PS Audio 4.6 preamplifier

666ps46.1.jpgChoosing a moderately priced preamp has traditionally presented the audiophile with a host of serious problems. Most attempt to be all things to all listeners, expending resources on bells and whistles which would have been better expended on basic performance. Few have anything resembling a decent moving-coil stage. But there have always been a few designers (and companies) willing to expend much of their effort at the "low end of the high end." PS Audio has been such a company. Their new 4.6 preamp, an update and cosmetic clone of the earlier, well-received 4.5, is not at the top of their preamp range—that honor belongs to the 5.5—but it is clearly designed to be more than a price-point product.

The design characteristics and features of the 4.6 follow those of the 4.5 closely. In fact, my sample of the 4.6 had a very thorough 4.5 instruction manual. Either low-output (moving-coil) or high-output (moving-magnet or high-output moving-coil) cartridges can be accommodated. The same inputs are used for both, with internal switch selection and sockets for adjusting the input load resistance; several useful values of plug-in resistors are included. Full dubbing for two tape decks is provided, with the ability to record from one source while listening to another (there must be a use or demand for this, although to my thinking it falls a bit below year-ahead VCR programming in social significance).

But do heed PS's warning: don't put the tape feed switch into the tape 2 position when tape 2 is recording. No, I didn't try it; I didn't want to see the opening scene of Back to the Future in my listening room. I am more than a little uncomfortable with such a design quirk; someone, somewhere, is guaranteed to do it (corollary 15 to Murphy's Law, I believe). The warning should be plastered across the first page of the manual (the only page that most owners read until something goes wrong), not hidden on the bottom of p.8.

Besides tape, two other high-level inputs are provided. The output control provides two options which deserve comment (besides the mono position, which is, I hope, self-explanatory). First, the off position. This does not actually turn off the power to the preamp, but disconnects the outputs. PS recommends using this position at all times when the preamp is not in use (it is also handy as a mute function), but they don't explain why. The "why" is the hazardous (to your loudspeakers) turn-on pulse of the 4.6 when driving a DC-coupled amplifier (such as PS's own 200Cx); it is a step pulse which will drive your woofer cones to an alarming offset before it gradually (about 20 seconds) stabilizes and returns to neutral. When you first turn on the 4.6, and when it's not in use, I strongly recommend either disconnecting your speakers or turning the output control to Off. Leaving it at Off when the system is not in use will prevent inadvertent loudspeaker damage from a power interruption, if you normally leave your power amp on.

The second significant position of the output switch is the so-called "straightwire" setting. This has nothing to do with the cable manufacturer of the same name, but is a line-stage bypass. That is, the power amp is driven directly from the high-level program source (ie, CD player) or directly from the phono preamp stage, with only the volume and balance controls of the 4.6 in the circuit (plus, of course, the selector switches). PS recommends using this position whenever possible. They have a point: if adequate gain exists in the system, the subtraction of a gain stage may well be a plus. But there is a downside: assuming there is enough gain (and that may not always be the case), the output impedance of the preamp will be a function of the volume-control setting (and possibly the balance control as well). This may have adverse effects on the system frequency response, depending on the length and characteristics of the preamp-to-power-amp interconnect and the input impedance of the power amp. It is impossible to predict how the bypass mode will function in any particular reader's system.

Therefore, most of my listening to the 4.6 was done with the line stage in-circuit; in general, I preferred this setting. As with the Sumo Athena, which also has this capability, the main changes on switching to straightwire were a slight sweetening of the high end and a reduction in dynamics. My comments generally refer to the sound of the 4.6 in its standard, non-bypassed mode. Bypass was sampled, where enough gain was available, to be certain of my observations and ensure that bypassing would not mitigate or eliminate any aberrations noted. I look on bypass capability as a bonus—it's there for the trying. But I don't believe a purchase decision on any preamp should be made with use of only the bypass mode in mind, unless that preamp is strictly passive. The bypass mode may well suit your present system, but do you want your future choice of components limited to those which are compatible with the bypass mode (or be forced to buy a new preamp if they aren't)?

Both the level and balance controls of the 4.6 are stepped potentiometers. Normally I prefer non-stepped volume and balance controls, although the controls on the 4.6 can be set between steps. Stepped controls seem to be popular—it must be that "professional" feel. Incidentally, the knobs used on the 4.6's controls strongly resemble the knobs on a line of far more expensive preamplifiers from a certain well-known Minnesota manufacturer of tube gear.

In fact, all of the parts of the all-discrete–designed 4.6 appear to be of very good quality. Circuitry is all class-A (pretty much a fixture in preamps these days). The power-supply transformer (referred to by PS as the HCPS—the high-current power supply) is mounted in an external case and is designed to be placed well away from the preamp and turntable. The optional M-500 power supply, although meant primarily as frosting on the cake for PS's top-of-the-line 5.5 preamp, can also be used with the 4.6, although the latter then competes in a completely different price class. The cleverly named M-500 is a 500VA high-current supply with an extremely large transformer. In the past I would have been skeptical of the need for such a power supply in a preamp, but having heard a demonstration in the VMPS room at the last winter CES of a Threshold preamp driven by a Godzilla-capable, John Curl–designed custom power supply, I was anxious to try the M-500 with the 4.6.

My one reservation is the M-500's price; internal inspection revealed a suitably impressive power transformer (it would not have been out of place in a power amplifier), but little else save two very small filter capacitors (1µF, 250V), a small rectifier, a small resistor, fuse, and internal wiring. My rough estimate of parts costs, estimating the transformer price from the prices of comparably large and heavy units with high amperage capacity, indicates a possible parts/retail cost ratio of over 6 to 1, higher (but not dramatically higher) than the often bandied-about figure of 5 to 1. I have no quibble with the performance of the M-500, however, as I hope to make clear.

The choice of cartridges to be used to evaluate the PS preamp presented an interesting problem. Finally, two were chosen: the Grado MCX was the best high-output cartridge I had, and was a natural choice to drive the moving-magnet stage. Its relative insensitivity to capacitive loading was a plus—one less variable to be concerned with. But its relatively low (for a non–moving-coil) output was a minus: the straightwire (bypass) mode would not generate sufficient output with this cartridge.

The moving-coil cartridge chosen was not an obvious choice. My trusty Monster Alpha-two was looking forward to entering a well-earned retirement from the record-groove wars, but it would not have been a good choice for another reason: its rising high-end response, for which my reference Klyne SK-5A is able to compensate, might lead to erroneous conclusions about a preamp, such as the 4.6, without such compensation. A van den Hul MC-One was on hand for imminent use in the reference system, but it was not yet fully broken-in or familiar (to me). It might also be argued that a $1000-plus moving-coil pickup is unlikely to be used with a preamp of this price (but the opposite tack might also be taken—feed the device under test the best signal you can generate).

PS Audio
4826 Sterling Drive
Boulder, CO 80301
(720) 406-8946

Bill Leebens's picture