The X-Altra phono preamplifier can be used with both Moving Coil (MC) and Moving Magnet (MM) pickup cartridges and separates the MC and MM input circuits to optimize overall noise performance. It features an ultra-low noise LSK389B JFET from Linear Systems in the MM front end with switchable gain and resistive loading. The MC front-end utilizes “current injection” input stage with Zetex ZTX851/951 bipolar devices for use with cartridges down to 3Ω coil resistance.
The MC preamplifier stage is best suited to cartridges with output currents in excess of 12μA. The design incorporates a 20Hz high-pass filter that can be switched to 45Hz to reduce “acoustic” rumble evident on some classical recordings. Surface-mount device (SMD) components are used extensively, and the result is a compact unit that fits into a Modushop “Galaxy” half-width housing. All DIP switch settings are accessible on the unit’s rear panel (Photo 1).
PCBs are available should builders wish to try out the design. The rear panel is made from drilled and milled double-sided, matt-black silk screened and gold-plated THP PCB, which replaces the aluminum rear plate supplied with the Modushop housing. A complete unit will cost about $300 to $350 in construction costs, including the housing and PCBs (Photo 2).
Read the complete Part 1 article available here.
Read the complete Part 2 article available here.
Part 2 of this article will discuss the MM stage design, the system gain amplifier, the filters, and the measurements. aX
This article was originally published in audioXpress, February 2021.
Project Files
To download additional material and files (updated BOM, user manual, and more), visit our Supplementary Material page.
Resources
“RIAA Equalizer Amplifier Design,” Hifisonix.com, July 2018, http://hifisonix.com/riaa-phono-equalizer-amplifiers
“Richard Lee’s Ultra-Low Noise Head Amp” thread, diyAudio, June 2019,
https://www.diyaudio.com/forums/analogue-source/339246-richardlees-ultra-low-noise-mc-head-amp.html
S. Yaniger, “RIAA Noise Calculator,” SYclotron Audio, January 2012,
https://syclotron.com/riaa-noise-calculator
