On Power Amplifier DC Offset…

By Bryan Geyer

Heads up, please!  It’s time to consider an important power amplifier parameter that a great many audiophiles overlook. It’s known as DC offset, and it has direct bearing on the dynamic range capability of your low frequency loudspeakers.

Fluke 87V Multimeter

When a high quality audio power amplifier is in a quiescent state (operational, but no input signal), its output terminals will normally measure nearly zero AC and zero DC volts. A tiny residual AC voltage (some ~ 40-80 microvolts) will reflect the sum of the internal noise generated within the power amplifier. And a small residual DC voltage (generally on the order of ~ 2 to 15 millivolts DC) will represent the amplifier’s implicit output imbalance; a.k.a. its DC offset. (In the case of vacuum tube power amplifiers there’s no DC voltage present because the output is normally isolated by a transformer. Similar DC isolation applies if the power amp is coupled to the load through a capacitor, an archaic compromise that’s taboo in modern hi-end power amps.)

The loudspeaker system connects directly to the power amplifier, so any offset that’s present at the amplifier output will also appear at the inputs of the speaker system’s passive crossover network, and then feed directly to the DC-coupled low frequency woofer. Higher bandpass speakers will be DC-isolated by the network’s coupling capacitors; it’s only the woofers that will see the DC offset.

When quiescent, the woofer rests in the neutral middle of its magnetic field. Optimally, the cone is not displaced—forward or backward—from that mid-point rest position until it sees an input signal. Of course, the presence of any DC bias will slightly offset that ideal mid-point rest position, and this shift could potentially impact the cone’s full range of linear excursion. If the offset is very slight, that impact will be entirely negligible. As offset increases, its influence can become significant. Any major offset (e.g. ~ 100-200mVdc) could measurably (audibly?) degrade the woofer’s dynamic range.

It’s commonly promulgated that some small (~ 10 or 20mVdc) offset is both harmless and inevitable. Since DC offset will increase as operating temperature rises, it becomes more difficult to hold offset within desirable limits in the case of class A (or partial class A) power amplifiers. They operate at higher chassis temperatures than class A/B amplifiers, so owners of class A (or pseudo-class A) power amplifiers should be especially vigilant. DC offset is a very easy measurement to monitor, and accurate DC millivolt reading multimeters are readily available. (Select a meter with input impedance ≥ 10MΩ that can read 600mVdc full scale, with 0.1mVdc resolution. Fluke’s products are highly recommended. Here is an excellent hi-end model: https://www.myflukestore.com/product/fluke-87-5-industrial-multimeter)

Modern solid-state hi-end power amplifiers typically apply symmetrical input stage circuitry using complimentary bipolar, Jfet, or MOSfet discreet devices that are carefully matched to minimize potential DC offset. It’s especially critical to minimize this imbalance at the input stage because, in a DC-coupled amplifier, that offset will then be magnified by the voltage gain (generally +23dB to +29dB) of the product. As a result, it’s only reasonable to tolerate some modest (e.g. ~20mVdc) output offset, and one supplier (Pass) of partial class A power amplifiers specifies a 50mVdc maximum offset limit for their respected “Point 8” series (e.g. X250.8) of pseudo-class A power amplifiers.

Many makers don’t disclose any DC offset specification, although they might maintain an internal screening limit that’s never published (hence not guaranteed), so some snooping could prove helpful. The company’s Service Department can sometimes be a good place to start when it’s a domestic operation. In some cases (e.g. Parasound Products Inc., of San Francisco), the company president, Richard Schram, is both technically savvy and personally accessible. I like that kind of company!

My recommendation is that you personally measure your power amplifier’s DC offset. Know what the offset is at moderate operating temperatures, in normal use, and know what happens to DC offset after you’ve pushed your power amplifier through a heavy listening session. If you are in the course of considering the purchase of a new power amplifier, research the DC offset specification. If it’s not published, contact the maker; seek reliable information. High quality audio power amplifiers with negligible DC offset are readily available, as are identical models with excessive DC offset. It’s up to you to discern (and reject) the lemons*. Leave the latter for the lazy buyers—the audiophiles that always evaluate everything exclusively by listening.

BG (October 23, 2019)

*Moderate DC offset, e.g. ~ 50-150mVdc, is extremely difficult to diagnose by ear. Imbalance of that nature becomes apparent only when the amplifier is approaching full output, a condition that invites other sundry (and more likely) imperfections. Those issues will normally mask the subtle evidence of moderate offset error, so aural screening—even for those who profess exceptional sensitivity—will probably prove ineffective. To be more precise, measure the DC offset.

Haydn: String Quartets, Opp. 71 & 74 (CD review)

The London Haydn Quartet. Hyperion CDA68230 (2-disc set)

Let me begin with a story, and I apologize that I've told it before. If you recognize it, you may safely skip to the third paragraph.

OK, I've always loved album covers. Especially classical covers that put me in a mind of the recorded music. A good illustration of this is a Philips album I bought many years ago on LP. It was the augmented Beaux Arts Trio doing Schubert's "Trout" Quintet. The bucolic cover painting showed an old mill and waterwheel on a stream in the country. It was lovely, and I enjoyed gazing at it while listening to Schubert's music. But when Philips issued the recording on CD some years later, they changed covers, giving it a mundane, almost nondescript booklet picture. Likewise happened when Pentatone released it on SACD. So what I did was go on-line and find a picture of the original LP cover, which I saved, resized, sharpened, and color corrected. Printed out on glossy photo paper, it looks beautiful, and slipped in front of the SACD booklet, I can again enjoy the pleasures of a day in the country while listening to the music.

Of course, it takes more than a pretty cover to sell me on a record album. Certainly, the music counts for a lot, the musicians, their performance, and the sound of the recording. All of which this Haydn album has going for it. The performers, the London Haydn Quartet, are superb. Their playing of the string quartets is above reproach. The Hyperion sound is about as good as it gets. And the cover painting, "The Naval Dockyard at Depford" by Samuel Scott (c1702-1772), puts one in mind of Haydn's environs at the time he wrote the music and, yes, contributes to my enjoyment of it.

The players, as I say, are the London Haydn Quartet, comprised of Catherine Manson, violin; Michael Gurevich, violin; John Crockatt, viola; and Jonathan Manson, cello. Founded in 2000, the players perform on period instruments, and their release of the present two-disc album is a part of their complete Haydn quartet recordings for the Hyperion label.

The material embraces the Opuses 71 and 74 string quartets, the so-called Apponyi quartets because Haydn publicly dedicated them to Count Anton Georg Apponyi (for a price). They are sometimes called the "London" quartets, too, owing to Haydn's having composed them for London premieres. Each opus contains three quartets of about twenty-five minutes each. Haydn wrote them in 1793, and in addition to being referred to as Op. 71, Nos. 1-3 and Op. 74, Nos. 1-3, they are also simply numbered Nos. 54-59.

London Haydn Quartet

Austrian composer Franz Joseph Haydn (1732-1809) didn't invent the symphony or the string quartet, but he refined and popularized the genres. Not only did he write 106 symphonies, earning him the nickname "Father of the Symphony," he wrote some 70 string quartets, making it fair to call him the "Father of the Quartet" as well.

Anyway, the quartets themselves are a delight, as we would expect from a genius like Haydn in his late middle period. So, what about the playing? It's obviously a historically informed performance, coming at us on period instruments. However, it is not one of those hell-bent-for-leather affairs that leave our sensibilities in the dust. The interpretations are completely charming, carefully judged, well paced, and judiciously measured. Nothing is too fast or too slow; there are no exaggerated contrasts or prolonged pauses. The fast movements are temperate and mostly joyous rather than helter-skelter. The slow movements are lovely, reflective, contemplative, without ever dragging. This is music-making to be appreciated and savored rather than admired solely for its virtuosity.

And the playing is immaculate and, yes, virtuosic.

I can't tell you how much I enjoyed this album. Some two-and-a-half hours of music went by before I knew it, and I enjoyed every minute. Each succeeding quartet seems better than the last, culminating in perhaps my favorite, Op. 74, No. 3.

As an aside, I also enjoyed a booklet note informing us that Haydn included a loud introductory gesture at the beginning of each quartet, intended to let the audience know the music was starting and to quiet down. It appears people never change.

Producer and engineer Philip Hobbs and editor Julia Thomas made the recording at Potton Hall, Dunwich, Suffolk, England in October 2018. How good is the recording? It's the proverbially reach-out-and-touch-it good. Clean. Clear. Close but not objectionably so. Smooth. Radiantly atmospheric, with lifelike imaging and a realistic separation of instruments. It's about as good as a chamber ensemble can sound without their being live in your living room.

JJP

To listen to a brief excerpt from this album, click below:

Giuliani: Complete Guitar Concertos (CD review)

Pepe Romero, guitar; Celdonio Romero, guitar; Sir Neville Marriner, Academy of St. Martin in the Fields. Philips Duo 454 262-2 (2-disc set).

Quite a few years ago the editor of The $ensible Sound magazine, our own Karl W. Nehring, asked each of the magazine's reviewers to come up with a list of ten favorite recordings, "Desert Island Recordings" if you will. I was a little reluctant at the time to find ten among hundreds of favorites. Today, however, I am not so hesitant, and I included the First Guitar Concerto by Mauro Giuliani (1781-1829) in my current listing. You'll find it here: https://classicalcandor.blogspot.com/2009/12/desert-island-classics.html.

It was a recording previously available only on a full-price Philips disc, coupled with the Third Concerto, but in 1996 or so, for the same price as the single disc, Philips gave us all three of Giuliani's guitar concertos, plus over half a dozen others of his guitar works on a pair of CD's in Philips's impressive, and long lamented, Duo series. I can't begin to tell you how enjoyable this set is. And as far as I can tell, it's still available.

Pepe Romero

The First Concerto, which premiered in 1808, was a popular favorite in its day and deserves its popularity to our own time. It was to the nineteenth century what Rodrigo's Concierto de Aranjuez became to the twentieth century and beyond in terms of public acceptance. One could not ask for a lighter, happier, sunnier, sweeter, more charming piece of music, seductively performed by a master of the guitar, Pepe Romero, and accompanied by the ever-reliable Neville Marriner and his Academy of St. Martin in the Fields.

Giuliani's Second Concerto is darker in tone, and not nearly so much fun, but it's worth one's time because it's still better than almost anything else composed for the guitar. By the Third Concerto in 1820, Giuliani had become, for better or worse, far more sophisticated in his composition and arrangement; the Third Concerto's main attractions are its enchantingly beautiful slow movement and its brilliant finale.

Closer to the stylistic simplicity of the First Concerto, however, is Giuliani's three-movement Introduction, Theme with Variations, and Polonaise, a thoroughly enchanting piece that comes close to challenging the First Concerto for all-out charisma. Philips also include on the set the Gran Sonata Eroica, Grande Ouverture, La Melanconia, Handel Variations, and Variazioni concertanti. It is a pleasurable mix, to say the least.

Originally issued on three full-priced LP's, Philips recorded the pieces between 1974 and 1978. The sound is typical of Philips's late analogue work, not analytically clear but well focused, warm, and smooth. The guitar is prominently displayed but not too far forward.

On the whole, I can't think of a better way to while away a little over two and a half hours with pleasant listening than with these "Duo" discs. Highly recommended, obviously.   

JJP

To listen to a brief excerpt from this album, click below: