■ FIGURE 7. Here’s the output of an ADSR
envelope generator derived from gate
and trigger inputs. Notice the righthand
portion; although the gate remains steady,
a new trigger restarts the attack cycle.
get a trigger for each note. But the gate
only lasts as long as a key is held in
place. The combination of a gate and
trigger tells the ADSR to begin its
cycle. A capacitor will charge in the
usual exponential manner during the
attack phase. As mentioned, a front
panel potentiometer lets you adjust
the amount of time this will take.
When it reaches the apex (+5V on
most modern synth gear), the decay
phase begins. The charge on the cap
will bleed off in the amount of time set
by a second pot. The voltage will drop
to an intermediate level indicated by
the sustain pot and remain there as
long as the key is still depressed (and
the gate is still present).
When the key is finally let go and
the gate signal evaporates, the enve-
lope generator enters the release
phase. The output voltage will drop at
the rate set by yet another front panel
potentiometer until it hits zero.
Now consider the right half of
Figure 7. It begins the same way, with
the attack and decay phases. But
notice that a second trigger has come
along. In other words, another key has
been struck while the first one is still
held down. The ADSR enters new
attack and decay phases. This is
called retriggering and most musicians deem it a very desirable feature.
ADSRs are extremely easy to design
and build. Essentially, all it boils down
to is being able to route some currents
to and from a timing capacitor at the
right moments. There are no critical
components, no high frequencies, and
no temperature drift to worry about.
As for the LFO, these are even
easier to make. The common Schmidt
trigger/integrator type of function gen-
While there are lots of schematics and plans for synthesizer
circuits kicking around, many are disappointing. Finding well
worked out, high quality designs suitable for a pro quality instrument can be quite a challenge. To point you in the right direction,
here are some recommended books, articles, and websites to
turn to, broken down by category. A number of the items listed
here appeared in Nuts & Volts Magazine. If you’re missing one,
be sure to go to www.nutsvolts.com to locate back issues.
■ Front Panels
Thomas Henry, “Secrets of Making Attractive Rack Panels,”
Nuts & Volts Magazine, December 1998, pp. 72 – 75.
■ Power Supply
Thomas Henry, “Power Supplies for Electronic Music,” Nuts
& Volts Magazine, January 1998, pp. 7 – 11.
If you’ll be using an analog switching bus keyboard, refer
to the first entry below. In any event, Hutchins’ collection is
essential reading for the serious electronic music synthesizer
builder. See the Electronotes URL below. The second article
shows how to build an inexpensive interface if you’ll be taking
the MIDI keyboard approach
Bernie Hutchins, Electronotes Builder’s Guide and Preferred
Circuits Collection, (Ithaca, New York: Electronotes, 1980), no
page numbers shown.
Thomas Henry, “Build Your Own MIDI-to-Synthesizer
Interface,” Nuts & Volts Magazine, December 1997, pp. 52 – 56.
Again, refer to the Hutchins’ collection mentioned above for
a number of superior VCO designs. For more recent circuits, visit
the two websites listed here. Fritz was one of the early guys in
electronic music synthesizers and has come up with an extremely
precise VCO, giving a detailed analysis of its tuning and temperature characteristics. Ray Wilson’s Music from Outer Space site is
very friendly and chock full of synth circuits and tips. He’s even
included free printed circuit board artwork for his VCO circuits.
Ian Fritz’s Electronic Music Site:
Music from Outer Space
The first design here is one of mine that seems to keep popping up on the Web after two decades, but it’s still a good one.
The second is an interesting and more modern alternative. Both
sites include free downloadable printed circuit board artwork.
A golden oldie
Yves Usson’s ADSR
Here’s Ray Wilson again with an excellent design for a
four pole low pass filter. Then the second website shown offers
several discrete versions emulating some of the classic VCFs
from the past.
Ray Land’s VCF
Synthesizer DIY pages of René Schmitz
It’s pretty easy to create a high quality VCA using the NE570,
CA3080, or LM13700 chips. Here is a good source on how to do it.
Ray Marston, “Understanding and Using OTA Op-Amps,”
Nuts & Volts Magazine, April 2003, pp. 58 – 62, and May 2003, pp.
70 – 74.
■ Noise Generator
Thomas Henry, “Build a Tunable Noise Generator,” Nuts &
Volts Magazine, November 1999, pp. 25 – 27.
The N&V articles, by Thomas Henry, mentioned here are
available as PDF downloads from www.nutsvolts.com
January 2006 61