Description: Solid-state stereo power amplifier. Power output (continuous): 150W into 8 ohms (21.8dBW), 250W into 4 ohms (21dBW), 325W into 2 ohms (19.1dBW), 500W mono into 8 ohms (27dBW). THD at rated power: 0.0015% up to 2kHz, 0.005% at 20kHz. Slew rate: 35V/µs stereo, 70V/µs mono. Voltage gain: 26dB stereo, 32dB mono (no load specified). Frequency response: 20Hz–20kHz, +0.0, –0.04dB; –3dB at 0.015Hz and 200kHz. Peak current: 50A. Damping factor: 800 below 1kHz (equivalent to an output impedance of 0.01 ohms). Input impedance: 33k ohms unbalanced, 12k ohms…
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Sidebar 2: Transient Intermodulation Distortion
TID is due to the active devices being unable to slew the voltages they are asked to handle sufficiently fast to keep up with the demands of the signal. Slew rate, expressed in V/µs, is the speed with which the output voltage of an active device is able to change and is a critical factor in the behavior of a negative feedback circuit: if slewing can't keep up with the changes in input level, the fed-back signal will not be able to properly attenuate the input signal, which will then increase to the point of amplifying-device overload.
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Sidebar 3: Complementary Symmetry
The complementary-symmetry circuit is sometimes called complementary push-pull, but strictly speaking, push-pull operation always means two identical output devices handling the same audio signal in relative inverted polarity. The output from these devices is their difference signal, which means out-of-phase signals—the music—produce maximum output while in-phase signals—even-order distortion and common-mode noise—cancel to produce zilch. Fig.1 shows how this works (in theory).
Fig.1 Simplified circuit of a push-pull transistor output…
Sidebar 4: Measurements
The Boulder's input impedance measured 11k ohms into its balanced inputs (the only type of input furnished on our sample). This is a fairly low figure; care should be taken to insure that the source fed into the 500AE has a low output impedance. The output impedance of the Boulder itself varied between 0.07 ohms and 0.11 ohms, depending on load and frequency, indicating that its frequency response should not vary with the load to any significant extent. Gain into 8 ohms measured 25.97dB at 1kHz, and the S/N ratio was 85dB referenced to 1W into 8 ohms. The DC…
Robert Harley wrote about the Boulder 500AE in April 1992 (Vol.15 No.4):
I was taken aback when I read J. Gordon Holt's rave review of the Boulder 500AE power amplifier last October. In comparing the 500AE with other high-end amplifiers (specifically Levinson, Krell, and Audio Research), JGH suggested that "...the Boulder 500AE will make acoustically produced music sound more realistic than any of these can." He continued: "After having lived with the 500AE for several months now, and attending symphony concerts locally from time to time, I contend that the 500AE has more of the ring of…
Lewis Lipnick reviewed the Boulder 500AE in June 1992 (Vol.15 No.6):
As JGH suggested in his glowing review of the $3999 Boulder 500AE, this manufacturer has not (until now) been considered a serious contender in the high-end audio marketplace. I've seen scads of Boulder amps in studios while playing on recording sessions, but never took the time to really listen. So when I ran into the people from Boulder at last January's CES in Las Vegas, they were quick to accommodate my request to sample their wares in my own system. A pair of 500AEs arrived while I was preparing a review of the…
Steven Stone compared the 500AE with the Pass Aleph 1.2 in November 1997 (Vol.20 No.11):
I spent several days comparing the Pass Labs Aleph 1.2 to my reference amplifier—the $9900/pair Boulder 500AE monoblock, using my Dunlavy Signature SC-VI loudspeakers. Both models share the ability to produce copious amounts of volume without stress, but each excels in dramatically different sonic parameters. My DATs of the Young Philharmonic Orchestra of Munich performing in Boulder's Chatauqua Auditorium, which I had recorded with J. Gordon Holt, had superior three-dimensionality through the Alephs…
We all have biases. The trick is knowing your biases so they don't get in your way. Mine are pretty obvious. I don't like "fussy" gear that demands special care and feeding. I'm lazy—I want to just turn stuff on and begin listening. Perhaps that's why I have a positive bias toward Nelson Pass's designs. They're reliable, untweaky, and usually sound good.
The Aleph 1.2 is the third piece of Pass Labs electronics I've lived with. I own a pair of Aleph 0 amplifiers, and I reviewed the Aleph P preamp in the February '96 Stereophile. Because of my past experience with Pass gear, my…
So they're hot, they're happening, and they're pretty damn awesome. But exactly what qualities make the Aleph 1.2 so sonically stupendous? My listening notes include copious references to the 1.2's lack of any sort of electronic signature. This is a midrange to die for—clean without a trace of an electronic signature. In addition to its nonelectronic nature, the 1.2 also has unflappable suavity. Nothing ruffles its calm. Even the most dynamic material in my arsenal, like the JGH/SS private recording of the 1812 Overture, complete with howitzers, didn't rock the 1.2's world. This sense of…
Sidebar 2: Associated Equipment
This review was the first full Equipment Report conducted in my alternate listening space, a room 16' square with an 8' ceiling. These are terrible proportions—all the dimensions support the same standing-wave frequencies. I needed to install lots of damping—heavy velvet drapes on three windows, a large overstuffed sofa, an area rug—before the room became tolerable.
The main stereo speakers, Paradigm Reference Studio/60s, flank a library table bearing a 27" television, the speakers farther forward of the table. The center speaker, a Paradigm…