of the LEDs. First check that the LEDs go
into the holes in the aluminium panel as
we have had some reports that the hole
diameter may be a bit tight. If necessary,
run a 5mm drill through the LED holes
to make sure and deburr afterwards.
The long leg of the LED goes into the
pad marked with a ‘+’. After soldering
the LEDs, do not clip their legs just yet
in case you need to adjust their height.
The fi nish height of the LED is around
16mm above the PCB (see Fig.60).
LFO circuit description
The two LFOs are identical in operation,
so only LFO1 will be described here. The
LFO has two frequency ranges selected
by S602. ‘High’ gives a frequency range
of a few Hz to about 300Hz, ‘Low’ from
0.1Hz to 30Hz. VR601 sets the frequency
within the range selected. Two switches
are used to select the output waveform,
S601 selects ‘Ramp’ or ‘Square’, if ramp
is selected, then S603 further selects
what type of ramp waveform, ‘Saw’,
‘Inverted Saw’ or ‘Triangular.
This LFO integrator/comparator
design is not unique; Jake Rothman
described a similar circuit he used
in his Gen X-1 Stylephone (see: EPE,
August 2018, Fig.13) and again in his
article from October 2018 (see Fig 61).
Referring to the LFO1 circuit di-
agram in Fig.61, U601.2 forms an
integrator and U601.1 a comparator.
The output of the comparator U601.1
feeds back into the integrator U601.2
via the frequency pot VR601 and R607
or R607 in parallel with R605 if the
range switch has ‘High’ selected. The
setting of the potential divider VR601
determines the current fl ow into/from
integrator U601.2 and how quickly the
integrator capacitor (C602) is charged/
discharged, and thus the rate at which
the output of the integrator ramps
up and down. This ramping voltage
is fed into the comparator’s non-in-
verting input; the inverting input is
connected to ground. The compara-
tor has signifi cant hysteresis due to
feedback resistor R604.Oscillation
When the comparator output is high
at 11V, the integrator capacitor (C602)
is charged up via VR601 and the in-
tegrator output voltage ramps down
towards –11V (inverting). When this
voltage falls below about –5V, the com-
parator output switches to –11V. Now
the integrator capacitor is discharged
via VR601 and the integrator output
begins ramping up towards +11V. When
it reaches around +5V, the comparatoroutput switches to +11V and the pro-
cess repeats, resulting in oscillation.Square operation
When ‘Square’ is selected on S601,
the LFO waveform is the output of the
comparator reduced to ±5V by potential
divider R601/R602, which is toggling
between high and low and thus pro-
duces a square waveform.Ramp operation
When ‘Ramp’ is selected on S601 and
‘Triangle’ on S603, the output via R603
is now from the integrator output which
is as described above, a ramp up and
ramp down waveform already at ±5V.Sawtooth Ramp operation
To generate the sawtooth and ramp
(inverted sawtooth) waveforms, diode
feedback from either D601 or D603
and R610 is introduced. This feedback
has the effect of cutting short either
the rising or falling ramp to around
1ms, depending on the waveform
selected. So, when S603 is selected
to ‘Sawtooth’ and with the compara-
tor output low, the integrator ramps
high in 1ms through D601 which in
turn forces the comparator output to
go high. With the comparator output
high, D601 is reversed biased so the
normal ramping through VR601
occurs, as described above. When
‘Ramp’ is selected, D603 is in place of
D601 in the feedback loop. As D603 is
reversed in relation to D601, you get
the opposite effect to the sawtooth,
which is a slow ramp up, and a 1ms
ramp down. This is a fairly simple
circuit, which has one side effect,
when either sawtooth or ramp is se-
lected, the LFO frequency is doubled
because one half of the ramping cycle
is cut short to 1ms. An LED, D602
driven by Q601 illuminates in sync
Fig.62. LFO1 and LFO2 assembled. with the output waveform.
Fig.61. MIDI Ultimate LFO1 circuit.