Krell K-300i integrated amplifier Designer Dave Goodman

Sidebar 1: Krell Designer Dave Goodman on iBias and Other Features

The K-300i uses modern techniques that we've implemented in the recent designs and upgrades we've issued since the split. First, it has a differential current-mode input stage that drives a fairly conventional output stage. Though we've done such designs before, I discovered that by fine-tuning parts of the gain stage I could significantly reduce distortion—specifically the second-harmonic component. The predominant characteristic is now the third harmonic, which we've found makes the K-300i sound much better.

In the amplifier circuitry, I then utilized a much lower output impedance than is typical, which produced a radical improvement in sound quality. It's more difficult to implement this because of potential mismatches between output transistors, which tend to be amplified more in such a design. Matching of devices was not as difficult as I thought it might be because we have a very tight thermal package, with everything mounted very close together on a very heavy-based heatsink, so each device tracks very closely to the temperature of the other. Because temperature remains stable, and there's a very short thermal path between each of the devices, you never get a big temperature difference. The other problem with implementing lower output impedance is potential instability, which, after designing amplifiers for a long time, I knew how to address.

The K-300i's other major design component is its iBias circuit, which monitors the bias current that flows through the output stage differently. Bias means there is a certain amount of current flowing through the output transistors in order to keep them turned on. The output stage is a push- pull design, which means you have one device for the positive half of the waveform and another device for the negative half of the waveform. In push- pull, they are connected together to produce the entire waveform.

In a typical class-AB amplifier, a small amount of bias current runs through both halves of the output stage. If you had more, it would generate a lot of waste heat, as does pure, passive class-A, which requires huge heatsinks and fans to not have relatively low output power. In class-AB, you don't have quite enough bias current to cover the full range of the signal. (It's usually quite low, and enough for only a couple of watts of class-A.) As the signal gets bigger than the bias current flowing through one side of the output stage, that side turns off. This process creates classic switching distortion.

With iBias, we monitor the current directly through both the positive and negative halves of the output stage. Through a control loop, we make sure that the inactive half of the output stage, which is the half that would start to turn off in a typical class-AB design, always stays on enough so it never switches off entirely and creates switching distortion. Because we maintain just the bare minimum of current through the inactive half, we never have more bias current than is necessary for a given signal and speaker load. We never throw away more power than we need to.

The other unique thing about iBias is that speaker impedance doesn't affect performance, even with speaker curves that are all over the place. Because we monitor the current directly in real time in each half of the output stage, we can make sure it's always on, no matter what. The iBias circuit ensures that the output stage is always operating in class-A, but only just enough; it's much more efficient than typical class-A designs.

Another difference between iBias and a lot of other sliding bias schemes is that many of them monitor the input signal to determine how the bias gets set. That has no bearing on speaker impedance and will result in either inaccurate class-A operation or inefficient operation by forcing more bias current than you need. Even if you're monitoring the output current that's going to the speaker, you still have to translate that into what the bias current should be for that output current level. With iBias, you don't need to worry about any of that because we're looking at the current through the output stage directly and maintaining class-A operation to the exact definition of class-A.

COMPANY INFO
Krell Industries, LLC
45 Connair Road
Orange, CT 96477-3650
USA
(203) 799-9954
ARTICLE CONTENTS

COMMENTS
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.
https://audio.com.pl/testy/stereo/odtwarzacz-cd-wzmacniacz/3021-marantz-sa-pm-ki-ruby
https://www.musicdirect.com/integrated-amp/Marantz-KI-RUBY-Reference-Bundle

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.

https://www.google.com/amp/s/www.ainonline.com/aviation-news/aviation-international-news/2013-01-01/main-gearbox-remains-helicopters-achilles-heel%3famp

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.

https://www.marinecorpstimes.com/news/your-marine-corps/2019/02/04/the-marine-corps-new-ch-53k-is-a-mess-this-is-why-its-operational-date-could-face-delays/

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.

Still,

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 :-) .........

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