If dedicated headphone listening is required, then just the positive speaker cables should be unplugged. NVA tried all forms of headphone switching, both mechanical and relay types, and they all compromised the sound quality.
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 power amp stage has minimal capacitive and inductive coupling, and has been designed to operate with the variable output impedance of the potentiometer. A current mirror operates at the pre-driver stage to ensure that the voltage rails track each other. The driver stage has both current and voltage amplification using hefty devices, while the output transistors themselves are 10 amp Darlingtons and there are two per channel.
In order to ensure no audible effects, there is no protection circuitry or filtering on the output of this amplifier, so some care needs to be taken in its usage, such as never short-circuiting the output. In addition, always choose low- capacitance speaker cable. Suitable cost-effective speaker cables include solid-core or low capacitance stranded types. However, high quality silver-plated cable such as NVA LS1 is best. Avoid bi- or tri-wiring or high capacitance or Litz type loudspeaker cables -- these could damage the amp because they create a virtual short-circuit at very high frequencies. This requirement may not be trendy, but the overall sonic results speak for themselves.
The basic circuit of the amplifier is very stable but it is not unconditionally stable -- anybody can make an unconditionally stable amplifier: you just put capacitors everywhere, but it will sound terrible.
NVA amplifiers have minimum capacitors, no inductors, no adjustable internal pots to set quiescent levels or balance, and a low negative feedback class AB circuit design which is unique. The power supply is based around a 30 VA toroidal 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 amplifier has an output of 15W per channel into 8 ohms.
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, diode bridge and output transistors 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.
Compared to most amplifiers, the AP10 requires that its owner pay some attention to the type of speaker cables for best sound and stability. The case design is unique and contributes absolutely minimal acoustic signature to the output. The payoff is high-end sound and dynamics for a very reasonable investment.
The AP10 Personal is small, being approximately 10 x 3 x 8 inches (whd), or 250 x 75 x 200 mm.