Practical_Electronics-May_2019

ADSR circuit description
The ADSR circuit shown in Fig.63
is perhaps the most complex in this
synth. (You can get simpler ADSR cir-
cuits that do not perform as well, or use
dedicated ADSR chips). A philosophy
for this synth was to use discrete com-
ponents to ensure longevity through
servicing with ‘off the shelf’ compo-
nents. (Even that approach is now
being tested by the decreasing availa-
bility of through-hole versions of many
common chips that we have used for
many years due to the move to sur-
face-mount construction.)
Both ADSRs are identical in opera-
tion; the circuit in Fig.63 is for ADSR1.
The ADSR has two modes of operation,
namely gate and trigger. These terms
are sometimes used interchangeably,
but at Soundtronics, like many other
synth designers we adopt the conven-
tion as best explained though the use
of a keyboard:

● Gate – goes high when a key is

pressed and remains high for as

long as the key is pressed. The rising

edge indicates that a key has been

pressed, a falling edge that the key

has been released

● Trigger – goes high when a key is

pressed but almost immediately

goes low again even though the key

has not yet been released. The pulse

width depends on the trigger source,

but could be in the range of 50μs to

5ms, ideally less than 1ms.

Various manufacturers have taken dif-

ferent approaches to implementing

gates and triggers, and their polarity.

Some modular synth ADSRs respond

simultaneously to both gate and trigger

signals where the trigger can re-trigger

the attack phase. For the MIDI Ulti-

mate, we kept things simpler due to

some constraints through the use of

the toggle patching switches, but did

include the mode option. The ADSR

circuit is capable of re-triggering but not

in this particular implementation. This

is worth noting in case you decide to

expand the MIDI Ultimate in the future

with the addition of external modules.

Now that you have an understand-

ing of the output envelope from the

ADSR with its four distinct stages we

shall move straight on with the cir-

cuit description.

Starting with the ADSR in Gate mode

at S703 and with no gate pulse pres-

ent, the ADSR is in its quiescent state

with the output of U701.4 – point

‘R1’, high. Point ‘R1’ is high because

both of its inputs are low. The high

state of point ‘R1’ closes the bilater-

al switch U705.3, which discharges

C715 to ground through the Release

pot (VR704).

U702.1 is a comparator with the in-

verting input set at 2V by R706/R710

which determine the voltage threshold

Fig.63. MIDI Ultimate ADSR1 circuit.