Krell K-300i integrated amplifier Measurements

Sidebar 4: Measurements

I measured the Krell K-300i using my Audio Precision SYS2722 system (see the January 2008 "As We See It"). I usually precondition amplifiers by running them at one- third power into 8 ohms for an hour. The Krell almost made it, turning itself off after 55 minutes, with the front-panel display showing the message "Over temperature left channel." The top panel over the heatsinks was hot, at 142.8°F (61.6°C), but cooler over the transformer at 113.1°F (45.1°C). This is a tough test for an amplifier with a class-AB output stage, as one-third power results in the maximum dissipation in the output devices. The Krell has just sufficient heatsinking for its power rating but should be given plenty of space for ventilation.

After the K-300i had cooled down, I was able to continue with the testing. Looking first at the line-level analog inputs, the Krell's voltage gain into 8 ohms is specified as "25dB, referenced to 2V RMS in balanced and full power output." (The specified maximum output of 150W into 8 ohms is equivalent to a gain of 24.77dB ref. 2V.) However, with the volume control set to its maximum of "100," I measured a gain of 43.5dB for both balanced and unbalanced inputs into 8 ohms. While higher than the specification, this gain is typical for an integrated amplifier. The gain measured at the unbalanced preamplifier outputs was 21.25dB, which is on the high side.

The amplifier preserved absolute polarity (ie, was noninverting) for both XLR and RCA inputs at both the loudspeaker and preamplifier outputs. The single-ended input impedance is specified as 8k ohms, the balanced impedance as 16k ohms. However, my measurements indicated 12.7k ohms for the unbalanced input impedance but a low 4.4k ohms, ie, 2200 ohms per phase, for the balanced impedance.

The output impedance is specified as a very low 0.035 ohms from 20Hz to 20kHz. By contrast, I measured 0.131 ohms at 20Hz, 0.133 ohms at 1kHz, and 0.159 ohms at 20kHz, these figures including the series impedance of 6' of loudspeaker cable. Even so, the response with our standard simulated loudspeaker varied by less than ±0.1dB (fig.1, gray trace).


Fig.1 Krell K-300i, frequency response at 2.83V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green), and simulated loudspeaker (gray) (0.5dB/vertical div.).

The audioband response into impedances of 8 and 4 ohms (fig.1, blue, red, cyan, and magenta traces) is flat up to 20kHz, and the two channels match very closely. The output into 2 ohms (green trace) was down by just 0.125dB at 20kHz. This graph was taken with the volume control set to "100." Commendably, the ultrasonic rolloff didn't vary at lower volume-control settings, or from the preamplifier outputs. The K-300i amplifier has a wide small-signal bandwidth, and its reproduction of a 10kHz squarewave (fig.2) featured very short risetimes and no overshoot or ringing.


Fig.2 Krell K-300i, small-signal, 10kHz squarewave into 8 ohms.

Channel separation was excellent, at 90dB in both directions at 1kHz, though it reduced to 67dB, R–L, and 75dB, L–R, at 20kHz. The wideband, unweighted S/N ratio, ref. 2.83V and measured with the volume control at "100" and the unbalanced input shorted to ground, was 63dB (average of both channels), which improved to 76.5dB when the measurement bandwidth was restricted to the audioband, and to 78.8dB when A-weighted. Spectral analysis of the Krell's noise floor (fig.3) revealed that the random noise level varied with the setting of the volume control, suggesting that this was related to the higher-than-usual preamplifier gain. The spuriae related to the power-line frequency were all very low in level, however.


Fig.3 Krell K-300i, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms with volume control set to "80" (left channel cyan, right magenta) and "100" (left blue, right red) (linear frequency scale).

The Krell's maximum power is specified as 150W into 8 ohms and 300W into 4 ohms, both equivalent to 21.8dBW. With "clipping" defined as when the THD+noise reaches 1%, I found that the K-300i exceeded its specified power, clipping at 190W into 8 ohms (22.9dBW, fig.4) and 320W into 4 ohms (22dBW, fig.5). Fig.7 shows how the Krell's THD+N percentage varied with frequency at a moderate output level, 8.975V, into 8, 4, and 2 ohms. Commendably, the distortion in the top audio octave is only slightly higher than it is at low frequencies. The distortion is predominantly third harmonic in character (fig.7), though the second harmonic is only slightly lower in level (fig.8). High-order intermodulation products with an equal mix of 19kHz and 20kHz tones at a peak level of 40W into 4 ohms were relatively low in level (fig.9), and the second-order difference product lay at –86dB (0.005%).


Fig.4 Krell K-300i, distortion (%) vs 1kHz continuous output power into 8 ohms.


Fig.5 Krell K-300i, distortion (%) vs 1kHz continuous output power into 4 ohms.


Fig.6 Krell K-300i, THD+N (%) vs frequency at 8.975V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), and 2 ohms (gray).


Fig.7 Krell K-300i, 1kHz waveform at 20W into 8 ohms, 0.0077% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).


Fig.8 Krell K-300i, spectrum of 50Hz sinewave, DC–1kHz, at 40W into 4 ohms (left channel blue, right red) (linear frequency scale).


Fig.9 Krell K-300i, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 40W peak into 4 ohms (left channel blue, right red) (linear frequency scale).

Turning to the digital inputs, Apple's USB Prober utility identified the Krell amplifier as "Krell K-300i" from "KRELL" and indicated that the USB port operated in the optimal isochronous asynchronous mode. Apple's AudioMIDI utility revealed that, via USB, the K-300i accepted 32-bit integer data sampled at all rates from 44.1 to 384kHz. All the digital inputs preserved absolute polarity (ie, were noninverting).

With the Krell's volume control set to its maximum and the level trim set to "0dB," a 1kHz digital signal at –30dBFS resulted in a level of 779.1mV at the preamplifier outputs but 20.23V into 8 ohms from the speaker outputs. The latter is equivalent to 51W, which is just 4.7dB below the specified maximum power. As often appears to be the case with integrated amplifiers, the K-300i's digital inputs have too much gain. I wanted to continue the testing from the preamplifier outputs using the menu to set the output mode set to "Fixed." Peculiarly, however, the volume control was still active in this mode. To avoid blowing up the Krell's output stage with high-level digital test signals, therefore, I continued the testing with the volume control set to "53," which gave a level of 20V at the loudspeaker terminals.

The K-300i's impulse response with 44.1kHz data was a fairly long minimum-phase type, with all the ringing following the single high sample (fig.10). With 44.1kHz-sampled white noise (fig.11, red and magenta traces), the Krell's response reached full stop- band attenuation at 24kHz, suppressing the aliased image at 25kHz of a full-scale tone at 19.1kHz (blue and cyan traces) by almost 110dB. The harmonics of the 19.1kHz tone can be seen in this graph, with the third the highest in level, at –74dB (0.02%). The K-300i's frequency responses with data at 44.1, 96, and 192kHz followed the same basic shape but with a sharp rolloff above half of each sample rate (fig.12).


Fig.10 Krell K-300i, digital inputs, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).


Fig.11 Krell K-300i, digital inputs, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan) into 100k ohms with data sampled at 44.1kHz (20dB/vertical div.).


Fig.12 Krell K-300i, digital inputs, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel green, right gray), 96kHz (left channel cyan, right magenta), 192kHz (left blue, right red) (1dB/vertical div.).

When I increased the bit depth from 16 to 24 with a dithered 1kHz tone at –90dBFS (fig.13), the random noise floor dropped by 18dB, meaning that the Krell's digital inputs offer 19 bits' worth of resolution. However, a lot of low-level AC supply–related spuriae can be seen in this graph. With undithered data representing a tone at exactly –90.31dBFS, the three DC voltage levels described by the data were well resolved and the waveform was perfectly symmetrical (fig.14). With undithered 24-bit data, the result was a superbly clean sinewave (fig.15).


Fig.13 Krell K-300i, digital inputs, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with: 16-bit data (left channel cyan, right magenta), 24-bit data (left blue, right red) (20dB/vertical div.).


Fig.14 Krell K-300i, digital inputs, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).


Fig.15 Krell K-300i, digital inputs, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red).

Intermodulation distortion via the digital inputs was even lower than it was via the analog inputs (fig.16), with the second-order difference product produced by equal-level tones at 19 and 20kHz with the combined waveform peaking at 0dBFS, lying at –93dB (0.0027%). Finally, tested for its rejection of word-clock jitter with 16-bit USB data, all the odd-order harmonics of the LSB-level, low-frequency square- wave were at the correct levels (fig.17, sloping green line). However, a pair of sidebands at ±120Hz to the sides of the peak that represents the high-level tone at one-quarter the sample rate can be seen, these power supply-related. Repeating the jitter test with TosLink data gave the same result, as did using 24-bit J-Test data (fig.18)


Fig.16 Krell K-300i, digital inputs, 24-bit data, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 100k ohms, 44.1kHz data (left channel blue, right red; linear frequency scale).


Fig.17 Krell K-300i, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit USB data sourced from MacBook Pro (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.


Fig.18 Krell K-300i, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit USB data sourced from MacBook Pro (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

Overall, the Krell K-300i offers excellent measured performance. The only shortfall was the digital input's very high gain, which probably led to the introduction of low-level supply-related spuriae.—John Atkinson

Krell Industries, LLC
45 Connair Road
Orange, CT 96477-3650
(203) 799-9954

Ortofan's picture

not enough heatsink.

Suppose that the designer of the Krell amp was still working at Sikorsky.
Would their helicopters overheat and shut off in mid-flight after running at one-third power for 55 minutes?

For $7K one could have instead the Marantz PM-KI Ruby integrated amp and SA-KI Ruby SACD player/DAC combo.

JRT's picture
ortofan wrote:

Suppose that the designer of the Krell amp was still working at Sikorsky.
Would their helicopters overheat and shut off in mid-flight after running at one-third power for 55 minutes?

The lubricant in a helicopter transmission is not only to address the tribology of friction, wear, and lubrication, but also serves to cool the working surfaces of the gear teeth and transport that heat to an oil cooler. With that in mind, read the brief article at the following link.

And more recently, "accelerated wear" has plagued the main rotor gearbox in the new CH-53K, the US Marine Corps new heavy lift helicopter, and has been one of the sources of delays in replacing old fatigued and worn CH-53E.

Bogolu Haranath's picture

Even with all that grease applied, helicopter motors don't sound musical anyway :-) .......

Ortofan's picture

... in a product, such as a helicopter, where simply pulling over to the side of the road, so to speak, is not an option. Every attempt is made during the development process to design out such potential failures, and/or include redundant back-up systems, and then test prototypes to beyond the expected operating conditions and life.

The measurements performed by Stereophile, including the one-hour at one-third rated power preconditioning test for amplifiers, are not a secret. Thus, one might imagine that any company submitting an amp for review would confirm that it can pass that test - unless they would rather debate its usefulness.

Bogolu Haranath's picture

They forgot to add 'Prestone' before sending the review sample :-) .......

TJ's picture

... for your very fine and interesting review. Great to see Krell "back" again, kudos to Dave Goodman! Hope you get a chance to review one of their Duo XD amps.

dial's picture

Krell is a good brand and always will be. They keep quality going, see them at audio salons (only separates in fact).
A part of the hifi legend, at least in North America. It's good they still are in, in Europe a lot have ceased operation (UK, France and Germany come to mind). I hope they'll last forever.
Only minor complaints. The design could have been better without this bump in the middle. Looks like an old Naim, isn't it ?
I also miss a knob volume control, well you are not by 3D LABS, right ?
But the rear panel shows poor cinch and speakers sockets, a shame for price asked.
Measurements are good to very good, nothing surprising.
Thanks for your article.

Bogolu Haranath's picture

Primare I35 Prisma integrated amp ($5,000, reviewed by Hi-Fi news) and Hegel H390 integrated amp ($6,000, reviewed by S&V magazine) are also in this price range :-) .......

Kursun's picture

First 90W in class A ?
With that heatsink?
Absolutely no way!

rwwear's picture

The iBias allows 90 watts class A.

tonykaz's picture

What perspective can describe this box of parts as "high value" ?

Certainly not the High Values of the Schiit designer types.

But then again...

If this device does a ton of Class A from that smallish chassis it's an engineering marvel.

Dubious claims aside...

Should we be comfortable with a Company using someone else's Name, isn't buying Krell also buying Dan ?

Spending serious money for Krell and not getting Dan D'Augistino seems a little empty. ( like buying Chinese ROLEX Watches )

Tony in Venice

dc_bruce's picture

I don't think that's very fair. Dan has been --ahem-- divorced from Krell for quite some time. Moreover, Dan was not the only guy responsible for Krell designs when he was there. I suspect that the lesser products, like the KAV-300i (which I owned back in the day) were not the recipients of a whole lot of the chief designer's time. The item under review is at least the third version of this product to bear the name KAV-300i.
It's good to see Krell trying to serve this end of the market; so many high-end companies have retreated "upstairs."

I believe Krell had its issues with its "SACD Standard" and subsequent generation of "ultimate" disc players. Through no fault of the company's, their optical drive supplier shipped them 2 generations of failure-prone drives. I'm sure customers were unhappy and warranty claims must hav been significant.

tonykaz's picture

You're probably right, I'm not being fair ( sort-of ) with my comparison.


I think of those superb Krell Amps that I could never seem to get my ( Esoteric Audio ) hands.

Of course...

I keep thinking of Dan & Krell in 1985 terms ( like an old geezer I am ) pining for those "good old days".

Krell had powerful magic. Dan still has powerful magic.

So, If we're gonna spend Big Bucks for a Brand Name there should be the Name accompanying the deal, shouldn't there ?

Anyway, I was hoping to read about something wonderful, something breakthrough, something having Krell-like spectacular performance and appearance, something that makes the reviewer feel like the singing artist is kissing him with pillowy embraces.

It's probably all Dan's fault, dammit, he set-us-up to expect new highs in everything.

Tony in Venice

ps. I was never one of Dan D'Augistio's Dealers, probably because I couldn't come up with the $$$$$ to stock the entire line with "one to show and two to go" Plus have the Apogee Loudspeaker Line. Phew !! I would've loved to have Dan prancing around my Store meeting Customers.

dc_bruce's picture

and dealers who sell this super-expensive stuff, like d'Agostino. Cash flow management is torture. I was a director of a small scientific instrument company that sold devices used to quality control silicon chips. Typically, these sold for about $1.5M each and the company would sell 2 to 4 of them a year. While that was not the company's only revenue source (it sold much smaller cryogenic devices and service/support contracts on the big stuff), missing an anticipated sale really screwed up the books for that year.

It's easy to rail at the stratospheric prices of this stuff, but when you figure in all of the costs other than materials and labor -- financing inventory (at either the dealer or manufacturer level), maintaining adequate working capital to deal with wide swings in cash flow -- they're not always so exorbitant. Whether they are a good value (as compared to less expensive products) is, of course, another matter about which I don't think one can generalize.

audiodoctornj's picture

Been following these comments so I have to chime in with a few points:

The mythos of Dan Dagastino is just silly, Dan was never a trained electrical engineer, Dan came up with a design that was over built and had the ability to drive a very low load to power the Apogee full range and that was the beginning of Krell and the KSA 50.

Dan's leadership of Krell was more strategic in running the company and overseeing the final designs of the product line which had grown quite a lot into amps, preamps, integrated amps, CD players, dacs, transports, and phono stages, and eventually speakers and even a subwoofer.

Most of Krell's modern designs for the last 20 years were designed by Dave Goodman who came up with the Ibias design and the new XD topology.

As an original Krell KSA 250 owner, as well as a FPB 450 MCX mono blocks and selling Krell for years, the new gear is by and far the best sounding Krell gear they have ever made.

The older Krell amps did have a more pronounced bass slam, than the newer XD amplifiers, the newer XD amplifiers have a far greater sense of involvement, midrange liquidity, and sense of space, than the original designs.

The K300i is a fantastic integrated amplifier which can easily stand aside any competitive integrated amplifier for the price range including the Hegel and the Primare.

Neither of these amplifies has the unique feature set of the K300i which includes Blue Tooth, HDMI inputs, and a great built in streamer along with a massive amount of Class A power, the Primare is a Class D design and the Hegel although excellent does not have the tube like liquidity that the XD circuitry provides.

All told the new Krell K300i is a fantastic performer, as per the gentleman who is decrying value lets look at that statement

Start with a good $5,000.00 integrated amplifier

add a good Dac $2,000.00

add a streamer $ 500.00

add a digital cable $ 200.00

add a set of interc $ 500.00

you would have $8,200.00 vs the $8,000.00 all in one K300i with dac

so the K300i is really quite reasonably priced for all it does and the superb sound quality it delivers.

Audio Doctor NJ

Bogolu Haranath's picture

It would be interesting to see a comparison review with the more expensive D'Agostino Progression integrated amp ($25,000) ......... May be JVS could do the comparison review :-) .........