Fig.59. Switches mounted into their corresponding positions, pots clipped into position and LEDs ready for inserting into the PCB.
The LFOs can also modulate the
VCA to create a varying amplitude
(volume) of the sound, better known
as ‘tremolo’. Some manufacturers of
guitars, for example, have incorrectly
called the vibrato arm or bar a ‘trem-
olo arm’. So, to be clear:
● Vibrato is a change in pitch
● Tremolo is a change in volume.
Finally, the LFOs can be used to mod-
ulate the cut-off frequency of the VCF
and thus alter the timbre of the sound
between two frequency cut-off points.
The envelope generators can be used
in AR or ADSR mode (see Fig.20 to
Fig.22 in Part 1). In ADSR gate mode,
on a gate pulse, the envelope generates a
voltage that initially climbs to the maxi-
mum voltage level over a period of time
called the attack period (A). When the
maximum voltage has been reached, the
voltage will reduce to a voltage level
set by the sustain (S) potentiometer in
the decay (D) period. The output will
now stay at the sustain voltage until the
gate pulse is removed and the output
voltage decays to zero over the release
(R) period. This is why the envelope
generator is frequently just called an
ADSR. Variations on the ADSR – de-
pending on mode and potentiometer
settings – can also achieve an envelope
just consisting of the AR stages. Enve-
lope generators can modulate the VCOs
(ADSR2 only), VCA and VCF just like
the LFOs, although they are not often
used for VCOs. Their principle function
is with the VCA for amplitude mod-
ulation; imagine striking a piano key
where you would hear the note imme-
diately but it would take time for the
note to diminish. Many small synths
like the MIDI Ultimate only include one
AR let alone an ADSR, so why do we
have two ADSRs? Having two ADSRs
means you can simultaneously control
both the VCA and the VCF with dif-
ferent envelopes, offering a far greater
range of sounds and enables the user
to better emulate what happens with
traditional instruments. Triggering an
ADSR can be from a MIDI key being
pressed or from the dedicated repeat
gate generator, or even from the S&H
gate generator.
Last but not least is the S&H; feed
in a sawtooth waveform from LFO2
and the synth generates a staircase of
increasing voltage levels. This control
voltage can not only be used to mod-
ulate the VCOs for a sequencer effect,
but also creates excellent timbre effects
when used with the VCF. Select other
source waveforms for different effects,
including the white noise source for
random voltage levels.Construction
Techniques used for this month’s con-
struction are similar to that in Part
2 and Part 3, so do refer back to theFig.60. Finish height of LEDs is 16mm
above the PCB.previous articles as needed. Addition-
ally, two pushbuttons and four LEDs
are mounted on the pot/switch side of
the PCB. We now have six sections on
the main PCB to populate with com-
ponents, these are:
● ADSR1 and ADSR2
● LFO1 and LFO2
● Repeat gate generator
● S&HI recommend soldering in the compo-
nents for all six sections before moving
onto the switches, pots and LEDs (see
Fig.59). As before, do not insert the
chips until you are ready to test, and
then only one section at a time. Test-
ing is covered at the end of this article.
The LEDs require spacing off the PCB
by 8mm, we achieved this by cutting
some sleeving and sliding over each leg