A low-feedback triode amp with zero output impedance
audioXpress has republished a DIY project article by Tim Mellow and Peter Farrow, first printed in May 2024, that tackles an old problem in tube amp building. Part 1 introduces the Zero Output Impedance (ZOI) amplifier concept and walks through the design of a single-ended input version.
The problem is this: low-feedback triode amps have been popular since the 1990s for their open soundstage, natural tonal color, clean transients and soft overload. But feedback is also what normally brings the output impedance down so that the changing impedance of a real loudspeaker does not alter the sound. Speakers are not a pure resistance. There is a big impedance peak at the fundamental resonance, the coil inductance makes impedance climb with frequency, and drivers with different nominal impedances or steep crossovers cause more irregularities.
Without feedback, the result depends on the output device. Semiconductors, pentodes and beam tetrodes have a high output impedance and act almost like current sources, so output voltage follows the speaker impedance, which can make the bass boomy and poorly controlled. Triodes behave better, with an output impedance roughly a quarter to half of the nominal load, but still not zero.
The authors ask whether you can keep the low-feedback character and still get zero output impedance. They present two designs, one for single-ended input based on a topology from the 1950s, and one for balanced input that is a new configuration. Part 2 covers the balanced version, measurements and design calculations.
I like that it explains the why before the schematic. It is a good read if you build or plan to build a tube amp.
Source: audioXpress
The problem is this: low-feedback triode amps have been popular since the 1990s for their open soundstage, natural tonal color, clean transients and soft overload. But feedback is also what normally brings the output impedance down so that the changing impedance of a real loudspeaker does not alter the sound. Speakers are not a pure resistance. There is a big impedance peak at the fundamental resonance, the coil inductance makes impedance climb with frequency, and drivers with different nominal impedances or steep crossovers cause more irregularities.
Without feedback, the result depends on the output device. Semiconductors, pentodes and beam tetrodes have a high output impedance and act almost like current sources, so output voltage follows the speaker impedance, which can make the bass boomy and poorly controlled. Triodes behave better, with an output impedance roughly a quarter to half of the nominal load, but still not zero.
The authors ask whether you can keep the low-feedback character and still get zero output impedance. They present two designs, one for single-ended input based on a topology from the 1950s, and one for balanced input that is a new configuration. Part 2 covers the balanced version, measurements and design calculations.
I like that it explains the why before the schematic. It is a good read if you build or plan to build a tube amp.
Source: audioXpress
Comments 0
Leave the first comment, in any language.
Log in to comment