I will try to explain as simple as possible the difference between Amati, and other expensive 211 amplifiers widely heralded as the holy grail in the audiophile world. The only way to design the world’s most technologically electron tube amplifier is by designing every single component for their specific goal, and the hardest part is to orchestrate them in a 100% synchronization manner. The use of exotic core material in transformer is just one of the many parameters in the design process, and one shall not amplify the importance of it due to marketing forces. Other parameters matter equally important in the process.
The only technical reason why most top amplifiers from the far-east do 211 amplifiers instead of the 845 tubes I can think of is because of the complexity to design a matching driver stages. The 211 with an amplification factor of “12” is much easier to drive than the 845 with an amplification factor of “5”. But that should not be a reason to choose the easy path if the goal is to design the most technological advanced electron tube amplifier.
The big problem with the 211 is the plate resistance of 4400 ohm (845 = 1700 ohm) which makes it almost “impossible” to design an output transformer working over a bandwidth over 20hz – 20,000 Hz. The schematic of famous 211 amps shows there is a “treble lift” engineered in the driver stage to compensate for the shortcoming of the “leaky” transformer.
For example, suppose the output transformer for a typical 211 amplifier is primary 10,000 ohm and secondary 8 ohm. The output impedance would be 10000 / 8 = 1,250. Tube plate resistance is 44,000 / 1,250 = 3.2 ohm, which in turn would yield a damping factor of 8 / 3.2 = 2.5 (with a 4 0hm load the damping factor would be 4 /3.2 = 1.25). A damping factor of 2.5 has absolutely no control over the low (bass) driver and negative feedback is essential to increase the damping factor to an acceptable level of minimum 10.
To compare, the Amati Amplifier with two parallel 845 tubes has an output impedance of 0.34 ohm and a damping factor of 24 with ZERO negative feedback. Based on history, the 211-tubes were designed for Class C operation in radio transmitters where the 845-tubes were intended as an audio tube.
Also, the heavy utilization of electrolytic capacitors, absence of chokes for filament supply, and insufficient filtering for anode voltage in other design of 211 amps are all detrimental to sound quality particularly in low frequencies. Electrolytic capacitors definitely have a huge negative impact on overall sound and insufficient filtering of H+ will result in hum.
Amati uses solely paper in oil capacitors and double “Pi” filtering for anode voltage and chokes for all filament supplies. Our chokes are all custom- designed for the specific voltage and currents. Our rectifiers are super modern super fast low noise.
The use of capacitors (however precious is the material) to couple the different stages is impossible to accept based on scientific reasoning, if the claim of the manufacturer is to build the ultimate tube electron amplifier. Even though efforts have been put forth to improve the quality of this capacitor for coupling application, I have done extensive scientific research in this field and have written a white paper to prove otherwise.
Amati’s driver stage is unique because it only uses one tube working on the same high voltage (1250-volt) exactly like the 845 power tubes. This tube provides a voltage swing over 800-volt! So the only (correct) way to couple the driver stage to the 845 tubes is to use an inter-stage transformer especially designed to deal with the high voltage and with the exact air-gap to deal with the DC saturation from the bias setting of the driver tube. The huge voltage swing allowed me to design the inter-stage 2:1 bringing down the voltage swing to a perfect matching 400-volt. AND most importantly, the 2:1 brings the output impedance down to very low levels which makes it easy to drive the 845 tubes fully with undistorted output power.
Other amps using electrolytic capacitors for decoupling the bias setting for the driver stage, once again, has a huge negative influence on overall sound. For the negative voltage setting in the 211 tubes, others typically use an adjustable power supply or fixed bias. In contrary to general belief (even by tube amplifier designers), fixed bias “IS IN THE AUDIO PATH”! And the first thing they see is indeed an electrolytic capacitor!
Amati is using cathode bias for both driver and power tubes bias decoupling. And they are done with newly designed paper in oil capacitors. Cathode bias is very reliable and safe. It automatically adjusts for tube aging and in case of replacing the tubes there are NO setting or adjustments required.
Amati output transformers are designed with the exact air-gap for the bias setting for both 845 tubes. We use an Amati proprietary alloy for core material. For maximum flux change with minimal losses, the secondary windings are made from 99.99% pure silver wire resulting in an increased reproduction of low level information.
To compare, output transformers must have very high primary impedance to compensate the high plate resistance intrinsic to the 211 tubes. Output transformers are very complex devices. When they are designed for high primary impedance, numerous compensations have to be accepted making them less than ideal. The application of negative feedback is absolutely necessary to make them practical.
Amati’s chassis are CNC machined out of solid blocks of aluminum. The tube sockets are CNC machined out of “solid Teflon blocks”. For maximum safety, visual aspect and to avoid component resonance or transformer ringing, all transformers, chokes and high voltage capacitors housings are CNC machined out of solid blocks aluminum filled with a resin that can withstand 20Kvolt insulation. The massive use of aluminum also assists heat dissipation in an effective manner. Absence of external screws elevates the aesthetic in a timeless and elegant manner.
Pursuing such route makes the design of Amati very expensive but my goal is to design the best of the best ever tube electron amplifiers based on new technology that simply cannot be surpassed now or for the next 20 years. I understand this is a bold statement. The Amati is an 8-year long project. Please contact Audio Exotics for an audition after they finish 200 hours of run in.
Dr Erwin Verelst