Active a/r envelope – Erica Synths EDU DIY EG Eurorack Module Kit User Manual

Page 11

ACTIVE A/R ENVELOPE

Thankfully, fixing that is really straightforward.

We’ll simply set up op-amp bu

ﬀ

ers both

at the in- and output

. Bu

ﬀ

ers measure a voltage and provide an identical copy at their

output, while being able to supply a decent amount of current. Because of that, it no

longer really matters how much current the input can provide

—

and how much the next

circuit eats up.

Cool! But why the 1K resistor at the output, then? And what’s all this additional stu

ﬀ

the input? Well unfortunately, there are other worst case scenarios we need to consider.
The first one of which being a classic user error. Imagine that user plugs our envelope’s
output into some other module’s output by accident. If that other module also uses a
bu

ﬀ

er there, we’d basically create a short circuit, since bu

ﬀ

ers can not only source (i.e.

send out), but also sink (i.e. absorb) plenty of current.

So by placing a 1k resistor before

the output socket, we make sure that in this scenario, the maximum amount of
current flowing is limited

. Saving our op amp from a potential early grave.

Okay, easy

—

so now, let’s tackle the additional op amp on the left. What kind of problem

does it fix? Well, while we did make sure that our circuit gets enough current, we haven’t
thought about the voltage we’re feeding it yet. We should, though, because that voltage
will determine the voltage range across which our envelope is operating.

Think of it this way:

if all our envelope does is take the input signal and make the

rising and falling edges less steep, then the maximum „height“ of the resulting curve
is completely determined by that input signal

. Why’s that a problem? Because if the

input signal would, for example, just swing between 0 and 1 V, that curve would be really
small. And a smaller curve means a reduced range of e

ﬀ

ect. When controlling our filter,

for example, this small curve would barely be able to move the cuto

ﬀ

point. So basically,

our envelope would behave very di

ﬀ

erently depending on what kind of circuit we use to

You can try this chapter’s circuits in a circuit simulator. I’ve already set them up for you right

here: https://tinyurl.com/ycz4qc3b –

you can change all values by double clicking on

components.