AJH Synth MiniMod VCF - Manual (Page 3)

8 Freq CV Input : Connect an external control voltage to this Input for voltage control of the filter cutoff
frequency. The amount of signal passed to the filter core control circuitry can be adjusted
with the Freq. CV Level control (9)
9 Freq. CV Level : Sets the amount of control voltage (between 0% and 130%) which is sent from the Freq.
CV Input (8) to the filter cutoff circuitry. It has a greater range than the 1 V/Oct Input so
that overmodulation effects are possible. 1V / Octave from the Freq. CV Input occurs
with this control knob pointing to position 7 ( 2 o’clock).
10 1 V/Oct Input : A voltage applied to this Input changes the filter cutoff at the rate of 1 volt per octave, or
if the filter is in full self oscillation (Emphasis Control fully clockwise) it will control the
pitch of the resulting sine wave at the rate of 1 V/Oct. When compared to the Model D this
emulates having “Keyboard Control” switches 1 and 2 “On”.
11 1/3 CV Input : A voltage applied to this input changes the filter cutoff at the rate of 333 mV per octave.
When compared to the vintage Model D this is the equivalent of having “Keyboard
Control” switch 1 “On” and switch 2 “Off”.
12 2/3 CV Input : A voltage applied to this input changes the filter cutoff at the rate of 666 mV per octave.
When compared to the vintage Model D this is the equivalent of having “Keyboard
Control” switch 2 “On” and switch 1 “Off”.
13 Emphasis Level : This control manually regulates the amount of internal feedback applied to the filter core.
“Emphasis” is now more commonly known as resonance in the modular synth world, they
are just different words for the same effect.
At high levels of feedback (between positions 8 and 10) the filter will self-oscillate, so that
even without any audio input to the filter a sine wave output is generated and the
frequency can be controlled by applying control voltages to the 1V/Oct, Exp. CV, 1/3 or
2/3 inputs.
Like the Model D, when using the 1 V/Oct the sinewave can be played in the same
manner as a VCO with a tracking range of 3 - 4 octaves.
The CV scaling is not temperature corrected so it may drift with changes in ambient
temperature. Under normal circumstances it should track reasonably accurately over a
four octave range. Again, this correctly emulates the behaviour of the vintage Model D
filter.
14 Emp CV Input : Applying a control voltage to this Input will vary the Emphasis at the level set by the
Emp. CV Level control (15). Acceptable input voltage range is +/- 5V
15 Emp. CV Level : This varies the amount of control voltage passed from the Emp. CV Input (14) to the filter
core and it’s range is 0% to 100%. The manual Emphasis Level control (13) is still active
when using external CV control of Emphasis and acts as an offset control.
16 Audio Output : This is the audio output for the signal after passing through the filter core and would
typically be connected to the Audio Input of a VCA. The output level is dependent upon
the input level, with a single input of +/- 5V to (for example) Audio Input 1 and Audio 2
Level at 100% the gain would be unity, i.e. output would also be +/- 5V.
Removing this jumper increases the gain of the signal by around 70% BEFORE it is fed to
the filter core, which can give a nice overdrive effect. This can be used to emulate the
vintage Model D “trick” of plugging the headphone output back into the VCF External
Input.
With this jumper removed there is a proportionate (70%) increase in VCF output level and
this should be compensated further down the signal chain. So, if being fed into a VCA the
VCA Input Level control will need to be turned down to compensate for this higher signal
level.
E Gain Jumper :
(J1 on PCB)