Tube Power Amp Reviews

We mentioned in the last issue that we were becoming increasingly disturbed by "a certain manic quality that is creeping into this pursuit of sonic perfection." We were referring then to a manufacturer's announcement of the imminent availability of a speaker system weighing over 1000 lb per channel, but we could just as well have been speaking of this behemoth from Audio Research.

The Dual 150 looks like a piece of professional broadcast equipment. It contains 8 output tubes (6550s), three slow-speed (but efficient) cooling fans, two front-panel meters for monitoring AC supply line volt age, output power in each channel, and (by switch selection) the state of adjustment of the bias of each output tube. Its front panel is a ¼"-thick slab of aluminum, it is equipped with two beefy carrying handles, and the whole thing weighs in at a gut-busting 115 lb. It is probably safe to say that more than half of that weight comes from one of the most massive power supplies ever built into a tubed amplifier, and indeed that power supply can, on initial turn-on, draw enough line energy (almost 3000 watts) to pop a 15-amp house fuse or melt practically any control unit's "convenience AC power" switch.

For this reason (and also, presumably, to extend the potential sales area to European countries), the D-150's AC power switch has five On positions, for fine voltages of 240, 220, 120, 110 and 100 respectively. If you live in the US, you note the incoming line voltage as monitored by the meter when the switch is Off, then advance the voltage selector to 220 for 10 seconds, to partially charge the power-supply storage capacitors. (It is the almost-instantaneous charging of these capacitors that causes most amplifiers to draw their greatest amount of AC power during the fraction of a second following turn-on.) Finally, turn the switch to the setting that is nearest to but above the voltage you originally read on the meter.

For example, if the meter reads 112 volts with the D-150 off, you should end up with the switch in the 120-volt position. The 110-volt setting would be closer to the line supply voltage, but would result in slightly excessive operating voltages that would seriously curtail component life. (A 5% excess does more damage than a 10% deficiency, footnote 1.)

The D-150's power consumption will vary from moderately high (around 600 watts) when idling, to around 1000 watts under full power. ARC recommends powering the D-150 from its own wall AC outlet, through a heavy-duty extension cord if necessary. We concur in that recommendation. If your preamp's AC cord and plug appear to be un usually heavier-duty than conventional lamp hardware, though, you might try powering the D-150 from one of the preamp's unswitched outlets, if these are not fused in the preamp (or if the rear panel rates the outlets at 25 amps or more). Check periodically, though, to make sure the AC plug to the preamp is not heating up; if it is even perceptibly warm to the touch, we would advise going to the trouble of connecting the 150 directly to the wall outlet.

The D-150 is rated at 150 watts per channel, which sounds piddling in comparison with what a mere $700 will buy you in a solid-state amplifier. The figures are not however directly comparable in terms of listening experience for, like many tubed amplifiers, the D-150 overloads so gracefully that it sounds like a good 300 watts per channel. We clocked the output from our relatively inefficient FMI J-Modular speakers at 110dB (from 10 feet away) on some signals before we threw in the sponge and cut the gain back. We did not detect any evidence of overload. (The same Radio Shack meter that we used has never read above 102dB when measuring the loudest passages from a large symphony orchestra from a distance of 10 feet, and it clocked most fortissdmos at a mere 97dB!)

Sound Quality
And so to the nitty-gritty: What does this betubed monster sound like? Nothing. Simply nothing at all. If it has any sound of its own at all, we were unable to hear it, on the most revealing speaker systems we could round up.

As a matter of interesting fact, the reactions of several critical listeners, who were not told what they were hearing, was that it sounded like the best solid-state or tubed amplifier they had ever heard, depending on whether they were partial to solid state or tubes. The D-150, in other words, seems to have finally bridged that long-narrowing gap between the musical liquidity of the best tube amps and the openness, crispness and solidity of the best solid-state amps. It made everything else on the premises sound sick, including, we are sad to say, the Audio Research D-76A.

On the J-Modulars, which now have the distinction of being the most revealing speakers we know of, a single D-150 was clearly superior to biamplified D-76As. It was also the first amplifier we have found that did as well with electrostatic speakers as with dynamics. In the best systems of either type, the depth, transparency, definition, naturalness, openness and low-end solidity have to be heard to be imagined. In short, we find it difficult to imagine how this thing could be bettered at any price. Even if it is, ultimately, we are convinced that the D-150 is destined to become a classic, in the manner of those early Marantz power amplifiers whose performance has never really been substantially bettered until, 20 years later, the D-150 came along.

Speaking of the potential longevity of tube equipment, we are told that a lot of dealers, pushing solid-state components, have been telling customers that the supply of replacement tubes is due to start drying up in a few years and that prices for them will start climbing precipitously as a result. We asked several tube manufacturers about this, and while one admitted that their future plans would be predicated on US consumer demand, four informed us that they had not considered discontinuing tube manufacture and could not foresee doing so as long as the present demand for them continued! One pointed out that their biggest market for amplifying tubes was in Europe and Asia, and said they would continue making them even if sales in the US came to a virtual standstill. That does not sound to us like the incipient demise of the vacuum tube.

Conclusion
Unfortunately, the price of the D-150 is murderous. Perhaps now, Audio Research can rest on their laurels for long enough to see if they can match this kind of sound from amplifiers that some of us can afford to buy.

We must also point out that the D-150 has yet to prove its durability. It does run very cool—more so than many solid-state amplifiers—but only time will tell that story. We hope the story has a happy ending.

Manufacturer's Comment
Editor: We appreciate the Stereophile's review of the D-150; however, we disagree with the perspective. We still believe the D-76A is a fine amplifier and a logical choice for most audio perfectionists.

When a state-of-the-art manufacturer contemplates putting a product like the D-150 on the market, several serious questions must be answered. For example, will a product like the D-150, which is beyond the financial capability of most audio perfectionists simply frustrate the consumer? Will the consumer decide that since he can't afford The Best that he might as well buy a cheap, temporary product and wait until The Best becomes affordable?

This question was raised at Audio Research when we compared the D-150 prototype to the D-76A. We felt that while the D-150 was superior to the D-76A, most audio perfectionists would still view the D-76A as the more sensible choice.

One important feature on the D-150 is the front panel meter and bias adjustments. With these controls, the D-150's optimum performance can be easily verified. We realize that many D-76A owners are missing the full potential of their amplifiers because of improper bias adjustments. We encourage D-76A owners to have their amplifiers rebiased after a few weeks use at the line voltage at which the amplifier is normally used. If this practice is followed, we believe most audio perfectionists will find the differences in sound quality between the D-76A and the D-150 to be significantly reduced.—E. Wendell Diller, Sales Manager. Audio Research Corporation

Reviewer's Addendum
We did not wish to give the impression that the D-150 had suddenly made the D-76A a hunk oú junk. The 76A is still, in our opinion, the second-best power amplifier available for use with full-range electrostatics and other speakers that don't need the extra crispness and low-end control of a solid-state amplifier. Our main point, which we admit probably was a mite overstated, was that the difference between the D-76A and the D-150 is not at all subtle.

We have ascertained that the biases in our D-76A are correct, and the unit sounds identical to a second D-76A we compared it with, so it would seem safe to assume that it is functioning properly.—J. Gordon Holt

---

Footnote 1: If the relationships here have you confused, remember that the output voltage that a power transformer feeds to a component (for a given AC line voltage) is the result of the ratio be tween the number of wire turns in its primary (incoming) winding and its secondary (output) windings. Thus, if it is required to deliver 480 volts output from a 120-volt AC supply, it will have a step-up ratio of 4:1. If we then need provision for operating from a 110-volt line, we must switch in 37% more turns of wire into the primary in order to get 480 volts from the secondary. Now , if the actual incoming-line voltage is 112 and we switch to the 110-volt set ting, it may appear that we are selecting a lower (and therefore safer) voltage setting than 120. But we have actually increased the transformer's turns ratio and, hence, the amplifier's power-supply voltages. The correct setting would be the one higher than 110, which will be 120.