Why tubes—objectively

If cries of fraud and derisive comments about "magical sound" sometimes greet the use of tubes in audio equipment, there are also highly competent electrical engineers who see definite advantages in tubed equipment. An example is John Atwood, consulting engineer and owner of One Electron Co., Santa Clara, Calif. The erstwhile designer of application-specific ICs and other solid-state logic circuits has managed to transform his hobby of tube audio design into a full-time consulting business.

In Atwood's opinion, "Some of the differences in the audio qualities between tubes and transistors have to do with the inherent physical properties of the devices and with the circuit topologies and components used with each type of device. There is no way around it: linear [triode] vacuum tubes have lower overall distortion than bipolar transistors or FETs, and the distortion products are primarily lower-order...the clipping characteristic of tubes is actually not much softer than transistors, but feedback tends to 'square-up' the clipping. Thus, the heavy feedback in most solid-state designs gives them worse overload performance.

"A low- or no-feedback design can be driven harder without audible distortion," Atwood continued. "High feedback also can lead to transient intermodulation distortion (TIM), caused by clipping or slew-rate limiting within the feedback loop." See following table for a comparison of the attributes of tubes and transistors in audio applications.