Chord CPM 3300 integrated amplifier
Michael Fremer, July, 2001
With its high-end heart and home-theater brain, Chord's powerful CPM 3300 integrated amplifier ($9500 with the aluminum-cylinder Integra leg option, $8950 without) is a uniquely fascinating audio product well worth considering. High-tech innards and magazine-cover good looks don't hurt either, but what originally got me interested was the superlative sound Chord products have consistently delivered at trade and consumer shows when paired with Wilson-Benesch loudspeakers.
Those combos always seem to produce exciting and, to my ears, musically honest sound from familiar demo discs, including CD compilations I've made using the Rockport System III Sirius turntable. A journalist has to keep moving at such shows or the coverage will suffer, but when I hit the Chord/W-B room, I inevitably end up sitting through and reconnecting with some overfamiliar demo disc in its entirety.
Game Plan
While Chord's penetration of the American audiophile market has so far been modest, the company's consumer line is expansive: preamplifiers, two- and multichannel amplifiers, DACs, and phono sections. According to John Franks, Chord's chief designer, artists including Paul McCartney, Ray Charles, and the Wu Tang Clan use amplifiers from Chord's professional line in their studios. Well-known venues include Abbey Road studios and London's Royal Opera House, which uses them for sound reinforcement.
I'd recently reviewed Musical Fidelity's M3 integrated amp (Stereophile, February 2001), so Chord's new variation on that theme seemed like a good model to choose from the company's long lineup. Like the M3, which combines much of the circuitry of Musical Fidelity's Nu-Vista preamp and 300 power amp in one chassis, the CPM 3300 is based on two extant and well-received Chord products: the C3200E preamplifier and 1200B amplifier. While twice the M3's price, the CPM 3300 actually offers somewhat less RMS power (220W vs 275W) into an 8 ohm load. I was curious to see and hear how Chord felt it could justify the CPM 3300's seemingly steep price.
Audiophiles comforted by the straightforward operating systems of most high-end audio products—turn on, select input, adjust volume—will be surprised and perhaps confused by this model's complexity. While the CPM 3300 is on the surface a simple, two-channel integrated amplifier, lurking beneath its smooth aluminum skin is a microprocessor-controlled environment somewhat like a home-theater receiver, complete with dual signal buses for independent tape monitoring, source-controlled gain settings, and dual outputs. The last allows you to route the signal to the built-in amplifier or to a set of RCA jacks for external amplification—or to both, should you choose to biamp your speakers.
Complementing the home theater-like operating menu are the two usual rancid side dishes: inadequate and confusing instructions. a complete lack of labeling of the rear-panel input and output jacks (though a key is included with the handbook) and a remote control from hell (neat, unlit rows of same-sized, multi-function buttons). If you've ever read one of my reviews in Stereophile Guide to Home Theater, you know that I readily admit to possibly having a learning disability when it comes to operating remotes. There were times when I got no sound at all from the CPM 3300 and couldn't figure out why.
The instructions emphasize the CPM 3300's capabilities while neglecting to tell you clearly how to access them (especially regarding the remote control), or in what order. The how-to section is long on jargon-filled lists and short on user-friendly prose.
I'm told that the instructions are being revised and that a new remote, designed just for the CPM 3300, will replace the impressive-looking but unwieldy and button-crowded one supplied with the review sample, and which can control up to six Chord audio and video products, your lawn sprinkler, your garage door, and probably your home's thermostat.
Design
Despite its complexity and the impressive amount of power it can deliver, the CPM 3300 is surprisingly light and compact, thanks to the switching power supply at the core of Chord's approach to building amplifiers. Designer John Franks came out of the aerospace industry, where such lightweight, compact designs are a necessity. Switch-mode supplies have been used in audio with varying degrees for years, beginning, I believe, with Bob Carver's much-vaunted The Cube amp, back in the early '80s. [Sony released an integrated amplifier with a switch-mode supply in 1979, if I remember correctly. Many in Linn's current range of amplifiers use such supplies, and they are now ubiquitous in PCs.—Ed.]
In conventional power-supply designs, a heavy transformer, rectifiers (silicon diodes or tubes), and large filter capacitors are used to drop wall voltages, convert alternating current to direct, and store energy needed by the amplifier to reproduce music. Unfortunately, according to Chord, the demands are so great that these supplies must be large and heavy, and in the best of them, efficiency in the audioband is relatively low.
In Chord's view, these bulky, inefficient designs are sluggish, put distortion back on to the AC mains, and can add noise. Countering Chord's view are all of the fabulous-sounding (if bulky) amplifiers that use just such old-fashioned power-supply designs. As with everything else in life, greatness is a matter of both design and implementation—a proposition with which Chord's John Franks would probably not take issue. It took him 10 years to overcome such problems in the audioband power supply as switching noise, poor reliability, and high costs before he felt his first amplifier design, the Chord SPM 900, was ready for the High End, in 1989.
Chord's current power supply, the fifth generation, is common to all of the company's amplifiers (in 600W, 2kW, and 4kW sizes, depending on application). This supply is a self-contained, self-monitoring module that Chord claims is extremely well-shielded and immune from interactions with the AC mains at one end and the amplifier circuitry at the other. Incoming AC is filtered and rectified to produce 300-350V DC, which is stored in high-voltage capacitors. High-voltage MOSFETs running at 80kHz "chop" the DC, which is then passed through an HF transformer. The higher the frequency, the smaller the transformer can be compared to the standard 50Hz or 60Hz design.
There's another rectification stage, a Dynamic Coupling system, and, finally, a bank of high-voltage storage capacitors. Dynamic Coupling keeps the supply's positive and negative rails tied together via a strong magnetic flux. When there is a demand for power on one rail, it is drawn from both, which keeps the system in balance so the zero crossing point doesn't get pulled one way or the other.
The CPM 3300's output stage runs in a "class-AB sliding bias design," which operates in class-A (always on) in most normal situations, and switches to class-B only in situations that demand high power. The output devices are custom-built, dual-chip 200V/300W MOSFETs; Chord claims that these ensure "perfect thermal matching," eliminating the need for temperature-balancing resistors for improved efficiency and stability.
Output protection is based on the Dynamic Coupling circuit. When the system senses flux-density differences between the two rails over time, an algorithm determines whether the cause is a need for power, or a fault such as a short circuit or a speaker-damaging DC offset. If it's a fault, the CPM 3300 switches itself to standby mode.
The preamp section is fully balanced dual-differential with a combination of balanced and single-ended inputs that are converted to balanced, though other than its output stage, the power amplifier section is single-ended. The volume and balance controls are four-gang motorized ALPS potentiometers. Each preamp channel has its own independent power supply, as do the microcontroller and the 32 hermetically sealed relays.
Connection, Operating Features, and (Ab)use
Connecting inputs and outputs is straightforward: two Disc inputs offer single-ended and balanced operation, and there are four more single-ended inputs, two of which are tape loops associated with a pair of tape outs. There are also RCA-jack line-level and speaker-level outputs. All jacks are high-quality, gold-plated chassis mounts, and the speaker-level outs are high-quality WBTs. Vinyl playback requires an outboard phono section.
On the front panel are solid brass Volume and Balance pots, a power switch (Standby/On), three buttons marked S, B, and A, and a custom blue fluorescent display. The hard-to-see slots on the rotary Volume and Balance controls make it almost impossible to gauge starting positions until you move either one, in which case the screen display kicks in. Unfortunately, while moving the balance control one way or the other causes arrows to appear on the screen letting you know in which direction you're going, there's no indication of when you've reached equal output in both channels. This will create high anxiety for anal-retentive audiophiles.
When I switched the unit on (this can't be done via the remote) I was greeted by one of the coolest cosmetic displays ever seen in consumer electronics: the outlines of the amp's innards, visible from a circular top grate, began glowing black-light style, resembling (depending on your history): 1) a 1960s head shop, 2) a night landing in Dallas, or 3) Tron. The light show is courtesy 160 tiny blue LEDs plus a few red ones, each the size of a grain of sand.
Once you're mesmerized by the lights, the fun begins! From the remote or the front panel you can choose the A or B button to select which bus you want to listen to. Bus A is connected to Tape 1 Out, Bus B to Tape 2 Out. Tape 2 In can be selected only with Bus A and Tape 1 In only with Bus B. The purpose of this ball of confusion is to allow you to record and monitor two different sources simultaneously. It's not that difficult once you understand it, and when listening to non-tape sources you can ignore it, but really—how many audiophiles need dual simultaneous analog-domain recording and monitoring? Or even single? Even digiphobes like me are mostly recording to CD and MD these days, which makes standard tape monitoring (listening off the playback head) moot.
In any case, it's there if you need it. But if you're just using one tape deck, you might as well connect it to Tape 2 so you don't have to change to Bus B just to listen. I hooked up an open-reel deck to Tape 1 and couldn't get any sound from it because I was using Bus A. I know—when in doubt, read the instructions. My advice with the CPM 3300: Read the instructions even if you're not in doubt.
More generally useful features are the four gain settings (x1, x2, x5, x7) available for each input. This allows you to come close to matching levels and prevent unwanted sonic blasts when you scroll through your sources. The CPM 3300 will remember your settings, even if it's been switched off for years. You can also reset all inputs to the lowest setting (x1) when you follow a procedure in the instructions that you probably won't memorize, so keep 'em handy! And on power-up, the CPM 3300 reverts to your last input.
Remote Oddities
Say you're listening to a CD and the phone rings. What do you do? Hit Mute, of course, and the sound mutes. When you're finished with the phone call and you want to hear music, hit Mute again, right?
Wrong. Nothing happens. You have to hit Volume Up to unmute. Then you'll get music again, right?
Wrong. For some reason, when switching from Mute, you have to select an output—not the bus, but the output. There are two outputs as well as two buses. Output 1 is the speaker output, Output 2 the line output. The procedure going from Mute is: Hit Volume Up or Down, then hit the Glas/DVD1/Op1 button (remember, it's a multi-use remote).
Another oddity: Among the many source buttons are some repeated ones: two each of Disc 1, Disc 2, Video, and Radio input. Turns out one set is for Bus A, one for Bus B. Red brackets on the remote are intended to delineate the two sets of buttons, but not until John Franks explained it to me did I understand what was going on, thanks to my learning disability or the instructions (or lack thereof). The button layout plus the less-than-forthcoming remote created a great deal of confusion and frustration in the Fremer household. Hopefully, the new remote and instructions will make using the CPM 3300 as pleasant as listening to it was.
In short: Setting up and using the CPM 3300 is quirky, but once you've got the hang of it, it's pretty simple. Chord just has to do a better job of helping people get that hang.
Listening
Before switching from the Hovland HP-100/Musical Fidelity Nu-Vista 300 combo, I sat down and listened to my usual suspects. On LP: Davey Spillane's Atlantic Bridge, Janis Ian's Breaking Silence, Nat King Cole's Love is the Thing, Mel Tormé and Friends at Marty's, The Weavers' Reunion at Carnegie Hall, and various RCA Living Stereo recordings of Jascha Heifetz (original and Classic reissues). On CD: Olu Dara's In the World, various JVC XRCDs, Mobile Fidelitys, and DCC Compact Classics, etc. I also mixed in some open-reel tapes played back on a recently acquired Viking 88 tubed deck, including The Byrds' Sweetheart of the Rodeo, The Fantastic Expedition of Dillard and Clark, and Herbert von Karajan and the Berlin Philharmonic's early-'60s reading of Beethoven's Symphony 7.
I spent the better part of a day and that evening locking that combo's sound into my brain, then switched to the Chord CPM 3300, which remained in the system for more than a month of intense listening. (I used the Audio Research Reference phono section for vinyl playback.) The LP I played right before and after the switch was DCC Compact Classic's stupendous-sounding reissue of Nat Cole's Love is the Thing. This 1956 three-track stereo recording puts a gigantic, hovering image of Cole in the center of the stage, surrounded by a dryly recorded, somewhat boxy-sounding string section augmented by a harp, double bass, and just a hint of brushwork.
The Hovland/Nu-Vista combo delivered Cole's overwhelming voice in a sweeter, richer, more laid-back fashion compared to the Chord, which presented him in greater three-dimensional relief, and so far forward that I thought he was going to land in my lap. I'm exaggerating, but when Cole first entered on the opening of "When I Fall in Love," it was so startling that my head snapped back reflexively. The Chord's presentation was bigger, more vivid, and more exhilarating by far. If the Hovland/Nu-Vista sounded liquid and warm, the Chord sounded positively gelatinous (not a criticism) but also somewhat sandier, while offering up more body and solidity to images and greater transient snap.
The Chord sounded definitely faster, with an intense rhythmic drive and a lean, taut bass presentation that added excitement to a very familiar recording. Some will prefer that sound, and not be bothered by the increase in sibilant grit and the greater emphasis on Cole's vocal cords at the expense of the mellowing effect of his chest cavity. It would be foolish to describe one presentation as "right" and the other as "wrong." Clearly, this was a closely miked recording, with Cole popping consonants all over the place—some so intensely that your walls will shake if you have a subwoofer—so why wouldn't there be an emphasis on the vocal cords and gritty sibilants? (If you listen carefully, you can hear Cole changing s and t to f in an attempt to limit the recorded damage.)
The Musical Fidelity M3's presentation of Love is the Thing couldn't have been more different. Sibilants were smooth and pristine, and Cole's voice was warm and rich, with more emphasis on the chest and less on the vocal cords. The string sound had more body and less bow-scrape, and the overall picture was more finely drawn: delicate, relaxed, and pure. But, as I noted in the review, the picture was somewhat lacking in body, solidity, and weight. The Hovland/Nu-Vista combo occupied the middle ground between the M3 and the Chord. "Right" and "wrong" didn't enter into this comparison: each of the three presented a coherent, appealing aural picture. Your preference will depend on your musical taste and associated equipment.
The musical excitement I'd experienced in the Chord/Wilson-Benesch rooms at audio shows was brought home by pairing the CPM 3300 with the Sonus Faber Amati Homage speakers. On such pristine live recordings as The Weavers' Carnegie Hall Reunion, the sense of experiencing the live event was heightened by the Chord's taut, up-front presentation. The extended, well-controlled bottom end decisively delivered the hall sound, as well as the onstage foot-tapping and the resulting sensation of the vibrations traveling along the floorboards. Voices and guitars had a deliberate yet natural focus, well-delineated from the rear boundary of the backstage wall, and cleanly recorded sibilants were delivered flawlessly.
Playing symphonic music and the Heifetz recordings, the Chord produced silky-smooth string sound while preserving the natural grit of the bow. Massed strings never glazed-over or hardened, though the emphasis was more on the strings than on the body.
Black backgrounds; fast, sparkling transients; taut, rhythmically adept bass; dynamic ease and superb resolution of low-level detail—the CPM 3300 reproduced it all without sounding mechanical, bright, harmonically bleached, or antiseptic. It produced organized musical excitement and a big, airy, dramatic sonic picture without phony spotlighting or amusical etching. Did it deliver the warmth and harmonic fullness of a good tube amp? No. Do most tube amps deliver the Chord's transient speed, dynamic drive, and taut, well-damped bass? No.
While I had the Chord in the system, there were times I felt as if I would prefer the greater warmth, liquidity, and suppleness of the Hovland/Nu-Vista combo. When the Chord was gone and I had the Hovland/Nu-Vista back in the system, with some LPs and CDs I wished I could re-experience the Chord's tight, energetic grip on the music. Welcome to the spectacularly imperfect world of electrically reproduced music.
Conclusions
While the Chord CPM 3300's operating system is complex and can be quirky to operate, once it's set up and you've gotten a handle on its idiosyncrasies, it's actually easy and convenient to use. It offers a great deal of flexibility and packs high power in a dramatic-looking, fairly compact, and surprisingly lightweight chassis. Build quality appears to be extremely high and very high-tech. Some of the features—such as dual independent taping and monitoring and line- and speaker-level outputs—will be of limited use to many audiophiles, but for some will be welcome options. As for the sound...
If you like your musical presentation big, dramatic, dynamic, fast, and exciting, the Chord will give it to you without adding the usual negatives associated with that sort of sound. If you prefer relaxing into a warm, laid-back musical picture, the CPM 3300 is not for you. It's reassuring to know not only that there's room in this marketplace for 2W single-ended tube amps and high-tech engineering marvels like Chord's CPM 3300, but also that open minds can find the musical truth in both.
Sidebar 1: Specifications
Description: MOSFET-powered integrated amplifier. Power output: 220Wpc RMS into 8 ohms, 300Wpc RMS into 4 ohms. Frequency responses: preamplifier, 2.5Hz-200kHz, -3dB; power amplifier, 0.8Hz-46kHz, -1dB, into 8 ohms. THD (x1 gain): 10Hz, <-91dB; 1kHz <-93dB; 10kHz <-90dB; 20kHz <-87dB. Power amplifier gain: 30dB. Preamplifier S/N: 93dB, all inputs. Input impedance: 47k ohms unbalanced, 94k ohms balanced. Input sensitivity (for full output): not specified. Power consumption: not specified.
Dimensions: 16.4" (420mm) W by 5.9" (152mm) H by 13.8" (355mm) D. Weight: 8.6 lbs (19kg).
Serial number of unit reviewed: Not noted.
Price: $8950; add $550 for Integra leg option (as reviewed).
Approximate number of dealers: 6.
Manufacturer: Chord Electronics Ltd., The Pumphouse, Farleigh Bridge, Farleigh Lane, East Farleigh, Kent ME16 9NB, England, UK. Tel: (44) (0)1622-721444. Fax: (44) (0)1622-721555. Web: www.chordelectronics.co.uk. US distributor: The Sound Organisation, 11140 Petal Street, Suite 350, Dallas, TX 75238. Tel: (972) 234-0182. Fax: (972) 234-0249. Web: www.soundorg.com.
Sidebar 2: Associated Equipment
Analog source: Simon Yorke turntable; Graham 2.0, Immedia RPM, Tri-Planar Mk.VI tonearms; Lyra Helikon, Helikon mono, Clearaudio Insider cartridges.
Digital source: Marantz SA-1 SACD player.
Preamplification: Hovland HP-100 preamplifier, Audio Research Reference phono section.
Power amplifier: Musical Fidelity Nu-Vista 300.
Loudspeakers: Sonus Faber Amati Homage.
Cables: Phono: Hovland Music Groove DIN/RCA, XLO Signature 3.1. Interconnect: Harmonic Technology Pro-Silway II, Magic. Speaker: Harmonic Technology Woofer. AC: JPS Labs, Electra Glide Fatboy, PS Audio Lab, Wireworld Electra Series III.
Accessories: PS Audio Power Plant P300, Sounds of Silence Vibraplane active isolation platform, Symposium Rollerblocks (Tungsten), Grand Prix Audio Monaco amplifier stands, Walker motor drive, Finite Elemente Pagode and Zoethecus equipment stands, A.R.T. Q dampers, Walker Valid Points, ASC Tube Traps, Shakti Stones and On-Lines, RPG BAD and Abffusor panels.—Michael Fremer
Sidebar 3: Measurements
My usual practice before measuring an amplifier is to run it at one-third power into 8 ohms for one hour. This severely stresses a design with a class-B output stage, as it results in the maximum heat being dissipated in the output transistors. However, I cut short the preconditioning with the Chord CPM 3300 after 30 minutes. This was partly because the rear heatsinks were far too hot to touch (footnote 1), as was the black mesh grille on the top, but also because the measured distortion had risen slowly and steadily throughout that time, reaching 0.53% from a cold start of 0.3%. (I made sure the bottom vents weren't blocked during the testing.) This preconditioning is admittedly a very severe test for an amplifier, but the Chord's behavior suggests that it has inadequate heatsinking for sustained high-power use, such as at a party, particularly with low-impedance speakers. (This also implies that the Integra legs are hardly an option, if the amplifier is to be kept off the floor.)
The balance knob was loose on the shaft when I received the review sample for measurement. I equalized the channel outputs with the remote before performing any measurements. The Chord's architecture allows for four different gain settings to be applied universally across the inputs, or individually to allow for sources with different output levels. Set to "x1," the maximum level from the preamp output was 0.6dB higher than the input, with a corresponding voltage gain from the power-amplifier section of 30.6dB. Set to "x2," the levels increased slightly more than the expected 6dB, at 7.9dB and 37.9dB, respectively. The "x5" and "x7" settings resulted in gains from the preamp outputs of 11.9dB and 14.6dB, respectively, and 41.9dB and 44.6dB from the power-amp outputs. I performed all subsequent testing with the gain set to "1."
The XLRs are connected with pin 2 hot. With the positive speaker lead connected to the red WBT terminal, the amplifier inverted polarity via the unbalanced input jacks. The input impedance at 1kHz was a usefully high 55.9k ohms unbalanced and 86k ohms balanced. The output impedance from the preamp jacks was a low 100 ohms over most of the band, rising trivially to 115 ohms at 20Hz. From the Chord's speaker terminal, the source impedance was a very low 0.06 ohm, rising slightly to 0.08 ohm at 20kHz. As a result, any interaction between the amplifier's source impedance and the impedance of our simulated loudspeaker was below ±0.1dB.
The overall frequency response (fig.1) sensibly rolled off above the audioband, but some slight wrinkles can be seen disturbing the otherwise smooth rolloff. These wrinkles might be associated with the very slight amount of overshoot on the otherwise excellent shape of the 1kHz (fig.2) and 10kHz (fig.3) squarewaves. (I had to average 64 individual readings for these waveforms due to the presence of very-high-frequency noise. This noise could be reduced in level by grounding the negative speaker lead at the Audio Precision test set, but this increased the level of a 120Hz hum component; I thought this strange, given the Chord's use of a switching power supply. Unfortunately, a computer failure meant I couldn't investigate the Chord's noise behavior any further.)
Fig.1 Chord CPM 3300, frequency response at (from top to bottom at 2kHz): 2.83V into dummy loudspeaker load, 1W into 8 ohms, 2W into 4 ohms, 4W into 2 ohms (0.5dB/vertical div.).
Fig.2 Chord CPM 3300, small-signal 1kHz squarewave into 8 ohms.
Fig.3 Chord CPM 3300, small-signal 10kHz squarewave into 8 ohms.
Channel separation via the balanced inputs is shown in fig.4. Below 1kHz, the crosstalk reading is dominated by the ultrasonic noise mentioned above. Above that frequency the crosstalk starts to rise due to capacitive coupling between the channels, but there is a strange minimum between 9 and 10kHz in both directions before the crosstalk resumes rising at the top of the audioband.
Fig.4 Chord CPM 3300, channel separation (10dB/vertical div., R-L dashed).
Plotting the THD+noise percentage against frequency for various loads at 9V gave the traces shown in fig.5. The measured distortion level doesn't change with frequency but does almost double each time the load impedance halves. The THD appears high, but this is due to the rather peculiar way the Chord's distortion changes with frequency (see below). Looking at the distortion waveform on the oscilloscope at this level (9V) was fruitless, as all that could be seen was noise. I had to increase the output level to an enormous 282W into 4 ohms to raise the distortion components out of the noise (fig.6); the distortion is strongly second-harmonic in nature, which will be subjectively benign at much higher levels than it appears in the CPM 3300.
Fig.5 Chord CPM 3300, THD+noise (%) vs frequency at (from top to bottom at 5kHz): 9V into simulated loudspeaker load, 40W into 2 ohms, 20W into 4 ohms, 10W into 8 ohms.
Fig.6 Chord CPM 3300, 1kHz waveform at 282W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.7 looks at the harmonic distortion in a different way, by calculating the spectrum of the CPM 3300's output spectrum while it drove a pure 50Hz tone into 8 ohms at two-thirds power. The only harmonic component that can be seen is, again, the second (100Hz), at a low -80dB (0.01%). A spectral line can be seen at 120Hz, but this is below the -90dB mark. The generally hashy-looking nature of the noise floor is due to the noise mentioned earlier.
Fig.7 Chord CPM 3300, spectrum of 50Hz sinewave, DC-1kHz, at 206W into 8 ohms (linear frequency scale).
Though a number of 1kHz-spaced intermodulation components can be seen in a similar but wider-bandwidth spectrum taken while the Chord drove a punishing 1:1 mix of 19kHz and 20kHz just below visible clipping into 4 ohms (fig.8), these are all well below -80dB (other than the 1kHz difference component, which hovers at around -83dB, or 0.007%). But again, note the rather hashy-looking noise floor in this graph, particularly at lower frequencies.
Fig.8 Chord CPM 3300, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 150W into 4 ohms (linear frequency scale).
I mentioned earlier the rather unusual manner in which the Chord's distortion+noise percentage changed with output power. This can be seen in fig.9, with the amplifier driving 8 and 4 ohm loads with both channels driven and 2 ohms with just one channel driven. Below 2-3W, the percentage is dominated by noise, hence the linear decrease in the THD percentage with increasing power. There is a fairly abrupt rise in the percentage between 3 and 30W, which, as I mentioned above, seems to be due to increased noise rather than to actual distortion. This, again, decreases linearly with increasing power output until the clipping point is reached, indicated by the sharp knee in these traces.
Fig.9 Chord CPM 3300, distortion (%) vs continuous output power into (from bottom to top at 10W): 8 ohms, 4 ohms, 2 ohms.
For consistency, we define clipping power as the level at which the measured THD+noise reaches 1%. Into 8 ohms, this was well above the Chord's specification, at 290W (24.6dBW). Despite the amplifier's very low output impedance, the current didn't double when the load impedance halved, resulting in outputs of 395W into 4 ohms (23.0dBW) and 500W into 2 ohms (21.0dBW) (footnote 2).
The Chord is indeed a powerhouse, and, with its blue- and red-LED-illuminated innards visible through its perforated top cover, easily the most eye-catching amplifier I have had in my lab. However, I didn't like how hot it got during the testing and I am bothered by the way its THD+noise changed with output level and load impedance. It should also be noted that the control circuitry locked up a number of times during the testing, necessitating powering the amplifier off and on and again to reboot the software.—John Atkinson
Footnote 1: As a rule of thumb, the temperature at which something becomes too hot to keep your hand on it is 60 degrees C (140 degrees F).—John Atkinson
Footnote 2: Our usage of "dBW" follows that established by UK reviewer Martin Colloms in the early 1980s, whereby an amplifier that acted as a perfect voltage source would give the same dBW figure into any load. Shortfalls from voltage-source behavior are thus easily seen as a reduced figure compared with the reference 8 ohm figure.—John Atkinson
