Frequency Central System X Filter Manual

Build documentation for:
System X filter
Based on the Roland System 100M VCF
Frequency central
Version 2 / April 2013
B100K B100K A100K B100K A100K A100K
Key to PCB screen print:
n: This signifies NPN BC547 transistors. Note the correct pinout as shown by the half circles.
p: This signifies PNP BC557 transistors. Note the correct pinout as shown by the half circles.
f: This signifies 2N5485 FETs. Note the correct pinout as shown by the half circles. D, S, G is
signified should you want to sub alternatives.
Gnd: Ground
Please observe the correct polarity of the 2 diodes and 12 electrolytic capacitors. The 10 x
22uF are configured in sets of 2 back to back (negative to negative).

The PCB shows the correct orientation for BC547/BC557/2N5485. Other transistor types can
be used (eg 2N3904/2N3906/BF245), but please observe the correct pinout.
Thermistor (next to 5K6 resistor)
I use a 10K NTC available from Tayda, seems perfect:
http://www.taydaelectronics.com/thermistors/10k-ohm-ntc-thermistor-5mm.html
…but, you can just use a 10K resistor if 100% temperature stability isn’t so important to you.
I can assure you, it’s pretty accurate anyway without the thermistor.
Trimmers
Offset - Trims the frequency up/down, should be just about fine in the mid position, but
adjust to taste.
V/oct –This filter can be set up to track V/oct fairly accurately over around 5 octaves.
1. Send 1V/oct keyboard into CV input 1 and turn it’s attenuator fully clockwise.
2. Play octaves, while adjusting the trimmer until you’re close.
3. Play 2 ocatves apart, adjust trimmer a little more…
Bill of materials
100R x 1
560R x 8
1K5 x 1
1K8 x 1
4K7 x 1
5K6 x 1
6K8 x 1
10K x 5
12K x 1
15K x 5
22K x 4
27K x 1
33K x 3
47K x 1
68K x 6
100K x 5
150K x 1
270K x 1
470K x 1
1M x 1
470pF x 4
22uF x 10
47uF x 2
10K Thermistor
LF351 x 2
LM13700 x 2
BC547 x 3
BC557 x 1
2N5485 x 4
1N4148 x 2
A100K x 3
B100K x 3
All pots are 16mm
Alpha.
10K trimmer x 1
100K trimmer x 1
All trimmers are
6mm (Tayda)

Underside of the PCB showing:
-Ground bus between sockets and PCB
-Connections between inputs/outputs and PCB
I use solid core for all of the above.
RDH 17th May 2013
http://www.frequencycentral.co.uk/