PrimaLuna DiaLogue Premium Integrated Tube Amplifier

A Tube Benchmark

Equipment report
Categories:
Integrated amplifiers
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Products:
PrimaLuna DiaLogue Premium
PrimaLuna DiaLogue Premium Integrated Tube Amplifier

As a product category integrated amplifiers make a lot of sense. Digital components such as CD players and stand-alone DACs already output a nominal 2V while power amp sensitivity is typically 1V or better. Given this context there is little reason to insert a preamp with from 18 to 20dB of gain into the signal path and then attenuate the input signal in order to avoid overdriving the power amp. Combining preamp functions with a power amp on one chassis makes both economic and sonic sense. There are fewer interconnects and active devices in the signal path, and no ground loops to worry about, the end result being a simpler and more direct signal path.

The new DiaLogue Premium from PrimaLuna retains a fair amount of preamplifier functionality including control over five line-level inputs, one home-theater pass-through, one mono subwoofer output, and remote volume control. Most notably missing are a balance control and tape loops. An optional phono board ($199) is available (though not reviewed) which is said to be compatible with both moving-magnet and high-output moving- coil cartridges and can either be pre-installed or added at a later time by anyone with basic soldering skills. A “premium” phono board with higher gain for low-output moving-coil cartridges is in the works at a price yet to be announced.

As you might expect, the Premium version of the DiaLogue features premium parts such as an ALPS volume control, and Takman resistors and SCR tinfoil coupling capacitors in critical signal-path locations. The front end is now all 12AU7-based, and that’s a good thing from a practical standpoint, since relative to the 12AX7 good-sounding new old stock 12AU7s are easier to find at reasonable prices. The first 12AU7 is configured as a conventional cascade voltage amplifier. The other two preamp tubes form a long-tailed-pair phase-splitter, with each dual triode connected in parallel for improved drive capability. The output stage is connected in Ultralinear (UL) mode with the screen grids tied to taps on the output transformer primary via relays. The output stage can be switched to triode mode using the remote control. The LED indicator on top of the chassis changes from red (Ultralinear) to green for triode.

Of course, all of the features of the DiaLogue Series have been retained. To recap, the output transformer is protected against a catastrophic internal tube short. The power transformer incorporates a thermal switch that opens the primary winding to prevent overheating. An adaptive auto-bias circuit is used to keep the idle current of the output tubes within an optimal range. The bias current is sampled continuously in real time at the cathode of each of the four power tubes and fed to a four-channel comparator chip. A corrective signal is generated and used to keep the bias voltage at each power tube’s control grid near its optimal value under most dynamic conditions. The circuit includes an EL34/KT88 bias-selector switch to accommodate both tube types.

Note that the Premium Series, like all PrimaLuna designs, is compatible with the KT120, which should be used in the KT88 bias setting. The EL34 is supplied as standard, but customers can order the amplifier with the Gold Lion KT88 for an additional $120, or the Tung-Sol KT120 for an additional $80. The auto-bias benefits to the user are twofold: freedom from the chore of manual bias checks and adjustments as tubes age, as well as the flexibility to roll in a variety of power tubes. Finally, the power supply includes a soft-start circuit and an “AC Offset Killer” circuit whose function it is to keep DC offset on the AC mains from reaching the power transformer. Without it there would be the potential for increased transformer vibration.

Experimentation with the 4- and 8-ohm taps revealed large sonic differences between the two in the context of a 96dB- sensitive speaker-load. In general, and this is also PrimaLuna’s advice, one should experiment with both sets of output taps to determine which sounds best with a particular load. There are simply no short cuts available. The savvy audiophile is well aware of the difficulty inherent in trying to assign a nominal impedance value to a speaker when typically impedance magnitude varies by over a factor of 10. The nominal impedance quoted by a speaker manufacturer is often tied to the minimum impedance in the upper-bass range, but matching it isn’t sufficient to assure optimal sound quality. The speaker impedance is reflected through the output transformer and modifies the amplifier’s performance relative to a purely resistive load. Therefore, as far as tap selection, let your ears be the final arbiter. Switching from the 8- to 4-ohm taps in UL mode brought about an immediate gain in rhythmic drive and soundstage transparency.