Introduction: SPKR MiK: How to Make a Microphone From a Speaker.
How to make an inexpensive microphone capable of picking up low frequencies that doubles as a speaker and direct box.
The large diaphragm of this microphone will pick up more of the low frequencies when recording a kick drum or bass guitar.
Sound recording engineers have been using this trick for years, and Yamaha has also made a commercial speaker microphone called the SubKick, that usually retails for around USD$300.
I was able to build this mic for under $20 by "scrounging" various parts out of old junk. Even if you need to buy all the components, you should be able to build this mic for a fraction of the price of the retail version.
This design goes slightly beyond the SubKick, as far as electronics are concerned, with a dual coil design, and internal direct injection (DI) box.
You should be comfortable using a power drill and a soldering iron, and be able to read a schematic diagram. There is a little sewing, but it isn't too difficult.
Step 1: Materials and Tools
- 6.5" dual coil woofer speaker (4ohm). I pulled this one from an Altec Lansing multimedia system that had a blown amplifier.
- 10" drum. Cheap is okay, but you'll want something with lugs that screw into the shell, not springs or T-rods that screw into couplers. I found this one for $2 at a second hand store.
- Two miniature bungee or elastic cords. I got a 4 pack for $2.50
- Crimp on terminal rings. I paid $2.50 for a dozen
- Adjustable hose clamp (the same diameter as the magnet of your speaker). This was about $1.50 at the hardware store.
- Female mounting flange for 3/8" microphone stand and small bolts to mount it. I got this at Parts Express
- 2 sq. ft. of speaker grill cloth. Also at Parts Express
- Male XLR 3pin panel mount connector and mounting screws
- Two 1/4" TS (mono) female phone jacks (at least one needs a tab for a normalling connection)
- A six-pole four-throw rotary switch (I scrounged my switch from a 4-way printer "data" switch box), or you can use Mouser part no. 105-SR2921F-34S
- 100 ohm potentiometer, also called a variable resistor
- two knobs (for pot and rotary switch)
- Two DPDT toggle switches (on-on)
- A SPST toggle switch
- Resistors: 100k ohm, two 10k ohm, 10 ohm
- 100nF capacitor
- 1:1 ratio audio isolation transformer (pulled from a second hand 270-054 I got for $1)
- metal container to hold and shield transformer, and mounting hardware
- heat shrink tubing or electrical tape
- connecting wire. 22ga or 24ga is fine.
- short microphone stand (another second hand store find)
- Small adjustable wrench
- Soldering iron and solder
- Wire stripper/crimper
- Sewing needle
- Small hack saw
- Ruler, or other measuring device
- Sharp hobby knife
- Marking pen
- Rotary tool
- Drum key
- Pliers, tweezers, or other soldering aids
- Adjustable calipers
- Cutting mat
- label maker
Step 2: Dissasemble the Drum
Separate the pieces of the drum.
It helps to have a drum key to unscrew the T-rods from the lugs, but you can use a small adjustable wrench instead.
We'll need the basics: Shell, Lugs, T-rods, heads, and rims.
My drum is a cheap toy, but it has the necessary lugs that screw into the shell.
I removed the old mounting hardware and the brush snare that were in my drum, and left them out.
Step 3: Shock Mounting the Speaker
Prepare the shock mounting system using the bungee cords, the crimp-on ring terminals, and the hose clamp.
Pick the right size crimp terminal rings: The crimp ends should be just large enough for the bungee cord to fit through, and the screws that hold the lugs onto the shell should fit through the rings.
The two bungee cords will eventually end up as 8 total pieces. Four will mount to the four mounting holes of the speaker. The other four will be mounted to the magnet using the hose clamp.
It is handy to make a reference template from one of the drum heads to position the speaker. Place the drum shell on top of the head. Mark four points on the head that line up with the mounting holes for the lugs. Remove the shell, and use a ruler to draw two lines through the center, connecting opposite marks in an "X". Center the speaker on the X and align the speaker's mounting holes with the lines. When it is centered, trace around the speaker. Keep this reference template head under the speaker and shell for the rest of this step (3). We will use this template again in step 5.
First cut the two bungee cords in half. By folding them, you can get the midpoint without measuring. Remove the hooks. Each piece should have a staple or crimp thing that was used to keep it from slipping through the hook. Slip each of the pieces through the mounting holes in the speaker. They should hold. If not, use a washer to make the hole smaller, or find some other way to attach them.
Next, remove the insulators from 12 of the crimp on terminals. This is safe, since we're not using them for electricity, just for their mechanical properties. Slide one crimp terminal over the end of each of the bungee cords. With the speaker centered in the drum shell, adjust the crimp rings so the ring ends just touch the shell without stretching the cord, and crimp them in place. You should have just under an inch of cord between the speaker and the crimp. Cut off the remaining bungee cord to use for the second set.
Crimp a ring onto each of the four pieces. Clamp the rings onto the edge of the speaker magnet using the hose clamp. Tighten the hose clamp until it is close, and slide the rings between the magnet and the clamp. Make sure that they are aligned with the speakers mounting holes, then tighten the hose clamp. (alternatively, you could just clamp the ends of the bungee under the hose clamp)
Slip the last four crimp rings onto the ends of the bungee cords. Adjust them the same as before, with the tip of the ring just touching the shell without stretching the cord. Crimp them in place and cut off any remaining cord blocking the holes of the rings.
Now is a good time to test the shock mounting system, so you can make any adjustments while there is still plenty of room inside the shell. Fit the lugs back onto the drum, attaching the shock mounting system with the same screws.
Step 4: Drill the Holes in Shell.
Lay out your panel and drill the holes for the jacks, switches, and potentiometer.
Its best to make a template to make sure you like the layout before you commit to drilling.
Check to see that the components will have enough room on the inside of the drum without interfering with each other, or the speaker. Also make sure that the components won't interfere with the lugs or the shock mount system.
Mark the center of each hole on the template, and tape it down temporarily. Mark the center of the holes by using a sharp object like an awl or the point of a nail to make a dent in the surface below. Mounting holes for the XLR jack are best drilled with the jack sitting in its hole, so the alignment is perfect.
Pre-drill the holes with a small bit first (1/8" works well), and enlarge to the proper size with larger bits. This will keep the bits from "walking" when starting the holes.
Step 5: Mount the Components
Trim the shafts of the switch and the potentiometer to the right length to fit your knobs with a hacksaw or cutting disc with a rotary tool. File down any sharp burrs.
Mount the components in the shell and make them snug. Don't over tighten. You don't want to strip the threads. :) Also mount the container for the transformer. A metal container will help shield from outside interference.
Wait to mount the speaker until step 7.
Step 6: Review the Plans and Solder the Circuit.
I've provided a PDF file (below) of my hand drawn schematics so you can print them off and have them at your workbench.
Warm up your soldering iron and follow the schematic. Lines that meet in a dot are connected. Lines crossing in a little "jump" are not connected.
Point to point wiring is actually simpler in this case. This is when components are soldered directly to each other without using a circuit board.
It may help to highlight different sections (paths that are connected together) of the circuit with different colors of ink, then wire one section at a time.
Solder leads to the transformer, and insulate the connections with electrical tape or heat shrink tubing. Then put the transformer into the container and run the leads through a hole drilled in the side. You may want to pad the transformer inside the container with a scrap of foam, cotton balls, or a plastic shopping bag so it doesn't rattle around inside. The connections on the transformer will be less likely to come loose if the transformer does not move.
Solder in the leads for the speaker and label them. Wait to solder them to the speaker terminals until after you have mounted the speaker in step 7.
Circuit ExplanationThe configuration of the coils can be changed by the rotary switch (S1).
- Position 1 Single: A single coil of the speaker is used (4ohms), both 1/4" jacks are wired in parallel with the coil.
- Position 2 Series: Two coils are wired in series(8ohms), with the two 1/4" jacks wired in parallel.
- Position 3 Parallel: Two coils are wired in parallel (2ohms), with the two 1/4" jacks wired in parallel.
- Position 4 Damping: Coil A is normalled to the 100ohm potentiometer, which will electromagnetially dampen coil B. When a 1/4" plug is plugged in, the potentiometer is disconnected, and the input directly drives coil A. Coil B is wired in parallel with the 1/4" output jack.
The filtered ground can be lifted by opening S4.
Step 7: Re-mount and Solder the Speaker
Mount the speaker back in the drum shell using the screws of the lugs. You will also be re-installing the lugs at the same time.
Solder the leads onto the speakers. Be careful to keep the polarity correct (+and-) or you will have phase cancellation of all the wonderful bass frequencies we want. Leave room for the speaker wires to move with the speaker when the speaker moves in the shock mounts.
If you like, you can put it on a stand now. You can put the knobs on too.
Step 8: Cosmetics
Your mic will look better with a nice cover.
We'll cut holes out of the heads, and cover them will speaker grill cloth.
Marking them is optional, but a good idea. You want to leave a strip all the way around that is slightly thicker than the shell of the drum. (if you cut out the whole head, the ring will just slip around the outside of the shell) The strip of head sits on the shell, and the rim keeps it tight.
To help me make a nice smooth cut, I used a washer. The difference between the inside radius and the outside radius of the washer was a little more than the thickness of the shell.
With the metal ring of the head facing up, set the washer flat inside the head, against the ring. Put the tip of the knife inside the washer. The washer will roll against the ring of the head, and keep the blade at the right distance.
Place the drum head "donut" on a corner of your grill cloth and cut out a square just a little larger. Then cut off the corners.
Take a needle and thread and sew some temporary holders in place. Connect each of the eight points to the point across from it by tying off the thread. This will temporarily hold the fabric while it is sewn on.
Take more thread and stitch around the edge. Roll up the cloth a bit to give it something to hold onto (grill cloth is a loose weave). Only stitch the cloth on the back side of the head, _not the front_. This stitch is similar to the way you would make the webbing of a "dream catcher." NativeTech.org has better instruction on the technique. For each stitch, instead of going around a ring of vine or metal, you stitch through the grill cloth. This thread won't really take any tension on the cloth until you get half way around the circle. When you make it once around, overlap a bit. Go back to the beginning, and tighten the stitches by pulling the thread with your fingers. You should be able to stretch the cloth a little and get it nice and even on the front side.
Then stitch a second round. It is fine to tie off the first round and use a second piece of thread. This time stitch in between your previous stitches, stitching around both the cloth and the thread of the first round. Tighten the second round, and tie off your thread.
Check your work. The front face should be nice and even, and the cloth should overlap the back side of the metal ring. When all is good, you can cut and remove the temporary thread on the crossing over the middle.
Repeat this step for the second head.
Step 9: Finishing Touches.
Put the grill cloth heads back on the drum with the rims and T-rods. (That drum key may come in handy again.) There is no need to get them super tight, they are just holding grill cloth. Just snug them up until they are firm. They will not be as tight as a real drum. If its too tight, you may snap the remaining plastic ring of the head.
Label your connections and controls before you forget what they do when you go to use it. It may also be handy to label which end is the front, and which is the back.
Stand back and admire your handy work.
Things to do with your speaker microphone DIMic a kick drum
Mic your bass guitar rig
Use as a speaker with your LM386 cracker box amplifier
Podcast in your warmest "radio theatre" voice
Use as a DI with bass for a different sound
SamplesHere are some quick samples of a kick drum. There is emphasis in the low end, and some resonance. It is usually best to apply a low pass filter to the SPKR MiK, and mix it with a normal microphone.
spkrmic.mp3 - The SPKR MiC by its self. There are no filters or processing, other than editing.
compare-spkrL-b52aL.mp3 - A split track: the SPKR MiK on the left channel, a SHURE Beta 52a kick mic on the right channel. Both are unprocessed, other than editing.
mix-in.mp3- The first half is a SHURE Beta 52A by its self, the second half is the Beta 52A mixed with the SPKR MiC.- in this sample the SPKR MiC has an EQ - a high roll-off starting about 400Hz, passing nothing above 2kHz.