AP70 Integrated Amplifier
This amplifier will not reach its designed sonic performance until it has been run for at least two weeks from new and it should be left on at all times if possible. No internal phono stage is available for this amplifier. For turntable use you must use a Phono 1 or Phono 2 or equivalent. All 6 inputs are identical and hardwired to the selector switch using silver alloy cable with PTFE cover. The output of the switch is routed to the potentiometer and tape socket again with the silver alloy cable. The potentiometer is a very high quality cermet type 100k ohms linear with 10k ohms precision metal film resistors as by-pass to simulate log law. The signal is then routed to the amplifier PCB again with silver alloy cable. the pre-amp stage is entirely passive, this combined with the cermet pot means that there can be surface noise from the pot when being moved, this is normal. Cermet track is not as smooth as conductive plastic film or carbon tracks but has a far superior sound quality.
The input to the power amp stage has minimum capacitive and inductive coupling and has been designed to operate correctly with the variable impedance output of a passive pre-amp stage. A current mirror operates at the pre-driver stage to ensure that the voltage rails track each other correctly. The driver stage has both current and voltage amplification using devices that could be used as output transistors. the output devises are 12 amp darlingtons and there are two per channel. The power supply is based around two 160va transformers (one per channel) to provide dual mono operation. There is no protection circuitry or filtering on the output of this amplifier so care must taken in its usage. Do not short circuit the output. Do not use bi or tri wire or high capacitance or litz type loudspeaker cables as these could damage the amplifier as they create a virtual short circuit at very high frequencies which they will create instability in the amplifier. as a rule of thumb avoid cables with a capacitance higher than 250pF per meter. Simple solid core or low capacitance stranded cables are cost effective but high quality silver or silver alloy cables such as NVA LS1 are best. Avoid loudspeakers with an average of below 4 ohms load impedance or loudspeakers using high frequency notch filters in their crossovers. This amplifier has minimum capacitors, no inductors, and is a low negative feedback class AB circuit design that is unique. The amplifier has an output of 70W per channel. The case design has been created for sonic reasons and is glued and insulated to stop induced circulating currents and static charge problems associated with normal case designs.