Tube Power Amp Reviews

Manufacturer's Comment
We did not design the H-3A specifically for use with the KLH Nine. In fact, monophonic versions of the basic design were in use 15 years ago, mainly with the RCA LC-IA loudspeaker, with which it gave superb results.

There is one coupling capacitor in each channel of the H-3A; computer-grade 800µF capacitors are used to isolate each loudspeaker from the output tube circuitry. In our opinion, direct-coupling a power amplifier to a loudspeaker (especially with transistor amplifiers) is inherently dangerous to the speaker system. In fact, some of the latest high-powered transistor amplifiers use elaborate and expensive circuitry to protect the loudspeaker from damage due to the direct coupling should some "unlikely" defect occur in one of the output stages. We emphasize this point because the subjective listening results that Stereophile obtained in the low-bass region may well be due to the size of the coupling capacitors used in the H-3A that was tested. We ore currently experimenting with much larger values of capacitance, as this is the only part of the H-3A's entire circuit that could affect the very low bass response.

We have found, and eliminated, the ground loop that was causing hum from three-terminal pickups.

Reviewer's Addendum
We, too, had wondered if the output coupling capacitors might not be responsible for the slight rolloff at the extreme bottom, but could find no conclusive objective evidence to support this, There was some tilt on bass squarewaves, but no more so than from other top-grade amplifiers, and the measured frequency response was within 0.5dB of flat down to a bit above 25Hz. On the other hand, since there isn't anything else in the circuit that could affect low end, it is quite likely that increasing the size of the coupling capacitors will indeed improve the low-bass response.