An All Discrete, Complementary Heaphone Amplifier

The Kumisa III

The Kumisa III amplifier was originally designed by Benny Jørgensen. Benny's article on this amp can be found in the projects section of Headwize. The Kumisa III is the third in a series of Benny's amplifiers. The first two used Opamps and in the Kumisa III Benny wanted to create an all discrete design.

As he describes in his Headwize article, Benny thought through the Kumisa III pretty carefully, including ensuring that it has a wide frequency response. His original power supply design is an unregulated 24V bipolar supply with various techniques applied to also give the power supply a wide bandwidth.

A small group of diyers built Kumisa IIIs based on Benny's original design. One review was posted by olseth and a working design thread was started by peter counsell.

The Cavalli Kumisa III

I started reading the design thread during one of my visits to Headwize. A few of the builders were having difficulty with the servo scheme that Benny had designed. Their Kumisas were showing subsonic oscillation that was changing the DC at the output of the amp. As I studied Benny's design I thought that I could improve on the servo scheme. At first we tried changing the component values in the original servo. Then we tried running the servo back to the input to get more control over the amplifiers offset. Several of the builders prototyped this version.

Running the servo back to the input, however, has the effect of lowering the input impedance of the amplifier and puts the servo into the audio signal path. So even after the new prototypes were built and worked, I continued to look for another, better way to servo the amp. In the end, the new servo scheme turned out to be very simple and is the scheme that is now in the CK2III amplifier.

CK2III boards and support are available from AMB Labs. Everything you want to know about building this amplifier can be found on AMB's website. After I had finalized what I thought was a really good revision of the Kumisa III I asked AMB if he would be willing to make, sell, and support boards. Diyers in the small community of headphone amp builders are very familiar with the exceptional job that AMB does creating new amplifier designs and supporting them. AMB was kind enough to take on the CK2III even though he was in the middle of introducing another new amplifier of his own design.

The Cavalli Kan Kumisa III

I sent my original Express PCB design to AMB and we then collaborated to make the final board layout. During the PCB collaboration AMB and I also collaborated on finalizing the design. We each ran extensive simulations on the amp alone and on the amp and power supply in combination. I noticed that AMB really liked the Toshiba jfets in his σ22 amp and I suggested that we could substitute these devices for the input bjts without any modifications to the design. AMB agreed. This was the final change before going into production with the new boards.

The currently active CK2III thread is here.