Tube Preamp Reviews

Whenever an audio high-ender thinks about tubes, he usually thinks about Audio Research. This is only natural, because Audio Research Corporation was almost single-handedly responsible for saving tubes from oblivion in the early '70s when everyone else switched to solid-state. But ARC was soon joined in its heroic endeavor by an upstart company called Conrad-Johnson, which entered the fray in 1977 with its PV-1 preamp, priced at an affordable (even then) $500.

Today, Conrad-Johnson is firmly established as the second most highly regarded manufacturer of tubed audio products. (In fact, ARC's recent ventures into hybridization—the combining of tubes and transistors—may have disqualified that firm from being counted as a "tube-electronics manufacturer," in which case C-J must now be considered the preeminent name in the field, footnote 1)

Until 1983, all of Conrad-Johnson's products were designed as "best value for the money," to provide much of the best that tubes had to offer at less-than-princely prices. Audio Research, for its part, has tried with each new product to redefine the state of the art in tubed design, as well as the amounts of money perfectionists would pay for it. But then C-J entered the no-holds-barred arena in 1983 with their first Premier product, the Model One power amplifier, which was very well received by critics. (When I reviewed this behemoth in Vol.6 No.5, I felt it to be the best amplifier at that time for driving big electrostatic panels.) The Premier Seven is C-J's latest cost-no-object product. It's also their most expensive preamp to date, and the costliest one on the market, apart from the most expensive version of the Cello Audio Suite. (As I write this, I'm mulling over the recent announcement of ARC's latest no-holds-barred, cost-no-object preamp, the SP15, which is expected to sell for $10,000. Where, oh where, will this madness end?)

Bill Conrad and Lew Johnson have pulled out all the stops in designing the Premier Seven. It appears that nothing which could possibly have improved its sound has been omitted. For example, we all know that so-called dual-mono circuitry enhances soundstaging, but the Premier Seven is dual mono like no other preamp I've seen. The power-supply "unit" contains two completely separate supplies—each on its own chassis, each with its own line cord and on/off switch, and each containing separate power transformers for the tube heaters and the B+ supply.

Even the volume-control circuitry is completely isolated. There is no master volume control, and no balance control; each channel has its own volume control, and there is not even a mechanical coupling between them. Of course, this means that there is also no mono operating mode. The volume control in each channel is a 22-position detented rotary switch. Under each volume knob is a horizontal row of LEDs, with panel markings indicating decibels of attenuation from 0 (at the far right) to infinity. (Unlike the usual LED bargraph, which displays a lengthening line of glowing lights as the signal level increases, only one light on each of these is lit at any one time.) The volume increments are in 2dB steps down to –32dB, below which the steps become increasingly large. One set of contacts on each volume switch connects the appropriate voltage-divider resistors, while the other set draws from a DC power source to light up the appropriate LED on the volume indicator array.

There are 12 gold-plated signal receptacles at the rear, connected to the appropriate traces on the glass-epoxy circuit board by short lengths of 300-micron linear-crystal, solid-silver wire. The Source and Record selector switches are at the rear of the chassis, near the circuits they control, with long shafts coupling them to the front-panel knobs. Power-supply connections are via hermetically sealed, collar-locking MIL-spec connectors and a pair of fat, 4' umbilical cords sheathed in black braided nylon sleeves.

The main circuit board in each signal chassis is shock-mount "floated" from the external case by rubber grommets, and all the 6GK5 tubes are fitted with damper rings. (Actually, they're large rubber grommets.) All of this is intended to minimize modulation of the signal by tube microphonics in response to external vibrations, but while I can see how the rings around the tubes might help here, the subchassis suspension seems too stiff to give effective isolation. The tubes are, in fact, incredibly free from microphony; at normal volume settings, even tapping them with a fingernail elicited nothing more than a dull and very brief "thunk." I suspect, however, that this is more a matter of their construction and the way they are used than of vibration isolation and damping.

The Circuitry
Each signal channel in the Premier Seven consists of three gain blocks (C-J's nomenclature), each comprised of a triode and a cathode-follower. The 6CW4, the first tube in the phono preamp stage, is a nuvistor—a thumbnail-sized tube introduced for ultra–high-frequency radio applications just when transistors were taking over electronics design. The input of the 6CW4 is bridged by a small 10-position rotary switch that provides a choice of pickup loading resistors, ranging from 70 to 47k ohms.

RIAA equalization happens between the first cathode follower and the second preamp gain stage, and is entirely passive. The second cathode follower feeds the Source and Record switches, and also provides a low-impedance source that should be immune to the loading effects of long signal leads to the recorders.

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Footnote 1: Dark horse Vacuum Tube Logic is probably C-J's real competition in the tube field now, but limited production and availability hinder VTL's recognition as a Name in the tube electronics field.