the pulse width even further. This en-
sures Q2 is only switched on for the
briefest of time and effectively con-
nects the output of U4.1 to C15 which
charges up to the same voltage level
as U4.1 output. C15 is a low-leakage
polystyrene capacitor, chosen because
we need it to retain or ‘remember’ that
voltage level for up to two seconds.
After the gate pulse, the gate voltage
returns to around –6V bias from R22
and R23, which effectively makes Q2,
open circuit between the source and
drain pins. This means C15 has no-
where to discharge to other than the
non-inverting input of U4.2. Howev-
er, U4.2 has a high input impedance
of 100s of megohms, thus preventing
any signifi cant voltage droop. The sam-
pled voltage from U4.2 is re-inverted
again by U4.3, which also has a gain
of 2 to restore the voltage to the origi-
nal level. The output of U4.3 is fed to
an RC circuit consisting of glide pot
VR10, R21 and C13 which is buffered
by non-inverting op amp U4.4 The
output of U4.4 feeds the S&H bus via
R30. D3 indicates the clock sample
rate and the clock is also available on
the S&H gate bus, which can also be
used to trigger the ADSRs.
You should now have completed the
assembly of the modulation sources,
and the pot side of your PCB should
look like Fig.69.
We have had a request for a PCB
layout drawing showing the compo-
nent identifi cation because the number
is hidden once the component is sol-
dered in. A PDF is available from the
Soundtronics website and from the
Practical Electronics website.Testing the modulation sources
Inserting chips for each new section
one at a time for testing before moving
onto the next is recommended but
not essential.
As the patch switches allow for nu-
merous routing options, the focus here
is to prove the functionality of the mod-
ulation sources assembled this month
using VCO1. Once proven, you should
then try out similar tests with the other
two VCOs, VCF and VCA.
Before you start testing, set all 3-way
switches to centre off, all 2-way switch-
es to the down position, all pots fully
counter-clockwise except the master
level, which you should adjust to a
comfortable level. You will also need
the headphones connected as described
in Part 3, Fig.58. In table 4, some pot
settings are referenced to clock set-
ting because at this point, you have
no front panel scales to use. Each pot
has a slot in the end that you can use
as a pointer.
None of the modulation sources
covered in this part requires calibra-
tion. The VCOs and VCF calibration
are best left until the MIDI interface isFig.67. MIDI Ultimate S&H (sample and hold) circuit.
Fig.68. S&H (sample and hold) ssembled.
Fig.69. PCB assembled and ready for testing.