zero feedback designs. The gain is set by two pairs of double pulse silver contact rotary switches connected to an array of hand selected precision resistors. These change the voltage on the input terminal of two pairs of complementary JFETs, so to change the current through the output terminal of these JFETs to the same ratio. In layman terms it changes the volume not by attenuation but by changing the gain of the amp. It maintains the full signal to noise ratio over the input signal, also when a lower playback level is set. The way the rotary switches are operated is remarkable too. A stepper motor is connected over a cock belt to the two axes of the double rotary switches. They of course need to be double since it's a balanced design. The mechanical construction is beautiful with two bearing mounted pressure rollers. Despite this it's somewhat noisy and actually surprised visitors for it sounds somewhat cheap. On the other hand it offers the option of working as a power amplifier for the left and right channels of a surround processor, as I mentioned in the beginning of this video. When you set the processor pass-through for an input, the amp simply turns the volume control full up. When switching back to another input, the volume control is set to the last level used on that input. Brilliant. Let me make clear that this is not my usual review video. Since I have not reviewed amps in this class for years, I would not dare to make absolute statements about the sound quality. As said, I bought this amp based on information in my network and reviews by well respected colleagues, topped off with listening sessions I did at the distributors or Wilson Audio loudspeakers. Due to Covid-19 I had to wait two months for delivery since there was a shortage of some parts. Once installed in my setup 1, the superiority over the audio note was immediately clear. The manual mentioned the burning period of 100 to 500 hours and although I did expect the sound quality to bloom somewhat over time, I did not expect the considerable sound quality improvement that presented itself over the first few hundred hours of playing. I used my setup 1 for long hours. Not only for my work but also privately for playing music and watching video. 10 hours a day on average I think. But if you just use the amp for listening music a few times a week for say two hours, burning will take months. So you better keep it on standby, consuming 40 watts an hour or even have it playing all the time if you can. High end audio comes with peculiarities. The Air AX520 will enable better and faster testing of digital front ends of higher quality. I will try to evenly divide my reviews amongst equipment in several price categories. That will not always be easy since getting equipment on loan on a certain date normally doesn't work, making proper planning a drag. But I'll try. The Air is a great pleasure to listen to and having an amp with a remote control makes comparing digital sources a lot easier too. Oh, if you wonder, the retail price in my country is €14,500 including VAT. Since my company bought the amp, I didn't have to pay retail price while tax deductions and investment subsidy made it more affordable. Good news to close this show with. There will be another video next Friday at 5pm central European time. If you don't want to miss that, subscribe to this channel or follow me on the social media so you will be informed when new videos are out. If you liked this video, give it a thumbs up. Many thanks to those viewers that support this channel financially. It keeps me independent and thus trustworthy. If that makes you feel like supporting my work too, the links are in the comments below this video on YouTube. I'm Hans Beekhuyzen, thank you for watching and see you in the next show all on theHBproject.com. And whatever you do, enjoy the music.