Conrad-Johnson PV9 preamplifier
Which brings me to the subject of this review. In a world that seems to be becoming increasingly frantic to junk the LP in favor of, yes, CD, but mainly cassette, Conrad-Johnson's $2950 PV9 appears to be a deliberate anachronism. It uses tubes. It is not particularly neutral. Its performance is optimized to get the most musical performance from LP. It was with some degree of anticipation, not to say trepidation, therefore, that I embarked upon this review.
Whereas the PV9's predecessor, Conrad-Johnson's Premier Three, was electrically quite complicated, with cascoded triodes acting as the basic voltage-gain element, the PV9 has a more direct signal path. Following an LP signal from its entry point, it first goes via twisted solid-core wire to the main printed circuit board where it is fed to both grids of a 12AX7 twin-triode tube. The use of both tube sections in parallel gives a 3dB reduction in noise power. The amplified signal is then buffered by half of a 5751 tube used as a cathode follower before being applied to a passive RIAA equalization network.
All capacitors used in the signal path, including the RIAA network, are polystyrenes. A physically large, 0.15µF, Conrad-Johnson polystyrene coupling capacitor then feeds the de-emphasized signal to a second voltage-gain block, this consisting of half of a second 5751 triode for voltage gain and half of a 5965 which acts as a cathode follower/buffer. In their literature, C-J claims that the PV9 is a zero-feedback design. However, I assume that they are only referring either to the triodes used for voltage gain or to the overall loop, as a cathode follower by definition has 100% negative voltage feedback.
The output from the phono stage is AC-coupled to the source selector switch (this appropriately mounted at the unit's rear adjacent to the line-input sockets) with a ginormous 2µF C-JD polystyrene capacitor. The selector switch is coupled to the front-panel tape output-select and source/monitor pushbuttons by ribbon cable and one twisted pair of thin solid-core flying leads to a small pcb mounted on the front panel, this carrying the balance and volume controls.
Balance is effected by a rotary switch, with five steps in each direction reducing the level of the appropriate channel by approximately 1.5dB—I measured between 1.2dB and 1.6dB—which may be a trifle coarse for some tastes. The volume control is a large (and expensive) ALPS 100k pot and its output is taken, via thin solid-core cable, to the line stage on the main pcb. This consists of a 5751 twin triode voltage amplifier followed by a 5965 cathode follower, this AC-coupled to the output via more C-JD capacitors. A small DIP-mounting relay mutes the signal for about 90 seconds on turn-on before it is fed to the twin output sockets.
C-J burns-in their tubes and warns in the PV9 owner's manual against substituting other tubes for those specified. Replacing the high-gain 5751 triodes with 12AX7s is said to seriously degrade the sonic performance, while substituting a 12AX7 for the 5965 will damage the replacement tube. The only alternative suggested, and this with a caveat that the inevitable degradation will be subtle, is to use a 12AT7/ECC81 for the 5965 cathode followers. The PV9 tubes are said to have a typical lifetime of between two and three years.
The phono circuit, having two gain stages, will be absolute-polarity–correct, but the PV9 line section, it should be noted, is polarity-inverting.
Construction is to a high standard. Nonmagnetic aluminum—yes, I know, there isn't any other kind—is used for the chassis, top plate, and front panel. Apart from in the power supply, precision metal-oxide resistors are used throughout and, with the exception of a 4700µF electrolytic used to smooth the filament supply upstream of its voltage regulator chips, and a couple of small electrolytics in the relay supply, all the smoothing capacitors are low–dielectric-loss C-JD polystyrenes and polypropylenes, with physically enormous 13.5µF, 20µF, and 30µF values used as reservoirs for the plate supplies. A Pi filter, consisting of two 20µF caps connected by a power resistor, feeds smoothed DC to three separate high-voltage regulators, one for each gain block. These regulators are constructed from discrete transistors and use totem poles of Zener diodes as their voltage references.
The bugbear of high-gain tube preamplifiers is, of course, microphony. C-J minimizes this in the PV9 by floating the main board, the one that carries all the tubes, on a rubber suspension. (The owner must remember to free this suspension by loosening two bolts before using the PV9.) This arrangement appears to be pretty effective at isolating the tubes from chassis-borne vibration: tapping the phono input tubes produced a slight bonk through the speakers, while virtually hammering at the chassis nearby produced no microphonic noise at all. Strange to say, once I had put the cover back on, hitting it vertically did produce some microphonic noise through the phono input, though lateral shock was still well isolated.
Footnote 1: All true audiophiles should check out "A Song of Reproduction."