Frequency Central Monograf Manual

Frequency central

Build documentation for:

Monograf synthesiser

Monograf is a fixed signal path monophonic synth voice with many unique features, including a

number of patch points for flexibilty within a modular setting. Our aim was to design the simplest

full featured and good sounding single VCO synth within as small a space as possible, while offering a

broad sonic palette and fast setup times. Monograf includes the following elements:

•Onboard LFO for PWM,

and onboard sub osc for

thick and juicy synth

lines

•Super stable AS3340

based VCO featuring

sawtooth, triangle and

square waveforms, as

well as rotary switch

control of octaves and

an onboard LFO for

PWM. Features highly

accurate 1V/octave

tracking, as well as

extenal CV control of

linear FM.

•Sub oscillator featuring

-1 octave and -2 octave

square waveforms. Add

a big bottom to your

sound.

•5 toggle switches for

fast waveform

selection, allows for

super quick changes.

Additionally, the

triangle waveform is

routed directly to the

VCA, bypassing the VCF, to maintain a rumbling low end with higher resonance settings, this

is a little known modular trick which was practised by the ancients (!?).

•12dB/octave OTA lowpass VCF based upon ARP Odyssey Mk1 ‘whiteface’. A very

characterful VCF with cutting resonance whose audio inputs can be driven into clipping via a

dedicated Drive control. The chosen level of drive can have a profound effect over the

character of the filter, and heavy drive will saturate the input while simultaneously reducing

resonance. Cutoff frequency can be controlled manually, from the LFO, from the ADSR,

normalised from a 1V/oct source, or from an external CV.

•Vintage ARP style VCA with gated/ADSR/drone options via two toggle switches.

•AS3310 based ADSR for super snappy envelopes. CV control of envelope output is available

for velocity controlled dynamics etc.

Monograf features a 3 PCB set:

•Main PCB

•Control PCB

•Octave switch PCB

The Main PCB contains the main elements of the

synthesiser: VCO, VCF, VCA, ADSR, PWM LFO, and

power section. Each section is labelled appropriately.

The Control PCB mounts all of the control elements,

potentiometers and switches, as well as interface

sockets.

The TL072 on the Control PCB performs waveform

mixing, and buffering for the octave switch circuit.

The sub oscillator is also mounted on this PCB

(CD4024).

The Octave PCB interfaces to the

Control PCB via a header.

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 FET. Note the correct pinout as shown by the half circles.

The PCB shows the correct orientation for BC547/BC557/2N5485. Other transistor types can be used

(eg 2N3904/2N3906), but please observe the correct pinout.

Please observe the correct polarity for the diodes and electrolytic capacitors.

CAUTION!!

(see text)

See text

Bill of Materials

100R

x 4

470R

x 5

820R

x 1

x 3

1K5

x 1

1K8

x 1

4K7

x 2

5K6

x 1

8K2

x 2

10K

10K 1%*

x 4

x 5

12K

x 2

15K

x 5

22K

x 5

27K

x 1

33K

x 1

47K

x 6

62K

x 2

68K

x 1

100K

x 13

150K

x 6

220K

x 1

270K

x 1

470K

x 2

x 5

x 3

All resistors ¼ watt

metal film.

100pF x 2

1nF x 3

10nF x 4

22nF x 1

33nF x 1

100nF x 3

10uF x 7

47uF x 2

AS3310 x 1

AS3340 x 1

LM13700 x 2

CD4024 x 1

TL072 x 4

2N5485 x 2

BC547x 1

BC557 x 2

1N4148 x 5

78L05 x 1

78L09 x 1

79L05 x 1

8 pin IC socket x 4

14 pin IC socket x 1

16 pin IC socket x 4

A100K x 1

B100K x 9

B100K x 3

(or these)**

4K7 trimmer vertical

(5K is fine)

10K trimmer x 3

100K trimmer x 3

Rotary switch

SPDT x 7

3.5mm socket x 6

Male 40 pin header x

Female 40 pin header

x 2

10 pin box header

Big knob x 2

Little knob x 9

* 10K 1% x 5 These are the five matched resistors for the octave switch voltage divider ladder.

They don’t have to be 10K exactly, they just have to be matched to each other, so for example if

they are all 9.98K that’s fine. Grab a bunch of 10K resistors, measure each one, make little piles of

10k, 9.99K, 9.98K etc. Before long one of the piles will have five resistors in it, that’s your matched

set! Should only take 5 minutes.

** I prefer the Song Heui tall trimmers because they have a longer shaft and a white notch.

Main PCB assembly

1. Solder all resistors and diodes

2. Solder all IC sockets

3. Solder all non-electrolytic capacitors

4. Solder all 3 voltage regulators

5. Solder all transistors

6. Solder all trimmers

7. Solder the power header –if you’re using box type, observe correct polarity

8. Solder all electrolytic capacitors

9. Cut male headers to size and solder them into place. Make sure that they stick out of the

bottom of the PCB.

Control PCB

1. Solder all resistors

2. Solder both IC sockets

3. Solder the trimmer

4. Place the 10 metal shaft pots on the PCB, and fold over the mounting tabs of the pots at the

rear of the PCB, then place the panel over them. This will assure that they are correctly

positioned. Flip the whole lot over and solder the pots into place. CAUTION!! –the rear tab

of the Fine Tune pot needs to be folded back (or cut off) to avoid fouling on the header for

the Octave PCB header.

5. Place all sockets on the PCB, making sure the ground tabs line up with the PCB’s ground

pads, then place the panel over them. This will assure that the sockets are correctly

positioned. Flip the whole lot over and solder the sockets into place. Don’t forget to ground

the sockets to the PCB using cut off resistor legs.

6. Place the 7 toggle switches, then place the panel over them. This will assure that the

sockets are correctly positioned. Flip the whole lot over and solder the sockets into place.

7. Place the three plastic shaft pots on the PCB, fold over their mounting tabs at the rear of the

PCB, then place the panel over them. This will assure that they are correctly positioned. Flip

the whole lot over and solder the pots into place.

8. Cut two female headers (to connect the Main PCB) to size and solder them into place. Make

sure that they stick out of the bottom of the PCB.

9. Cut one male header (to connect the Octave PCB) to size and solder it in place. Make sure it

sticks out of the bottom of the PCB.

Octave Switch PCB

1. Solder the 1P12T rotary switch.

2. Cut the plastic flange off.

Final Assembly

1. Mount the Control PCB onto the panel. Tighten all mounting nuts.

2. So, the 1P12T rotary switch needs to be adjusted to be a 1P6T rotary switch. Remove the

mounting nut and the washer, below them you will find another washer with a small flange

at 90°, the inner part of the switch has a number of slots, drop the flange into the slot

marked 6, then waggle the switch to make sure you’re getting 6 positions.

3. Mount 1P12T rotary switch onto the panel, making sure the relevant pads also pass through

the male header of the Control PCB. Tighten the nut. Solder the Octave Switch PCB to the

Control PCB via the male header. See photo.

4. Plug the Main PCB into the Control PCB

Calibration

•4K7 Octave (Control PCB): Set the octave switch to it’s highest octave position (2’) and

adjust trimmer until you get a reading of 7.5V at the Test on the octave rotary switch

daughter board.

•10K Scale (VCO):This trimmer sets the 1V/oct tracking of the VCO, and it’s really worth

spending some time to get it right. On first power up, the VCO should already be pretty close

to 1V/oct with the trimmer in it’s mid position, as I carefully chose associated resistor values

to make it so. To set it closer, play two notes an octave apart and adjust until it sounds spot

on. Then play two notes two octaves apart, and adjust still further until it sounds spot on. If

you have a frequency counter, still better!

•10K Duty (VCO): This sets the duty cycle of the square wave, should be just about spot on

with the trimmer in the mid position, as I carefully chose associated resistor values to make

it so. Set the Pulse Width knob to 50 (hard left), and twiddle the trimmer until you hear the

easily recognisable hollow sounding square wave.

•100K Offset (VCO): Sets up the initial frequency of the VCO. Set Fine Tune to it’s centre

position, play an A on your keyboard. Adjust the Offset trimmer until you can read a

frequency of 110 Hz, 220 Hz, 440 Hz, 880 Hz etc when playing octaves of A.

•100K Bias (VCA): set both VCA switches to the up position to send a nice snappy ADSR into

CV3 input. Adjust Bias trimmer to sweet spot, ie there is no DC thump. The chances are that

that the sweet spot is around the mid position.

•10K 1V/Oct (VCF): turn Resonance all the way to self oscillation. Patch a 1V/oct source into

CV input 1, with the attenuator fully clockwise. Play octaves and adjust the V/Oct trimmer

until they are spot on.

•100K Offset (VCF): you want to tweak this so that the filter is fully open when the Cutoff pot

is fully clockwise.

RDH 04/01/20

https://www.frequencycentral.co.uk/

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