AJH Synth MiniMod VCO User Manual

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VCO User Guide

Vintage discrete transistor core VCO

Thank you for purchasing the AJH Synth MiniMod VCO module, which like all AJH Synth Modules,
has been designed and handbuilt in the UK from the very highest quality components. We hope
that it will help and inspire you towards creating some great music and soundscapes!

The discrete all transistor core of this Voltage Controlled Oscillator is an exact emulation of the
original, and now very rare, R A Moog Model D Oscillator from 45 years ago. It features a fully
matched PNP transistor expo. converter and P-FET transistor buer amplier section but with the
advantage of much improved temperature stability and modern construction using reliable
precision SMT components along with Eurorack modular synthesiser expandibility.

It features front panel switchable octave selection and four independent waveform outputs
including the famous “Sharktooth” waveform, Sawtooth wave, Square wave and Triangle wave
and when used as part of a full MiniMod system can provide a very accurate emulation of the
Model D systhesiser and it’s highly acclaimed “fat vintage” analogue sound.

The Square wave output has fully adjustable Pulse Width Modulation, with manual and CV control
rather than the more restricting selection of square wave or two static xed pulse widths of the
original design.

Also included are Exponential and Linear modulation inputs and two dierent types of oscillator
sync which help to make this a very versatile Eurorack VCO in its own right.

Module width is 14 HP of EuroRack space and it is compatible with standard Euro Rack cases. The
height of the panel is 128.5mm, and depth is 38mm. There are four mounting holes at the corners
of the module and we provide 4 of M3 rack xing screws, along with a Eurorack compatible
power cable. Current consumption is 55mA from the +12V supply rail and 40mA from the -12V
supply rail.

All AJHSynth modules are covered by a one year guarantee against manufacturing defects.

Notes:

As with all vintage analogue oscillators, after powering up it is best to be allowed to “warm up” for 15
minutes or so before use. Tuning immediately after power up is not advised as it takes time for the
internal circuitry to get up to temperature and stabilise. After the warm up period it will maintain pitch
well - much better than the original on which it is based!

It is very important that the power supply ribbon cable is connected correctly, see the “adjustment and
calibration” section for an illustration of the correct orientation.

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Controls, Inputs and Outputs

1 Octave Switch

5 Exp. CV Level

9 Sync Threshold

4 Exp. CV Input

10 Sync Type

8 Sync Input

3 1 V / Oct. Input

6 Linear Input

Linear CV Level 7

Tune Control 2

PWM Control 16

PWM Shape 15

Shark Output 13

Saw Output 11

Square Output 14

Triangle Output 12

1 Octave Switch :

2 Tune Control :

3 1 V/Oct. Input :

4 Exp CV Input :

Use the Octave switch to quickly select preset octaves, the Lo setting reduces the oscilla­tor frequency into the Low Frequency Oscillator range so that it can be used as a modula­tion source.

The tune control varies the oscillator pitch by around +/- 8 notes. Larger variations in
pitch can be obtained by applying a positive or negative control voltage to the 1 Volt per
Octave input (3)

This is the 1 Volt per Octave control voltage input. Applying a positive voltage to this
input raises oscillator pitch at the rate of exactly one octave for each volt applied, and a
negative voltage will reduce the pitch by exactly one octave per volt. The acceptable
input range is +/- 7 Volts. A limitation of the vintage oscillator core design means that
tracking may not follow the 1V/Oct law above 10KHz

Distribution Buss 1V/Octave CV signal:

The Eurorack power distribution buss can transmit a 1V/Octave voltage from a Mini/CV
converter or similar source. The MiniMod VCO accepts this signal by default as it is “normalled”
through the 1V/Oct socket, however plugging a jack into the 1V/Oct socket unswitches /
defeats this control voltage and allows the VCO to run free.

This is the Exponential Control Voltage input. Acceptable input voltage range is +/- 5V.
When a voltage is applied to this input it varies the oscillator pitch in a similar fashion to
the 1 V/Oct input, however the amount of control voltage applied to the oscillator core
can be regulated using the Exponential CV Level control (5).

5 Exp. CV Level :

This control adjusts the amount of voltage from the Exp. CV input that is passed to the
oscillator core, its range is 0 to 120%.