|AP10H Headphone Amplifier|
There is a single stereo RCA line-level input for a dedicated single source which is hardwired to the volume control potentiometer using silver-plated cable with a PTFE cover. The potentiometer is a very high quality cermet type 100K ohms linear, with 10K ohms metal film resistors acting as bypass to simulate a log law. The signal is then routed to the amplifier printed circuit boards, again hard-wired with silver-plated cable. The use of a cermet pot means that there can be surface noise from the volume control when it is rotated, which is normal. Cermet track is not as smooth as conductive plastic film or carbon track but has a far superior sound quality, or less effect on the signal, depending on your viewpoint.
The input to the amplifier stage has minimal capacitive and inductive coupling, and has been designed to operate with the variable output impedance of the potentiometer.
The power supply is based around a small frame transformer with a 6 amp bridge rectifier, and four different capacitor networks in order to filter the power supply at progressively higher frequencies to create very smooth low-impedance power supply rails.
The case design has been created for sonic reasons, being made of wood, anodized aluminum and plexiglass, each being an electrical insulator and non-magnetic. No ferrous materials are used, only four screws for the cover, so the case's constituent parts are glued together to prevent induced circulating currents as well as high frequency and high voltage static charge problems associated with normal designs. The concept extends to gluing the circuit boards, power supply caps, and diode bridge to the thick anodized aluminum baseplate. All high current wiring is via heavy gauge uninsulated copper wire to avoid dielectric effects. The case is unvented.
Note that old-style 8 ohm headphones will not work properly, and may damage the amplifier. Headphones of 200 to 300 ohms and higher of the modern variety is the kind you should use, and are almost universal these days.
The AP10H is small, being approximately 10 x 3 x 8 inches (whd), or 250 x 75 x 200 mm.