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ChromaKnob (Chroma Knob Box)

By David Clarke [21030085++] <>

Note from the author: "The ChromaKnob is a one-off creation, and as such there really aren't additional ones available for sale. I briefly thought about making a series of them for sale, but I didn't pursue it as I thought that it would be cost prohibitive. (I didn't think too many people would be interested in paying $400US or so for a box, so I only ever made myself one.)"

David's creation was featured in Keyboard magazine in 2001; see the Bibliography for more information.


The Chroma Knob Box atop one of my Chromas (while it is shown sitting in the middle of the unit, is was actually designed to rest on the right-hand side.)

Design Goal

To provide a more intuitive and more easily used programming interface for the Rhodes Chroma (Rhodes Chroma Expander) synthesizer.

By design the interface:

  1. Should not require any modifications to the Chroma itself in order to function;
  2. Should not in any way negatively impact on current functionalities of the Chroma;
  3. Should provide the user full-text descriptions of the parameters and values being edited;
  4. Should allow for any parameters to be immediately edited without the need for the parameter to be separately selected; and,
  5. Should be aesthetically compatible with the Chroma.


Interfaces to a Rhodes Chroma, Rhodes Chroma Expander or Fender Chroma Polaris via the DB-25 "Chroma Port."

Has a DB-9 serial interface to allow communications with a PC. When the internal EPROM is replaced with a RAM + ROM board, the serial interface can be used for complete reprogramming of the box - allowing such changes as the functional reassignment of each of the knobs and modifications to the output text messages.

Powered by a 9Vdc power cube.

Measures approximately 9 1/4" deep x 21" long (5" high at the rear, 3" high at the front). The unit was specifically sized to sit on the top of a Chroma/Chroma Expander, directly above the existing parameter selection buttons (sits on the top deck, to the right of the centre rib).

Chassis of box is hardwood (Cherry). Work surface is steel.

Full two-way communications with the Chroma are implemented.

Using the Knob Box in no way interferes with the normal functioning of the Chroma. All front panel commands on the Chroma are still active (i.e., the Knob Box and the Chroma controls can be used in parallel).

Main processor is a 80C31 running at 11.0592 MHz (essentially any processor could have been used.)

"Reasonable Cost" was kept as a design consideration. As a result:


A medium close-up of the box itself (alone).


A close up showing the type of knobs used and the type of labels.


An interior view of the unit.


Samples of the types of messages that appear in the LCD display during use.