Memphis MC 1Discrete Solid State Moving Coil Phono Stage
Most MC phono head amplifiers are implemented as voltage controlled voltage sources with an ideally infinite input resistance and resistors across the input to optimize the input impedance for the specific cartridge in use. Operational amplifiers but also discrete circuits with low noise jFets suit this purpose well. Specifically operational amplifiers are gifts from heaven for the audio industry. Their „plug and play“ option, the need of few external components, excellent power supply rejection ratio (PSRR) and easy gain adjustment over a wide range make operational amplifiers universal and cost effective devices and obviously they are getting better and better. However, the effects of huge global feedback on the audio signal are still discussed controversially.Discrete designs JFets are considered an easy option for their almost infinite input impedance, easy operation point settings and seemingly tube like characteristics and I used them a lot. However, jFets exhibit relatively high gate capacitances, poor linearity and gain compared to BJTs (fig. 1a and b) and their electrical characteristics scatter over a wide range (fig. 2).The new phono stage Memphis MC 1will be a discrete no-(global)feedback concept with BJTs only in the direct signal path. It will comprise one single gain stage only that amplifies the MC signal to line level, followed by a passive RIAA equalization and a subsequent emitter follower at the output (fig. 3). A step-up transformer elevates the MC signal close to MM level. The first stage will be designed as a high gain transconductance amplifier. It injects the amplified audio signal current into a frequency dependent load. The voltage across the load drops with ascending frequency according to the RIAA equalization curve.