Paul Katz wields an unusual kind of authority. Even when his students act deferential, he addresses them as colleagues. At a master class, he’s usually the last person to comment, waiting so students will voice their perceptions freely. During a coaching session, he may ask, “If you were rehearsing without me, where would you start?” Perhaps the urge to collaborate developed during his 26 years as cellist of the illustrious Cleveland Quartet, which played more than 2,500 concerts between 1969 and 1995. Or maybe he already had the soul of an ensemble player when, as a teenager growing up in California, he heard a concert by the Budapest String Quartet.

“Mischa Schneider, the cellist, was my hero,” says Katz. “When I heard his sound at the bottom of the [Budapest], I wanted to be a floor like that. From the cello position, you can inspire the sound of the whole group.”

Now 65, Katz still inspires whole groups, but more often as a teacher than as a player. He puts in ten-hour days at the New England Conservatory of Music (NEC) in Boston, giving cello lessons and directing the Professional String Quartet Training Program he established during the tenure of Daniel Steiner, the late president of NEC, who recruited Katz in 2000. Katz remains “a floor,” giving students the foundation they need to build their own musical lives.

On a typical day, Katz opens the door to his studio wearing a sweater and corduroy slacks, holding his 1669 Andrea Guarneri cello by the neck. He smiles warmly but makes no small talk before introducing the members of the quartet he’s coaching. It’s 2006, so they are the award-winning Jupiter String Quartet: Nelson Lee and Meg Freivogel, violins, Liz Freivogel, viola, and Dan McDonough, cello. The Jupiters are having a final coaching session before graduating from the school’s high-powered training program for string quartets. (NEC also offers a professional training program for piano trios.)

For the next hour, Katz zeroes in on what he considers most important in string quartet playing: every little thing.

“Again?” he asks the group.

Positioning their instruments, the Jupiters send a series of fortissimo double-stops reverberating throughout the room. A ragged, very soft unison tone follows, with bows suddenly stuttering. From fortissimo to pianissimo, it’s a tough transition (bars 98–99 of the second movement of Beethoven’s Op. 59, No. 1). Beethoven allowed no time for players to leap from one dynamic extreme to another. “We don’t want to take any time,” Meg Freivogel says.

“We want to set the new sound, but we don’t want to lose time.”

“Okay, set it,” says Katz, “but take the fortissimo to the wall.”

They try again, digging into the loud chords, but wavering on the soft unison.

“Would a tiny diminuendo be okay?” Meg asks.

Always the catalyst, never the oracle, Katz says, “To my mind, the more subito you can play Beethoven, the better.”

McDonough, the cellist, asks Lee, the first violinist, to cue the change. They try it and then laugh when no one can follow the cue. Katz suggests they use rehearsal time to take turns playing the passage individually. That way, they can experiment with their own techniques and listen to each other’s solutions to the problem.

Later, Lee reflects on the quartet’s growth during its two years under Katz’s tutelage as the program’s student-quartet-in-residence. “He hasn’t babied us. He just gives us his opinion,” Lee says. “When we disagree as a quartet, we have to find a compromise that everyone feels sincerely.”