Chroma CPS Improvement

The CPS-Kit, available for the Chroma was an interesting thing, but has some drawbacks. The first thing I did'nt like, was the mounting with the self-adhesive band. I decided to not using this and made brass brackets to hold the CPS-stripe under the wooden bar above the keys. I've removed the self-adhesive stripe, it isn't difficult but needs some patience to not break something on the CPS.

After mounting the CPS, described below, I found that the aftertouch response was not really smooth and I tried to find out, what I can do. First I recognized that the action needs a lot of force and the start of the modulation was not smooth, more immediately like a switch. With two changes on the CPS I could it make working really better, smoother and with less force needed. Changes are described in detail below.

Mounting without self adhesive stripes

Two different brass L-profile has to be soldered together. With some pertinax parts, screws and distance holder, I formed the CPS mountig brackets. The pcb of the CPS is protected by a piece of a silicone-tube (cut in half).

This "sandwich" can be mounted now to the wooden bar, therefor I made aluminium brackets.

left: L-profiles, right: soldered together
all parts 
detail view of the "CPS-sandwich"
mounting brackets on wooden bar 
detail view, silicone tubes protects pcb from scratches 
mounted in CHROMA  

Aftertouch Response Improvement

square rubber feet (black squares) for a more efficient action to the FSRs

The area from the metal piece of the keyboard-action doesn't fit really to the force-sensing-parts of the CPS. So there is a lot of force needed to use the aftertouch action and it is not sufficient for a differenciated aftertoch-feeling. I looked for a way that the sensitive area of the FSR could be better matched from the keyboard action and tried a few different rubber feet (same as used for small cases). The best result I had with square feets (ca. 10x10mm, h=ca. 3mm). There was a new adjustment of the wooden bar needed after mounting the rubber feet, otherwise the key-action becames to flat. To find the "hot spot" of every key was a little bit difficult, but it was also the chance to equalize some "oversensitive" FSRs.

To measure the effect of the rubber feet and to find the "hot-spot" a DMM or an oscilloscope is very helpful 

CPS electronic with some parts removed

The last thing for better aftertouch-response was in the aftertouch electronic. Mesurements shows a amplitude of the aftertouch control-voltage from 0 - ca. 2.5 V. After consulting the schematic, I wasn't really surprised, the voltage was generated by a simple voltage divider formed by a fix resistor and the FSR. In addition to this, the FSR resistor repsonse to the force isn't linear in the first range, to reach the linear range a lot of force would be needed. I made a litte circuit around the TS912In Opamp, it's a dual single rail, rail to rail opamp.





With the first stage on the left side of the schematic, the theshold of the aftertouchresponse could be set with the trimmer. The diode needs about 0.6V forward voltage, with the first trimmer it can be set, when it will be reached this 0.6V. So the very unlinear first portion af the force responce of the FSR can be "skipped". The diode equalizes also the non-linearity of the FSR. The second stage amplifies the signal so it can reach nearly 5V. With the trimmer the required force to reach the highest value could be adjusted. The opamp is supplied by a 5V supply, I soldered a cable from a 5V point of the original CPS electronic to pin 6 of the original opamp position. The used opamp is rail to rail, so it can reach nearly 5V, but the voltage can never go higher than 5V, so the input of the following circuitry (ADC) can never get a voltage to high.

The schematic is very simple. the output is not exactly linear and it is also not temperature compensated. It means, it's far away from an exact expo to linear converter. But is absolutely sufficient here, the aftertouch reacts smooth and with the trimmers it is simple to adjust to your own preferences.

the pcb of the addition (blue part)

the pcb of the addition (blue part), I ve build it on a prototypeboard, the connections hand wired, it has only one resistor, one diode two trimmers and the dual opamp. The board is connected to the CPS electronic via a 8-DIP socket (mounted at the position of the original opamp) and the pins (golden points on the picture) on the protoboard

After these changes, the chroma polyphonic aftertouch is a pleasure and shows one time more how much power the chroma has.