RIAA Record Preamplifier

This record preamplifier is design to take a signal from either of a magnet cartridge, a coil cartridge, or a ceramic cartridge (each with differing impedance matching requirements and input stage factors) and amplify it while applying a mix of passive and active filter correction to the signal to match the RIAA standard for record cutting equalization. The circuit is built on TL072 low noise operational amplifiers and implements an active base and treble equalization with a passive top end curve.

The project spanned 3 weeks from inception, to research, to design, to digital verification and finally prototype manufacturing which was conducted in the Melbourne University Telstra Creator Space. The key challenge of the project was mathematically verifying a novel filter network design instead of copying one of the many designs online.

This process began with searches on Instructables and other online forums, and really kicked off when I came across a widely read paper written by Stanley P. Lipschitz’s. From this paper, I split on of the papers 4 pole active filter networks into a two pole active and one pole passive network that I was able then able verify with a calculus 2 level of mathematics. After mathematical verification, I recreated the design in LTSpice, where I verified the filter curve will be what I expected in practice.

The prototype constructed demonstrated a significant flaw in the build process, as the audio reconstructed lacked upper frequency clarity and consistency across the left and right channels. Upon troubleshooting and verifying component correctness and function with an oscilloscope and multimeter, it was concluded that the design implementation on stripboard introduced unintended resistance and capacitance in the long wire runs and many solder connections across the board. The design was considered too expensive for its functionality to warrant the order of a custom PCB.