You Can DIY! X-Altra MC-MM RIAA EQ Preamp - Part 1

May 12 2021, 17:10
In this DIY project, the author proposes the X-Altra, a highly refined Moving Coil/Moving Magnet RIAA EQ Preamplifier that we know our readers will treasure building (and owning). 
 

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).
 
Photo 1: The X-Altra RIAA design incorporates a 20Hz high pass filter that can be switched to 45Hz to reduce “acoustic” rumble evident on some classical recordings (a). SMD components are used extensively. All DIP switch settings are accessible on the unit’s rear panel (b).
 
Photo 2: A complete unit will cost about $300 to $350 in construction costs, including the housing and PCBs.

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.


About the Author
Andrew C. Russell worked for the first half his career in the electronics industry in product development, qualifications and marketing, During the second half of his career, Russell was in business management and business development in the semiconductor industry working for a global MNC, spending the last 10 years based in Japan, Taiwan, and China. He struck out on his own in 2015, founding Ovation High Fidelity and doing freelance audio design consulting and contracting with an emphasis on power amplifiers and preamplifiers. When not working in his lab, he can be found walking his dog or enjoying his vinyl collection.

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
Attachment
PhotoLead_AndrewRussell-X-AltraWeb.jpg
AX-Feb2021-ARussellXAltra.pdf
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