Integrated Amp Reviews

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.