TESTED: Conrad-Johnson ET2 Preamp & LP66S Stereo Power Amplifier

Entry-Level Magic

Entry-level may mean different things to different manufacturers, but at C-J it definitely does not denote a major sonic penalty relative to its ultra-high-end products. Cost containment may be implemented through circuit simplification, judicious passive part substitutions, or watered-down cosmetics. Well, since C-J’s design philosophy already emphasizes simplicity—“circuits should be kept as simple as possible”—that mostly leaves the other two cost-cutting avenues open. The ET2 with its acrylic tube cage is surprisingly far more upscale-looking than one would expect from its price tag. The LP66S power amp, on the other hand, does project a Spartan appearance, especially with its tube cage off. But as you will soon discover, component quality—a major factor in any C-J product—is still extremely high.

A major part-selection decision was necessary in the case of the ET2’s volume control. According to C-J’s Lew Johnson, a discrete stepped attenuator would have ideally been the top choice in a pecking order in which a standard potentiometer defines the lowest step on the totem pole. He opted for the middle ground, a high-performance Burr-Brown volume-control-chip. One advantage this chip affords is the ability to remotely control volume without the need for an on-board motor. Another is the ability to adjust volume in 0.5dB steps, while most discrete stepped attenuators can only do 1dB steps. Because this is a stereo control, it is possible to adjust left and right channel volume independently to obtain balance control. The preamp’s sensible front-panel layout displays volume settings with sufficient size to be easily discernible (at least with my glasses on) from across the room. I found the modest-looking remote control to be perfectly adequate. Two external processor loops are provided, one of which is designed expressly for the addition of a surround-sound processor to a two-channel system. The other is conventional and allows the connection of a tape deck or equalizer.

Lew Johnson was kind enough to describe for me the basic circuit topology for both products under review, and what follows is based on this information. The ET2 linestage features a single gain stage. Following the volume control, the signal is applied to the grid of a Mullard M8080 medium-mu triode, which was billed by Mullard as a reliable RF power triode. The gain stage is direct-coupled to a high-current MOSFET buffer circuit to achieve low output impedance, and this arrangement comprises C-J’s Enhanced Triode (ET) circuit.

The optional phonostage’s input is a 12AX7 dual triode operated in parallel for reduced noise. It is coupled through a passive RIAA equalization network to a second gain stage which deploys a single section of either a 12AX7 (high-gain version) or a 12AU7 (low-gain version). As with the linestage, the signal is direct-coupled to a MOSFET buffer stage for low output impedance. The high-gain phonostage is best suited for low-to-medium-output moving-coil cartridges with a rated output of 1.0mV or less. The low-gain phonostage is recommended for cartridges with a nominal output above 1.0mV. No global loop feedback is used in this product. Due to the linestage’s single gain stage, the ET2 inverts signal polarity on all of its outputs.

Separate discrete regulated power supplies are used for the linestage and phonostage plate circuits. It’s worth emphasizing that resistor and capacitor choices are top-notch—no skimping here! A peek inside the chassis is worth a thousand words! Plate resistors for all stages are large Vishay metal-foil resistors, while all other resistors are precision metal-film types. Plate power supplies use polypropylene capacitors with Teflon bypasses (0.15uF) exclusively. Output coupling capacitors are a composite of polypropylene and Teflon, while the RIAA network capacitors are polystyrene types.

The LP66S is rated at 60Wpc into 4 ohms, and the output transformer is wired that way by default. There is only a single set of binding posts, so there’s no selection of impedance taps for a particular speaker load. However, the amp may also be ordered with 8- or 16-ohm load connections. This amplifier’s circuit is also about as simple as can be. The input voltage gain stage (half of a 6922) is direct-coupled to another 6922 which is configured as a coupled-cathode phase splitter and also provides the drive voltage for two pairs of Russian 6550 beam power tubes operated push-pull. The output stage is connected in ultralinear (UL) mode to the output transformer. Even over 50 years after its invention, UL remains a popular alternative to pure pentode mode, and that’s what the LP prefix in the model name refers to: Linear Pentode. A limited amount of loop feedback is used to obtain a reasonable damping factor and to minimize distortion levels. Plate supply voltages for both the input and phase-splitter circuits are regulated. You won’t find electrolytic caps anywhere in this amplifier. This is most unusual (and an added expense) for any amplifier, let alone an entry-level product. All caps are polypropylene and polystyrene types, including the main power-supply storage reservoir, which is polypropylene. All resistors are metal film. The output transformers are the same wide-bandwidth designs used in the more expensive LP70S amplifier. Bias adjustment is a piece of cake due to built-in LED bias indicators which allow the user to properly set the bias using only a supplied screwdriver.