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These Noise-Canceling Headphones by Jules Ryckebusch

by Jules Ryckebusch

In today’s hectic and noisy world, we are all searching for a little
peace and quiet. Well, you might not be able to slip off to a tranquil
forest for an hour or two, but you can block out background noise with the
Noise-Canceling Headphones. The theory behind this project is that by
picking up ambient sound with a microphone and reproducing it out of
phase, we can actively cancel or "null" out background noise. In fact,
several commercially available devices perform the same function. However,
by building your own headset, you can add features not otherwise available
and have fun while doing it!

Along with noise-features, the Active Noise-Canceling Headphones let
you mix in an auxiliary line-level signal from a CD or tape player. That
allows you to minimize background noise while quietly listening to music.
The project also has a phase switch that will let you keep the microphone
signals in phase, thus amplifying background sound. In addition, the
design of the Noise-Canceling Headphones lends itself to several other
interesting functions, which we will look at later.

How It Works. The electronics consist of three op-amp circuits;
each built around one half of an NE5532 dual op-amp. Each circuit uses
that op-amp in a different configuration. The first circuit is a
non-inverting pre-amp, the second is a unity-gain phase-inverter, and the
third is an inverting headphone amplifier. Since the Noise-Canceling
Headphones is a stereo device, the circuit is actualty two identical
circuits side-by-side. Only one channel will be described; the second
channel works in exactly the same way.

Fig. 1. The Noise-Canceling Headphones is a simple
phase-inverting amplifier. All inverted sounds played back through the
headphones cancel out the original sounds, leaving nothing but
silence. The amount of canceling can be adjusted for different
situations. A CD player or cassette tape can be listened to if you
want to "fill the quiet."

The schematic diagram in Fig. 1 shows the design of the electronics
portion of the project. A headset-mounted microphone is connected to J1, a
1/8-inch stereo jack. Electret-condenser microphones need a 2- to 10-volt
bias voltage for their internal FET pre-amps. That is supplied by R2. A
voltage-dividing network, which also decouples the bias volt-age from the
power supply, is provided by Rl and Cl. That is necessary due to the high
gain of the entire signal chain.

The signals from the microphone then go to ICl-a. an NE5532 set up as a
standard non-inverting pre-amp. The gain is set to one plus the ratio of
R8/R6 in the feedback path. The total gain for that stage is about 31 dB.
Resistor R4 provides a ground reference for the pre-amp. A pair of
high-pass filters is formed by C2/R4 and C4/R6. Those filters block any DC
that tries to slip through the pre-amp.

From the output of the pre-amp, the microphone signal is sent down two
different paths. It feeds both one pole of Sl-a and the phase-inverter.
The phase inverter is nothing more than a second NE5532 configured as a
unity-gain inverting op-amp (IC2-a). The output of IC2-a is connected to
the other pole of Sl-a. That way, Sl-a can select either the inverted or
the non-inverted signal. The selected signal on Sl-a’s common pole goes to
potentiometer R14-a. That potentiometer sets the level of the microphone
signal feeding the headphone amplifier.

The headphone amplifier is built around IC3-a, a third NE5532 wired as
an inverting op-amp stage. The gain here is set by the ratio of R19/R15.
That type of op-amp configuration can be easily modified to add a summing
feature by the inclusion of R17. The second input comes from an auxiliary
line-level input that is attenuated by potentiometer R23-a.

There is a reason why 10K-ohms was chosen for the value of R15 and R17.
Besides keeping the values of R19 manageable, the 10K-ohms resistors
interact with the 100K-ohm linear potentiometers. The potentiometers then
behave in a logarithmic fashion.

This is how that feature works: One end of the potentiometer is tied to
ground because we are using it as a voltage divider. Because the sum-ming
junction of an op-amp is at a virtual ground, the 10K ohm resistor is also
essentially tied to ground. That affects the response of the
potentiometer, As the potentiometer is rotated, there is a more pronounced
increase in the output as the end of the potentiometer’s travel is
reached. That causes a smooth increase in perceived loudness of the
signal. Potentiometers with an audio taper are, of course, available, but
a linear-taper unit is easier to obtain and costs less.

The output of the headphone amplifiers is coupled to output jack J3
through R21. That resistor provides overload protection to IC3-a in case
the output is shorted. If you have never used an op-amp for driving
headphones before, you are in for a nice surprise. The NE5532 will supply
a 10-volt rms signal into a 600-ohm load with very little distortion. That
works out to 166 mW of power. Most personal stereos only supply 20 to 30
mW of power to headphones.

A final note on using operational amplifiers as headphone amps: Most
generic ones will not supply enough current to function properly. Some
substitutes for the NE5532 that are known to work include the 0P275 from
Analog Devices, the OPA2604 from Burr Brown, and the LM833 from National
Semiconductor. Those components are available from several sources,
including Digi-Key, Allied Electronics, and Jameco.

Construction. There are two parts to this project: building the
electronics and modifying a pair of headphones. The circuit is relatively
simple and can easily be assembled on a perfboard. One style of perfboard
that simplifies construction is one having a pre-etched copper pattern on
its solder side that connects groups of holes together. One example of
that type of perfboard is Radio Shack #276-150.

The etched pattern on that board has a pair of buses that run the
length of the board. Those buses are very convenient for power
distribution. If you use that type of board for the Noise-Canceling
Headphones, it is best to start by spacing out the three ICs on the board
so that they straddle the buses. Then attach the power supply leads from
each chip to the buses. It is then a simple matter of point-to-point
wiring the rest of the circuit.

Check your work often while building the circuit. A common mistake many
hobbyists make is not checking their work thoroughly enough. Often a few
components are accidentally wired in backwards. The usual result is that
the circuit will probably not work, the ICs could be damaged, and the
electrolytic capacitors might explode!

Fig. 2. The circuit board and batteries fit neatly into a
simple project box. Keeping the wiring neat and following the layout
shown here makes assembling the unit and changing the batteries
easier.

When wiring the jacks, it is a good idea to follow the audio industry
standards as to which jack connection is for which stereo channel. Normal
standards for stereo connections are to connect the right channel to the
ring and the left channel to the tip. The board and batteries are mounted
in a suitable enclosure. A suggested layout for the components and control
panel is shown in Fig. 2.

When selecting a case for the project, be sure that it is large enough
to hold the circuit board and the two 9-volt batteries comfortably. After
the front panel is laid out and drilled, check to make sure all the
controls and jacks will fit. One method for labeling the front panel is to
spray the entire panel with a flat color such as white or yellow. After
applying transfer let-ters, seal the panel. Use several thin coats of a
clear coating such as Crystal Clear by Krylon. The results are worth the
effort. While the front panel is drying, we can start on the headphones.

Fig. 3. The microphones are mounted on the earpieces of the
headphones with a dab of silicone sealant. Tie both wires together in
order to make the headphones more comfortable to wear (A). Follow the
diagram in (B) when wiring the microphones. The left mic is connected
to the plug’s tip and the right is connected to the ring. The ground
connection on an electret microphone cartridge is easily identified by
the solder connection between the terminal and the mic’s case (C).

Headphones with Ears. The headphones are a standard pair of
aftermarket Walkman-type units. They sell for about $20 at most record or
electronics stores. The headphones are modified by mounting two small
electret-condenser microphones on the head-phones, one on each earpiece.
That modification is shown in Fig. 3.

The key to making the headphones wearable is to use thin wires running
to the microphones. An excellent source of thin audio cable is to buy
another set of cheap headphones - the cheapest you can find. Cutting the
wire off them will yield a shielded stereo cable that is thin and
flexible. As an added bonus, the wire will have a l/8-inch stereo plug
molded on to it already!

The best way to strip that type of wire is to roll a razor blade very
carefully over the insulation without cutting the wire underneath. Once
the insulation is cut, carefully pull it away from the wire. That method
works especially well on Teflon-insulated wire. After you have prepped the
wire, mark the wire that is connected to the ring and the one that goes to
the tip of the jack. An ohmmeter makes that task easy.

Carefully solder the wires to the microphone elements. The easiest way
to do that is to pre-tin the wires and melt the little dab of solder on
the microphone element with the tinned wire beneath the soldering iron
tip. Look carefully at the microphone. As shown in Fig. 3C, the terminal
that is connected to the case of the microphone is the ground connection.
The other terminal is the actual microphone output. Holding the microphone
element in an alligator-clip holder will make the job much easier. After
soldering on the microphone elements, it is good idea to test them prior
to gluing them to the headphones. The wiring should follow Fig. 3B.

Mount the microphones on the headphones as shown in Fig. 3A. One way to
attach the microphones to the headphones is to use a dab of silicone
sealant. Using a toothpick or other suitable substitute, mold the silicone
around the edges of the microphone element to smooth everything off. Be
careful not to get any on the black felt surface - that is where the sound
enters. Obviously, the left and right microphones should be attached to
the left and right sides of the headphones, respectively. Trying to cancel
out a sound on the right with a sound from the left will not work.

After the glue is dry, gather and bundle the wires together with
several nylon tie wraps along the length of the wires. With the headphones
complete, it is time to experiment with the Noise-Canceling Headphones.

Creating A Quiet Zone. For testing purposes, you should be in a
quiet room with just a little background sound, such as a heater or
air-conditioner fan. Plug in the microphone jack and the headphone jack,
and put on the headphones. Turn both controls all the way down and turn
the power switch on. Slowly turn up the microphone level. You should
either hear the background sound increase or start to fade. If it
increases, change the position of the phase switch. At some point, you
should reach a "null" point where the background sound is at a minimum. If
you adjust beyond the null, background sound will become louder as the
out-of-phase signal exceeds the ambient sound level. Try talking aloud. If
it sounds like you have a massive head cold and can barely hear yourself,
the circuit is functioning properly.

Note that it is impossible to eliminate all incoming sound. Many things
affect the ability to cancel out noise. The loudness of the incoming
sound, the specific frequencies involved, and the position of the sound
source all play a part in how well the headphones do their job. Feel free
to experiment.

If everything is working fine, try connecting a CD player to J2. You
will need a 1/8-inch -to- 1/8-inch patch cord similar to the ones used to
connect portable CD players into a car stereo. After connecting the CD
player, slowly turn up R23. It should sound clear with no distortion.
Experiment with combining low levels on the CD player and canceling out
room noise. The Noise-Canceling Headphones is the perfect device for
environments that have a loud ambient sound level, such as rooms with loud
ventilation systems.

Beyond Peace and Quiet. This project lends itself to many other
uses. Several interesting applications will suggest themselves that do not
require any additional hardware. For example, by switching the microphones
to "in-phase," the unit can be used to assist hearing or improve hearing.
Areas that can benefit include outdoor activities such as hunting or just
observing nature.

Another unusual application for the Noise-Canceling Headphones is in
binaural recording. Since we already have two microphones mounted in
essentially the same place human ears are, all we have to do is send the
headphone output to a tape recorder input. Binaural recordings put the
listener directly in the sound field. The two microphones capture the
exact phase and timing relationships of sound as we hear it. Those are the
clues our ears use to determine the location of a sound.

Try this little experiment: record a person talking to you while you
are wearing the headphones and have them walk around you in a circle. Then
listen to the recording on the headphones. You will hear the person walk
around you! The microphone elements used in this project feature full
20-Hz to 20-kHz frequency response. They provide a signal with
surprisingly high fidelity.

Fig. 4. Mounting both microphones angled apart at the end of a
long stick makes an audio probe. It is very useful when you need to
listen at a location that you can’ t reach.

Other interesting tools can be created by building different types of
housings for the microphones. If two microphones are mounted on the end of
a length of 1/2-inch dowel, an audio probe is the result (Fig. 4). It is
wired similarly to Fig. 3B. That device lets you listen to things up close
that you wouldn’t normally hear. It can be used to "sniff" out problems in
mechanical equipment or to record things like hamsters chewing on
cardboard. With the microphones mounted at an angle between 90 degrees and
120 degrees, you will have a stereo image of the sound source too!

Fig. 5. A parabolic dish or lamp reflector makes a usable "Big Ear"
microphone. The microphone is mounted at the focal point facing in
towards the dish. Either one or two microphones can be mounted in the
dish. If you build two, you can pick up stereo sounds.

An extension of the shotgun-style microphone is a "Big Ear." The
general arrangement is shown in Fig. 5. The main component is a small
parabolic dish. Place one or both microphones at the focal point of the
dish and experiment away. Sources for parabolic dishes can be as close as
a local hardware store. A simple reflector for a light bulb can be found
at a very reasonable price. Another source of true parabolic dishes is
Edmund Scientific, 101 East Glouchester Pike, Barrington, NJ 08007. For
some advanced experimenting, use two dishes (one for the left microphone,
one for the right) and experiment with stereo reception of a distant
sound.

Parts List

Resistors (1/4W, 1% metal film):

R1 - 4.7Kohm
R2, R3 - 2.2Kohm
R4, R5 - 1M
R6, R7 -
1Kohm
R8, R9 - 33Kohm
R10-13, R15-18 - 10Kohm
R14, R23 -
100Kohm pot, dual-gang, linear taper
R19, R20 - 100Kohm
R21, R22 -
47 ohm

Additional Parts and Materials:

IC1-3 - NE5532 dual audio op-amp
C1 - 33uF, 25WVDC, electrolytic
capacitor
C2, C3 - 0.01uF Mylar capacitor
C4, C5 - 10uF, 25 WVDC,
electrolytic capacitor
J1-3 - Audio jacks, 1/8-inch, stereo
S1, S2
- Dpdt toggle switch
B1, B2 - Battery, 9 volt
Microphones
(Digik-Key P9967-ND or similar), headphones, PC board, case, wire,
hardware, etc.

Addendum

10/5/1998: The following sentence: "The signals from the microphone
then go to ICl-a. an NE5532 set up as a standard non-inverting pre-amp"
originally identified an NE5522. Corrected.

7/8/2000: Marc Goodman built a
noise-cancelling headphone amplifier for his Sennheiser HD580 headphones
based on the circuit in this article. He writes:

Just thought I'd report some results on building a noise cancelling
headphone amp for my Sennheiser HD580's. I started with the Ryckebusch
project from this site, but I ended up making a fair number of changes.
My goals were, in order of importance, 1). get the best audio quality
possible driving my HD580's from my Pioneer portable DVD player
(PDV-LC10), 2). cancel enough noise, on demand, to make airplane
listening/movie watching possible, 3). maximize listening times between
battery recharging, 4). make the resulting project with enclosure small
enough to fit into a camcorder bag along with the DVD player,
headphones, and a selection of DVDs (i.e., not too big but no need to
make it as small as, say, a CMOY amp).

1. Optimizing Audio

The Ryckebush NC project uses a single NE5532 inverting op amp stage
for line-in amplification. I didn't like the fact that the audio signal
gets inverted, so I first tried changing the circuit to use a
non-inverting amplifier. It sounded OK to me, but when I went to a
unity-gain inverting op-amp as a first stage with an inverting amplifier
as a second stage, it sounded cleaner, so that's what I went with. I
also used LM6172's for both stages. As other people have noted, the
LM6172 requires a fair amount of voltage. More about this in section
three.

2. Noise Cancellation

Ryckebush feeds the output of electret condenser microphones through
a non-inverting preamp and allows switching of this output through a
unity-gain inverting op-amp to select between cancelling the noise or
amplifying the noise. The inverted or non-inverted signal is then summed
with the line-in signal through the final inverting amplifier. I was
more concerned with conserving power than I was with being able to
amplify external sounds, and feedback was a real problem with the Senns
in any event, so I opted to drop the inverter from the circuit and to
separately switch the power on/off for the entire mic/preamp stage.
Since the preamp is non-inverting and the final audio stage is an
inverting amplifier, the output of the preamp can be fed directly (via a
potentiometer) to the input of the final stage. I would call the result
"noise reducing" rather than "noise cancelling," but the effect is quite
noticable especially for lower frequency sounds. There is also a faint
but noticable hiss when the noise cancelling circuit is enabled, so I
wouldn't want to use this stage unless the environment was pretty noisy
to start with.

I wasn't thrilled with the notion of gluing microphones to the
outside of my Senns, both because it would look ugly and also because I
carry them around with me everywhere and they'd be bound to get
snagged/knocked off. Fortunately, the Senns have removable covers over
the speakers with plenty of space between the speaker and the cover. In
order to both reduce higher frequency noise and to provide a secure
mounting for the microphones, I cut and shaped pieces of low-density
foam (squishy) to fit the inside of the cover and give the speaker a
little room. Though this closed the speakers off a little, the effect
was not significant enough to be noticable (to me, anyway). It also
teneded to acoustically isolate the microphones from the speaker
elements which reduced the likelihood of feedback. I found the best
results to be when the mics were placed to one side of the foam cover,
directly in front of my ear canals.

3. Batteries

My first implementation of the circuit used dual 9V batteries as in
the original Ryckebush circuit. However, I was completely bummed by how
short a charge lasted. So, I tried using 6 C rechargeable batteries
instead with a ground tap in the middle. These batteries held around
1200mAh as opposed to around 140mAh for the 9V's, so they lasted a long
time. But, I was driving the circuit with only around +/-4V instead of
+/-9V, and while this turned out to be enough for the LM6172's to power
up, there was a huge amount of clipping at higher volumes with my
HD580's.

My solution was to build a separate DC voltage doubler using the
ICL7662. The 7662 is a higher-voltage version of the 7660 that avoids
latch-up on startup. It operates at 10KHz, which is smack in the middle
of the audible range so ripple is a big issue. I found that by using
22uF low-ESR tantalum caps for the reservoir capacitor and by carefully
routing all wires away from this capacitor to minimize inductive
coupling, the ripple was beneath the threshold of my hearing. Note that
getting the wires positioned just right was extremely important, as was
putting the voltage doubler on a separate circuit board. Also, I had to
shorten the wire lengths to all of my input/output jacks. It was kind of
a pain, but not as bad as listening to a barely audible high-pitched
whine ;).

4. Enclosure

It's about the size of a paperback novel with roughly half the space
taken up by the 6 C batteries in three rows of 2 lying in a Z and the
other half taken up by the PCBs and controls (two DPDT switches, two
pots, three 1/8th" jacks and a power on/off LED).

Conclusion

All in all, kind of fun and only around three times as expensive as
if I had bought something from a store ;).

7/15/2000: Laura
Balzano, Anna Huang, Eric Rombokas and Yasushi Yamazaki at Rice
University built noise-cancelling headphones based on the Ryckebusch
design for their project: "Sound Cancellation by Signal Inversion."

As you know, we were using low-quality headphones, and when we
mounted the mics on the outside of the headphones, we got little
beneficial effect. We found that we could get better results using nice
headphones, but they still were not very effective. We moved mics inside
headphones to decrease sound delay effects. This greatly improved
performance, but introduced feedback until we rigidly pointed the mics
away from the headphones' speakers and covered the mics with headphone
foam (from the pair of headphones that we had to sacrifice).... We found
that due to sound propogation delay, non-inverted output (in our case)
produced better effect than inverted output. We also observed that
headphones were ineffectual for high frequencies, so to reduce noise
produced by the circuit, the input was put through a simple RC lowpass
filter with Wo=1K.

Results: The apparatus heavily attenuates sounds of very low
frequencies, and somewhat attenuates all frequencies <= 1Khz. The
headphones also get rid of the part of your voice that "echoes" in your
inner ear-- the extra echo that you typically hear when your ears are
covered and you speak.

2/10/2002: Value of C3 in figure 1 corrected to 0.01uF. Also posted
larger version of images and cleaned up figure 1.

c. 1997, Gernsback
Publications (Popular
Electronics and Electronics
Now).
From Electronics
Now, September 1997. (Republished with permission.)

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© Chu Moy, 2001