Integrated Amp Reviews

Integrated amplifiers are hot. I don't mean in the literal sense—although having a preamplifier and stereo power amplifier in the same chassis usually results in higher running temperatures—but in the metaphorical one. Once viewed as the type of component that no serious audiophile would consider buying, integrated amps have made a comeback in popularity and prestige. Consider: the October 2006 "Recommended Components" issue of Stereophile listed 29 integrated amps, whereas the October 2011 issue lists 40. Stereophile's 2010 Amplification Component of the Year award went to an integrated amp, the Audio Research VSi60, beating out a host of heavy-hitter preamps and power amps. The 2012 Stereophile Buyer's Guide lists 400 integrateds.

What accounts for the renewed popularity of this product category? Cost is certainly a factor, but it's not the only one. You could buy a separate preamp and power amp for the cost of some integrated amps. I suspect that a major part of the appeal is the desire for simplicity: one box instead of two (or, in the case of monoblock power amps, three) cluttering up the living/listening room, and fewer power cables and interconnects—another cost saving.

When I consider the selection of products for review, I don't normally look for ones that are in the state-of-the-art, cost-no-object category, but rather ones that promise to offer high sound quality at a moderate price. Based on my experience with the PrimaLuna ProLogue Three preamp and ProLogue Seven power amp (see December 2009, Vol.32 No.12), that pretty much describes PrimaLuna's design philosophy. So when I read PrimaLuna's announcement of the ProLogue Premium—"the best integrated amp in its class just got better"—I was keen to check it out.

Description and design
It's said that an amplification component's weight can tell you a lot about its quality. Other things being equal, amplifiers that weigh more tend to sound better. Two factors are involved here: transformers and chassis construction. Higher-capacity transformers weigh more and usually result in better sound. Having a more solid chassis helps suppress resonances, which negatively affect sound.

For a 35Wpc integrated amplifier, the ProLogue Premium is surprisingly heavy: 46.3 lbs. The chassis has a very solid feel, and the fit'n'finish is of a quality that suggests a much more expensive piece of equipment. The tubes are covered by a cage that can be easily lifted off rather than requiring a tool to remove it. The front panel has two knobs: one for volume, the other for source selection. There is no balance control. Source selection has an unusual logic: no matter what source you've been listening to, when the ProLogue Premium is turned off and then turned on again, the source defaults to CD. On the left side panel, near the front, there's a rocker switch for power on/off, and on the right side another rocker switch allows selection of bias optimized for EL34 or KT88 power tubes. Accidentally setting this switch to a tube type not installed does no harm; in fact, the user manual suggests that you experiment with the switch position for personal taste. I explored matching/mismatching the tubes and bias switch positions, and preferred the matched positions.

The rear panel sports five pairs of RCA jacks for inputs, one pair labeled HT (for Home Theater). This is not just unity gain, but actually bypasses the preamp section, and allows volume to be controlled by a surround processor/receiver. The ProLogue Premium normally comes with EL34 output tubes, but can also accommodate KT88s, a set of which was included with the review sample. Except as noted, I did my listening with the EL34s. There are connections for speakers with nominal impedances of 4 and 8 ohms. The ProLogue Premium is available with an internal moving-magnet phono stage ($199), but the review sample didn't include this option.

The ProLogue Premium was designed in Holland by Herman van den Dungen, and, like most audio products these days, it's manufactured in China. According to Kevin Deal of Upscale Audio and PrimaLuna USA, PrimaLuna's North American distributor, the engineering of the Premium began with a circuit design in which nothing was taken for granted, and the components selected are of a quality not normally seen in cost-conscious designs. These include:

• Nichicon and Solen polystyrene and tin-foil capacitors

• Alps motorized potentiometer (said to be 10 times the cost of lesser parts)

• toroidal power transformer, manufactured in-house

• capacitor-plus-choke power supply, designed to eliminate both low-frequency ripples and high-frequency hash

• custom-designed output transformers, manufactured in-house

• heavy-gauge, ventilated chassis with a five-coat finish

• construction by hand and point-to-point wiring

The tubes supplied with the ProLogue Premium are SilverLabel, according to Upscale Audio's classification, and are said to be already "special" compared to standard tubes on the market. Other tubes, including cryogenically treated ones, are available from Upscale.

The ProLogue Premium has a number of specific, trademarked circuits designed to optimize performance and prevent damage to the amplifier in the event of a malfunction:

• Adaptive AutoBias (AAB) monitors and adjusts bias constantly. AAB was present in previous PrimaLuna amplifiers, but the latest version is said to include more extended output-stage protection circuitry, to safeguard the output transformers, resistors, and high-voltage power supply in case a tube fails.

• Bad Tube Indicator (BTI) is what PrimaLuna calls the red LED next to each output tube, though BTI is not quite what its name implies. A lit BTI LED indicates that the tube is drawing more power than it should; however, this condition may be only temporary, and not necessarily indicate a bad tube. If the BTI comes on, you're advised to turn off the ProLogue Premium, wait a few minutes, and turn it on again. If no BTI LEDs light up, you're fine. It's only when the indicator stays on that the cause is likely a bad tube, which must then be replaced. During the review period, there were a few instances of a BTI coming on, but each time, turning the amplifier off and then on again fixed the problem.

• Power Transformer Protection (PTP) is a thermal switch built into the AC transformer. If the internal temperature gets too high, the AC is switched off automatically, coming back on again when the problem is resolved. This never happened during the review period.

• Output Transformer Protection (OTP) protects the output transformers from high-voltage transients. Power in my system is supplied by a PS Audio PerfectWave Power Plant 5, which is designed to protect against transients of this sort; in any case, there were no problems in this area.

• SoftStart is yet another protection circuit, designed to extend the life of sensitive components and reduce the chance of tube failure from thermal shock.

The basic design of the ProLogue Premium follows tried-and-true principles: the output stage runs in Ultralinear mode, and tubes run well below their maximum ratings. Like other products in PrimaLuna's Premium range, the ProLogue uses double 12AU7s in each channel rather than the earlier combination of 12AX7 and 12AU7. This is said to result in much lower distortion in the first stage.

The ProLogue Premium includes a solidly built remote control that also controls the basic functions of PrimaLuna CD players. I found the remote to work extremely well to set the desired volume: a very brief depression of the Up or Down button resulted in a correspondingly minimal change in volume, with no overshoot. Like most remotes, this one has buttons of identical shape and size. Since there are only six buttons, remembering the most frequently used ones, Up/Down/Mute, is not that difficult, but I wish they'd made the Up and Down buttons differently shaped or sized from the others, so that they could be identified by touch.

Setup
Setting up an integrated amp like the ProLogue Premium should be pretty straightforward: connect the source interconnects, speaker cables, and, last, the power cord; make sure the volume control is turned all the way down; turn it on.

Sounds simple enough, but I ran into a problem at the very first stage: plugging in the interconnects from the CD player. The ProLogue Premium's RCA jacks have plastic covers on them, and I had a difficult time trying to remove the ones over the CD input pair. (The user's manual makes no reference to these protective covers; they may have been a late addition.) Pulling on them didn't seem to work, so I started twisting one of them—and then I felt it give, and the entire input jack started turning! Not good—I'd inadvertently broken the internal connection to the CD input jacks. To repair the damage, with Kevin Deal's approval, I engaged the services of a highly experienced audio technician, Roger Sherman, who came to my home and was able to resolder the connection without difficulty. I eventually found out that there's a trick to removing the protective caps. There's a part that provides for a grip (it was facing the bottom, so I didn't see it); once you take hold of that, you can easily pull the cover off. This problem solved—and both channels of the CD input working properly—I was ready to do some listening.

According to the folks who view measured electrical performance as the only criterion for the assessment of an amplifier, once an amplifier's internal components have reached operating temperature, the amplifier is performing as well as it's ever going to perform: "break-in" and "warm-up" (beyond a minute or so) are simply myths.

But most audiophiles and manufacturers of audiophile equipment know that break-in and warm-up phenomena are real, and that amplifiers vary in these characteristics. In discussing the effect of replacing tubes, PrimaLuna notes that "break-in yields improvements." In the case of the ProLogue Premium, I found that over a period of several weeks, with the amplifier on 9–10 hours a day and playing music 3–4 hours a day (I leave the CD player on all the time), there was an improvement: the music sounded more detailed, and somehow more "relaxed" and free-flowing. It also seemed to me that, playing music each day for several hours, there was a similar sonic improvement, suggesting a warm-up effect beyond the few minutes it takes the tubes to reach their normal operating temperature. I noted these effects while using the Avantgarde Uno Nano speakers; the warm-up effect was also apparent with the GoldenEar Triton Twos. I decided to evaluate it in a more systematic way.