AP50 Integrated Amplifier
This amplifier is designed to provide an extremely high quality output for speakers of normal to lower sensitivity. The unit should remain powered-up indefinitely for best sound -- power consumption is minimal.
An optional Phono stage is available which can be configured to MM or MC, but as standard there is no analog disc input. One input is labelled as DISC for your convenience if you possess an outboard RIAA pre-amplifier, and is also used when the optional Phono stage is installed.
Otherwise, all six stereo RCA inputs are identical and hardwired to the selector switch using silver-plated wire with a PTFE cover. The output of the switch is routed to the potentiometer and tape socket using the same cable. 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 using hard-wired 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. However, it has less sonic effect than conductive plastic film or carbon track pots.
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 160 VA toroidal transformer with a 25 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 50W 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 AP50 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.
Dimensions are 17 x 12.5 x 3.5 inches (wdh) or 430 x 320 x 90 mm.