Workshop #2
Amplifier Listening Comparison

Few have ever been able to participate in a real-time, level-matched comparison of two power amplifiers driving the same speaker system in the identical acoustic environment. Two amplifiers were auditioned in real-time listening comparisons using a line-level matching scheme that does not degrade audio quality or introduce impedance-dependent colorations (e.g., as opposed to the use of L-pads that kill the damping factor). The listening comparison involved a 35 Watt per channel vacuum tube amplifier and a very well-built solid state Denon amplifier rated at over 200 Watts per channel.

This was an interactive workshop, where one participant held a remote switch and could select either the vacuum tube or the solid state amplifier. Some people came away from this presentation amazed at how hard it is to hear variances in obviously different components, while others thought they could perceive subtle audible differences.

The vacuum tube amplifier was one of "classic" late Sixties type design. It was a DIY design originally built in 1966 that was recently completely updated. Each channel employs a pair of KT-88's in a Class-AB pentode arrangement with fixed bias to deliver 35 Watts per channel (yes, the KT88's were loafing at this power level). The amplifier employs a modest degree of negative feedback typical of amplifiers of the time, and achieves a damping factor of about 20. The amplifier is described in more detail elsewhere on this site.

As an aside, we knew we needed to disable the Peak/Average power display during this comparison so that clipping-limited passages would not be a dead giveaway as to which amplifier was the tube design.

Switching between amplifiers was done both at the line level input side and the speaker level output side of the amplifiers. The amplifier not selected received no input signal. Moreover, that amplifier was loaded with a dummy load, as it is well-known that some vacuum tube amplifiers may go unstable with no load.

The comparison was level-matched to within 0.1 dB, as read at the speaker terminals with a 1 kHz test tone. The level matching was done using the input level controls available on the Denon amplifier. Of course, the output impedance of the tube amplifier driving the varying impedance of the loudspeaker could have caused tenths of a dB variation at other frequencies across the band. This could have led to some coloration differences between the tube amplifier and the solid state amplifier that would distinguish their sound, but this is an inherent part of the whole tube vs solid state array of differences.

One participant sitting in the sweet spot in a comfortable chair was given a remote switch to select which amplifier to audition. The switch was in a very small plastic project box and included a green LED that was illuminated when one of the amplifiers was selected. There was also a second box atop one of the speakers for all to see with a larger pair of green LEDs providing the same information. A variety of music could be selected for the comparison.

At the end of the session the attendees were polled in a very informal and unscientific way as to which amplifier they thought was the tube design. They could answer one amplifier or the other, or they could answer that they honestly could not tell which was which. Attendees were cautioned before the comparison that the amplifier that was selected when the green LED was ON might not be the same from session to session. This was done to discourage the influence of hallway banter during the show. At the end of the show it was disclosed that the vacuum tube amplifier was always the one being played when the green LED was ON.

There were no "night and day" results. Indeed, for most attendees the differences were difficult to hear. Moreover, those who perceived a difference were just as often wrong in selecting which amplifier they thought was the tube amplifier. This shocked all of us.