Manley expanded on the design, adding: “Generally, with zero global negative feedback, the shorter, faster negative feedback loop offers less deleterious sonic impact, and allows the use of the Damping Factor switching, which allows adjustment of the applied negative feedback in approximately 2dB steps, lowering the output impedance in approximately ½-ohm steps. Output impedance can be set to improve control of the loudspeaker loads, and to suit the listener’s taste, with increasing loudspeaker control as the Damping Factor setting is increased, with slight loss of musical flow.
“We feel that this is a better approach than using output transformer taps, as we want to utilize the whole transformer for an efficient transfer of power without the high-frequency problems that typically result from tapping, and leaving part of the output transformer secondary unterminated.”
The transformer itself is a proprietary interleaved-and-coupled balanced unit, designed for wider bandwidth (from –0dB at below 10Hz up to –1dB at 100kHz), stability, and zero ringing.
During his visit to my house, Luke and I experimented with the S-200’s three damping factor settings: low, medium, and high. Given that the Mylar diaphragms of my Magnepan 1.7s are ultra-lightweight and unusually fast, the low setting proved to be the most musically natural, with the medium and high toggles adding levels of what I perceived as over-control and tightness to the sound that were immediately obvious and distinctly less musically satisfying. That said, although I’ve not had the chance to prove my guess, it’s not much of a leap to imagine speakers with large dynamic drivers benefitting greatly from the ability to apply a tad more negative feedback to the circuit. Which is of course why it’s an important key to this amp design’s flexibility to begin with.
Manley further explained that the other differences between the Performance Series ST-150 and the Signature S-200 is that the S-200 has separate power supplies for the input and driver stages, which he says keep the image stable even under heavy load conditions and modulation at the output stage.
Furthermore, in order to ensure a constant output tube operating point, and to stabilize the critical power supplies—even under AC and main power supply fluctuations—the S-200 employs adjustable, precision-regulated bias and screen supplies, which are designed to increase tonal stability and overall sonic integrity, most especially under complex dynamic signal conditions.
Manley and his team have also done a most welcome job of making the S-200 relatively idiot-proof for listeners—ahem, like me—who may want to enjoy the sound of vacuum tubes without having to geek-out or otherwise futz around with them.
No doubt, some traditional tube-ophiles will balk at the S-200’s auto-bias feature, which may add another layer to the circuit, and hence, some slight sonic compromise. Frankly, I can live with that (though you may not wish to). Because until the day when I have a lot more time on my hands, I prefer to listen to my gear rather than tinker with tubes or otherwise babysit the damn things.
Moreover—further aiding us non-technical types—in addition to the auto-bias feature, the S-200 offers comprehensive output-tube fault sensing in three redundant layers that protect the amp against bum output tubes. Deck-mounted LEDs signal to the user which tube may be causing the fault, and handy side-mounted (and easily removed) hatches allow easy tube access without the need to remove the entire top cage. Although I’ve experienced only a few tube glitches during my lengthy evaluation period, I can’t begin to tell you how grateful I was for this thoughtful feature.
Another important design aspect of the S-200 is the ability to switch, via a front-panel Mode button, between tetrode and triode modes for the output tubes. This switching is managed by a microprocessor so that switching can be performed “on the fly” (unlike the ST-150 which required turning off the amplifier before switching between tetrode and triode). Although I’ve already stated my natural aversion to tinkering with gear, I must confess that I became a bit obsessed with listening back and forth to a wide range of music—from Parsifal to Hendrix, Sinatra to Bach—to hear exactly how each mode affected the music.