Solid State Power Amp Reviews

The '331's rear panel features four custom-made, gold-plated, high-current speaker-cable binding posts per channel in two columns, one for each channel, with two positive and two negative terminals in alternating fashion to allow for biwiring. These binding posts are the same 100-ampere–rated posts used on the No.33 Reference amplifier. This insulated connector satisfies the IEC-65 safety standard. The connector has a large curved wing nut. Using these posts, I was able to achieve tight, high-contact pressure connections with only modest finger pressure, and did not need my usual set of wrenches. These binding posts are now my favorite, easily the equal of the elegant speaker posts found on Classé amplifiers.

What's new internally?
Whereas the No.27's top cover was made from a single sheet of undamped sheetmetal, with countersunk holes, fastened to the chassis with 12 small Allen nuts. The top cover for the No.331 is a well-damped, lightweight, nonresonant piece of aluminum that slides into two grooves at the top of the chassis' lateral heatsinks and is secured with only two Allen-head nuts on the rear panel.

Underneath its top cover the No.331 is a densely packaged amplifier: visual inspection revealed only a large steel tunnel running down the center of the chassis, flanked by two long printed circuit boards (pcbs). This tunnel covers the two toroidal power transformers—801VA in the '331 vs 729VA in the '27.5—toroidal transformers than found in the 20 series, and, which are mounted on their sides within the steel tunnel.

The No.331 shares many features found in earlier Mark Levinson amplifiers: dual-mono design with completely independent power supplies, voltage, and current gain-stages packaged in one compact chassis; two toroidal transformers, one for each channel, oriented to cancel stray magnetic fields; a soft-clipping circuit to reduce the subjective effects of amplifier overload and clipping; and all voltage gain-stages independently regulated. However, the current-gain output stage is unregulated; only the No.20.6 amplifier (now out of production) uses regulation for its output stage.

The '331 includes refinements not found in the No.20 series. First, better manufacturing and engineering technology are used in production. The No.331 utilizes "wire-free" design elements to reduce losses and nonlinear artifacts in the signal path. From the supply electrolytics to the output devices to the loudspeaker binding posts, all high-current connections are made using solid bars of oxygen-free plated copper. For voltage-gain stages, all inputs are directly soldered to their respective pcbs—and subassembly connections are made with modular connectors—to avoid the use of wire. This modular design approach reduces labor costs during manufacturing, resulting in an amplifier with improved ratings over the No.27.5 but costing about $1000 less.

Madrigal believes that these improvements in mechanical design also greatly reduce the subtle unit-to-unit sonic variations (due to different crimp pressures, different dressing of the harness, and different solder heat), reduce the number of sometimes unreliable solder connections, improve product reliability, and facilitate in-the-field service. For example, the manufacturer claims that an experienced technician using conventional tools can disassemble a No.331 completely in 15 minutes, and reassemble it in the same amount of time.

The design of the '331's signal circuitry is based on the No.33 Reference amplifier. It features a balanced signal path, three voltage gain-stages, and replaces the No.27.5's active input buffer with a different topology. This is said to yield even greater common-mode rejection performance, a better match between the inverted and non-inverted signals, lower noise, and lower distortion. An important aspect of the design was the implementation of a new sliding-bias output-stage topology which Madrigal calls "Adaptive Biasing." The design goal was for the No.331 to function as a true voltage source for any loudspeaker load between 8 ohms and 2 ohms. (The No.33 Reference extends this ability down to 1 ohm.)

As discussed in one of the company's white papers, an amplifier rated for this range of impedance loads needs a high level of output bias to minimize crossover distortion and to avoid any possibility of reverse-biasing an output device. In a traditional class-A design, this leads to a large quiescent current draw, hence the production of a lot of heat. Madrigal's solution was to modulate the bias level as a function of the input signal, using an algorithm that includes both the input signal and the level of output current being demanded by the loudspeaker.

The No.331's power supply features two triple-bypassed, 44,000µF, low-ESR electrolytic filter capacitors per channel. The intended capability to continuously deliver power ranges from 100W to 400W requires a large number of output devices to source the required current. Each channel uses 16 TO3-can bipolar transistors in matched, complementary pairs.

Protection circuitry protects the amplifier against short circuits at the speaker terminals, overheating, excessive AC supply current, AC mains over- or under-voltage conditions (±10%), or excessive phase-angle output-stage power dissipation. If any of the these fault conditions is sensed while the amplifier is in Standby or full on, the amplifier turns full off. In addition, the 331 employs a new DC servo to protect the loudspeakers against voltage offsets. It can compensate for up to 1V of DC at the amplifier's input. If the DC is larger than that, the amplifier will shut down. The muting relays operate in the shunt mode, with no signal contacts in the direct signal path.

The No.331 is built from the best assets of the No.20-series circuitry, coupled with design innovations from the No.33 Reference amplifier, and further refined through listening tests. In designing the 331, the full design team participated in listening tests of all the circuit features—a process they refer to as "comping"—including optimizing the use of passive components (resistors, capacitors, etc.). There is no point, for example, in using, say, a $5 Vishay resistor in a place where it has no effect on the amplifier's sound.

All mechanical bolts and fittings are brass, so that electrochemical reactions between different metal types do not occur to cause corrosion and aging of the mechanical connections, which can give a increasingly harsh quality to an amplifier's sound over time. For example, a brass screw holds the copper bus bar to the electrolytic capacitors, and is threaded into a brass fitting.