Mark Levinson No.38 preamplifier Page 2

The No.38 is the first Mark Levinson preamplifier to incorporate remote control. Via the remote handset (machined from heavy aluminum), the listener can adjust the volume, change sources, adjust the display brightness, invert absolute polarity, adjust the left/right balance, select which source is routed to which record output jacks, mute the output, and switch the No.38 into mono.

The cumulative effect of all these features is a preamplifier that adapts to your system and your specific needs. Once the No.38 was set up, its functionality and "smart" operation made it a joy to use. Further, the ability to repeat a volume position, and have precise (0.1dB) control over the relative levels between inputs, made the No.38 a reviewer's dream.

Technical description
The No.38's uniqueness extends beyond features to the unit's design philosophy. The 38 was optimized for today's phono-stage–less audio systems (or those that have an outboard phono stage), high-output source components that need no gain, and sometimes a surround-sound processor.

The ideal preamp for such a system would have no gain unless the source component needed it—the preamp would function merely as an attenuator and buffer between the source component and the power amplifier. The No.38 does just that, providing attenuation at volume positions below #73.1 on the display, and gain (up to 18dB) when the display is above volume setting #73.1. With high-level source components—most balanced-output digital processors, for example—the No.38 attenuates the signal.

The volume knob and gain circuit are software-controlled; when the transition from attenuation to gain is made, 6dB of gain is added, then instantly attenuated by 5.9dB so that the volume control continues to move in tenths of a dB. This transition is accompanied by a slight "tick" sound.

The amount of attenuation is controlled by a multiplying digital-to-analog converter (MDAC). The MDAC has a resistor network on a silicon substrate; a signal generated by an encoder reading the volume-control knob's motion controls which resistor is switched-in (and thus how much attenuation is provided). Movement of the volume control changes the signal input to the MDAC, which is like a stepped attenuator with more than 4000 attenuation positions. Precise resistance values are realized by laser-trimming the resistors.

The relative balance between left and right channels can be easily controlled by changing the left- and right-channel control signals. This removes a resistive balance control from the signal path. Note that this technique uses digital control for affecting an analog signal—the No.38 doesn't digitize the audio signal (as does the PS Audio Reference Link). Moreover, the digital control and analog audio signal circuits are optically isolated to prevent corruption of the analog signal by digital noise.

The No.38 is a fully balanced preamplifier from input to output. As explained in my review of the Audio Research LS5 elsewhere in this issue, most "balanced" preamplifiers convert a balanced signal to single-ended for the gain stage, then back to balanced just before the output jacks. The preferred method is to keep the signal balanced all the way through the preamplifier.

One problem with fully balanced preamps is that they need identical circuit conditions for each half of the balanced signal, and perfect matching between the volume-control elements. A fully balanced preamplifier has four volume-control elements (± left, ± right) that must perfectly track at all volume-control positions. Madrigal believes that conventional potentiometers, no matter how good, aren't matched well enough to avoid gain errors between phases of the balanced signal.

This is where the MDAC volume control comes into play again; it perfectly complements the No.38's fully balanced topology. Because the dual MDACs can have a volume precision on the order of magnitude of a thousandth of a dB (Madrigal's claim), the volume control is no longer a source of gain errors between halves of the balanced signal. (See "Measurements" sidebar for some hard data on volume-control tracking.)

Single-ended input signals are converted to balanced at the input. Madrigal claims some degree of common-mode rejection (between signal and ground) with their input circuitry for single-ended signals. In addition, the balanced inputs are processed by a circuit that provides common-mode rejection before the gain stages; most fully balanced preamps provide differential gain paths with the common-mode rejection occurring in the power amplifier. The input stage is Madrigal's BIBO circuit (Balanced In, Balanced Out), which provides common-mode rejection before the signal gets to the attenuator or output buffer. The latter is the same high-speed discrete stage used in the No.30 digital processor's output section.

The power supply and digital electronics occupy the center third of the chassis. A metal box isolates the audio circuits from radiated digital noise and power-supply hum. The No.38's software is contained on an EPROM next to the microprocessor, and can thus be updated to accommodate future refinements in operation, or greater sophistication in the communication link with other 30-series components.

Mark Levinson division of the Harman Consumer Group
1718 W. Mishawaka Road
Elkhart, IN 46517
(516) 594-0300