Mark Levinson No.32 Reference preamplifier Page 3
Each channel is connected to the Controller by a 10-conductor, heavily shielded DC cable. Four conductors deliver regulated DC power to the preamplifier's audio circuits; the other six carry DC control signals. More important, each channel's microprocessor is active only when receiving control signals—otherwise they're in "sleep mode" and entirely silent. A low-voltage, differential-drive control signal, buffered by optoisolators, issues commands. Except for the fraction of a second when the microprocessor is actually transmitting one of these commands, there's nothing in the DC cables between Controller and preamplifier except pure DC.
And that means pure. The DC flowing from the Controller is further regulated with another low-noise, high-speed voltage regulator. Critical blocks such as the volume control and the optional phono modules receive additional local supply regulation. "Liberal use of local bypass capacitors assures an optimal electrical environment for critical audio circuit components."
Why is it so easy to believe them?
The No.32 sports eight inputs: three on balanced XLR connectors and five on single-ended RCAs. The optional phono modules include two additional inputs, either balanced or single-ended. As alluded to before, unused inputs have both their signal and ground connections open to prevent noise from seeking a path through the preamp's well-grounded chassis. An R/C network hinged at 1MHz provides RF filtering for each input. An instrumentation-grade T-switch topology keeps the No.32's inter-input crosstalk "exceptionally low." Even with adjacent inputs unassigned and unterminated, isolation is greater than 120dB. With other source components connected, as would usually be the case, isolation improves to a rather startling 140dB. Madrigal proudly points out that that's about 100 times quieter than their "already excellent" No.380S. The special relay switching scheme, circuit layout, and shield bars tied to the system ground are responsible for the high level of performance.
The No.32 is the first Mark Levinson product to use Arlon 25N circuit boards in the active preamplifier and phono-module circuits. Madrigal explains that this material offers superb dielectric properties and was chosen after extensive listening tests. A quick visual inspection of the left- and right-channel circuit boards reveals them to be symmetrical, but only in a general way—individual parts are not mirror-imaged to the opposite channel.
"The layout was developed by hand for electrical rather than visual symmetry. We have learned that this is the only way to approach circuit layout, using exhaustive listening tests as the measurement of success—to understand this point is to recognize the beauty of the circuit itself." That's devotion.
Single-ended signals have their common-mode noise (ie, noise present on both signal and ground) rejected at the input stage. Fully balanced instrumentation amplifiers are used in the gain stages, and each input can be separately adjusted for 0, 6, 12, or 18dB of gain. This allows the volume control to work in its optimum range even with sources of substantially different output levels.
Optional phono modules
Phono modules can be installed in the No.32 for analog diehards like me. The channels are separately enclosed in mu-metal shielded boxes that slide into the preamplifier chassis on either side of the channel divider wall. The positioning is anything but accidental; it's optimized for the power supply and signal path, and being close together allows for easy connection of tonearm cables. Each phono module is fitted with two independent input connectors to accommodate systems in which two tonearms or turntables are in use. Phono modules are available with each input configured for either single-ended (RCA) or balanced (XLR) connectors. The review module had two pairs of RCA inputs so I could run the Forsell and the SpJ/La Luce turntables!
Each input's settings are independent of the other, allowing custom setup and optimization for two different analog sources. A defeatable infrasonic filter, resistive and capacitive loading, and gain—all can be selected independently for each input. A fine trim adjustment for balance allows the rapidly-getting-spoiled owner to compensate for the small channel imbalances found in many cartridges. Couch potatoes rejoice! It's all doable from the remote control while you're sitting on your can in the listening chair!