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REP Magazine Feature on Magnesaurus

MAGNESAURUSTM, the 351.5TM All-Tube AnalogTM Master Recorder

by James Boyk

The Ampex 351 recorder, designed in the '50's, had a stable transport and excellent sound from all-triode tube electronics. I used it in the '80's for my concert albums pr-3, -4 and -5, whose All-Tube AnalogTM sound quality received rave reviews.

To improve the 351, I wanted to use 1/2-inch tape for better signal-to-noise ratio, but to stay at 15 ips for its superior low end compared to 30. I hoped that I could still get good high-frequency performance by careful design. I also wanted to eliminate the 120-Hz hum of the 351.

Wanting was simple! But it was seven years before the 351.5TM—"point five" for 1/2-inch tape—made its first successful tape. Over this time, many experts contributed to the project. John French modified a head nest and did the beautiful installation of Greg Orton's custom heads. Steve Haselton improved the signal circuitry. Richard Maurer designed and built the outboard power supply. Shelley Herman performance-tuned the transport with me and was a fount of information about the entire machine. Jeff Gilman rebuilt motors and all rotating parts. Doug Sax gave advice from his experience with master tapes of all descriptions. Along the way, the machine acquired a nickname from producer and polymath Chris Greenleaf, who dubbed it "Magnesaurus."

MagnesaurusTM makes a quiet, steady tape with no hum. The sound is effortless and wideband, with excellent integration across the band. The first MagnesaurusTM recording is my concert album of "Pictures at an Exhibition," Performance Recordings® pr7lp and pr7cd, released May, 1991. The All-Tube AnalogTM Lp was made direct from the two-track master tape, and the same tape was transfered to digital for the CD. The CD also carries the entire performance from a digital master made from the same mike feed. Listeners may thus hear pure analog on the Lp, while the CD gives pure digital and digital-from-analog.


Description: 1/2", 2-track, 15 ips line-level master recorder.
Transport: Rebuilt Ampex 351, tuned with FFT analysis.
Heads: Custom Record and Play heads. Custom head nest.
Electronics: Modified Ampex 351 (all-triode).
Power supply: Custom-built; B+ energy storage >100 joules.


Wow & Flutter: <0.03% DIN peak weighted, <0.06% unweighted.

Frequency Response: ± 1/2 dB, 23 Hz to just under 15 kHz;
        -3 dB at 18 Hz and 22 kHz,
        -6 dB at 13 Hz and 30 kHz, at 15 ips at 0 VU (230 nW/m ANSI, 250 DIN). Recorded on 351.5TM, played back on mastering deck at The Mastering Lab, Hollywood, as for Lp and CD transfers.

Dynamic Range: 78+ dB (Signal-to-Noise + Headroom)
     Signal to Noise Ratio: 64 dB, from 0 VU (230 nW/m ANSI) down to CCIR/ARM-weighted noise. Tape played back on Ampex ATR.
     Headroom: 14.6 dB above 0 VU was measured on true peak meter from master tape for album pr7.
     Note: Tones can be heard down into the noise. This extension, which ranges from 16 to 25 dB, is not included in the above figure. If it were, the rating would be between 94 and 103 dB. It is this figure that should be compared with those for dithered digital systems.

The first MagnesaurusTM tape is heard on album PR-7, "Boyk Plays Mussorgsky."

© 1990-1993, 1998 Performance Recordings®