A phonograph preamplifier

Objective

The objective of this coursework is the design of a monophonic phonograph preamplifier, i.e., the preamplifier that converts the signal from the cartridge on the arm of a turntable to a suitable audio line level. You have to design both the circuit, using LTSpice, and the printed circuit board (PCB), using DesignSpark PCB (or any alternative software you are familiar with).

Background

There are two different types of cartridges: moving coil (MC) and moving magnet (MM). MC cartridges are generally considered to have better performance, but they provide a lower signal than MM cartridges and they also require careful selection of the preamplifier input impedance.

After the signal from the cartridge is amplified to a suitable line level it must be filtered. In order to minimise the groove width and the high-frequency noise in a vinyl disk, the high-frequency components of the audio signal are emphasised before recording. This results in a more balanced structure in the groove features as well as a reduction in the stylus wear. The phonograph preamplifier must de-emphasise the high-frequency components using a specific response standardised by the Recording Industry Association of America (RIAA).

Circuit Specifications

Your preamplifier must be able to support both MM and MC cartridges. You can either use separate inputs and/or a suitable switching mechanism. The required gains and input impedances are shown in Table 1:

Table 1: Gain and input impedance.

The low-pass RIAA filter has the following transfer function:

where τz = 318 μs, τP1 = 75 μs, and τP2 = 3180 μs.

There is not a minimum required value for the total harmonic distortion, noise, and power consumption, but you will have to simulate (or estimate) their values – the lower the better.

Assume that a dual power supply +12V/ground/-12V is available. If your circuit requires different voltages, they must be generated from these levels.

You are only allowed to use operational amplifiers and discrete MOSFETs as active components

Printed Circuit Board Specifications

The PCB should be designed using a 2-layer board with mil (1/1000 inch) as the base unit. The board size should be as small as possible, and the design should be optimised for milling manufacturing. The minimum features are:

• Track width ≥ 10 mil (254 μm).
• Component-to-component spacing ≥ 10 mil (254 μm).
• Hole diameter ≥ 20 mil (508 μm).