Sidebar 3: Measurements
I used Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (see the January 2008 "As We See It" and www.ap.com) to examine the NAT Symmetrical preamplifier's measured behavior; for some tests, I also used my vintage Audio Precision System One Dual Domain.
The NAT Symmetrical line preamplifier is beautifully constructed, but I had some problems measuring its performance, as the right channel's balanced output was half that of the left's. I reseated all the tubes and ribbon cables, but this didn't fix things. The fact that the…
Denon Anechoic Orchestral Recordings
Music and Test Signals for Evaluation of Room Acoustics
Mashahiko Enkoji, Osaka Philharmonic Orchestra
Denon CD PG-6006 (CD only). Katsuhiro Tsubonou, Yoshiharu Kawaguchi, dirs.; Norio Okada, Katsuhiro Miura, engs. DDD. TT: 58:42
If this is supposed to be the age of communication, why aren't people communicating? If we can bounce a TV show off a satellite into the Soviet Union, or send an architectural plan along with the designer's signature from New York to London in 30 seconds by facsimile, why the hell haven't we been able to explain…
Way back in the mists of time, around 1980 to be exact, the Marantz company in Europe introduces a range of ostensibly cost-no-object solid-state electronics under the "Esotec" banner. Manufactured in Japan, but apparently designed in the USA, these ruggedly constructed components are noteworthy in that the power amplifiers are capable of being operated with the output stages running under class-A bias as well as class-B. The relatively expensive Esotec amplifiers sell in small numbers in the UK—remember that this is before the rebirth of the British high end—and pass into the history books…
Atkinson: Do you think that the correlation between good sound and low loop feedback is due to the basic linearity of the circuit? Or is it due to other factors? For instance, if you have very high amounts of loop feedback, you can then have time-domain problems.
May: Well, the basic amplifier is more linear. But there are a lot of aspects of it. You have to look at what the open-loop frequency response is with phase compensation. There you want to keep the energy storage in the loop as small as possible. In other words, get the widest bandwidth open-loop. But the phase-compensation…
May: I've got to say that it's pretty well broken up, distributed over the whole amplifier. It's a pretty stable power supply—not that any unregulated supply doesn't have its limitations—but it's a good transformer, a good power supply. I think the real breakthrough on Polaris was the linearity correction circuit for the MOSFET output stage. Basically what we do is compare an idealized output signal with the actual output signal of the amplifier. And introduce local error-correction networks within the output stage proper. It's a small feedback loop existing right around the output stage.…
Atkinson: You mean an IC's characteristics change as the output current modulates its temperature.
May: Yeah. And this will show up at low frequencies. this is a thing which can get you into trouble playing a phonograph record with the inherent 6-cycle warp component. That can come right back and haunt you.
An excellent article by National Semiconductor points out that, hey, thermal limitations are the real problem in the amount of gain in IC op-amps. Not electrical problems. To give an example, the open-loop gain of some of their earlier ICs is considerably less than the gain of…
Atkinson: You're changing the way the RF garbage is induced into the low-level circuitry.
May: And even the board layout is extremely critical. Minimizing the amount of loops within the grounding circuitry—not ground loops per se, but how the signal flows in the board—is a major problem. A fully differential amplifier, if well designed, is immune to things floating around on the ground because the amplifier is only measuring the differences between two inputs.
Atkinson: Professionals have worked balanced for a long time. Why is it only now that this is being worked into consumer…
While headphone listening remains secondary to that of loudspeakers for most serious listeners, it's still an important alternative for many. And while good conventional headphones exist, electrostatics are usually considered first when the highest playback quality is required. As always, there are exceptions (Grado's headphones come immediately to mind), but most high-end headphones are electrostatic—such designs offer the benefits of electrostatic loudspeakers without their dynamic limitations. Last year I reviewed the Koss ESP/950 electrostatics (Vol.15 No.12), a remarkable set of…
My experience of the Orpheus was very much of the "boy gets girl, boy loses girl, boy gets girl" variety. I actually auditioned it first, prior to the Stax SR-Lambda Pro Classic, and was very impressed with its performance, particularly its shimmering, sweet, clear top and superior overall definition. It was supremely comfortable to use over long periods (one characteristic about which my opinion never changed). But it also sounded a bit shy on the bottom—it lacked sufficient body and weight, seemed rather lean through the midbass, and was without a fully natural warmth, particularly on…
The Orpheus was, however, unusually effective in keeping the soundstage from clustering inside my head. It wasn't completely successful in this—no headphones in my experience have been—but the soundstage seemed less of the clothesline-between-the-ears variety than is typical, extending instead from my left ear, across an arc just in front of my nose, then around to my right ear. This is more a matter of spectral balance than of any signal manipulation. The diffuse-field compensation referred to in Sennheiser's specifications refers to the design of the earpieces themselves—no electronic…
