Practical_Electronics-May_2019

available, but we can now set up the
VCA as we have the LFOs available.

VCA Calibration
In Part 3, we constructed the VCA
(Fig.53), now we can calibrate it using
the two presets P501 and P502 and LFO1.

1. Turn all mixer controls to mini-
   mum, LFO1 set to square wave, its
   frequency set to maximum with the
   range switch set to ‘high’. VCA mod-
   ulation pot set to maximum. VCA

modulation switches, LFO1 select-

ed, other two switches centre off.

2. Attach your scope (see Part 2, page 41
   on how to make your own PC sound-
   card oscilloscope) to the output of
   U501.1 pin 8 where it links to the
   top of R501, or plug your head-
   phones in with the volume level set
   to maximum. There are numerous
   0V ground points but I find clipping
   a lead to one of the mounting holes
   works just fine.
   3. The adjustment we are making is to
   ensure the minimum breakthrough
   of the control voltage onto the audio
   output. Adjust the CV Null preset
   P502 until the amplitude of the
   LFO1 square wave is at a mini-
   mum, which should be less than
   5mV. If using headphones, adjust
   P502 until the audible tone from
   LFO1 is at its quietest.
   4. Switch the VCA modulation switch
   from LFO1 to centre off. Connect
   a DC voltmeter to the VCA output
   at the same point as the scope was
   connected to and adjust P501 until
   the output voltage is as close to
   0V as you can make it. P501 can
   easily adjust to around 0.1mV, but
   that depends on the resolution of
   your voltmeter.

Last, but not least this month, here is

some inspiration for constructors – a

link to a patch which I have called

‘Pulsing filter’: https://youtu.be/kjMu-

VyiLIP8 – worth playing back through

some big speakers.

Next month

You now only have one section re-

maining to build on the main PCB,

which is the MIDI interface – so you

can now really start to find your way

around the synth.

Next month, is all about MIDI, both

as a general topic and how it is im-

Fig.70. Test points for setting up P501 and P502 presets. plemented within the MIDI Ultimate.

No. Test Pots Switches Result

1 Establish a base point

VCO1 coarse frequency 12 o’clock, mixer for VCO1

square 100%, VCF filter cut-off 100%, VCA initial

level 100%

You should hear a tone from VCO1

2 Prove LFO1 VCO1 modulation 9 o’clock, LFO1 rate 9 o’clock VCO1 – LFO1 Mod, LFO1 – Square ‘Nee-nah’ tone

Vary LFO1 rate for the speed of the nee-nah. Switch LFO1 square to ramps and try the different ramp waveforms

3 Prove LFO2 VCO1 modulation 12 o’clock, LFO2 rate 12 o’clock VCO1 – LFO2 Mod, LFO2 – Square Nee-nah tone at a faster rate than test 2

As for LFO1, carry out same tests for LFO2 then return switches and pots to their position as in test 1

4 Prove ADSR2

VCO1 modulation 9 o’clock, ADSR2 attack

3 o’clock, decay 12 o’clock, sustain 12 o’clock,

release 3 o’clock

VCO1 – ADSR2, Press,

hold then release ADSR

2 manual gate button

Tone will increase in frequency, then

decrease and remain steady until gate

button released, then the frequency

will decrease to the starting point

Return switches and pots to their position before test 1

5

Prove

ADSR1 and

Repeat Gate

VCO1 coarse frequency 12 o’clock, mixer for VCO1

square 100%, VCF filter cut-off 9 o’clock, VCA initial

level 100%. ADSR1 attack, decay, and sustain to 1 2

o’clock, release 3 o’clock. VCF ADSR mod fully clockwise

ADSR1 Repeat Gate,

VCF ADSR1 Repeating dull to bright sound

6 Prove S&H gate S&H sample rate to 12 o’clock ADSR1 gate to Trig mode, ADSR1 S&H Gate Pulsating sound

7 Prove S&H S&H input level to 3 o’clock. VCO1 to S&H Mod Random notes

Table 4: Schedule of tests to prove functionality of the modulation sources – LFOs, ADSRs, Repeat Gate and S&H.