Frequency Central System X Amplifier Manual

Build documentation for:
System X Amplifier
Based on the Roland System 100M VCA
Frequency central
Version 2 / April 2013
B100K B100K B100K A100K A100K
CV2 CV1 Initial Input 1 Input 2
Gain
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.
Gnd: Ground
Please observe the correct polarity of the 6 electrolytic capacitors. The 4 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.

Trimmers
Bias: Adjust Bias trimmer to sweet spot, ie a nice clean undistorted VCA output with no
thunk when a snappy ADSR is applied to a CV input. I do this without any audio at the
inputs. The chances are that that the sweet spot is around the mid position.
Linear: I tend to turn this to around fully anticlockwise.
Expo: I tend to turn this to around fully anticlockwise.
Bill of materials
560R x 2
1K x 2
3K3 x 1
6K8 x 1
8K2 x 1
10K x 2
15K x 3
22K x 1
33K x 2
47K x 1
68K x 1
100K x 3
150K x 2
180K x 2
1M x 1
47pF x 1
10nF x 1
22uF x 4
47uF x 2
CA3080 x 1
LF351 x 1
BC547 x 2
BC557 x 2
2N5485 x 1
A100K x 2
B100K x 3
All pots are 16mm Alpha.
22K trimmer x 1
100K trimmer x 2
All trimmers are 6mm
(Tayda)
SPDT toggle
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 31st May 2013
http://www.frequencycentral.co.uk/