Croft Acoustics Phono Integrated integrated amplifier Measurements

Sidebar 3: Measurements

I measured the Croft Phono Integrated using 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). Before performing any measurements on an amplifier, I run it for an hour with both channels driven at 1/3 its rated power into 8 ohms; this is the level at which the maximum amount of power is dissipated in the output devices with a class-A/B topology. At the end of the first 30 minutes running at 1/3 the specified 45Wpc into 8 ohms, the top panel over the Phono Integrated's output stage was very hot, at 118.4°F (48°C), though it got no hotter for the remaining 30 minutes of preconditioning.

Looking first at the phono input, I examined its performance at the Line Out jacks, which appear to be placed ahead of the volume control but after the Mute switch. These offer just under unity gain, with a very high output impedance: almost 10k ohms at low and middle frequencies, dropping slightly to 7.5k ohms at the top of the audioband. This suggests that there is no output buffer stage ahead of these jacks, merely a high-value series resistor. Signal polarity was preserved from these output jacks for both phono and line input signals. The phono input appeared to be optimized for moving-magnet cartridges, with a gain at 1kHz of 40.9dB and an input impedance that varied from 49k ohms at 20Hz to 44k ohms at 1kHz and 36.5k ohms at 20kHz.

The Croft's phono-input RIAA error is shown in fig.1. The two channels match superblyy well; however, the phono stage suffers from a severe rolloff in the treble, reaching –3dB at 10kHz and –6dB at 20kHz. The low frequencies are boosted a little, reaching +1.8dB at 20Hz. Phono-input channel separation at high frequencies was poor, at <35dB R–L and <41dB L–R above 10kHz. The separation did increase at lower frequencies, reaching 56dB R–L and 60dB L–R at 1kHz. Phono noise was relatively low, with unweighted, wideband signal/noise ratios (measured with the input shorted and ref. 1kHz at 5mV) of 52.9dB in the left channel, 50.7dB in the right. These ratios respectively improved to 55.4 and 56.3dB when the measurement bandwidth was restricted to the audioband, and to 75.5dB when A-weighted.

1013Croftfig01.jpg

Fig.1 Croft Phono Integrated, response with RIAA correction ref. 1kHz at 5mV input (left channel blue, right red; 1dB/vertical div.).

The phono-stage overload margin was good at high and middle frequencies, at almost 20dB ref. 1kHz at 5mV. But the margin was very poor at low frequencies: just –1.8dB at 20Hz. The phono stage's distortion signature was predominantly the subjectively innocuous second harmonic (fig.2). When it came to high-frequency intermodulation, however, the Croft's phono stage did poorly, an equal mix of 19 and 20kHz tones at 50mV giving a second-order difference product at –34dB (1.5%, fig.3).

1013Croftfig02.jpg

Fig.2 Croft Phono Integrated, spectrum of 1kHz sinewave, DC–10kHz, at 5mV input into 100k ohms (left channel blue, right red; linear frequency scale).

1013Croftfig03.jpg

Fig.3 Croft Phono Integrated, spectrum of 19+20kHz sinewaves, DC–24kHz, at 600mV into 100k ohms (linear frequency scale).

Turning to the Croft's performance via its line inputs, assessed at the speaker terminals, the Phono Integrated offered a maximum gain into 8 ohms at 1kHz of 34dB, the two channels differing by just 0.08dB. The input impedance was very high at almost 80k ohms at low and middle frequencies, dropping inconsequentially to 51k ohms at 20kHz. The amplifier as a whole inverted absolute polarity at the speaker terminals.

The output impedance was very high for a solid-state design, at 2.15 ohms at 20Hz and 1kHz, dropping slightly to 2 ohms at 20kHz. As a result, the variation in the Croft's frequency response due to the Ohm's law interaction of this impedance with that of our standard simulated loudspeaker was relatively large, at ±1.2dB (fig.4, gray trace). This graph also indicates that while the Croft has a wide small-signal bandwidth, there are the beginnings of an ultrasonic peak, suggesting an incipient instability. However, while the traces in this graph were taken with the twin unganged volume controls at their maximum—note the excellent channel matching—turning down these controls to 6:00 both eliminated the response peak and reduced the bandwidth to –3dB at 50kHz (fig.5). This graph was taken with the settings of the volume controls visually matched; note the channel imbalance of 0.7dB. This was typically the best balance I could achieve by matching the controls' positions by eye. Art Dudley likes this volume-control arrangement; I find it ergonomically unsettling.

1013Croftfig04.jpg

Fig.4 Croft Phono Integrated, volume control at maximum, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green) (1dB/vertical div.).

1013Croftfig05.jpg

Fig.5 Croft Phono Integrated, volume control at 6:00, frequency response at 2.83V into 8 ohms (left channel blue, right red) (1dB/vertical div.).

With its volume controls at 6:00, the Croft's reproduction of 10kHz and 1khz squarewaves was excellent (figs.6 & 7, respectively). Channel separation at 1kHz (not shown) was moderate, at 63dB R–L and 78dB L–R, decreasing due to the usual capacitive coupling between channels to 38dB R–L and 52dB L–R. The unweighted, wideband S/N ratios, ref. 1W into 8 ohms (2.83V), taken with the line input shorted but the volume controls set to their maxima, were good at 68.4dB, improving to 82.9dB with the measurement bandwidth restricted to the audioband, and 84.6dB when A-weighted. Fig.8 shows the spectrum of the Croft's output while it drove a 1kHz tone at 1W into 8 ohms. You can see that, other than the full-wave rectified spurious tone at 120Hz, all the power-supply–related spuriae lie below –90dB.

1013Croftfig06.jpg

Fig.6 Croft Phono Integrated, small-signal 10kHz squarewave into 8 ohms.

1013Croftfig07.jpg

Fig.7 Croft Phono Integrated, small-signal 1kHz squarewave into 8 ohms.

1013Croftfig08.jpg

Fig.8 Croft Phono Integrated, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (left channel blue, right red; linear frequency scale).

The Croft's output stage is not very linear, and is actually reminiscent of a tube amplifier. Figs.9 and 10 show how the THD+noise percentage changes with output power, both channels driven, into 8 and 4 ohms, respectively. The THD is already >0.2% at just 100mW, and rises with increasing power. Into 8 ohms (fig.9), the THD almost reaches our usual definition of clipping at just 8W, but drops a little above that power. It crosses the 1% line at 33W (15.2dBW), and the amplifier reaches its specified power of 45W into 8 ohms (16.5dBW) only at 3% THD. The picture is worse into 4 ohms (fig.10), the THD reaching 1% at just 700mW, and reaching 55W (14.4dBW) at 3% THD. One complicating factor with this measurement was that, into 4 ohms, the output with continuous drive slowly declined with time, the implication being that the Croft's power supply begins to collapse with sustained high levels of current delivery.

1013Croftfig09.jpg

Fig.9 Croft Phono Integrated, distortion (%) vs 1kHz continuous output power into 8 ohms.

1013Croftfig10.jpg

Fig.10 Croft Phono Integrated, distortion (%) vs 1kHz continuous output power into 4 ohms.

Fig.11 shows how the THD+N percentage changed with frequency at 2.83V into 8 ohms (blue and red traces) and 4 ohms (cyan, magenta). Though the THD didn't change at different frequencies, it is high overall. Yes, it predominantly consists of the subjectively innocuous third harmonic (fig.12), but at this level, 1.23% at 1W into 4 ohms, the distortion will certainly be audible with pure tones, if not music. (Try the test tones with various levels of second, third, and seventh harmonic that I included on Test CD 2, Stereophile STPH004-2.)

1013Croftfig11.jpg

Fig.11 Croft Phono Integrated, THD+N (%) vs frequency at 2.83V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta).

1013Croftfig12.jpg

Fig.12 Croft Phono Integrated, 1kHz waveform at 1W into 4 ohms (top), 1.23% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).

At low frequencies and an 8 ohm load, the second harmonic is the highest in both channels (fig.13), but at higher powers, the third and fifth predominate (fig.14). Intermodulation distortion is similarly high, even at a level well below clipping (fig.15).

1013Croftfig13.jpg

Fig.13 Croft Phono Integrated, spectrum of 50Hz sinewave, DC–10kHz, at 1W into 8 ohms (left channel blue, right red; linear frequency scale).

1013Croftfig14.jpg

Fig.14 Croft Phono Integrated, spectrum of 50Hz sinewave, DC–10kHz, at 10W into 8 ohms (left channel blue, right red; linear frequency scale).

1013Croftfig15.jpg

Fig.15 Croft Phono Integrated, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 10W peak into 4 ohms (left channel blue, right red; linear frequency scale).

Without any circuit board, the three-dimensional, hard-wired layout of the Croft's circuit is a thing of wonder. However, I must admit to being puzzled why Art Dudley liked the sound of the Croft Phono Integrated as much as he did. To me, it seems, at best, inadequately engineered, and at its worst—that nonflat RIAA response, the high levels of harmonic and intermodulation distortion—just plain inadequate. I therefore asked Stephen Mejias to take a listen to the Croft. His thoughts on its sound appear in this issue's "Follow-Up" section.—John Atkinson

Company Info
Croft Acoustics
US distributor: Bluebird Music Ltd.
310 Rosewell Avenue
Toronto, Ontario M4R 2B2, Canada
(416) 638-8207
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Comments
Rick Tomaszewicz's picture
What a contrast...

between this and the preceding review of Marantz's Network Audio Player.  Idiosyncratic old school vs bleeding edge new age.  This is why I love reading Stereophile!

Despite the measured flaws JA found in the Croft, Art and Stephens' emotional reactions (and greater focus on the music played than the player) spoke louder - particularily since they're both contextually well informed.  It's not the first time I've noticed such a discrepancy in these pages.  What's really going on here?  Does art trump sound engineering?  

Reminds me of a Japanese mantra; better to do a small thing well than a large thing poorly.

Rick Tomaszewicz's picture
Wait, maybe...

I'm not an electrical engineer.  But, when I look at the photo of the Croft's innards I'm struck by the apparent circuit simplicty, paucity of parts and what looks like point to point wiring.  Could this be the reason it sounded so good to Art and Stephen, despite JA's poor measurements?  Perhaps its flaws were lost in the light of what it did so well.

And, what are the pots on the rear panel's top left corner?  

LS35A's picture
Dealers?

The importer is in Canada.   I'm trying to find a list of U.S. dealers..... 

Doesn't Stereophile have some rule about how many dealers a product has to have before they will review it?   

 

 

 

 

Stephen Mejias's picture
Bluebird Music

The importer is in Canada.   I'm trying to find a list of U.S. dealers.....

Bluebird Music handles all North American distribution for Croft.  You can contact Bluebird for a dealer near you.

Doesn't Stereophile have some rule about how many dealers a product has to have before they will review it?

The Five Dealer Rule.

Rick Tomaszewicz's picture
BTW, JA,

I know this is well above my pay grade, but wouldn't it be fun if Stereophile held an internal competition once a year?  Pick a gear type, let's say speakers the first time out, invite companies to submit the product they're proudest of, and then run a controlled blind test with Stereophile's editors and reviewers.  Let the companies know in advance what the associated gear will be so they can send their most compatible product. (Yes, without regard to cost or category, let your staff listen to all the speakers blindly "on a level playing field".) There could be two categories; rank overall, and, rank vs cost ratio.  To motivate the companies, you could give the winner of the second category free advertising for a year!  

It would be a blast to read the results; probably even more fun than Mikey's cartridge shoot-out over at AnalogPlanet and would set the bar very high for the audio press.  Of course, the 1%'s would be interested in the highest rank overall.  The rest of us 99%'s would love the highest rank vs cost ratio. 

It was your recent comparison of the Marantz to the MSB which triggered this idea.  A similar test of historic (Strads and Guarneris) and modern viloins was done by one of the violin magazines a while back.  The results were surprising.

andy_c's picture
RIAA accuracy

With that kind of RIAA accuracy, it's a fair bet that the design was done by the proverbial "passionate artisan".

jgossman's picture
Adverts

Croft was an advertiser in Listener.  

Still, there's no snark intended so don't take it as such.  I usually don't remember where I put my keys.  And if it was after you sold publishing to another company, you may have never known.  I would be more surprised that you both knew and remembered than otherwise.

Great review.  Unfortunately for my taste, my signal path hasn't had a transitor in years now.  Unfortunately for my purchasing power, I'm about to be a new dad.  Maybe one of my fellow readers can enjoy this amp based on the review.

JayeColby's picture
Croft integrated

I have owned my Croft linestage integrated R for six months and enjoy it more and more each day. I don't listen to the measurements.

nunhgrader's picture
Dudley = my own views

I usually have very similar viewpoints as Mr. Dudley's! Great article - I dig Mr. Mejias's unique and youthful voice/ viewpoint as well - keep up the great work!

SET Man's picture
Less bling more performance...

Hey! After seeing so many expensive audios full of bling-bling, especially some of which I saw and heard at the NY Audio early this year.... some didn't sound good to me and made me want to throw up after they told me the price!

       This is just what the real world need today. Good audio piece at more affordable price for the 99%s like myself. For me I don't care much how the audio component looks if it sound good to me, but if it happened to look good too than that is a plus. Of course there will be people who buy audio with their eyes first still... well its their money.

     Man! Wish I could this Croft in my own system just to see and hear them.

killersax's picture
Sound vs. measurement

When a component's sound and its measurements diverge so much, we have a good opportunity to re-examine some assumptions. Two questions come to mind: (1) Is there something important about components that JA is not measuring? or (2) Do even expert listeners like their music better with added harmonic distortion and rolled-off treble? Very puzzling. (Although it is heart-warming to learn that Stephen Mejias likes Bruckner.)

SergioLangstrom's picture
Listening Vs Deaf Ears

Seems to me that if a component measures as badly as this one does, then those that liked how it sounds needs a hearing checkup. How can anyone trust reviewers that can't hear obvious faults in a component? Pretty soon everything will start sounding peachy.

Magnum Innominandum's picture
Measurements vs. Listening

I fail to remember how often a device greatly praised for it's sonic qualities by the reviewer in this publication is measured by John Atkinson and it turns out it measures worse than a turd.

Here, we have TWO reviewers agreeing "sounds great" even the reviewer who knew it "measures poor" just loved the sound.

So, we can go with the conspiracy theory and consider that both reviewers were paid off by the manufacturer (that one seems universally popular) or we can simply conclude that the standard measurements JA performas have comparably little, if any bearing on actual sound quality (this one is unpopular especially among those who love to believe in "measurements", so it would likely have the ring of truth to it).

Indeed, I would issue two challenges to John Atkinson:

1) Justify measurements performed and their weighing in terms how they relate to what we hear, taking into account the extant body of work on the subject. So if for example harmonic distortion is measured and the impression is given that "lower is better" it should be backed by evidence that provides proof that lower distortion reliably results in better sound.

2) Justify the measurements interpretation in a system context; e.g., should I worry about the given amount of distortion in a given amplifer, considering the know distortion levels in speakers and microphones (or indeed LP records, analogue mastertapes). One might say justify any interpretation with an impact analysis.

To ask more pointedly, for example, why does anyone "define clipping at 1% THD"? It has for example been shown that much higher levels of certain types of distortion are inaudible while other types are both audible and objectionable at levels of THD much lower than 1% THD [1] and when most speakers exceed double digit level of THD at rated power. 

Failing any support by solid science regarding their importance and impact, performing measurements and interpreting them has preciously little value.  When solid evidence that these methods are not reliable or able to give us information about the audible effects and sound quality is present, continuing with the same old methods is something that Richard Feynman once charaterised thusly:

"In the South Seas there is a cargo cult of people. During the war they saw airplanes land with lots of good materials, and they want the same thing to happen now.

So they've arranged to imitate things like runways, to put fires along the sides of the runways, to make a wooden hut for a man to sit in, with two wooden pieces on his head like headphones and bars of bamboo sticking out like antennas--he's the controller--and they wait for the airplanes to land.

They're doing everything right. The form is perfect. It looks exactly the way it looked before. But it doesn't work. No airplanes land.

So I call these things cargo cult science, because they follow all the apparent precepts and forms of scientific investigation, but they're missing something essential, because the planes don't land."

It would be a major step forward in Audio SCIENCE, if we could discard trappings of science and instead actually act scientific, so that, in a figure of that speech, "the planed land", which for audio measurements would mean measurements that reliably predict if a given item will sound good or not.

Magnum Innominandum

[1] E.R. Geddes and L.W. Lee, “Auditory 

Perception of Nonlinear Distortion-Theory,” Paper 

presented at the Audio Engineering Society 115th 

Convention - Paper 5890 (2005, Oct.) 

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