AudioPrism Mana Reference monoblock power amplifier
The Mana is a fine blend of good looks and contemporary circuit design, even though it sports a classic, if somewhat quirky, 6L6/5881 tube array. (The 6L6 is most often associated with guitar amplifiers and is said to be good for about 10,000 hours.) When I mentioned this to Victor at HI-FI '99, he screwed on his best whatever-do-you-mean-of-course-we-do-it-that-way look. And when he explained the what and why of it, I have to admit, it all made a lot of sense.
The Mana Reference is good-looking, almost architectural. All decked out in burnished golden hues, the monoblock pair makes a strong fashion statement. Despite all the brightwork, the amplifier looks anything but vulgar. There's a gratifying, unified wholeness to its design that I found very appealing.
Atop the broad expanse of the upper deck, in front of the slotted transformer covers, the tiny phase-splitter tube sits in its own beveled well at the back of a curved, raised console set between the tube arrays. (Cages to protect the tubes, not to mention little fingers and curious pets, are available.) Below the phase-splitter tube is the biasing meter, set into a gracefully radiused oval with a rotary control below it to select the tube to be biased. The front panel's nicely weighted Operate knob is set into a beveled strake with a subtle LED showing operating status.
Around back, Cardas speaker terminals sit in a convenient vertical array behind the output transformer. Speaker impedance can best be matched to 2, 4, 6, and 8 ohm taps, a neutral tap between each making wiring chores a snap. The neutral taps also facilitate biwiring; all taps are driven, so the user can tailor the output impedance for the associated driver. This is not so remarkable in itself, but it's one of the Mana's many thoughtful and well-implemented design elements.
Input is available on single-ended RCAs or balanced XLRs (pin 2 hot), selected via a toggle. Another switch tips the amps from ultralinear into triode mode, with another switch next to that for toggling between grounding options: Float disconnects the audio circuit from the chassis ground, Ground reconnects them. One setting should prove quieter than the other.
An IEC power-cord receptacle is tapped into the center of the rear panel. It's a chunky number meant for Hubbell 20A IEC-320 connectors rather than the 10A fittings found on most high-end gear. AudioPrism's own ACFX power cords with EMI/RFI filtering modules were supplied; I made good use of them running Poor Man's Balanced Power, as described in the May "Fine Tunes."
We begin at the power supply, where it's said the quality of any gain stage is ultimately defined. AudioPrism uses nine precision regulated supplies to energize all circuits, including the high-voltage and high-current output stages. There are two separate regulated DC supplies for the tube filaments, and the input stage gets its own filament supply.
Next, for those of you coming in single-ended, the signal is split into an opposite-polarity pair with zero gain. The Mana's splitter is based on a Raytheon 5842 single high-transconductance triode tube, with the Western Electric 5842/417A a $250 option (when available). The phase splitter sits right at the input but significantly outside the 6dB global differential negative feedback loop. Tiscareno explains that this reduces noise and improves signal integrity and linearity. Once the signal is split into two phases for push-pull operation, each phase is fed to two gain stages using the highly regarded 6SN7 dual-triode octal-based tube. A push-pull driver couples to the low-impedance grid circuit of the eight 6L6 power tubes. The 6SN7s supplied were Philips JAN (Joint Army Navy) WGTAs, the power tubes Sovtek 6L6WXTs.
Balanced input signals bypass the phase-splitter completely; that's one fewer tube and a more direct signal pathwhat could be wrong with that? Exactly nothing, and that's how I used the Manas for most of the evaluation period. When I did listen to a single-ended input, I switched between the standard-issue Raytheon and the WE phase splitter, and, no surprise, I preferred the WE. It's not something to get neurotic about, as the Raytheon is no slouch. If you can afford it, close your eyes, hold your nose, and go for the more costly tube. Better still, go in balanced with a differential signal and fuhgedaboudid.
The transformers aren't huge, but you can bet they're anything but run-of-the-mill. They feature proprietary dual ultralinear primary windings, a standard-configuration secondary, and a "special" tertiary winding. This last supplies a differential feedback circuit that, Tiscareno posits, works significantly better than the global-feedback topologies found in other designs. The Mana Reference operates in ultralinear "partial triode" at a conservatively rated 100W, and switches easily to all-triode operation, with the output dropping to 55W. Tipping the amplifiers into ultralinear to match the music or my mood was a snap. Just mute or lower the gain of the preamp, exhale, bend way over (ouff), and flip the toggle switch on the rear apron. No muss, no fuss, and a quick beeline back to the Ribbon Chair. (The gain rises by 1.5dB in ultralinear mode and the bias bumps up 2mA per tube.)
Each pair of Manas is burned-in at the factory for 100 hours and tested before the final biasing operation. All tubes are matched and set for lowest distortion in each amplifier, then marked for placement in the corresponding sockets on the top deck. The clearly marked tube positions and bias-adjustment legends so blend into the overall design that you don't notice them until you need themand suddenly, there they are before you. Setting the bias, if required, is a snap, even for those as heavily klutzed as I. The 6L6 proved a robust performer that held its 45mA bias with little or no drift.
Four operating modes can be selected with the front-mounted control knob: Off simply means B'bye, and Standby puts the amplifier into quiescent mode with the tubes in bias cutoff and the output muted. If you continue rotating the knob to the Operate position, a one-minute timer holds the amp in Standby to avoid unseemly tube-related thumpage. When the amps come up from Standby they make a low-level electronic burp of sorts. Tiscareno explains that this frisson of sound is a not a flaw but a feature. Hey, think positive. But the amps have impeccable manners; the relays, for example, are quiet and silky-smooth in operation.