ZEPPELINDESIGNLABS.COM • 2950 N. WESTERN, CHICAGO, IL 60618
BALANCED PIEZO CONTACT MIC
Assembly Instructions
thE CORTADO MKII
THE CORTADO MKII
Balanced Piezo Contact Microphone
103116
© 2016 ZEPPELIN DESIGN LABS. NO PART OF THIS DOCUMENT MAY BE REPRODUCED WITHOUT WRITTEN PERMISSION FROM THE AUTHOR. ZEPPELIN DESIGN LABS TAKES
NO RESPONSIBILITY FOR ANY DAMAGE OR HARM THAT MAY COME TO ANYONE OR ANYTHING THROUGH THEIR PRODUCTS. THE CORTADO IS UNDER THE CREATIVE
COMMONS SHARE-ALIKE LICENSE, WHICH MEANS YOU ARE FREE TO ADD OR BUILD UPON THIS CIRCUIT IDEA IN ANY WAY YOUR CREATIVITY ALLOWS, BUT ANY
DERIVATIVES MUST BE SHARED USING THE SAME LICENSE.
2
INTRODUCTION
Piezo sensors are quite incredible little things. They can be used to detect the slightest variation in
pressure, force, or strain and convert that energy into a voltage. Over the past several decades they’ve
been used in numerous applications in several fields including the audio industry. Unfortunately, they’ve
developed a bad reputation for sounding “harsh” and “brittle” when used as contact microphones
in acoustic instruments. This is due to improper impedance matching and the inadequate driving
circuits that are usually used with these sensors in audio applications. Piezo sensors in themselves are
capable of a very wide bandwidth, and when used properly, can achieve excellent results. So with the
intention of getting the most out of a piezo sensor, we here at Zeppelin Design Labs have developed
The Cortado -- a balanced piezo contact microphone. This mic contains a phantom powered circuit
that properly matches the piezo sensor input impedance and drives the signal via a balanced output,
which allows for wide bandwidth (~20Hz-20kHz), low signal losses, and high signal to noise ratio.
The Cortado’s circuit was originally designed by Alex Rice (http://www.zachpoff.com/diy-resources/
alex-rice-piezo-preamplifier/) for use in his contact mic hydrophone, but it can be used in countless
other applications with excellent results. In the recording studio or on stage the Cortado can be used
on pianos, percussion, guitars or other stringed instruments. It can be used for a variety of mic and
sensor applications such as a plate reverb pickup or wooden stomp box mic. We even created a tin
can vocal microphone out of stuff we found in the trash (as pictured on this manual’s cover). This is
the perfect accessory for a lofi geek or a field recording enthusiast. Let your imagination run wild...
discover what the pipes in your apartment building sound like, what bridge suspension cables sound
like, even what melting ice sounds like. Anything that vibrates or resonates with an audible frequency
can be captured by The Cortado contact mic. Several field recording enthusiasts around the web use
this circuit (or something similar) to record some pretty cool stuff. Just do a Google or Youtube search
for “contact mic field recording.”
The assembled Cortado kit can take several forms, suitable for many different applications.
• Standard Contact Mic. The circuit board is shielded in a metal box and the piezo disc and cable
are also shielded.
• Tin Can Mic. Use this configuration whenever the entire Cortado will be installed inside a shielding
container, such as a tin can, garbage pail, grounded metal sculpture, etc. The piezo is attached
with a fine wire; the piezo and circuit board remain unshielded.
• Instrument Pickup. Similar to the Standard Contact Mic, but the circuit is shielded not with a metal
box but with copper tape, to reduce size.
STANDARD CONTACT MIC
TIN CAN MIC
INSTRUMENT PICKUP
3
WHAT YOU WILL NEED
Here’s everything you will need to build The Cortado DIY kit as shown (Figure 2 on page 4).
TOOLS
1. Digital Multimeter, able to measure Resistance
2. #2 Phillips Screw Driver
3. Small flathead screwdriver
4. Ruler
5. Small awl, or metal poking probe
6. Soldering Iron (not a soldering gun, or a “cold heat” iron), good quality, 15-50 watt, with a good medium
or small sized tip, conical or “screwdriver” shape. One with a temperature control and a stand is best.
7. Wet sponge or dry solder-cleaning pad
8. Wire strippers
9. Flush cutters or small diagonal cutters
10. Needle-nose pliers
11. Clamp or vise to hold the printed circuit board while soldering (optional, but handy).
12. Solder sucker or solder braid – optional, but very handy if you have to remove / repair any components!
13. Scissors
14. Drill and bits:
9
/
64” (3.5mm) and
3
/
8” (10mm), plus several in between, or stepper bit
SUPPLIES
1. Solder, 60/40 rosin core, the smaller diameter the better (we prefer .032” diameter). Make sure it’s good
quality; we prefer Kester brand, but most brands will work fine.
2. Isopropyl alcohol, denatured alcohol, or rubbing alcohol
3. Rough grit sandpaper
4. Electrical tape
Figure 1: What’s In The Box
Figure 2: The Finished Kit
4
WHAT’S IN THE BOX
Table 1: The Cortado Balanced Piezo Contact Mic Bill Of Materials
(BOM) is a complete parts list of everything that should be present
in your kit, followed by photos of each part. Print the BOM and
carefully go through the kit, identifying every part. Please read
about the proper handling of FETs (Q1,Q2,Q3) on page 11
onto the cluttered workbench, or
onto the living room carpet! This
will protect you from losing tiny
before removing them from the static-protective bag. Note that
some of the components are difficult to tell apart. Compare them
carefully with the photos. Besides verifying that nothing is missing,
this will acquaint you with the parts and their names. If ANYTHING is
missing, first double-check; we double-checked before sealing the box at our lab! If it’s still missing,
EMAIL US right away at support@zeppelindesignlabs.com. If we goofed and shorted your kit, we will
get replacement parts in the mail to you as soon as possible. If you lose or damage anything, we will
be glad to sell you replacements. The unusual or custom components can be ordered directly from us
(contact support@zeppelindesignlabs.com). For more common parts, like resistors, caps, or screws,
you may just want to go to a local electronics or hardware store.
Table 1: The Cortado Balanced Piezo Contact Mic Bill Of Materials
Part # Description Notes Qty
HD-40-30 Male XLR Jack
TD-10-22 Piezo disc
CB-20-30 Microphone cable 30” (76cm)
3
CB-01-30 Hookup Wire 24/1 2-
CB-01-69 Wire-wrap Wire 30/1 12” (30cm) total, for piezo disc
TP-30-11 Double-Sided Tape ~1” (2.5cm) sq.
TP-20-14 Copper tape 11.5” (29cm) total
HD-08-01 Zip Tie
ST-10-23 Nylon Hex Standoff M3x12
FA-60-37 Phillips Machine Screw – Pan head M3x6
EN-30-01 Tin Box
PL-10-42 Cortado MKII Label Sticker
PL10-90 Serial Number Sticker
HD-20-02 Rubber Grommet
PC-41-01 PCB Printed Circuit Board
VR1 Potentiometer Trimmer 5K 1
R3, R6, R7, R8 Resistor 150R 4
R1, R2, R4, R5 Resistor 3.3M (or 3M3) 4
C1, C2 Film Capacitor 2.2nF/100V 3
C3 Film Capacitor 680pF/100V 1
Q1, Q2 Matched Pair FETs J112 - Painted Silver 2
Q3 Unmatched single FET J112 1
/
8” (7cm)
Empty the parts
of the kit into a bowl, NOT
parts.
1
1
1
1
1
1
1
2
1
2
1
1
1
2
1
5
HD-40-30 TD-10-22 CB-20-18 CB-01-30 CB-01-69
TP-30-11 TP-20-14 HD-08-01 ST-10-23 FA-60-37
EN-30-01 PL-10-42 PL-10-90 HD-20-02 PC-41-01
VR1 R3, R4, R5, R8 R1, R2, R6, R7 C1, C2 C3
Q1, Q2 Q3
6
THE PRINTED CIRCUIT BOARD
Whatever configuration you are building, all Cortados start with a circuit board. Your work space
should be well-lit, well-ventilated, and disposable; that is, don’t work on the nice dining room table!
Work on a utility surface that you can burn, drill and scratch. A piece of ¼” tempered masonite, or a
chunk of MDF, makes an excellent cover if you don’t have a utility work bench.
Solder fumes are not healthy for you. The fumes consist of vaporized flux,
which can irritate your nose, lungs, and even your skin. You MUST work in a space where
the air drifts away from you as you work, so fumes do not rise straight onto your face.
Solder residue usually contains lead, which is poisonous if you ingest it. Do not
breathe the fumes, do not eat the supplies, wash your hands after you handle solder, and
sweep and wipe up your work space after EVERY USE.
Figure 3: Component Values and Locations
7
The printed circuit board (PCB) holds the components in this circuit. All of the components will be
installed on the “component side” of the board, which is the side that has the part numbers on it. The
other side of the board is called the “solder side”, which, as the name implies, is the side on which
the legs of the components are soldered to the board. Proper technique for installing and soldering
components to a circuit board is demonstrated through several great resources on Instructables and
Youtube under the search “PCB soldering tutorial.” The general procedure consists of the following:
1. Install the part on the “component side” of the board, by threading the wire leads
through the appropriate holes in the board. For your convenience, the board has
silk screen outlines indicating where the components should be placed, along with
text indicating the part number and the component value.
2. Hold the component in place with your finger and turn the board over.
3. Gently bend the leads out at about 45 degrees to keep the component from falling
out of its holes.
4. Install all of one type of component, bending each of the leads as they are installed.
5. Flip the board over solder-side-up, and solder all of the components in one pass.
6. Clip the leads off with small diagonal cutters, right at the solder joint.
Let’s begin!
1. Resistors: The values of resistors are given by a series of colored stripes on their body. There are
several tutorials online describing how to decode these stripes, but we will identify each resistor
for you by simply naming the stripe colors, and giving you the value and the part number.
“Figure 3: Component Values and Locations” is a good reference. If you are color blind or can’t
see the stripes clearly, then you must use your digital multimeter to measure the resistance of
each resistor.
Resistors are not polarized, meaning they can be installed in their holes in either direction. It
doesn’t matter which lead goes in what hole.
The hole spacing of most of the resistors on the circuit board allows the leads to be (gently) bent
90 degrees at the body of the resistor (1). This allows most resistors to slip into their holes very
easily. Resistors R7 & R8 are exceptions (2). Note how those two components are bent.
1 2
8
a. Start with the 3.3M resistors (R1, R2, R4, R5), labeled ORANGE, ORANGE, BLACK
YELLOW, BROWN. Compare to its picture in the BOM. Find their locations on the circuit
board; install and bend the leads as described above (3). Don’t solder any of them until all 8
resistors are installed; just bend the leads to keep them in their place.
3
b. Continue with the 150 ohm (150R) resistors (R3, R6, R7, R8). The body of these resistors are
blue and are labeled BROWN, GREEN, BLACK, BLACK, BROWN. R7 and R8 stand upright
on the PCB so bend one lead nearly parallel with their bodies and install them standing up
(4). Bend the leads on the back so they won’t fall out.
4
c. You should have a whole forest of bent leads coming out the solder side of the board. Now
you can turn the board solder-side-up and solder each one to the board. Use a clamp or vise
if you have one; it makes soldering much easier (5,6).
5 6
9
d. Now clip each lead with your flush cutters at the solder joint (7).
7
e. Before installing any more components on the circuit board, double check the resistance
values of each of the installed resistors. Set your digital multimeter to the “ohms” or
“resistance” setting, and measure across all of the resistors. Compare the measured value to
the listed value in Table 1 and in Figure 3. Make sure they are all correct (within 1%) before
moving on!
2. Capacitors: There are three film capacitors of two different values in this circuit. Both values of
capacitors look very similar so please make sure you look closely at their markings and compare
them to the photos in the BOM. Like the resistors, these capacitors are not polarized. They can
be installed either direction; it doesn’t matter which lead goes in which hole.
a. The 2.2nF capacitors (C1,C2) are labeled “2A222J.” Place both C1and C2 in their holes
and bend the leads on the back (8).
8
b. The 680pF capacitor (C3) is labeled “.2A681J.” Place the C3 in its holes and bend the
leads out. When you have all three caps placed, turn the board over, solder and then clip
the leads (9,10,11,12).
10
9 10
11 12
3. Trimmer Potentiometer: Place VR1 in it’s location with the adjustment screw located toward the
PZ holes. Solder and clip the leads (13).
13
4. Transistors: This circuit contains three field effect transistors (FETs). FETs are quite sensitive to
static electricity and could easily be damaged if they are exposed to moderately high voltages.
The voltage rating of these FETs is only 35V, meaning that if over 35 volts is applied across
two leads on one of these components it will most likely be damaged to the point of not being
usable. Unfortunately, humans are not sensitive to static electricity at such low levels; in fact, most
people can’t even feel a static discharge less than 1000 volts! So it is very easy to damage these
components without even knowing it. Consequently, it is important to handle these components
as little as possible. When you do have to handle them make sure you are grounded, preferably
by touching something grounded to the mains like the metal chassis of a plugged in amplifier, or
a refrigerator. At the very least you should touch a large conductive object like a metal desk or a
filing cabinet. With this information in mind please proceed carefully.
11
Two of the three FETs have silver paint on their face. We painted them to indicate that these two
FETs have been “matched” to each other. This means that the gate-source turn-off voltage of these
transistors were measured to be within about 100th of a volt of each other. This allows both
sides of the circuit to be balanced very closely, achieving very high common mode rejection,
which results in a very quiet microphone.
Transistors are polarized, meaning it is important which way they fit into the holes. The
component side of the circuit board shows an outline of the transistor body in the correct
orientation.
a. Place the silver-painted FETs in the locations of Q1 and Q2. Either silver FET can be Q1
or Q2; it doesn’t matter which one goes in those locations as long as they are both silver
(14,15). On the bottom of the board bend the leads a bit so they won’t fall out of their holes.
b. Place the other FET in the Q3 location (16). Please be mindful of the correct orientation.
Bend the leads of Q3. Turn the board over, solder and clip all the transistor leads
(17,18,19).
14 15
16 17
18 19
12
1
5. Grounding wire (Part #CB-01-30): Strip about
/
4” off of both ends of the black grounding wire
(20). Place one of the ends of the wire in one of the holes in the corner of the board (note the
picture) (21,22). Solder the wire in place.
20 21
22
6. Use one of the black M3 screws to screw the standoff to the board. The standoff should be on
the component-side of the board (23,24,25).
23 24
25
13
XLR JACK
All Cortado configurations use the same XLR cable.
1. Prep the Mic Cable, part CB-20-30:
a. Cut the mic cable into two pieces, about 18” (45cm) and 12” (30cm) long, or to suit your
application (26). Note that if you are going to install the entire Cortado inside a shielding
container, like a Tin Can Mic, you will only need one piece of mic cable.
1
b. Strip off about
insulation be careful not to cut too deep, or you will cut through the thin copper wires acting
as the cable shield.
c. Pull all of these copper shield wires to one side of the cable and twist them together (27).
d. Pull the layer of foil and the cotton strings to the other side of the cable and cut all of them off
with your snips (28, 29).
28 29
/
2” (13 mm) of outer insulation from one cable-end. When stripping the outer
28 29
14
1
e. Strip off about
/
8” (3 mm) from the white and red wires (30).
f. Tin the twisted copper shield wire, the red wire, and the white wire (31).
g. Repeat Step 1a - 1f for the other three cable ends.
30 31
2. Disassemble the XLR jack (HD-40-30):
a. Unscrew the tailpiece, remove the strain relief, and push the pins out of the housing (32).
b. Place the tailpiece and the strain relief onto the 12” (30cm) cable (33).
32 33
3. Solder the wires to the pin insert:
a. Take a close look at the pin insert. Note how the pins are numbered (34). Mount the pin
insert in a clamp if you have one, with the pins oriented so they appear as three troughs
(35). Tin each pin with solder.
34 35
15
b. Solder the white wire to Pin 2 (36).
c. Solder the red wire to Pin 3 (36).
d. Solder the ground wire to Pin 1 (37).
36 37
4. Reassemble the Jack:
a. Slide the strain relief up to the pin insert. Make sure the key on the insert is aligned with the
key on the strain relief.
b. Carefully line up the keys on the insert and relief with the slot in the housing--it is opposite the
rectangular hole. Gently press the pin insert and strain relief into the housing (38). Be very
careful not to disturb your nice new solder connections.
c. Screw on the tailpiece (39).
38 39
You have completed all the steps that are common to all Cortado configurations. As you continue, look
for the sections particular to your project: Standard contact Mic, Tin Can Mic, Instrument Pickup.
16
PIEZO DISC
From this point onward, sections of the manual are specific
to your application.
The piezo disc and its connection to the circuit must be
carefully shielded. The procedure is the same for the
Standard Contact Mic and the Instrument Pickup: you
attach the piezo to the remaining piece of mic cable, then
wrap the piezo in electrical tape, and then copper tape.
You can also provide shielding by installing the whole
Cortado into a grounded, conductive enclosure, as with
the Tin Can Mic. In this case you attach the piezo to a fine
wire, with no further shielding.
Now drop down to the section appropriate to your project.
STANDARD CONTACT MIC &
INSTRUMENT PICKUP
Follow this section to prepare the piezo for use with either the Standard Contact Mic or Instrument
Pickup.
1. Solder the 18” (45cm) microphone cable to the Piezo disc (Part #TD-10-22): For your
convenience, we have tinned the disc to make soldering easier.
a. Solder the white wire to the ceramic part of the piezo disc (40).
40
The best way
to successfully solder wires to
the disc is to lay the tinned wire
on top of the tinned part of the
disc and very quickly touch them
together with the soldering iron.
17
b. Solder the red wire to the brass part of the disc (41,42).
41 42
2. Now you need to electrically insulate the piezo disc. When we make Cortados in the lab, we
use Plasti-Dip tool handle coating, but you can use electrical tape.
a. Wrap the piezo disc in electrical tape (43,44). Keep the tape only one layer thick,
especially on the back (the flat side) of the disc. Use two pieces of electrical tape butted sideby-side to wrap the disc (45).
43 44
45
18
b. Use your scissors to trim off the excess tape to within about a millimeter around the edge of
the disc (46,47).
46 47
3. Shield with copper tape:
1
a. Cut two pieces of copper tape about 2-
/
4” long. Use one piece of copper tape to wrap the
disc in a similar way as with the electrical tape (48,49). Once again, keep the tape only
one layer thick on the back of the disc.
b. Trim off some of the excess copper tape around the disc (50), but make sure to keep a fold
somewhere, to maintain a conductivity path all the way around the disc: the copper tape on
the front of the disc must be electrically connected to the copper tape on the back side of the
disc.
48 49
50
19
c. With as little heat as possible, solder the two pieces of copper tape together on the top of
the disc (51). Also solder the shield of the microphone cable to the copper tape. Make sure
the back of the disc remains flat so it can be mounted on a flat surface (52,53).
51
52 53
That’s it! Move on to the FINAL ASSEMBLY chapter, and find the section specific to your project..
TIN CAN MIC, OR OTHER SHIELDING CONTAINER
If you are building a ZDL Tin Can Mic, you should follow our Instructable from here on out. Follow
this section if you have your own Tin Can Mic design in mind, or if you are going to incorporate the
Cortado into some other conductive shielding container. In this situation, since the cable between the
piezo and PCB does not need its own shielding, we use a very lightweight wire. This may improve
response, and also makes it easier to install.
1. Cut the red wrap wire (CB-01-69) into two 6” pieces (if it’s not already that length). Strip about
1
/
8” (3mm) off of each end of each wire (54, 55).
54 55
20
2. Solder the wires to the piezo disc (56). The ceramic part of the disc is very sensitive to heat and
can be tricky to solder to. For your convenience, we have tinned the disc to make soldering
easier.
56
The best way
to successfully solder wires to
the disc is to lay the tinned wire
on top of the tinned part of the
disc and very quickly touch them
together with the soldering iron.
3. Clean all the finger oil and other contaminants from the bottom of the disc with rubbing alcohol
and a rag or paper towel (57). Make sure you don’t touch the surface you just cleaned!
57
4. Place the double sided tape on the bottom of the piezo disc (58,59). Make sure it covers the
entire bottom surface. Trim off the overhanging excess with scissors (60,61).
58 59
60 61
5. Gently Twist the wires of the disc together.
21
FINAL ASSEMBLY
Jump to the section appropriate to your project.
STANDARD CONTACT MIC
1. Prep the container:
a. Drill a
3
/
8” (10mm) hole on both ends of the metal tin,
17
/
64” (7mm) up from the bottom.
(62,63) This is more likely to be successful if you start with a smaller bit and re-drill a few
times with progressively larger bits.
62 63
b. Place the grommets in each hole (64).
64
9
c. Drill a
/
64” (3.5mm) hole in the bottom of the tin, roughly in the middle (65,66).
65 66
22
2. Attach the grounding wire:
a. Use a bit of sandpaper or your flat head screwdriver to scratch up a tiny patch in the corner
of the can. With your soldering iron, tin this area (67).
67
b. Solder the ground wire to the tinned area (68,69).
68 69
3. Attach the cables to the PCB:
a. Pinch together all three wires at the end of the XLR cable, and slide the cable through the
grommet from the outside of the tin (70).
70
23
b. Solder the XLR cable to the PCB (71,72,76). The red goes to the hole labeled XLR3 and
the white wire goes to the hole labeled XLR2 The copper shield wire goes in the round hole
labeled XLR1.
71 72
c. Slide the piezo disc cable through the other grommet (73). Solder the red wire to the hole
labeled PZ1. Solder the white wire to PZ2. Solder the shield of the cable to ground: use a
pad from either R2 or R7, in the corner of the PCB. Use whichever is closest (76).
73 74
75 76
4. At this point take a break from assembly, and set the bias by following the steps in the section
“SETTING THE BIAS” on page 30. After the bias is set, continue on from here to the end of
the section.
24
5. Install the PCB into the metal can:
a. Screw the PCB stand-off to the metal can via the hole in the bottom of the can (77).
77
b. Fasten the zip ties around each of the cables on the inside of the tin (78). This acts as a
strain relief and protects the solder connections. Press the tie snug against the can interior,
and pull the tail as tight as it will go (78). Trim off the tail with your snips (79).
78 79
6. Add stickers:
a. Clean the lid of the can, inside and out, with alcohol.
b. Place the serial number sticker on the inside of the lid (80,81).
80 81
25
c. Place the Cortado Label sticker on the top of the box (82,83). Your Cortado Standard
Contact Mic is done!
82
83
TIN CAN MIC, OR OTHER SHIELDING CONTAINER
If you are making the ZDL Tin Can Mic, you should be reading our Instructable from this point on.
When the entire Cortado is installed inside a grounded shielding container, like a tin can or garbage
pail or steel sculpture, it does not require additional shielding.
1. Solder the piezo disc wires to the PCB. Use a multimeter/continuity tester to identify the wire
attached to the brass part of the disc, and solder this to the hole labeled PZ1 on the PCB (84).
Solder the wire attached to the white ceramic part of the disc to PZ2.
84
2. Solder the XLR cable to the PCB (85). The red goes to XLR3. The white wire goes to XLR2. The
copper shield wire goes to XLR1.
85
26
It’s time to set the bias, so please turn to “SETTING THE BIAS” on page 30. Once the bias is set,
your Cortado is complete, and ready to install inside your shielding container. This can be made of
anything that conducts electricity. You must connect the grounding wire to the container itself; the PCB
must NOT touch the container. If necessary, insulate it with electrical tape, or mount it with the standoff
provided.
INSTRUMENT PICKUP
To permanently install your Cortado inside a musical instrument, or a non-metallic sculpture or art
installation, you may use the copper tape instead of the metal can to shield the PCB. This will keep the
over-all size of the Cortado small and easy to tuck inside a sound hole or instrument body. Unscrew
the standoff from the PCB.
1. Solder the XLR cable to the PCB (86). The red wire goes to the hole labeled XLR3. The white wire
goes to XLR2. The copper shield wire goes to XLR1 (88).
86
2. Solder the piezo disc cable to the PCB. The red wire goes PZ1. The white wire goes to PZ2.
Solder the shield of the cable to ground: use a pad in one of the corners of the PCB. Use
whichever is closest (87,88).
87 88
3. At this point take a break from assembly, and set the bias by following the steps in the section
“SETTING THE BIAS” on page 30. After the bias is set, continue on from here to the end of
the section.
27
4. Wrap the entire PCB and exposed cable wires in electrical tape (89,90,91).
89 90
91
5. Very carefully remove the backing from the remaining piece of copper tape. Very carefully wrap
the copper tape around the circuit board. The goal is to encase as much of the circuit board as
possible. It may be easier to handle the tape if you cut it into two equal lengths (92,93).
92 93
28
6. If you cut the copper tape into two pieces, put a solder joint spanning the edge of the two pieces
so they will be electrically connected (94).
7. Solder the ground wire anywhere on the copper tape (95). When soldering the copper tape
make sure you don’t use too much heat which could melt the electrical tape under the copper.
94 95
96
The copper shielded electronics of the Cortado can be anchored to the inside of the instrument with
Velcro®. The disc should be mounted to a flat, resonating surface inside the instrument, such as the
sound board of an acoustic guitar. Experiment with placement by using a temporary adhesive such as
sticky-tack, or poster putty. When you find the sweet spot, permanently attach the piezo with epoxy.
An alternate way to permanently wire the output of the Cortado is to replace the XLR jack with a
balanced female end-pin jack installed through the body of the instrument. You can plug the instrument
into a sound system using a
circuit.
1
/
4” TRS male to XLR male cable, in order to supply phantom power to the
29
SETTING THE BIAS
In setting the bias of your Cortado MKII, you are simply adjusting the amount of nominal current that
is passing through the FETs, Q1 and Q2. The more current that passes through the FETs, the more gain
the circuitry will have on the signal from the piezo disc. But on the other hand, the more gain the circuit
has, the more the small differences in the matched FETs will be magnified. The result is less commonmode rejection and thus a higher noise floor. So we are seeking a balance between output volume and
noise rejection. We found these FETs work well with the bias set in the range of .1mA to .5mA through
each FET.
Follow these steps to set the bias on your Cortado MKII:
1. First, you need to supply power to the Cortado by plugging the XLR jack into a mixing console or
other phantom power source (such as the Espresso, by Zeppelin Design Labs).
97
2. With phantom power applied to the circuit, use your multimeter to measure the DC voltage
across TP (test point, marked red on Figure 3) and ground (98.99). Make sure that the solder
joints on the PCB don’t short against the tin container. A convenient ground point to put your
meter probe is through the hole in the bottom of the container (100).
98 99
100
30
3. Turn the trim pot (VR1) until the voltage on TP is .075V (101). This will set the bias current to
.25mA per FET. In the lab, we always start out setting the bias to this current, because that seems
to give a good balance of output volume and common mode rejection.
101
4. Now you just need to listen to how it sounds. Stick the piezo to something and lend an ear. If
you’d like more gain turn VR1 (clockwise), so that the voltage on TP is slightly higher. Do not set
TP higher than 0.15V, which corresponds to 0.5mA going through each FET.
31
USING YOUR CONTACT MICROPHONE
There are about as many applications for the Cortado as there are sound sources in the world. Please
read this section carefully to learn how to get the most from your contact mic and how to use it properly.
With some basic supplies and a little creativity your Cortado contact mic should give you years of
great service.
MOUNTING
One of the most important things to know when using piezoelectric devices is how to mount them to
obtain the best pickup. If your piezo disc is mounted incorrectly most likely it will have a very narrow
bandwidth (meaning it won’t pick up a wide range of frequencies), have a low signal to noise ratio,
and in a worst-case scenario, get damaged. Piezo electric devices pick up the best when they are
very tightly coupled to a vibrating surface. The more securely mounted they are, the better. The best
place to mount the piezo disc is on a smooth, flat surface, where the surface area of the disc can make
the most contact with the vibrating material. The double-sided tape provided with your kit is excellent
for permanently installing the disc on a flat sound source. If non-permanent installation is required,
then just taping the piezo disc to a sound source with painter’s tape also works sufficiently; just make
sure it can’t rattle or vibrate against the surface and it is held securely. Sticky-tack (the stuff used to
hold posters on the wall) also works well for non-permanent installations. If you need to mount it on a
round or uneven object a plastic clamp works well, but do not allow the piezo disc to bend at all; this
will damage the disc. The disc is rather fragile and the ceramic element can crack if it is not handled
carefully.
The circuit board is intended to be mounted inside a shielded enclosure, such as the tin can furnished
with the kit. However, any conductive container to which you can attach the ground wire will work,
such as a tuna can or soup can. A die-cast aluminum box (like the Hammond 1590 series) would also
work well as long as you can figure out a way to attach the ground wire (such as a solder lug attached
via a nut and bolt). The PCB is easy to mount with the stand-off and the two screws it comes with, but
it’s not necessary to use them. If the container you use is too small to mount the board via the standoff,
it is possible to just wrap the board in electrical tape and leave it floating (not anchored to anything).
3
In the container you use, you must drill two
rubber grommets are provided in the kit for mounting in the holes around the cables. Two zip ties are
also provided in the kit to tie around the cables on the inside of the container to provide strain relief.
/
8” holes for the XLR cable and piezo disc cable. Two
USING MULTIPLE MICS AT ONCE
Some applications may require more than one contact mic being used on the same source. This works
very well for many musical instruments particularly stringed instruments and pianos. The main thing
to be mindful of in these situations is the phase of each microphone. In most situations the relative
phase of all mics should be the same. This means that a positive impulse from the sound source will be
picked up as a positive (not negative) signal from all the microphones. If you followed these assembly
instructions properly, then any Cortado you build should be in the same relative phase.
32
In some cases you may want to invert the phase of one of the microphones. One example of when you
might want to do this would be when you use two mics on opposite sides of a vibrating plane, such as
an acoustic guitar soundboard. With one mic on the bottom (inside the body) of a guitar soundboard,
and another mic on the top (outside the body) of the soundboard, these mics should be out of phase
with each other. This can be accomplished by reversing the two piezo disc conductors (the red and
white wires in PZ1 and PZ2) on only one of the mics. But manually reversing these wires may not be
necessary if the equipment you are using has a phase switch.
A WORD ABOUT CABLES
In some cases you may find it useful to extend the distance between the piezo pickup and the circuit
board. This is possible by replacing the 18” cable with a shielded balanced cable of your desired
length. The maximum cable length that you can use without experiencing signal degradation will
depend on the quality of cable used, but in most cases it should easily be over 100 feet. The length of
the XLR cable could also be extended, theoretically even further than the piezo cable (assuming it is
connected to a good quality receiving circuit). One of the main things to keep in mind when using long
balanced cable lengths is the quality of the shielding. Only use cable that has a tight shield. The more
area the shield covers around the internal wires, the better.
PHANTOM POWER
The Cortado microphone circuitry does need power to operate. This is provided through phantom
power via pins 2 and 3 of the XLR jack. The circuit works best at 48Vdc phantom power, but can
function with much less voltage. It is not recommended to use less than 18Vdc or the mics headroom
will be too low for most applications. If the device you are plugging the microphone into doesn’t
provide phantom power then you need to use an external phantom power supply (available at most
pro-audio stores). Zeppelin Design Labs is currently developing an external phantom supply DIY kit
which should be available soon.
33
1
1
2
2
3
3
4
4
D D
C C
B B
A A
1
ZEPPELIN DESIGN LABS
2950 N. WESTERN
CHICAGO, IL 60618
USA
1
CORTADO MKII
8/23/2016
Title:
Size:
Date:
Revision:
Sheet of
A
Engineer: Brach
1.0
2
1
3
Q1
J112
2
1
3
Q2
J112
2
1
3
Q3
J112
R1
3.3M
R2
3.3M
R3
150R / 1%
R7
150R / 1%
R6
150R / 1%
R4
3.3M
R5
3.3M
R8
150R / 1%
GND
XLR PIN 2 XLR PIN 3
PIEZO IN 1 PIEZO IN 2
C1
2.2nF / FILM
C2
680pF
C3
2.2nF / FILM
Q1 AND Q2 NEED TO BE
CLOSELY MATCHED
XLR PIN 1
THE ORIGINAL VERSION OF THIS CIRCUIT WAS DESIGNED BY ALEX RICE. PUBLISHED UNDER THE CC4 LICENSE.
https://creativecommons.org/licenses/by/4.0/
http://www.zachpoff.com/diy-resources/alex-rice-piezo-preamplifier/
5K
VR1
Loading...