AudioPrism Mantissa Line Preamp
Dick Olsher (May 2022)
Some months ago, I was fortunate to snag an AudioPrism Mantissa preamp. This turned out to be no ordinary Mantissa as it had been upgraded circa 2003 to the then current Red Rose Music model 3a standard. As the story goes, Mark Levinson liked Victor Tiscareno’s designs so much that he bought AudioPrism in 1999 and rebranded its products under the Red Rose Music label with a few upgrades along the way.
The basic topology of the Mantissa remained the same. One 6DJ8 dual triode per channel connected in parallel provides 20 dB of gain. It is direct coupled to the output stage – a 12AU7/5814 configured as a White cathode follower for low impedance drive. No global feedback is used. When purchased by Red Rose Music, the preamp’s standard color changed from Champagne to Silver as the obvious external change. Internal changes occurred as well: coupling capacitors changed from ASC to Multicap, some resistors were changed, and the 6DJ8 grounding was revised.
The Model 3a represented the last change to the preamp circa 2003. The output of the regulated 450VDC supply feeding both left and right channel 295VDC and 145DC regulators were split with RC networks to improve isolation between channels. Also, the external supply incorporated an additional transformer for the filament supply.
I decided to send my unit to Brian Hawkins at Raptor electronics (https://www.raptorelectronics.com/) who had spent nearly seven years with AudioPrism/Red Rose Music and specializes in repairs and upgrades for these products. As a minimum, my goal was to have Brian fully recap the unit, and if necessary, bring it up to model 3a status.
I’m very pleased with Brian’s work and professionalism. After Brian was finished, my unit ended up being slightly more advanced than a model 3a, with the exception of the silver contact volume attenuators:
· All of the electrolytic caps were replaced with better, 105c, long-life types
· The 4.7uF/450V electrolytics were replaced with 10uF/450V electrolytics
· Grid stopper resistors (392 Ohm) were installed on the 6DJ8 grids to prevent RFI interference
· The 12AU7 White cathode follower now has MultiCap PPFXS instead of the PPMFX caps of the model 3a (ASC brand was used in Mantissa; PPFXS used in Silver Signature)
The end product is exceptionally musical - detailed, transient perfect, and super dynamic. I now understand why Mark Levinson was so attracted to this design. It has become my go to line preamp for critical listening.
I should also mention that I installed Herbie’s Audio Lab tube dampers on both the 6DJ8 (SS-9) and 5814 (HAL-O III-9). I find Herbie’s dampers to improve image focus and lower the noise floor. Both of these, especially the SS-9 which sounds a tad sweeter to my ears, are well worth the investment and are highly recommended.