Pre/Power Amplifiers

A choice of 8ohm and 4ohm (and perhaps 16ohm or 2ohm) transformer taps are typically offered on the rear of many tube amplifiers, optimising power delivery into higher and lower impedance speakers, respectively. The secondary windings in VAC's output transformers are configured in parallel when in mono mode, halving the specified 4-8ohm, 2-4ohm and 1-2ohm taps. This influences the available power and, in this instance, the high frequency response realised by the chosen tap/speaker combination.

The right match will result in a very flat and extended response – remarkably so for a tube amp – [solid traces, inset Graph] while mis-matches can provoke a treble roll-off [dashed traces]. So, if you have a speaker with an impedance trend that's generally higher than 4ohm, then the 4-8ohm tap will realise the best result at ±0.0dB/20kHz and +0.7dB/ 68kHz into 8ohm [solid black trace], and +0.05dB/20kHz and +0.6dB/68kHz into 4ohm [red trace]. However, drive a '4ohm' speaker via the 2-4ohm tap in mono mode and the response droops to –1.2dB/20kHz [dashed red trace] just as a very low impedance '2ohm' speaker falls to –2.4dB/20kHz [dashed blue trace]. '1ohm' speakers are spectacularly rare but, should you own vintage Apogee Scintillas, then driving them via the 2-4ohm taps yields a soft –3.8dB/20kHz [dashed green trace]. To achieve the best results with very low impedance speakers then VAC's 1-2ohm taps are essential – reaching out to –0.6dB/100kHz with 2ohm loads [blue trace] and –0.9dB/100kHz into 1ohm loads [green trace]. PM