Technical Highlights

Highlights of the original Jørgensen Kumisa III

• All-discrete fully-complementary topology
• No overall global negative feedback.
• No integrated circuits used except for the DC servo opamp and voltage regulators.
• The fundamental Jørgensen Kumisa III topology:
  • Complementary input gain stage with complementary voltage sources to set the operating point.
  • Current mirror with gain.
  • Complementary push-pull output stage.
  • DC servo.

Enhancements in the Cavalli-Kan Kumisa III

• The input BJTs have been replaced with Toshiba 2SK170/2SJ74 JFETs, which increases input impedance, reduces input bias currents as well as reduce distortion and noise.
• The output stage topology has been changed from a Darlington emitter follower configuration to a Sziklai CFP (complementary feedback pair). The CFP has the advantage of some gain, thus 100% local feedback provides improved linearity. However, CFP is more prone to high frequency instability, thus in the CK²III, compensation capacitors were added to prevent potential oscillations. (Douglas Self reference). This change also required changes to the output stage bias circuit. Namely, the Vbe multiplier transistor's biasing resistors and trimpot values had to be changed to provide a suitable range of adjustment.
• The CFP output stage is also thermally more stable than a Darlington configuration, reducing the likelihood of thermal runaway. Thus, the Vbe multiplier no longer need to be thermally-coupled to the output devices, making possible a more logical PC board layout.
• The DC servo scheme is significantly modified:
  • The opamp was stabilized by adding rail bypass capacitors.
  • The servo was decoupled from the current mirror, and instead acts on the input stage without being directly in the signal path. It adjusts the constant voltage sources that bias the sources of the input JFETs in order to zero the offset. Under AC conditions, the bases of the constant voltage transistors are held at a constant DC voltage by the various capacitors to ground. Therefore the servo doesn't really impact the AC behavior of the amp but it still has plenty of gain to zero the output DC offset.
  • The DC servo's low-pass slope was also made steeper (a 2-pole filter rather than a single pole).
• All small signal BJTs were replaced with BC550/BC560 and the output transistors with BD139/BD140. These are more commonly available and provides excellent performance.
• Rather than an unregulated supply, a regulated dual rail DC supply using the 7815/7915 IC regulators were integrated onboard. This provides cleaner, more stable power rails while minimizes the number of hookup wiring and makes the CK²III very easy to build.

Cavalli-Kan Kumisa III circuit board highlights

• Glass epoxy 4.725"x4.200" printed circuit board, double-layer with plated-through holes, silkscreen and solder mask.
• Heavy duty 2oz. copper layers provide lower trace impedance.
• The layout of all parts and traces have been carefully considered for maximum performance.
• A star-ground configuration is employed on the circuit board. The star point is a screw hole that could be used to couple the ground point to the chassis via a machine screw and standoff.
• The board was designed to slide into a Hammond 1455Q series extruded enclosure, making the casing of this amplifier very easy.
• Mounting holes on the board allows it to be used in other enclosures.