SSM 2040 style VCF + discrete VCA (AMORE)

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

This AMORE module combines a VCF based on a discrete clone of the SSM 2040 filter IC with a two stage discrete VCA on one half size board. Both of these circuits were developed by René Schmitz, and you can find the schematics on his web site (links below). This module is fully plug compatible with the Minimoog clone VCF + VCA..

The filter is basically a discrete clone of the SSM 2040 IC, which were used in some commercial synths back in the 80's. This IC has been out of production for many years and is now very hard to find. Therefore, both Jürgen Haible and René Schmitz have done discrete clones of the IC. Their versions are naturally very similar but not identical. I chose René Scmitz version. The filter sounds a bit different than the classic Moog ladder filter and I notice that it doesn't track 1 V/oct very well above circa 2 kHz. Neither Jürgen Haible nor René Schmitz have a trimmer for adjusting the tracking in their schematics, so I suppose perfect tracking is not really a requirement. I added a trimmer so you can get it to track well up to 2 kHz, at least.
My version of the filter has voltage controlled resonance and this is controlled by a VCA that is based on René Schmitz VCA-2. My version uses two 0.1 % resistors instead of the offset trimmer, to save board space.
The VCA has two stages in series. In the Minimoog, the first stage is controlled by the envelope generator and the second stage is controlled by the foot pedal. I recommend to reverse this, as having the EG on the second input gives much better signal to noise ratio. The foot pedal input is used for any type of amplitude modulation in my module. The individual VCA sections are René Schmitz VCA-3+.

Bill of materials

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

5.6 kohm 0.1 % resistors (2)

Matching

You need to match all transistor pairs in both the filter and the VCA. That's why I made the Transistor Matcher before selecting the transistors for this module.

Trimming

Oct/V

The filter has only one trimmer. It adjusts the tracking of the cutoff frequency.
Feed the filter a triangle or sine wave of around 100 Hz frequency and set the filter to medium resonance.
Tweak the filter frequency knob to maximum amplitude and then increment the CV that goes to both the VCOs and the VCFs KOV inputs in 1 volt increments.
Adjust the trimmer until the filter outputs roughly the same amplitude for as wide a CV range as possible.

CV reject + offset null

The VCA has three trimmers for minimising the CV bleed-through and DC offset.
Start by measuring the voltage at the output of the fist (topmost) VCA's op-amp. Adjust the topmost trimmer so that there is as little DC shift when you sweep the VCA AM CV. Don't be bothered if this results in a constant DC offset.
Then set VCA AM CV to full and measure at the output of the seconds VCA. Adjust the middle trimmer in the same manner as the first.
Now you trim out the DC offset with the bottom trimmer. You might have to repeat step two and three to arrive at optimum results.

Skill level required: MEDIUM

This module needs matching of transistors, using the Transistor Matcher. Otherwise there are no particular difficulties and the circuit uses no hard to find parts.

Circuit board layout

Component placement

VCF schematics (link to René Schmitz homepage)

VCA schematics (link to René Schmitz homepage)

Connector pin

signal

on this module

1

1 oct/V

VCF keyboard tracking

2

in 1

VCA input

3

CV 1

VCA AM CV

4

CV 2

VCA envelope CV

5

CV 3

not used

6

-15 V

-15 V

7

out 1

output from VCA

8

-1 V

-1 V

9

gnd

gnd

10

key

-

11

switch 1

bypass VCA

12

switch 2

bypass VCF

13

out 2

output from VCF

14

+15 V

+15 V

15

+10 V

not used

16

aux output

not used

17

in 2

 VCF input

18

CV 4

VCF cutoff CV

19

CV 5

VCF emphasis CV

20

CV 6

not used