Similarly, the 840A did a spectacular job with the sound of keyboardist Tommy Washington’s electric organ. If you listen closely, you’ll observe that some electric organs (typically older Hammonds) produce a soft, scratchy “click” just as their keys are depressed. These clicks might actually be indicative of excess wear in the instrument, but experienced blues keyboardists like Washington use them to give the notes in fast-paced runs a bit more kick and definition. The Cambridge amp nailed the powerful sound of the organ, clicks and all, and it perfectly caught the eerie shimmer of the Leslie rotary speaker used to give the organ its voice (Leslie speakers feature a rotating horn tweeter whose sweep speed can be controlled by a foot pedal).
Finally, the Cambridge did a gutsy job with the sound of Tracy Mortimer’s electric bass, which sounds clean, clear, and thunderous on the Untapped Blues Festival disc. Even though four-string basses don’t reach down into true low bass territory, they are still difficult to reproduce, partly because they have deceptively complex timbres, and partly because they impose abrupt, large-scale power demands on amplifiers. But even when I cranked “Let’s Set the Time” up to very invigorating levels, the Cambridge took Mortimer’s chunky, propulsive bass lines in stride while keeping the rest of the band in sharp focus.
Thus far, we’ve focused on the 840A’s strengths, which are wonderful and exciting, but we should also discuss two areas where the amplifier’s performance is good, but not great. First, the amplifier’s treble response, though clear and well-detailed, is shelved downward a bit, at least relative to some of the more transparent-sounding ultra high-end amplifiers I’ve heard. But don’t get me wrong. The 840A’s highs never sound “soft” or diffuse; it’s just that they are very slightly recessed in the mix. Second, the 840A fails to achieve the sculptural three-dimensionality that competing amplifiers such as the YBA Designs YA201 provide. By A/V standards the Cambridge is an extremely detailed amplifier, but even so it can downplay small sonic cues that could reveal the acoustics of recording venues, or suggest the physical presence of instruments or vocalists. Listening to the 840A is like gazing at a high-resolution photograph, while hearing the YA201 is more like viewing a sculptural object. Good though the photograph may be it never conveys the substance and smooth, continuous shadow detail that the sculpture possesses.