Solid State Power Amp Reviews

My writing desk looks out over a large garden with chickens, bees, and feral cats. My chair sits only six feet from loudspeakers, playing softly on my left. Between the speakers sits whatever painting I am working on. That painting hangs no more than 10 feet from the oscilloscope and drill press in my kitchen. Best of all, my desk is only six feet from squadrons of ravenous sparrows attacking the suet cage on the fence outside my window. These real-world attractions keep my right and left brain in balance. Similarly, reviewing both analog and digital sources, as well as tube and solid-state amplifiers, keeps my review practice in balance. But not always. Sometimes my Apollonian self struggles to balance my Dionysian self. Especially when reviewing class-D amplifiers.

You see . . .

Class-D amplifiers are about quantities: like high power with high damping factors delivered at low temperatures in small, lightweight boxes at low dollars-per-watt prices. Class-D is a left-brain pleasure that declares, "Look what a smart shopper I am."

Class-D lovers venerate Bruno Putzeys, the Belgian engineer who kick-started class-D's current popularity, first with his UcD power amp module for Philips, and subsequently with his NCore amplifier modules and various switch-mode power supplies for Hypex.

I've been a forever fan of Bruno's class-D modules because they deliver exceptionally clean, dynamic power at very reasonable prices. Best of all, Hypex makes these modules available, not only to other manufacturers (like Bel Canto, NAD, and MBL) but also as kits made available to DIY people. Looking at the bigger picture, it's fair to say: Putzeys's inventions are currently reshaping the landscape of high-fidelity audio.

To me, the chief beauty of class-D is: It brings the specter of upper-class sound to the parlors of the merchant class.

Class-A, on the other hand, is about qualities. Long favored by gentry, class-A appeals to right-brain audiophiles with a penchant for sensuous excitements and leisure-class pleasures. Think stone fireplaces, linen sheets, and wines from Châteaux. Like aged beverages, class-A warms the listener's blood. Class-A amplifiers are typically low power, reside in heavy chassis, and operate at high temperatures.

But times are changing. Young Royals have started to abdicate. And class-D amp manufacturers are seeking Royal Warrants.

Class-D amplifiers are tiptoeing into the Lord and Lady's listening chamber. They are sneaking in because they are being presented in understated, fashionably sculpted chassis, at Oxford Street prices. The subject of this review, the $3495 Primare A35.2 stereo amplifier, is one of these new breeds of fashionably sculpted, "gentrified" class-D amplifiers. My plan is to see how it sounds while sipping Châteaux Margaux.

Sequence is everything
The only way I know to review a basic amplifier is to take out the amp that's driving whichever speaker is in the system and replace it with the amp I'm scheduled to review. My review deadlines are always the 10th day of each month, so usually, on the 11th or 12th day, I connect the new amplifier to whichever loudspeaker was in the system at the end of the review period. My first hope is that it works—that some form of relatively undistorted sound emerges in equal portions from both channels. If that happens, I can relax: As long as the amp doesn't catch fire or blow fuses, I will likely earn my shilling and three pence for the month.

On December 11, 2019, I connected the just-arrived (from Sweden) Primare A35.2 amplifier to the been-here-forever Magnepan .7 quasi-ribbon panel speakers. The .7s have a current-hungry 3–4 ohm nominal impedance, which I assumed would work very well being driven by the A35.2's 400Wpc into 4 ohms. And I was right. Immediately, the modest Maggies made clear, undistorted sound, from both channels, causing me to relax and listen closely to Alexandre Tharaud's faux-cabaret album Barbara (24/44.1 FLAC Erato/Qobuz). This recording, an homage to a chanteuse Française known only as "Barbara," is a 2019 favorite of mine. It's nicely recorded and, through the A35.2, sounded especially well-articulated.

The tone was good, and the A35.2's transparency was conspicuous, but the Primare amplifier sounded a lot different than the 20Wpc "class-A-like" Schiit Audio Aegir amplifier it replaced. With the Primare amp and Maggie .7s, this well-made recording presented Alexandre Tharaud with a very distinct outline. The lower register of his piano became more solid but less resonant. Tharaud's voice acquired a well-articulated but slightly insentient quality. More obviously, the Primare made the Aegir sound grainy and slightly blurry.

I was also surprised because every time I muted the Primare amp, to talk on the phone or answer the door, the damn thing shut off! It would start up again when I pushed the Standby button, but the ordeal was annoying. When I checked the A35.2's online owner's manual, it said, "By default, if no signal is present for 20 minutes the A35.2 will automatically go into standby to save power." The manual also said, "to disable automatic standby: press and hold down the front panel button for a few seconds, and release once the indicator light has flashed two times and become steadily illuminated." This was easier said than done. Later (see below) I discovered a switch on the back panel that turned this 20-minute-to-standby-scheme into a genuinely pleasant feature.

Description
The Primare A35.2 is a stereo amplifier that weighs 26lb, measures 5.7" high × 16.9" deep × 15.7" wide, and features what Primare calls their "proprietary UFPD2 analogue class-D amplification technology" to deliver 200Wpc into 8 ohms, and the ability to be easily bridged into mono for 800W (!) into 8 ohms.

On the back panel are two 3.5mm jacks for input and output of a 12V trigger; an RS232 input; one pair each of RCA and XLR (balanced) inputs; and of course, right and left speaker cable binding posts. There is also a row of four small toggle switches. The first switch turns on and off the input sensor that allows the amp to automatically come out of standby if an input is sensed. (Her Ladyship liked this.) The second switch toggles between balanced and single-ended inputs. The third switches between two-channel and bridged-for-mono operation. And the fourth allows for normal or +6dB gain (the latter in bridged mode only).

Class-D Primare style
According to the design brief I received via email from Primare rep Terry Madalen, "The main reason that class-D was invented was for efficiency, and many of the performance benefits derived from class-D amplification are the result of the significantly greater ability of the circuit to deliver more of the power fed into it as musical signal output. Some manufacturers of class-D based amplification do not embrace the full potential this technology can provide, but our amplifiers do so in combining state of the art efficiency together with state of the art audio performance." (When I read these SOTA claims, my urge to insert a snarky remark was high, but I restrained myself.)

The design brief went on to explain why Primare thinks the A35.2 is superior to other class-D amplifiers:

"A major source of distortion in class-D designs is the demodulation filter on the output that becomes unstable with variations in loudspeaker impedance unless it's controlled by sufficient feedback. The failure to provide enough feedback to cope with dynamic changes in loudspeaker impedance across the audio band results in rising THD with frequency. While sounding very dynamic and vivid initially, it can become tiring and uncontrolled in the long term, especially when driving complex loads.

"UFPD provides a consistent 26dB feedback loop gain across the entire audio range and is stable way beyond the audio band. Rather than have the amplifier and then the filter as discrete stages, the UFPD design integrates the two, making control with feedback much more immediate and accurate."

I asked Terry who designed the A35.2:

"We design our products by hiring consulting designers with specific talents in various building block components for each product's design. So, in the case of A35.2 it is a product of the particular expertise in the design of Class-D power output circuitry of Patrick Bostrom (Sweden) and in the case of power supply, Bjorn Sandberg (Denmark). Bent Nielsen (Sweden), our chief engineer since the company's founding, then works with their fundamental designs to allow them to work most sympathetically together within a produce-able, and elegant, design."

According to Terry, the UFPD2 (amplifier) and AFPC (power supply) modules, as well as the chassis, are manufactured by "Our OEM manufacturer in China." When I asked where the A35.2 is assembled, he said, "In one of the largest and most sophisticated OEM manufacturing facilities in China, with the final quality-control testing being done in our facilities in Sweden."

More listening via Magnepan .7s
After a week of casual listening, I began to get a good feel for the Magnepan .7s driven by the Primare A35.2. The beauty of Magnepan's quasi-ribbon panel speakers is that they are not wood boxes with dome tweeters, and they generate an enormous soundfield. Best of all, they disappear when I close my eyes. The silence of the A35.2 amplifier let the quiet Maggies disappear more than usual.