On transformers and multiple output-taps: "Multiple output taps are inherently a serious compromise in performance. Inactive winding-segments are inevitable in a tapped transformer and result in additional leakage inductance that affects high-frequency response. Also, in VTL amplifiers the negative feedback loop is designed to encompass the whole transformer; tapping the secondary would upset the feedback ratio and cause only parts of the secondary to be used, which would result in sub-optimal power transfer. Speaker impedances vary with frequency; in order to have the optimal match it is best to choose a secondary impedance at the minimum impedance of the loudspeaker. We have found that setting the turns ratio for maximum power-dissipation into 5 ohms is generally optimal for most speakers and provides amplifier stability down to 2 ohms."
On low negative-feedback and high output-impedance: "We have found that too little feedback yields insufficient control of the load and that too much feedback makes the amplifier sound flat and lifeless. Through listening evaluation we have chosen the best ratio between damping factor and harmonic resolution, with the result that the output impedance...has good bass control; and even though more feedback would improve the measured performance...sonic performance would be negatively impacted."
On tetrode and triode: "Triode mode typically offers a deeper soundstage, but not as much width and impact as tetrode mode. Triode might suit solo vocal, jazz, or classical quartets, where a sweeter sound and depth of soundstage is prized; tetrode might be preferred for more demanding programs, in particular large-scale classical or rock pieces of big-band jazz, where the amplifier needs to be able to resolve a large soundstage. Power output is halved in triode and there is some high-frequency roll-off. Different bias points are required for the different modes..."
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Last modified 17 Dec 2020