Minikorg filter (AMORE)

The prototype version of the board. There are a few differences compared to the final board.

This is a clone of the filter in the Minikorg 700, 700S and 770. The filter contains both highpass and lowpass sections with individual voltage control. On the Minikorgs, the filter cutoff frequency is controlled by two sliders named ”Traveler”. These have a mechanical interlock to prevent the highpass cutoff to be set higher than the lowpass and thus letting no sound through.
On the Minikorgs there is no knob or slider for filter resonance. Instead there is a two position switch (three positions on the 770) named ”Bright”. This switch selects between two degrees of resonance, with the addition of a position for no resonance on the 770. So on the 700 and 700S you can't turn resonance off completely and you can't control the resonance for the highpass and lowpass sections independently. This module mimics the 700/700s functionality, using AMORE switch 2. On the original Korg filter there is an input named ”Expand” that is fed by the envelope generator. On the AMORE module this is a separate input although it too controls the cutoff frequency, just like the Traveler input. The reason is that the Traveler input has a somewhat limited frequency range (mimicking the slider throw) and a smoothing filter. The Expand input is more sensitive and you can choose which input you want to use for controlling the cutoff frequency.

The Minikorg 700S

The Traveler sliders on the Minikorg 700S.

The Expand switch adds envelope control to the filter and the Bright switch increases the resonance.

This is a half size AMORE board and has been tested and should not contain errors. The original Minikorg filter runs on +20 and -10 volts. I mimicked this by using a -5 V voltage regulator on the module, for establishing the filter's ground potential at -5 V instead of 0 V.
Note that there are several wire connections on the board in the photo above. These are not shown in the component placement plan but instead marked with capital letters. You just solder a wire between the two points that have the same letter.
The four diodes for the diode ring of each filter section should be matched. Using the resistance measurement range of a DVM turned out to be erratic. Instead, you should connect every diode in series with a 100 kohm resistor between +15 volts and ground from the synth's power supply. Use lab cables and alligator clips for this. Remember not to touch the diodes with your fingers when you swap diodes for measuring. Measure the voltage drop over each diode with a DVM and select the four diodes that are most similar. There is no need to match the transistors used.
The Expand inputs are connected directly to the filter core and have somewhat higher sensitivity than the AMORE standard. Instead of the usual 100 kohm resistor you'll want to use around 150 kohm if you use these inputs.
Note that the 7905 voltage regulator is not equipped with a heatsink. With the normal current draw of this board this is OK – the 7905 will be just bearable to touch. Note however, that if something on the board draws exessive current, the 7905 might get very hot. This might happen if there are shorts between traces, parts are not soldered correctly or the 4066 is faulty. Therefore, pay attention to the 7905 temperature whenever you are working on the board.

Bill of materials

You should have access to the parts in the general bill of materials.
In addition, you need the following parts:

7905 -5 V voltage regulator (1)

Trimming

There are two trimmers in each filter section.

Resonance adjust
This is for setting the fixed resonance. You can adjust it to your liking. I don't know how they did the adjustment at the Korg factory.

Unmarked trimmer
There is another trimmer in each filter section that frankly doesn't do much. I retained it because the original has it. It adjusts the cutoff frequency very slightly, but you can just leave it alone.

Skill level required: MEDIUM

For this module you need to select diodes for matching the diode ring. Otherwise there are no particular difficulties.

Schematics

Circuit board layout

Component placement