A70 Monoblock Power Amplifiers

The input to this power amp has minimum capacitive and inductive coupling and has been designed to operate correctly with the variable impedance output of a passive pre-amp stage and should under no circumstances ever be used with an active Pre Amp. 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 devices are 12 amp Darlington's and there are two per channel.

The power supply is based around a 160va toroidal transformer with a 35 amp bridge rectifier and three different capacitor networks. 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 is rated at 70 W RMS. 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. To obtain design performance Sound Pipe or Sound Cord should be used.