Audio Research M300 monoblock power amplifier Page 2

I wish more amplifier manufacturers would provide input level-sets. If you feel they impair the sound, you can always leave them full-up, which, apart from the wiper/track contact, takes them out of circuit. But there are times when they come in handy, as when using unusually sensitive speakers, which may make latter-stage preamp noise obtrusive. They are also useful when using preamps with unusually high gain, which tend to crowd everything down into the lowest 45 degrees of volume-control rotation, or when some acoustical anomaly in your listening room causes a persistent channel-balance "pull" to one side of center. And they're invaluable for muting the power amp inputs, so you can unplug cables without having to go through the whole shutdown/fire-up cycle. (Tiffany plugs obviate the need for this, because they break the hot circuit before the ground, but not all interconnects use them.)The front panel sports an impressive lineup of three switches and five colored LEDs. The AC power switch works through a delay circuit and a power relay, to prevent turn-on/turn-off thumps. (There are absolutely none, but one of my samples makes a funny little squeak when turned off.) The second switch selects Standby or Operate modes. "Operate" is self-explanatory; "Standby" holds everything at full supply-voltage levels, but biases down the output tubes to cutoff so that they draw no current. You use this mode to keep things warmed up during periods of inactivity, while conserving tube life and electric power consumption. (LEDs show orange for Standby, green for Operate.) The third switch is for fan speed. You set this to Low for warm-up and low-volume listening, to High when listening at high levels.

The High setting generates a fair amount of wind noise (a gentle rushing), which can usually be tolerated when listening at loud levels. High-level listening is, in fact, the only time the High-speed fan setting is needed, for the class-AB1 output stages only draw large amounts of current at high output levels. There's no clear-cut rule as to when to use which fan setting, though; you have to use your own judgment as to how much fan noise you can tolerate, bearing in mind that, once the amp is fully warmed up, any appreciable increase in operating temperature will tend to reduce tube life. The cooler you can run the M300, the longer the tubes will last.

Installation: This is a massive amplifier, best moved by someone with a strong back. (The handles are almost a token gesture; they're so skinny, it hurts your hands to lift the amp by them.) Just removing an M300 from its shipping carton is a two-person job; I suggest you don't even bother trying it by yourself. Anyone who buys a pair of these can fully expect his dealer to deliver and unbox them, install the tubes (separately packed, for safety), and move the amps to their ultimate resting places in the listening room.

And that place should be carefully chosen, for at least two reasons. Although the M300s don't get all that hot to the touch, because of the internal fans, they do throw off a lot of heat and therefore need an unobstructed flow of open air from all directions but the front. These are not amplifiers you can tuck away in a small closet. And secondly, the amps are rather noisy on the High fan setting, which means they should not be placed right next to the listening seat. (One of my samples also produced enough mechanical noise, a 120Hz buzz, to be audible from 10' away.) Ideally, each should be right next to or behind its loudspeaker, with long, shielded interconnects between the preamp and the power amplifiers. And do go for the best cables you can find—Audio Research makes very good ones at moderate cost, or you can pick some off the top of DO's cable report (Vol.10 No.2).Since each of these amps can pull as much line current as a moderate-sized electric room heater, it is almost essential that they be powered from their own dedicated AC lines. At full power, a pair will draw about 18 amps from the line, and if both share the same line, particularly with a preamp and other equipment, the voltage drop in the line will be great enough to waste a lot of the performance you paid (dearly) for (see fig.1). Any electrical contractor can install this for you for a hundred bucks or so, or you can do it yourself if you're certain you know what you're doing. Just make sure, though, that your job conforms to the local building codes, or your cost-saving handyman project might void your household fire insurance.

Fig.1 Dedicated AC line wiring.

So How Does It Sound?: ARC recommends a 1½-hour warmup in Standby and low-fan mode, or ¼-hour warmup in Operate mode (also low-fan setting). I found no need to disagree, although it really takes a bit over an hour in Operate mode to start sounding its very best. (Interestingly, even from a cold start, the M300s sounded better than many amplifiers I've used after a week of warmup!) Just to play it safe, though, I allowed 48 hours in Standby and two hours in Operate before any serious listening. I figured, if they weren't up to optimum by then, they never would be. Other components used for my tests were the Versa Dynamics 2.0 turntable and arm, Ortofon MC-2000 cartridge with X-2000 step-up transformer, Threshold FET-10 preamplifier, and Sound Lab A-3 full-range electrostatic speakers. Interconnects were the new Monster M-1000 Laboratory Reference series, the speaker cables were by Straight Wire. Signal sources were from original 15ips tapes, CDs (reproduced from the Sony CDP-705ES player), and analog discs from Sheffield, Reference Recordings, Wilson Audio, and Opus 3 (the Depth of Image test record).Previous Audio Research tube amps (and, in fact, just about all other tube amps) have been criticized for not handling low end particularly well. No output transformer can pass DC, and all of them exhibit increasingly rapid rolloff as they approach their cutoffs, introducing phase shift audible as a thinning-out and lack of impact at the extreme low end. (Full-range electrostatic speakers have the same problem, because of their impedance-and-voltage step-up transformer, but because dynamic speakers usually have much the same low-end limitations, for different reasons, the difference is not all that conspicuous.) But not so with this amplifier! The M300 has the best low end of any tube amplifier I have ever heard, bar none—deep, powerful, and of considerable impact.Just to give you an idea of the LF extension of the M300, it is the first stable tube amplifier I've used that reproduces the subsonic pulses from disc warps. Although my electrostatic speakers don't have cone woofers that one can watch pumping in and out on subsonics, they do have light-reflecting diaphragms which are visible through the grille when properly illuminated. With the M300s, they can be clearly seen moving back and forth on discs that produce up-and-down motions of the cartridge.

Audio Research
3900 Annapolis Lane North
Plymouth, MN 55447-5447
(763) 577-9700