INTerior INTelligence. From my Classic Amp review, these comments set the stage, "…our MSA had always enjoyed a fine reputation for great transparency and finesse. It came with amazing measurements, an extremely low noise floor, negligible distortion and so forth. Really the only request we ever received for it was, could we make it more powerful. Though it might seem trivial, in Nagra terms it's not. When for us the HD DAC created a new chassis paradigm with its increased depth, we dared push that envelope further to now grow vertically. Whilst the Classic Amp remains reasonably compact, it provides far more interior space to accommodate larger parts and scale up dissipation. This allowed our engineers to go after a much larger power supply. Just like in the MSA, that's an SMPS with power factor correction to increase dynamic headroom. As a consequence, the amp behaves far more powerful than its specs suggest. The larger casing also supports more and larger capacitors and higher voltage rails. Then the Classic benefits from a completely new input stage with RCA and XLR balanced inputs. As our extensive R&D on the HD Amps showed, this stage is an essential contributor to the final sound. Applying advanced computer simulation software, we also optimized heat dissipation. This allowed for a dramatic increase in class A bias. The result is a very smooth transparent amplifier with more power and greater reserves than the MSA and a tighter grip over your speakers. It behaves rather more powerful than its measurements indicate. As any short audition demonstrates very quickly, this new amplifier is really a smaller HD Amp, not a bigger MSA." [Basic circuit schematic below.]

"With a PFC-driven power supply, the electrical current is always in phase with the voltage in a perfect sinusoidal curve. There are no interference peaks or distortion. From the power grid's perspective, this type of supply is seen as a pure resistance. That avoids polluting the grid. In a way such a supply behaves as though it were electrically decoupled from the wall AC. Another obvious advantage is the very clean current propagated downstream to other circuits. By precisely superimposing current and voltage, the PFC power supply generates little losses. It guarantees very efficient energy transfer and doesn't collapse with load increases. It is therefore capable of reacting extremely rapidly to on-demand stress situations from the output stages. The MSA power supply was built in ways which fundamentally distinguished it from more conventional switch-mode power supplies. In particular, it had no fly-back transformer or snubber circuit which can lead to hot spots on the PCB. Instead a sizeable 200VA toroidal transformer reduced the rail voltage to ±35V to suit the output stage and derive all other supply voltages from. This transformer ran at the power grid's frequency to avoid generating any residual HF noise."

The white arrows above point at the power Mosfets bolted directly to the aluminium stock beneath the motherboard. That's the heat sink which becomes the top cover from which all these bits hang like a sloth.

"On further specs, the MSA had a 10Hz-75kHz response at +0/-3dB, an A-weighted SN/R of 109dB, +100kΩ input impedance and -0.08% THD at full power. Its 115/230V acceptance windows of 90-132V and 180-264V demonstrated further SMPS benefits. For the Classic Amp, the rail voltage became ±48V (up from ±35V) with a 50% increase of class A bias and 141'000µF filter capacitance per channel (up from 84'000µF). The input board grew its own dedicated ultra low-noise linear supply. The already low noise floor of the MSA was pushed down by another 10dB. Bandwidth became 10Hz-80kHz +0/-3dB, THD at full power -0.05%. Max power draw is 400 watts which implodes to below 1 watt in standby and auto modes. Output impedance is 60mΩ in stereo, half that in bridged mono mode. Net weight is 15kg."

A thick shielding plate separates the transformer from the output stage beneath it.

Again, the Classic Amp's two-stage PSU starts with a traditional 400VA transformer ⇒ diode rectifier ⇒ filter capacitor type followed by a power-factor-corrected switching supply. Multiple polypropylene caps, self-inductance coils and six 47'000µF electrolytics show up for those tasks. The input circuit includes a signal-sensing mechanism to default to standby after 15 minutes of no signal in 'automatic mode'. The gain circuit combines a double current-transfer drive stage and a pair of class A/B common-source push/pull lateral Exicon Mosfets. This arrangement is said to be so precise as to require very little negative feedback even for low-impedance bridged mode. The security circuit monitors thermal and output-stage overload conditions to disable the power circuits via relays should protection be required. The same relays are active during the soft-start procedure. The micro-processor control circuit behind the front panel oversees start, stop, standby, automatic and mute modes plus the security circuit and also delivers the clock signal to the PFC circuits.

Exicon power Mosfets.

Now it seems fair to confess to a personal Exicon crush. My favorite amps—in order of more power, Bakoon's AMP-13, LinnenberG's Liszt and Kinki Studio's EX-B7 monos—all drive these parts. Even the $1'495 budget wonder that was Goldmund's Job 225 did. Incidentally, this was no self hypnosis, no mental pillow talk manipulating perception to suit expectations. This was an after-the-fact 'Aha' moment. I'd wondered what else our amps might have in common other than class AB, DC coupling and high bandwidth. It was a matter of spotting a pattern behind personal preferences to find them not at all arbitrary. It's still no guarantee though. As always, implementation trumps circuit type and parts. Still, there was a more than fair chance that I'd really like the Classic INT. If so, you can now cut me some slack; or claim expectation bias if you find subjective reviews all tosh no nosh.

(Note to self: for 33 days, repeat three times "I shall love the Classic INT because of its lateral Exicon Mosfets" before falling asleep. Mind over matter, dammit.)

To self-immunize against unconscious mental games, I asked regular Warsaw contributor Dawid Grzyb to share some of his own listening comments by way of a 2nd opinion. Those will be culled from his pending Classic INT review for HifiKnights.com.