Amplifying Acoustic Guitars with Inexpensive Condenser Microphones (C) 1994 Paul Marxhausen - all rights reserved [This article has been produced for free distribution to individual readers on Internet newsgroups. Under no circumstances may it be reprinted in a commercial or non-profit publication of any kind without the written approval of the author.] Why the article... ------------------ This is an article I promised readers of rec.music.makers.guitar.acoustic, with full details on wiring up an electret condenser microphone element for amplifying acoustic guitars. It's based on the motley grab bag of information at my disposal and my own experiments, but I think it sounds pretty nice and is a very economical way to amp up acoustic instruments without making major modifications to your instrument. I'm going to err on the side of providing more details than you might actually require to get it built, so wade through this and ignore any extra stuff you already know, or don't care about. If I have outstanding results with this mike I've considered building some to sell, but there's absolutely nothing here that is in any way patentable - it's all been done before. BE SURE TO READ THE UPDATE NOTES AT THE END OF THE ARTICLE, because there significant changes and suggestions there. Who can build it ---------------- This is a simple project, electronically speaking. Most of the skill comes in making a bunch of connectors and in mounting the mike. Anybody who can solder a decent connection should have no trouble at all. However, the mike elements are very small, with two little tabs to tack-solder to: if you are just getting started in soldering you could incinerate the mike (worst case) or glob the tabs together (easy to do). If you're a bit shaky with soldering or only have a big 100 watt iron or something, have someone who's better equipped or trained attach the cable to the mike. That's all I'll say on that since I don't want to spend time writing up a basic soldering lesson here. What you need... ---------------- Here's the parts list. Some items depend upon how you want to implement your microphone; the ones you've got to have, period, are marked with a "*" in this list. Further details about each item are found in the text of the article. * Electret condenser microphone element * Skinny coaxial microphone cable wire * Phone plugs and jacks * Small electronics chassis / project box (able to hold chosen battery) * 15K 1/4 watt resistor (see text) * 10 ufd capacitor (see text) * Battery, battery connector (9 volt recommended) Mounting items - doublestick carpet or foam tape, rigid insulation foam, open-celled foam, plexiglass, duct tape, 12 penny nails, drywall screws, plastic tape, poster putty, etc... Let me comment on the minor items first: you can get everything but the mike element at almost any Radio Shack or other electronics supplier. In particular, the Shack has a number of small plastic project boxes to choose from, including a "Transmitter Project Case" that has a handy 9-volt battery compartment. Also, they sell some nice, thin (1/8") gray microphone wire that's strong and flexible, just right for this project. What size of phone plugs and jacks to use is up to you - I used the smallest (3/32") for everything except connections from the electronics box to the amplifier, where I wanted to be able to use regular guitar cords. The key item you need is an electret condenser microphone element. For the technically obsessed, this kind of mike is using what is actually a permanently charged capacitor to pick up sound. Although the charge is permanent, this kind of condenser still requires power because it also contains a Field Effect Transistor on its output for built-in amplifying and impedance matching. None of this is important except to note that we have to add a bit more fiddly stuff to power the mike...especially because they run power for the element on the _same wire_ as the _output signal_. Another noteworthy aspect of these mike capsules is that they are _omni- directional_, meaning they pick up sound from all sides equally well, not just from in front of them. In addition, they do NOT exhibit the "proximity effect" of many microphones, where sound becomes fuller, deeper, and considerably louder when the source is very close to a mike (as in moving up next to a vocal mike.) These attributes can help or hurt, depending on the application. It makes the location of these mikes less critical but obviously can mean that they may pick up sounds you don't want to pick up, and may contribute to feedback in amplified situations. (A drummer in an old band of mine wired one up as a headset mike: it also picked up his entire drum set very nicely!) Even so, I found these are not overwhelming problems. Where to Get Them... -------------------- Now, these kinds of elements are as common as mud: almost any tape recorder you can lay your hands on with an internal mike has one. But the garden variety ones have some shortcomings: a peaky response curve, for one, and they roll off at about 10 kHz - 12 kHz. For a bit more money, you can get ones that go the whole 20 - 20 kHz range, and are flatter in their response. I buy mine from: Digi-Key Corporation 701 Brooks Avenue South P.O. Box 677 Thief River Falls, MN 56701-0677 Ordering Phone Number: 1-800-344-4539 FAX: 218-681-3380 You want either: Digi-Key Part #9932 (Panasonic #WM-063T), $2.57 in singles -64dB+-3dB sensitivity, >38dB S/N, 6 x 5 mm, .5 mA max. current OR Digi-Key Part #9959 (Panasonic #WM-60AY), $2.04 in singles -62dB+-3dB sensitivity, >38dB S/N, 6 x 5 mm, .5 mA max. current Just a couple dB difference in response, and a few cents in price - either will be fine. I don't think Digi-Key has a minimum order. Not the cheapest place to get stuff, but they have a lot of parts, and are easier to work with than some O.E.M. type suppliers. Easiest to order from them if you've got a charge card. How to Wire It Up... -------------------- OK, what's this weirdness with the power and hooking it up? Here's a wonderful ASCII-art schematic that gives the gist of what needs to happen to make these things happy. Like many other electrets, the case and one of the two electrodes is ground for both the signal and the power, and the other lead is the "hot" or positive for both the battery and the signal. Whoa, how do we avoid the battery shorting out the signal, since a battery is a short circuit for AC? Like thusly: |+V DC supply / \ 2.2K resistor (use 15K with 9 volt battery) mike / _____ \ || | |-------------+------||--------> Output signal | | || .1 ufd cap (suggest using 10 ufd cap) | |---------------+ ----- | (Ground tab is ----- Ground common to case) --- - Pretty simple, huh? The schematic in the catalog shows a .001 ufd capacitor and 2.2K ohm resistor, when it's driving right into an IC amplifier. I've used a 10K resistor before, and for good low end response I figure the capacitor ought to be at least as big as .1 ufd, even bigger if you're going to drive a low impedance input and not a high impedance input like a buffer amp or guitar amp input. Otherwise the component values aren't too critical, and the voltage supply can go from about 2 volts to 10 volts. (Even down to 1.5 volts with a resulting 3 dB loss in sensitivity.) I use a 9 volt battery, both because it's easy to get and because you get more "headroom" for transients that way. Now, the schematic shows all this stuff wired together in one spot, and you _could_ put it all together in one enclosure. Some lavalier mikes do just this, with little button cell batteries. However, in my case I wanted the mike as small and light as possible, so I carefully soldered my skinny microphone cable directly to the mike element, and located all the other components off in the project box. Here's one place you can make your own choices: you could solder a 25 foot cable with a plug right to the mike, and have the junction box off on your amp or effects pedals. But then you have to either make the mike removable each time or you have a cable permanently gaffed to your guitar. My own approach was to put a 12" wire on the mike and terminate it in a 3/32" phone jack, one that has a square plastic case on it. This can be fastened just inside the soundhole with doublestick tape. Then, I made a patch cord out of the same skinny mike cord, with 3/32" plugs on each end, to go from there to the junction box. My plan is to have the junction box located on my guitar strap near the neck strap button, so this is just a short patch cord. I then put a full sized 1/4" phone jack on it so I can use a standard guitar cord to plug from my junction box to the amplifier. mike capsule O short cable | | phone jack ^ phone plug ^ short cable | | phone plug v phone jack v junction box +----+ | | (contains battery, resistor, capacitor, any other +----+ additional electronics) 1/4" phone jack ^ | guitar cord w/ | 1/4" phone plugs | v amplifier v +-----+ This way I can mount my mike and it's jack all inside the guitar and unhook the rest when it's not in use. Now that I think about it, I may make the patch cord 20' long, so I can leave the junction box sitting on my amp instead, since there are no active electronics to fiddle with in the junction box just yet. This mike puts out plenty of signal with no preamplification necessary. It's low impedance. How to Mount It, Take One ------------------------- At this point, you're all done electrically, and mounting it becomes a creative process where you can follow your own whim. You could lay the thing on it's side on top or inside your guitar and just slap a chunk of tape on it; or maybe dangle it down in the sound hole with a bent paper clip to hold the cord to the rim....and quite possibly end up with as good or better results than the involved rigmarole I outline below. I did what I did because I was looking for 1) a PZM-type mounting close to a boundary on the body and 2) a semi-permanent internal mounting. So take my two mounting suggestions with a grain of salt, and use your own good ideas, par- ticularly if you're miking a different acoustic instrument. Just remember that it's an omnidirectional mike and depending upon where you mount it, you may end up picking up other sounds you don't want. How to Mount It, Take Two ------------------------- I wanted to mount this mike much like a Pressure Zone Microphone element, which is to say pointing down into a flat surface at a separation of a millimeter or less. So the first one I built used a piece of 1/4" plexiglas about 1/2" wide by 1.5" long, with a hole in it for the mike. Side view: +--+--================================== microphone cord ---^ | | | | 1/4" gap -> +--+---+-----+____________+ v mike I machined a gap on the bottom side where the mike faces down, but I could have just made the mike flush with the bottom and then shimmed the little bar up with something to provide a gap. This is essentially the assembly I see on Crown PZM mikes, stuck on a big aluminum plate. I stuck it to the inside of my guitar with foam double-stick tape, (more on mounting locations in a moment), but when I hit certain loud, deep notes I believe the whole assembly vibrated and made an unpleasant crunching sound. Better mounting might eliminate this. I also tested this assembly with chunks of plexiglas as a PZM recording mike (just stick it in the middle of a big square of plexi) and it works very nicely in that application. The 3/32" jack on the end of the cable from the mike is worth mentioning: I got one that has a square plastic case and used doublestick foam tape to stick it just inside the soundhole near the neck, about at 10 or 11 o'clock relative to the neck at 12 o'clock. At this location there's a big, heavy structural member right behind it to brace against it when plugging in the external cable, and this external plug can go in parallel to the strings, out of the way. I then double back with this cable and intend to run it alongside the neck to the strap or strap button, where I'll keep my electronics box. Just to confuse the issue, here's a baffling ASCII art representation of this. "H" is location of jack. \ : inside | --------- :< brace |=D =========================-+-----+ rigid foam -> | | | | +--------------------+--+-----+ <- doublestick tape soft opencell foam -> +-----------------------------+ <-+ So we have a fairly solid assembly, and the thin foam gives us a good close spacing to a surface. The whole thing is very light and shouldn't contribute any coloring to the sound. Where to Mount It ------------------- This is the BIG ITEM. I haven't exhaustively researched this, but I did spend an hour or two fiddling around with the mike plugged into my stereo system, listening to the sound both live and also listening to playback of it recorded on my stereo cassette recorder right off the system. I tried five locations before I quit. The "o'clock" references use the neck as "12 o'clock" on the soundhole, and all but the last are on the INSIDE surfaces of the guitar body. 1) AGAINST THE BACK OF THE GUITAR UNDERNEATH THE SOUNDHOLE. Not too bad. Pretty good bottom end, but I think my Alvarez has a reso- nant peak that coincides with the bass A (110 Hz) that seems a bit louder, but even so, not too boomy. But there's a somewhat boxy sound, which I found was true to varying extents almost _anywhere_ inside the guitar. 2) 2 INCHES FROM SOUNDHOLE AT 4 O'CLOCK, NEAR TREBLE END OF BRIDGE. Sort of under where the pickguard is....this is a very crisp location. A newsgroup reader said this was a location recommended by an internal mike manufacturer. However...here, and other places on the top, I found there was kind of a peaky response, resembling to my ears the sound of piezo pickups. I think this is the result of hearing mostly the wooden top sound near the bridge. Worth trying, lots of signal and highs, but I was trying to get away from the piezo sound, and there was a bit of "boxiness" here too. 3) 1/2" FROM SOUNDHOLE AT 2 O'CLOCK. This is up against a cross brace that connects to the neck, near where I stuck the jack. At this location, A) the top shouldn't be moving much, and B) I put the element near the very edge of the soundhole, looking to pick up the soundhole's sound, which I hoped would be less boxy and more balanced. It was. I think there was a slight diminution of bass, and it's not the "perfect" sound - still a touch of boxiness, a hard-to-describe midrangy sound. But I thought it was less colored than any of the other locations. 4) 1" FROM SOUNDHOLE AT 7-8 O'CLOCK - hoping for more bass. Got it, but not a very balanced sound. Somewhat like location #2, but more "mud". 5) ON THE OUTSIDE OF THE GUITAR NEAR THE BRIDGE - not so great. Not very much bass. So, at the moment location #3 is the winner, which is handy because it's very easy to get to - you don't even have to loosen your strings to mount the thing, just slip it under the rim of the hole. It occurred to me that since this is essentially the same place as I stuck the jack, only across the neck from it, I could probably make a mike mounting that had the jack built in to it and just have one thing to stick under there. Location #2 has a lot going for it too. A note about feedback and unwanted pickup: When taping my tests, I started each tape segment by putting my mouth close to the soundhole and announcing the location under test in a clear, loud voice. On playback, this voice sound was distant and had far less level than what the guitar was kicking out when I was playing it. In addition, I had the stereo up as much as I dared without waking up my 6 month old son upstairs, and I got no squeals or howls, (from feedback, I mean) even with location 3 where the mike is almost outside the guitar. So this is a definite plus to this setup - two of it's potential problems are controllable. On the down side: I guess I expected to get the total, balanced, uncolored sound of my Alvarez with this. I didn't get it: there's still a particular kind of tonal coloration associated with this. But, far less so, in my opinion, than with my cheap piezo pickup. (And now I realize part of the piezo's sound derives from being located on the bridge and picking up _only_ the sound of the top/bridge, as noted in location #2.) And better than a magnetic, too. I'm anxious to try this thing out with my church's good Peavey KB100 keyboard amp, and with my chorusing pedal. I fully intend to use this in concert in a couple of months. Also, hope to try one in a classical guitar soon. More ideas? ----------- Except for the basics of wiring this, there's a lot of room for experimenta- tion along the way with these little mikes. I'd very much like to hear back from people who try using these, or who have used small mikes in a similar fashion. 2 January 1994 Paul Marxhausen University of Nebraska-Lincoln Engineering Electronics Shop (402) 472-1969 FAX (402) 472-4732 Internet mpaul@unlinfo.unl.edu ADDENDUM, 31 January 1994 I could re-edit a lot in this article but I'm trying to sell a similar article to a couple of print magazines, so I'm going to leave it as it stands..but I did want to add a couple comments based on further experiments. Construction: the unit I'm actually using on my guitar now is pretty simple. I got a piece of narrow, flat wooden trim about 1/4" thick by 3/4" wide by 1.25" long, drilled a 1/4" hole in it, and cut a groove from the hole to one end for the cable. I stuck a mike in so it protrudes just a bit from the other side than the cable. This facing side is then mounted facing the surface with a piece of foam doublestick tape, which is thick enough to provide a gap and keep the mike from actually touching the surface. In addition, this hole apparatus is low-profile enough that it can go where I eventually mounted it, my SECRET SPOT..... Mounting: All the inside locations mentioned above had a certain amount of boxiness or resonance. I played around with external locations until I found that the "perfect" place was between the A and D strings, between the sound hole and bridge, about 1.5" from the hole (right over the last ring of purfling.) At this point, there appears to be a good overall balance of tone, and it's juuuussst out of the way of my thumb and fingers when I play - I use a fingerpicking style with heavy rhythmic damping, and a lot of locations that were close by that sounded good were in my way one way or another. Now, this is the perfect place to get high frequency feed- back if you're running a big PA or hot monitors, so you have to be careful, but I'm real happy with the tone. Availability: I've been told that the DigiKey Cat #9932 is no longer available from DigiKey. Makes your decision simple, doesn't it? Miscellaneous: I've tried using the PZM configuration for recording now, experimentally. Very crisp. Very good sound. Have been told by other electret users that these elements can handle SPL levels in excess of 130 dB, and are used in places like inside kick drums on trap kits!!!! Wow. - Paul Mx More addendum, 17 February 1994 The values of the resistor and cap are different than when I first tried this project: if you are going to use the 9 volt battery to get good headroom, you really need to use a 10k-15k resistor to bias these mikes right. Since this raises their output impedance I recommend increasing the DC blocking capacitor to 10 ufd as well, with the positive terminal connected to the mike output. Also: have had pointed out to me that Digi-Key sells their cat.#9933, which is a Panasonic WM-62A, and which is even thinner than what I suggested: only 2.5 mm deep, or roughly 1/8"!! This should be even better for the kind of mountings this article suggests. The frequency response is supposed to be just as flat but "only" runs from 50 - 16kHz. Since a guitar doesn't go below 80 Hz or so and I've found these mikes and these mountings have treble to burn -- I'm always turning down the highs on the amp when I use them -- these models should work just fine. Cost a bit more, $4.24 each, but I'm going to buy a couple to try out. Also have seen that Mouser Electronics carries electret mikes, not panasonics but a couple of them look like they'd work OK. ADDENDUM, 11 November 1994 I can see, as I post this on our server, that I left out one item of interest: how'd it work in concert? I played a one hour solo concert from the front of a church using this mike into a Peavey KB100 keyboard amp. It sounded very nice: not the "perfect" acoustic tone, but very usable, and I played at a pretty fair volume level and used a flanger/choruser on some stuff, all without any feedback to speak of. -Paul 25 July 1995 - A reader of this article did some more experiments with classical guitar miking, with interesting results, so I add his messages here . . . From: drobin@bigtex.ci.dallas.tx.us Comments: Authenticated sender is To: mpaul@unlinfo.unl.edu (Paul Marxhausen) Date: Mon, 24 Jul 1995 17:31:37 +0000 Subject: electret mic After some experimentation here are my thoughts..... You were right,,,,, a 3 volt lithium is not enough voltage to drive without some minor distortion. I went to the Pre-amp version which made it easy to use a 9 volt battery. The results are spectacular. The mic is very faithful to the guitar. I tried both the pc mount 2 conductor, and 3 conductors they sell at radio shack, but ended up using a 3 connector version. The extra shield actually improves the signal. I have found out that it is hard to get thin enough wire at local stores, so, I bought the cheapest set of headphones I could get( made by Labtec and sold at Circuit City for 70 cents a pair) and sacrificed them to obtain the micro-shielded wires, which already have a 3.5 mm plug. I attached the mic to the cables by using the two conductors for leads, and braiding toghether the shields to make one shield common. At the pre-amp, one 3.5 mm STEREO jack, and one 1/4 mono foroutput to the amplifier. The connections were just as you had experienced, a 10 K resistor to the DC, and a 10 micro farad capacitor on the signal cable. Smaller values first published don't work as good. The advantage of using such microthin cable is that I dont have to worry about attaching the mic inside the guitar, which is always in the mind of a classical player... I just bend the tip with the mic and suspend by the soundhole. I have experienced no feedbacks, and perfect fidelity to the real sound of my guitar. However, this is possible because classical technique does not allow as much guitar movement while playing. However, folk guitarrists or other may have a problem if the move too much while playing. If that is the case, they would have to fix the mic inside the guitar. oh, if you want to get a little fancier, you can add a small led and a microswitch so that you can turn the guitar on and off at the seat instead of having to walk to the amp location. .... I tried this also and it works well. However, you must make sure to use a very small led so that you dont pull down the available current from the battery. I cannot thank you enough, and hope that as you improve upon the mic, you will continue to share with us... David Robinson, "The Sensei" To: mpaul@unlinfo.unl.edu Date: Tue, 25 Jul 1995 10:00:57 +0000 Subject: electret again.... these are the materials I used from the shack part # 272-999 10uf @ 50 V capacitor $ 0.99 270-092 Electret Mic Element $ 2.99 271-1335 pk(5) 10K resistors $ 0.49 274-249 3 conductor 3.5 mm jacks $ 1.59 274-252 2 conductor 1/4-in jacks $ 1.99 270-220 project box (4x2x7/8) $ 1.99 9 volt battery $ 2.19 9 volt terminals $ 1.00 Labtec innexpensive headphones $ 0.70 patching wires for the connections...... free from just about anywhere at the most, less than 15 dollars!!..... what a bargain!! PIEZOS Labtec phones $0.70 273-064 Piezzo Buzzer $1.79 3.5 mm phone jack $1.5 1/4 female to 1/8 male connector $1.79 1-inch of double sided tape was donated at the office.... By the way, I figured out a way to attach inside the guitar, use a wooden dowel sawed to a wafer of approx1/2 inch... Drill through with 3/8ths bit. Sande smoothly. cut groove/drill on one side for recessing of the cable... oncefitted, use hot glue to attach mic at desired depth. cover top with male velcro. attach female to the under side of guitar soundboard, bending the headphone wire in a hairpin manner... Voila, cest la meme chausse !!! the spot inside in my classical really does not matter as long as its close to the soundhole edge. the deeper in to the body of the guitar, the more echo/boxy/bassy sound I get. however, i really don't know what could happen with metal strings..... If a piezo, its harder but you have to route the inside of the dowel as to create a cave that will hold the wires inside the wood piece, with a notch for the cable. Then you glue from the inside the piezo so that its flat, and covered by the wood. Then the fun part.... sand it very well, ( oh yeah, you only need a slice of dowel of approximately 1/4 inch thinckness, and the same diametyer as the piezzo.) and with the hot iron brand it with your signature, this will make it officially your creation. varnish, its a work of art.... it CAN look so good that you may opt for attachment near the bridge outside the guitar...... thanks... Sensei From: drobin@bigtex.ci.dallas.tx.us To: mpaul@unlinfo.unl.edu (Paul Marxhausen) Date: Tue, 25 Jul 1995 10:04:36 +0000 Subject: P.s. electret regarding the prior message, if its a piezo, instead of a dowel, use a piece of hardwood, cut with the saws used to drill door holes, commonly available at home depot. then you sand it to the right size... can't use dowels because the grain of the wood would be perpendicular to the top, and would never look good! I would never attempt to do anything on a guitar to make it look tacky. Its not my style.... sensei ===================================================== 11/3/2000 More follow-up from Paul in response to an e-mail: >Your 1994 article on guitar microphones was most helpful and, as a matter of fact, will be using my prototype (radio shack $4.00) tomorrow to play in front of 200 kids in a local church. > >The reason I am writing is to ask you if you have published further articles on your microphone experiments. There might be something I am missing. >Also, I am interested in making my own pre-amp/equalizer for a magnetic pickup. The soundhole pickup sounds good with the microphone and rounds out the sound but I could use more bass. I did do some editing on the earlier article with thoughts of publishing in print but never did so. Up until very recently, I have been using one of these original mikes every Sunday in church. My "ultimate" position for the mike was to stick it on the top, just barely peeking over the edge of the soundhole at about the 10 o'clock position, facing down into the hole. (Held in place with poster putty.) I put my battery box on the side of my guitar, also with poster putty, and added a "kill" switch to silence the mike between songs or in case of runaway feedback. This sounded really nice but the putty business got old, and when they gave me some church accompanist stipend money, I purchased and had installed a Trinity pickup system, ( http://www.gollihur.com ) which combines internal piezo sensors and an internal condensor mike, with an end-pin jack and external preamp. Actually, the piezo part of this product is not terribly impressive to me, but the mike is good, and it's all nicely tucked away inside. But I'll tell you: the tone is not radically better than the home-built mike, and I rarely even dial in any of the piezo. So I'm still fond of the homemade approach. Always meant to build a preamp but haven't yet. There are a number of guitar circuits archived at: file://nyquist.ee.ualberta.ca/pub/cookbook/audio/guitar/ esp. file://nyquist.ee.ualberta.ca/pub/cookbook/audio/guitar/preamps.gif Hope this helps!