VAC Sigma 170i Integrated Amplifier Tap Tuning

Tap Tuning

As we saw with the Signature 200iQ [HFN Mar '21], VAC offers numerous transformer taps to step-down the high voltage/high impedance output of its KT88 tubes to better match the lower impedance and higher current demand of the attached loudspeaker. In this case the 8-16ohm, 4-8ohm and 2-4ohm taps offer a source impedance of 7.0-7.75ohm, 3.45-3.8ohm and 1.75-1.9ohm, respectively, from 20Hz-20kHz. Conceived to 'match' an 8, 4 or 2ohm load, respectively, in practice very few speakers provide this linear impedance. Instead the amplifier 'sees' a load that swings up and down with frequency as the various reactive components of crossover and drivers come into play, causing the system response to rise with increasing speaker impedance and fall with decreasing speaker impedance.


Arguably, my reference B&W 800 D3 loudspeakers [HFN Oct '16] might not be the Sigma 170i's likeliest partner, but they still perfectly illustrate how its response is modulated by the impedance trend of a 'real' loudspeaker. The inset Graph shows the native, unloaded response of VAC's 170i [cyan trace] versus that driving the 800 D3s via its 8-16ohm tap [black trace], 4-8ohm tap [red] and 2-4ohm tap [green]. Note how the 'lift' to the mid/presence – caused by an inductive upswing in the 800 D3's impedance – is moderated by choice of tap. For most speakers with a nominal 4, 6 or 8ohm load, VAC's 4-8ohm tap [red trace] yields the best balance between power output, distortion and response. PM

Valve Amplification Company, Inc.
Florida, USA
Supplied by: Signature Audio Systems, UK
07738 007776