Your car is full of computers that make it easier to operate and maintain; why shouldn’t your hi-fi follow suit? Increasingly, that’s what’s happening. One of the more recent computerized hi-fi circuits sets the bias current of the output tubes on tube amplifiers. Those circuits make it feasible to switch easily between different types of tubes. So if an amplifier is shipped with EL34s, you can change them to KT88s and the automatic-bias circuit will adjust the bias to suit the KT88s with no adjustment from the user. Just insert the new tubes and the bias circuitry changes the necessary parameters. Rogers High Fidelity’s new Corona KWM-88 integrated amplifier’s automatic bias circuitry makes it possible to switch between KT88s and KT150s. It’s your choice. Just specify when you buy the amp which output tubes you want. (Both types produce the same power.)
The Corona deliberately leaves out features that can be found on several other integrated amps. There’s no DAC, no phono preamp, no headphone amp. This makes sense to me; if you’re willing to shell out $15,000 for the Corona, you’re probably a very serious listener who wants an advanced stand-alone DAC, phono preamp, and headphone amp. You know exactly which of those units suits your system, and don’t want to waste money on the type of limited or compromised integral DAC, phono preamp, or headphone amp that’s typically included in some integrated amps. Rogers makes a very interesting phono preamp, the PA-2, which is definitely worthwhile considering to match to the Corona, but doesn’t offer a DAC or headphone amp. The Corona has the typical integrated amp advantages: It requires no preamp-to-amp interconnect, and it occupies just a single shelf, albeit a very sturdy, well-ventilated shelf.
The Corona is built on a wide, flat chassis with a silver, brushed-aluminum faceplate in the front, tubes in the central area, and in the rear a transformer that spans the full width of the chassis. In a departure from the norm, the chassis is painted red instead of the usual Boring Black. The red color is not a garish, bright hue, but sort of a candy apple red. I thought the combination of the brushed aluminum faceplate and transformer trim plate with the red paint on the chassis looked quite elegant and attractive. (I’d welcome more color departures from black chassis. Maybe not lime green, though.)
The faceplate has, from left to right, four toggle switches (power on/off, standby, mode, and input) along the lower part, with a blue information window on the upper part. The labels for the switches are engraved in the front panel, which looks nice but in my view is harder to read than painted labels. To the right of the switches and information window is the silver, serrated, single-knob volume control, CNC-machined from aluminum. The right half of the faceplate is reserved for two VU meters. I don’t know that the VU meters serve a functional purpose, but they surely look cool, with their blue illumination matching the status window on the right. There’s no mute control, balance control, mono switch, or phase-inversion switch, either, all of which might have been useful. However, there’s a fair chance none of those controls would ever be used, so leaving them off makes financial sense.
The tube choice is a bit unusual, comprising two output tubes per channel--normally KT88s or KT150s (KT120s will also work)—and one 6SJ7 and one 6SN7 low-level tube per channel. The 6SN7 is a very common dual triode, but the 6SJ7 is rather unusual in current amplifiers. It’s a pentode with a metal rather than a glass envelope. The 6SJ7 tube serves as the input stage and provides all the gain for the amplifier. I asked Rogers’ President and Chief Engineer Roger Gibboni why the 6SJ7 was chosen instead of a traditional triode and he said, “I use the 6SJ7 in the front end for very specific reasons. First, they are still in production. They are available from Sovtek and a Chinese manufacturer. In general, we prefer the Russian versions for reliability. The 6SJ7 was in such frequent use that there are many sources of NOS tubes still available and affordable, and we will use them as long as we can get them. You were supplied with the 5693—a special red version of the 6SJ7. These were designed for the military and are high-reliability versions. They have extra structures inside that allow them to be employed in environments of shock and vibration up to 10Gs. This extra rigidity also gives them much lower noise characteristics than standard 6SJ7s, and we sell them as an upgrade for $125 each.
“I use the 6SJ7, which is a sharp-cutoff pentode, in the front end rather than the traditional triodes because you can achieve much higher gain in a shorter signal path. We get 35dB of gain in one stage that with traditional triodes would require two or three stages of amplification. This yields much lower phase distortion because the gain is achieved in a shorter signal path.
“In almost all cases, the small signal tubes—6SJ7 and 6SN7—have almost infinite lifespans. They run very little current and are not under stress in the amplifier. The user should expect 5000 to 10,000 hours with these tubes.
“The KT88 or KT150 power output tubes on the other hand run significant current—approximately 125mA per tube. This operating condition achieves the dynamic range and transient response you hear in the Corona. These tubes typically last 2000–3000 hours, about two years of normal use. The beauty of the Corona is that the bias condition of these power tubes is managed by the on-board processors. When a tube exceeds its normal self-bias range due to aging or failure, the processors shut down the affected channel and indicate which tube requires replacement on the screen and app. Because the output tubes are auto-biased, the user does not have to replace the power tubes as sets. Only the failed tube requires replacement. We supply tested and burned-in KT88s for $135 each and KT150s for $175 each. While we have very few requests, the Corona will also run the KT120.
“The Transparent power cable you were supplied is a $75 upgrade to the normally supplied power cable.”
The 6SN7s drive the output tubes for a 100Wpc rating in Ultralinear or 80Wpc in triode mode. The output tubes sound different, and since I was provided with a set of each type of tube, I’ll explore the sonic differences. I’ve experimented with both types of tubes in my Audio Research VT80SE amplifier and can see why some people would prefer the KT88 sound. KT88s are popular tubes in production by several manufacturers, so you can pick the ones you like, but, again, Rogers rigorously tests the tubes it provides, so it’s not a bad idea to buy their replacements. The same goes for the 6SN7 tubes, which are in production from several manufacturers. The Corona shipped with Gold Lion tubes, except for the KT150s, which were made by Tung-Sol. Those are all high-quality, current-production Russian tubes.