Oct 30, 2019

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.
Labels: ,

3 comments:

  1. AnonymousOctober 3, 2021 at 1:15 PM

    Why do you use a multimeter instead of an oscilloscope?. A multimeter measures Volts rms or peak or peak to peak which tells nothing about the symmetricality of the waveform. The ideal waveform should have a 0 volts offset which means the peak value of the top matches the peak value of the bottom. An multimeter cannot tell you this

    ReplyDelete
  2. UnknownNovember 11, 2021 at 3:08 PM

    Measuring DC offset, not the waveform.
    DMM gives perfect measurement for this, it's cheaper and easily obtainable.

    ReplyDelete
  3. BGNovember 13, 2021 at 4:15 PM

    DC offset is commonly measured when the amplifier is operating WITHOUT any signal input (input jacks shorted), so the only AC waveform appearing at the output will be hum + noise. The latter is of no interest when seeking to measure the amplitude of any DC offset that's present, so the optimum instrument of choice is a sensitive (0.1 Vdc full scale) DC millivoltmeter, e.g., Fluke's 87v multimeter.

    ReplyDelete

Thank you for your comment. It will be published after review.

Subscribe to: Post Comments (Atom)

Meet the Staff

Meet the Staff
John J. Puccio, Founder and Contributor

Understand, I'm just an everyday guy reacting to something I love. And I've been doing it for a very long time, my appreciation for classical music starting with the musical excerpts on the Big Jon and Sparkie radio show in the early Fifties and the purchase of my first recording, The 101 Strings Play the Classics, around 1956. In the late Sixties I began teaching high school English and Film Studies as well as becoming interested in hi-fi, my audio ambitions graduating me from a pair of AR-3 speakers to the Fulton J's recommended by The Stereophile's J. Gordon Holt. In the early Seventies, I began writing for a number of audio magazines, including Audio Excellence, Audio Forum, The Boston Audio Society Speaker, The American Record Guide, and from 1976 until 2008, The $ensible Sound, for which I served as Classical Music Editor.

Today, I'm retired from teaching and use a pair of bi-amped VMPS RM40s loudspeakers for my listening. In addition to writing for the Classical Candor blog, I served as the Movie Review Editor for the Web site Movie Metropolis (formerly DVDTown) from 1997-2013. Music and movies. Life couldn't be better.

Karl Nehring, Editor and Contributor

For nearly 30 years I was the editor of The $ensible Sound magazine and a regular contributor to both its equipment and recordings review pages. I would not presume to present myself as some sort of expert on music, but I have a deep love for and appreciation of many types of music, "classical" especially, and have listened to thousands of recordings over the years, many of which still line the walls of my listening room (and occasionally spill onto the furniture and floor, much to the chagrin of my long-suffering wife). I have always taken the approach as a reviewer that what I am trying to do is simply to point out to readers that I have come across a recording that I have found of interest, a recording that I think they might appreciate my having pointed out to them. I suppose that sounds a bit simple-minded, but I know I appreciate reading reviews by others that do the same for me — point out recordings that they think I might enjoy.

For readers who might be wondering about what kind of system I am using to do my listening, I should probably point out that I do a LOT of music listening and employ a variety of means to do so in a variety of environments, as I would imagine many music lovers also do. Starting at the more grandiose end of the scale, the system in which I do my most serious listening comprises Marantz CD 6007 and Onkyo CD 7030 CD players, NAD C 658 streaming preamp/DAC, Legacy Audio PowerBloc2 amplifier, and a pair of Legacy Audio Focus SE loudspeakers. I occasionally do some listening through pair of Sennheiser 560S headphones. I miss the excellent ELS Studio sound system in our 2016 Acura RDX (now my wife's daily driver) on which I had ripped more than a hundred favorite CDs to the hard drive, so now when driving my 2024 Honda CRV Sport L Hybrid, I stream music from my phone through its adequate but hardly outstanding factory system. For more casual listening at home when I am not in my listening room, I often stream music through a Roku Streambar Pro system (soundbar plus four surround speakers and a 10" sealed subwoofer) that has surprisingly nice sound for such a diminutive physical presence and reasonable price. Finally, at the least grandiose end of the scale, I have an Ultimate Ears Wonderboom II Bluetooth speaker and a pair of Google Pro Earbuds for those occasions where I am somewhere by myself without a sound system but in desperate need of a musical fix. I just can’t imagine life without music and I am humbly grateful for the technologies that enable us to enjoy it in so many wonderful ways.

William (Bill) Heck, Webmaster and Contributor

Among my early childhood memories are those of listening to my mother playing records (some even 78 rpm ones!) of both classical music and jazz tunes. I suppose that her love of music was transmitted genetically, and my interest was sustained by years of playing in rock bands – until I realized that this was no way to make a living. The interest in classical music was rekindled in grad school when the university FM station serving as background music for studying happened to play the Brahms First Symphony. As the work came to an end, it struck me forcibly that this was the most beautiful thing I had ever heard, and from that point on, I never looked back. This revelation was to the detriment of my studies, as I subsequently spent way too much time simply listening, but music has remained a significant part of my life. These days, although I still can tell a trumpet from a bassoon and a quarter note from a treble clef, I have to admit that I remain a nonexpert. But I do love music in general and classical music in particular, and I enjoy sharing both information and opinions about it.

The audiophile bug bit about the same time that I returned to classical music. I’ve gone through plenty of equipment, brands from Audio Research to Yamaha, and the best of it has opened new audio insights. Along the way, I reviewed components, and occasionally recordings, for The $ensible Sound magazine. Most recently I’ve moved to my “ultimate system” consisting of a BlueSound Node streamer, an ancient Toshiba multi-format disk player serving as a CD transport, Legacy Wavelet II DAC/preamp/crossover, dual Legacy PowerBloc2 amps, and Legacy Signature SE speakers (biamped), all connected with decently made, no-frills cables. With the arrival of CD and higher resolution streaming, that is now the source for most of my listening.

Ryan Ross, Contributor

I started listening to and studying classical music in earnest nearly three decades ago. This interest grew naturally out of my training as a pianist. I am now a musicologist by profession, specializing in British and other symphonic music of the 19th and 20th centuries. My scholarly work has been published in major music journals, as well as in other outlets. Current research focuses include twentieth-century symphonic historiography, and the music of Jean Sibelius, Ralph Vaughan Williams, and Malcolm Arnold.

I am honored to contribute writings to Classical Candor. In an age where the classical recording industry is being subjected to such profound pressures and changes, it is more important than ever for those of us steeped in this cultural tradition to continue to foster its love and exposure. I hope that my readers can find value, no matter how modest, in what I offer here.


Bryan Geyer, Technical Analyst

I initially embraced classical music in 1954 when I mistuned my car radio and heard the Heifetz recording of Mendelssohn's Violin Concerto. That inspired me to board the new "hi-fi" DIY bandwagon. In 1957 I joined one of the pioneer semiconductor makers and spent the next 32 years marketing transistors and microcircuits to military contractors. Home audio DIY projects remained a personal passion until 1989 when we created our own new photography equipment company. I later (2012) revived my interest in two channel audio when we "downsized" our life and determined that mini-monitors + paired subwoofers were a great way to mate fine music with the space constraints of condo living.

Visitors that view my technical papers on this site may wonder why they appear here, rather than on a site that features audio equipment reviews. My reason is that I tried the latter, and prefer to publish for people who actually want to listen to music; not to equipment. My focus is in describing what's technically beneficial to assure that the sound of the system will accurately replicate the source input signal (i. e. exhibit high accuracy) without inordinate cost and complexity. Conversely, most of the audiophiles of today strive to achieve sound that's euphonic, i.e. be personally satisfying. In essence, audiophiles seek sound that's consistent with their desire; the music is simply a test signal.

Mission Statement

It is the goal of Classical Candor to promote the enjoyment of classical music. Other forms of music come and go--minuets, waltzes, ragtime, blues, jazz, bebop, country-western, rock-'n'-roll, heavy metal, rap, and the rest--but classical music has been around for hundreds of years and will continue to be around for hundreds more. It's no accident that every major city in the world has one or more symphony orchestras.

When I was young, I heard it said that only intellectuals could appreciate classical music, that it required dedicated concentration to appreciate. Nonsense. I'm no intellectual, and I've always loved classical music. Anyone who's ever seen and enjoyed Disney's Fantasia or a Looney Tunes cartoon playing Rossini's William Tell Overture or Liszt's Hungarian Rhapsody No. 2 can attest to the power and joy of classical music, and that's just about everybody.

So, if Classical Candor can expand one's awareness of classical music and bring more joy to one's life, more power to it. It's done its job. --John J. Puccio

Contact Information

Readers with polite, courteous, helpful letters may send them to classicalcandor@gmail.com

Readers with impolite, discourteous, bitchy, whining, complaining, nasty, mean-spirited, unhelpful letters may send them to classicalcandor@recycle.bin.

"Their Master's Voice" by Michael Sowa

"Their Master's Voice" by Michael Sowa