Manley Snapper monoblock tube power amplifier Measurements
The Manley Snapper's single-ended frequency response measured -0.04dB at 10Hz, -0.01dB at 20kHz, and -0.14dB at 50kHz. The balanced response at the 15kohms input setting measured -0.05dB at 10Hz, +0.06dB at 20kHz, and +0.34dB at 50kHz. The Snapper's gain measured 31dB single-ended and 25dB balanced. The A-weighted signal/noise ratio at 1W into 8ohms was -96dB using the balanced inputs (15kohms setting). THD+noise at 1W into 8ohms measured 0.076% at 20Hz and 0.015% at 20kHz with a balanced input. At 2W into 4ohms, balanced, the figures were 0.2% at 20Hz and 0.025% at 20kHz. The output impedance measured 1.5ohms.
Into an 8ohms load at 1% THD+noise the Snapper delivered 49W at 20Hz and 37W at 1kHz. Into 4ohms, the values were 13W at 20Hz and 12W at 1kHz. Into an 8ohms load at 2% THD+noise, the output increased to 101W at 20Hz and 104W at 1kHz into 8ohms (into 4ohms at 2% THD+noise), the respective values decreased to 31W and 24W. The results here are to the nearest watt, at a line voltage of 120V.
These measurements reflect some of the compromises often found in tube amplifiers. Their distortion is often far higher at a given power output than that of a good solid-state amp, but they distort more gradually as power demands increase. Tube enthusiasts correctly note that tube distortion is often heavily second-harmonic, which is known to be audibly pleasant in comparison to odd-order harmonics, but it's still distortion. And a good impedance match between load and amplifier can be difficult to achieve with tube amplifiers, most of which provide several output taps to better match a given speaker. The VTL has a single tap.
We've also measured the Snapper's output impedance, for an important reason. A high output impedance affects the frequency response of a speaker-amplifier combination in ways related to the speaker's impedance (magnitude) curve. The deviations are often large enough to be clearly audible. In other words, a tube amp may well sound different with different speakers. Solid-state amps almost never have an output impedance high enough to have this effect—which is why we don't usually measure this parameter. The Manley's output impedance, while not unusually high for a tube amp, is still high compared with typical solid-state designs, and may therefore have some effect on the frequency response of the speaker-amp combination.—Thomas J. Norton