You Can DIY! X-Altra MC-MM RIAA EQ Preamp - Part 2

For the second half of this article, we will begin with the Moving Magnet (MM) EQ amplifier design. The junction field effect transistor (JFET) plus op-amp topology used in the X-Altra MM preamp stage borrows heavily from Denis Colin’s excellent LP797 design published in audioXpress in 2007 but in this implementation, all active EQ is applied around the first stage rather than Colin’s active/passive split EQ architecture.
 
The key gain element is U21, a dual ultra-low noise N-channel JET from Linear Systems, the LSK389B featuring 0.9nV/"Hz typical (1.9nV/"Hz maximum) input noise voltage. By paralleling the two devices, the equivalent input noise of the JFETs is reduced by 1/"2 to 0.63nV/"Hz typical. In the LP797, the JFET source resistor was set to 10#, which contributed just 410 pV/"Hz noise (57nV RMS over the audio band).

In an all active EQ arrangement such as the X-Altra, if the gain resistor is set much below 100# (191nV RMS wideband noise contribution), the feedback equalization components quickly become large and unwieldy. At least 1% tolerance capacitors will be required for acceptable EQ accuracy, which are expensive at high values such as 0.47$F or 0.1$F. These are likely to be wound film types, so they will be large and prone to noise pickup. With the gain resistor set to 110 #, the EQ capacitors are readily available in 1% tolerance NPO/COG surface mount device (SMD) types and they are inexpensive.

By using a JFET front end in this MM amplifier, the current noise × RSOURCE term discussed in Part I (audioXpress, February 2021) is effectively removed and results in a >3dB SNR improvement using the standard cartridge compared to a NE5534A op-amp.

For readers wishing to buld the X-Altra preamplifier, PCB sets can be ordered at http://hifisonix.com/x-altra-phono-eq-preamp. aX

Read the complete Part 1 article available here.
Read the complete PDF in Part 2 of the article available here.

This article was originally published in audioXpress, March 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

D. Colin, “The LP797 Ultra Low Distortion Phono Preamp,” audioXpress, September 2007.

diyAudio  “Mechanical Resonance of MMs,” February 2017

G. A. Galo “Disc Recording Equalization Demystified,”
The LP Is Back!, The Audio Amateur, 1999,

“Very Simple, Accurate RIAA Phono EQ Amp,” Hifisonix.com, January 2018, 

S. Yaniger, “RIAA Noise Calculator,” SYclotron Audio, January 2012,