Bob Carver VTA20S “Black Magic” Power Amplifier

Magic, Indeed

Equipment report
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Tubed power amplifiers
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Bob Carver VTA20S
Bob Carver VTA20S “Black Magic” Power Amplifier

Bob Carver LLC’s latest addition to its power amplification family is a 20Wpc stereo amplifier that has been affectionately dubbed the “Black Magic.” If you were expecting another beefy, high-power tube design from an engineer known for muscle amps, then you would be as surprised as I was at a diminutive amp that could comfortably be carried under one’s arm. You could look at it as an entry-level product, but part of the motivation for its addition to the line was Bob Carver’s love for the El84/6BQ5 pentode. He says: “It’s easy to get great sound from them—I don’t know exactly why, but it just is. More amplifiers on the face of our planet have been made with 6BQ5/ El84s than any other tube. And not by a little bit—a full ten times more.” Indeed, the El84 became wildly popular in the late 50s and into the 60s, partly because it was less costly to manufacture, but also since its higher gain characteristic relative to an El34 or KT88 power tube allowed for a simpler circuit topology that did not necessarily require a driver stage. And simpler as most of us know is usually better. This was an age when 10 to 15Wpc from a Class AB push-pull stage were considered adequate for domestic consumption. Two well-known hi-fi examples, which I happen to own, are the leak stereo 20 and the Tl12+. Fender, Gibson, Marshall, Vox, and Peavey all offered El84 guitar amps, which in some models pushed in excess of 20 watts.

Carver’s circuit topology is fairly simple. The input signal first passes through a 250k ohm volume control and is then amplified by one half of a 12AX7. The second half of the 12AX7 is configured as a cathodyne phase-splitter. That’s it—no driver stage is used. The push-pull output stage is pentode connected and uses a pair of self-biased El84Ms per channel. I estimated the anode dissipation to be about 11 watts, which is well within the 14W maximum rating of an El84M. Bob Carver indicated that the bias is actually set for a nominal plate dissipation of 10.5 watts to allow for tube variation. Note that the El84M is a mil- spec version of the standard sovtek El84 with extended voltage tolerance, improved plate dissipation, and rugged construction.

I would not recommend substitution of a standard El84, as it is only rated for a maximum dissipation of 12 watts.

As with other Carver designs, I’ve come to expect at least one interesting design twist, and the Black Magic did not disappoint. A common cathode resistor is used for all of the output power tubes. The voltage across this resistor is held constant by a Zener-diode clamp at about 14.5V. The helps minimize distortion during high-drive conditions. Otherwise, Carver explains, “As the current increases, so does the bias voltage, which makes the grids more negative, allowing distortion to go up. By using a common cathode resistor, the bias voltage is regulated even better because, on average, with speech and music the two channels are not driven exactly the same. This allows the least-driven pair of output tubes to act as shunt regulators for the other two, helping to keep the bias regulated. The effect is quite significant, and flies in the face of what I have always been taught—that is to use separate cathode resistors. Not a good idea, actually. A brief inspection of many 6BQ5/El84 amps reveals every possible way under the sun of arranging the cathode resistors. Separate ones for each tube, a separate one for each pair of tubes, bypassed, not bypassed. How many combinations is that? A lot anyway. That tells me that there is no known consensus. I like my way the best, especially with the clamp.”

The Carver is officially rated for a minimum load impedance of 4 ohms. There’s only one set of speaker terminals on the rear panel, which gives no options as far as impedance-matching is concerned. However, loads with a nominal impedance in the range of 4 to 16 ohms should work well with this amp. You may double your pleasure by switching the amp over to monoblock operation and using a pair of amps for stereo playback. Doing so doubles the power output to 40 watts. Although the Black Magic is promoted as a basic power amplifier, it can be effectively used as an integrated to control a single line input via its volume pot, but do keep interconnect runs fairly short in such a configuration. A current feedback loop is used to raise the source impedance to a respectable 1.5 ohms and confers a decent damping factor. The power supply is fairly basic and includes a full-wave solid-state rectifier, AC filament supply, and Zener-diode regulation for the power-tube screen voltage. The output transformers are of good quality and internal wiring is point to point.

Noise and hum weren’t issues even when driving a high- sensitivity loudspeaker. Residual hum was only audible with the DIY Basszilla Feastrex Edition with my ear positioned a few inches from the full-range driver. It took on the order of 15 minutes from power up for the amp to settle down and gel sonically. Soundstaging and image focus improved significantly during this warm-up period.

My first listen was with the Acoustic Zen Crescendo loudspeakers and I was immediately impressed by how huge a soundstage the Black Magic was able to create. In this respect it clearly belied its physical size, generating a spatially expansive soundstage with a nicely layered depth perspective. A surprising amount of transient detail was being retrieved with commendable clarity. For example, a ride cymbal in a dense jazz mix was readily resolvable. In fact, the entire top end was clean and well extended. The bass range was also surprising with a good measure of bass definition—an uncommon finding for a pentode amp.

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