The EternalArts OTL headphone amplifiers are the application of the brilliant Futterman-idea on a tube amplifier for headphones of highest quality. The compact amplifier module is designed to connect two high-impedance HiFi-headphones. The board works with its electronic components inside the two-piece powder-coated sheet steel housing with high gloss black acrylic front, gold, nickel or rubber feet and a volume knob made of solid metal or acrylic.
The amplifier family - KHV, "basic line" and "professional" - was developed and optimized on the high-end headphones HD800 by SENNHEISER. After the first tests with a variety of low-impedance headphones it appeared that with a tiny modification an equally outstanding interaction with other types was given. Consequently a new board was designed for the unmodified circuit, which received a small switch with the optimal setting for low-and high-impedance headphones to be made â€‹â€‹from the outside. You recognize the new board by the blue EPCOS film capacitors in the cathode outlet.
The circuit design is the "basic line" with a total of 2 and the KHV and "professional" is designed with 4 tubes: 2 triode pentode at the "basic line" and in addition 2 stabilizer tubes for KHV and "professional" for gain-independent stable anode voltages of the amplifier tubes. As with the other OTL amplifiers of the house also a board-supported design with gold-plated tracks laminated on both sides of 70Î¼ was selected. Along comes a grouted TRANSTEC voltage transformer used in toroidal winding and protection technology with Mu-metal shielding. Only high quality matched components, including a classic RUWIDO-Poti in cast aluminum housing (first series) and the legendary blue ALPS potentiometer were used. Gold-plated headphone jacks and gold-plated Neutrik RCA jacks complete the equipment. The circuit is absolutely secure and stable for maximum safety and very high tube life. All in all, a phenomenal transmission bandwidth and pulse sequence was reached.
OTL-amplifier, board-supported, 70Î¼ gilded double laminated boards
|Tube Placement||14GW8 (PCL86) per channel, KHV and additionally "prof" STV108/30 per channel|
|Input Resistance||25 / 50 kOhm|
|Output Impedance||300 ohms / > 20 ohms|
|Frequency Response||10 - 375.000 Hz - 0,3 dB|
|Distortion Factor (THD) (1 kHz, 400mVs)||< 0,5%|
Technical changes without notice
Through a potentiometer the signal reaches the grid of the triode in the combi-tube 14GW8, which combines a triode and a pentode in a slender cylinder tube. At its cathode to an RC circuit is an adjustable R to control the gain of the triode. The anode then controls via an RC circuit, the control grid of the pentode. Through its cathode follower circuit, a low internal resistance is achieved. At the cathode, then the signal will be coupled out, while dc components are being eliminated by a capacitor. Along a fuse and two Z-diodes for overvoltage protection, the signal then reaches the headphones.
A special feature is a stabilizer tube in the circuit, the anode voltage of the triode-pentode keeps constant regardless of its amplitude. The latter is DC heated via RC-element and double buffering. Its screen grid is powered by an RC-screening. The goal was an absolute hum-free operation of the headphone amplifier. For the amplifier stages and the power supply we use new tubes from old stock of such Valvo, Philips, Telefunken and Siemens recourse that remain available.