Introduction: Modify a Cheap LDC Condenser Microphone
Second Prize in the
DIY Audio and Music Contest
I have been an audio guy for a long time and an avid DIY'er. Which means my favorite kinds of projects relate to Audio. I am also a firm believer that for a DIY project to be cool there has to be one of two outcomes to make the project worth doing. It either has to be something you can't get commercially, or something you can build your self that is way cheaper than buying what is available commercially. This project is of the second kind. Build a cheap but good LDC microphone. LDC stands for “Large Diaphragm Condenser”. This project can be built for about $50 in parts and rivals microphones costing way more. It is quiet, sounds very neutral, and will handle large SPL (Sound Pressure Levels).
First a little history of microphones.
There are three basic types in use for studio and live sound use; dynamic microphones, ribbon microphones, and condenser microphones. A dynamic microphone is like a speaker but in reverse. A small diaphragm is coupled to a coil of wire that moves when sound hits diaphragm. The coil is in a magnetic field. When it moves a small electrical signal is generated that you can then amplify or record that represents the sound. A ribbon microphone is similar except the ribbon, a thin strip of foil, usually aluminum, is placed in a magnetic field. Sound waves cause the ribbon to move in the field and an electrical signal is generated. Read more here: Microphones
A condenser microphone starts with a very thin membrane that has metal sputtered onto it so it conducts electricity. The membrane is stretched and placed very close to a backplate to form a capacitor. Grandpa Ryckebusch used to call capacitors condensers and now you know that we should really call them capacitor microphones... When sound waves hit the diaphragm and it moves, the capacitance changes. If there is a charge on the capacitor, there will be a change in voltage that corresponds to the sound. Like the other two microphone designs above, if you amplify or record the voltage, you get the sound. There are two styles of condenser microphones. Some use a high voltage (50-70 volts) to charge the condenser capsule and others use what is called an Electret Capsule. The Electret (Electrostatic) has a permanent charge associated with it read here: Electret.
What this means for us is that if we use an Electret capsule there is no need to apply 50-60 volts to it, which means simpler circuitry.
one of the benefits of a condenser microphone is that the diaphragm can be very light and it is easier to get a smoother frequency response with one. The downside is that you have be very careful when getting the signal off of the diaphragm without adding noise which brings us to the electronics.
To pull the signal off of the capsule you need a very high impedance device. Tubes have this one covered and were the main way this was accomplished 40 years ago. Not to get into a debate on sonic quality of tubes vs anything else, you have to admit; using a tube inside a microphone body does not lend itself to simplicity. Or normal DIY skills! After the tube the Field Effect Transistor or FET was invented. This is how most condenser microphones work today. Even the really inexpensive mic capsules have one internally mounted. A German company Schoeps. arguably one of the top microphone manufacturers in the world, designed a circuit for condensers microphones that defined how this was done a long time ago. See the Schoeps Circuit for details. (If you google “Schoeps circuit” this is what you find!) The circuit runs off of phantom power from the mic pre-amp. Part of this circuit is used to generate a stable high voltage to charge the capsule. In our case we wont need that. The DIY community simplified this circuit down to its basic form for electret capsules that is almost identical to the original Schoeps Circuit. Scott Helmke designed a version of this circuit for his “Alice” microphone. I am using the same circuit with slightly different values and a different FET transistor. I chose the J305 which is used by several of the high end manufactures. I located it here. You can certainly use the parts list from Scott. His latest list is from 2013 and the parts are available from both Mouser and Digikey. I built the circuit on a small perfboard which is perfect for fitting inside the microphone body.
Here is how the circuit works; let's look at the signal path then the power:
The 1Gig (Yes one gigohm...) resistor develops the signal coming off of the capsule. The FET and the two 2.43K resistors form a phase splitter and impedance converter. The two .47uF capacitors couple the signals to the two bipolar transistors. These are PNP transistors setup as emitter followers. The two 100K resistors bias the transistors. Uber simple. If you are wondering about the 1gig resistor, it is key to a condenser microphone. It is also the most expensive component, coming in at around $2 each from Digikey. On the powering side, we connect the microphone to phantom power form a mixer or preamp. That brings 48 volts into pins 2 and 3 of the XLR connector and the two transistors. UPDATE October 2015: I added two 22nF capacitors at the XLR jacks and two 49Ohm 1% resistors on the inputs to the transistors for RF noise suppression. I didn't realize this until I used a different mic preamp when in a "noisy" environment. Schematic Updated! The 6.8K resistor and the zener diode take that and drop it to 12 volts. The 10uF and 68uf capacitors along with the 330Ohm resistor filter this and provide a stable voltage to the FET circuitry. Once again, very simple and elegant. The critical component and one we haven't talked about yet is the capsule itself. I am using the TSB2555B from JLI electronics. it is a Transound capsule and is what makes this project what it is. It costs $12.95 and uses nickel instead of gold on the diaphragm. It is also used commercially in at least one microphone I know of, the CAD e100s.
Now that we have the capsule and electronics all set, you could actually build one of these into whatever housing you want. I have tried this and learned a couple things. Because of the high impedance of the capsule and the FET electronics, the wire between the two acts like an antenna and unless the whole thing is fully shielded by metal or metal screen, you will have all sorts of noise.Both 60hz hum and white noise from all the RF leaking into it. In essence you need to put the capsule and electronics inside a Faraday cage.
I found an easier way than building my own. It turns out that there are several Chinese manufactured really cheap mics that actually have great metal cases somewhat decent electronics (very similar circuit...) and a small capsule. And the cost about $20 bucks. They make a great donor body, which is what we are using it for. Search for them on eBay by searching for “BM700” and “BM800” microphones. I got mine for about $22. Interestingly as you can see form the pictures it doesn't say BM800 on it. It also came in a paper mailer with the foam casing but no box. OK, now that we have covered the background, lets build one!
Edit: 9 October: Here is some audio with these recording my kids high school orchestra: Guyer HS Intermezzo Orchestra
Step 1: Step One: the Electronics
The electronics section is easily built on some perf board. I cut mine to 1” by about 1.5” then populated it from the PNP transistors working towards the FET end. The critical part here is the junction of the FET Gate and the 1gig resistor. Notice I am “floating” the leads. This is where the FET gate to capsule wire connects. We don't want that touching anything or using the circuit board that my have flux residue or attract moisture in a high humidity environment. Also look at the positioning of the FET. See the data sheet in the article. I had my pin 1 of the FET backwards until I realized the position mentioned in the data sheet was the top view of the transistor, not the bottom. If you use Scotts recommended FET, download the data sheet and read it! I left a spot to one side that lat me drill a hole large enough for the mounting screw to hold it to the chassis. I actually lucked out here... I built this before I thought through how I was going to mount it.
Step 2: Step Two: Disassemble the Original Microphone
Take the microphone body and unscrew the base. This will let you slide off the metal sleeve that covers the circuit area. Note: Your mic may vary. I bought tow of these from different vendors and they were similar but definitely different. After the sleeve is off take out the two little screws holding in the original circuit board. Then un solder the lower three wires. We will reuse these to attach the new board to the XLR connector. You can cut or unsolder the capsule wires. We will replace those.
Now remove the two screw holding the basket to the housing. The basket comes off and exposes the original capsule. This original is mounted in a bit of foam and pressed into the plastic capsule holder. Save the screws!
There are two screws that hold the plastic capsule holder to the metal frame. Remove those and separate the two. You now have a fully disassembled microphone.
Step 3: Step Three: Prepare and Install the New Capsule
I have built two of these and the capsule holders were both different. In this one you can carefully push out the old capsule and then remove the foam. The other one did not have the foam but little plastic side extensions every 90 degrees. I cut those out with little snips and then used a drop of hot glue to hold the new capsule in place. In this mic I cut a small piece of the foam and used it to press the new capsule it. Before doing this you will want to solder on short leads to go from the capsule to the electronics. I used some 24 gauge stranded wire I already had. You can reuse the original capsule wires if you like. I like teflon insulated wire. The insulation does not melt when accidentally touched by a soldering iron.
Step 4: Step Four: Reattach the Capsule Mount
Using the two small screws and reattach the capsule mount. There are four little holes but only two of them are threaded. This was the same on both of my microphones. Be carful to not where the tab on the base of the metal frame. The tab faces the direction of sound. It lines up with the metal sleeve that is printed with the microphone's name. Now this may vary! One of mine was not labeled at all. You can read the brand name on this one. Don't think it will become a household name any time soon. Once that is mounted feed the little wires for the capsule through the other holes in the metal frame.
Step 5: Step Five: Mount and Connect the Electronics, Then Reassemble
In my case I built my circuit board before I figured out how I was going to mount it. This necessitated drilling a hole in it with all the components already on it. Not the best way to do this. I had a couple small 4-40 angle brackets for mounting circuit boards in my project bin. Using one of those I mounted the circuit board to the metal frame. You could mount the baord directly as long as you don't create any shorts.
Once mounted connect the XLR connector per the schematic. Then connect the capsule. Take care on the main capsule positive lead as it connects to the junction of the 1gig ohm resistor and the gate lead of the FET. This floats in the air to ensure a very high impedance connection.
Slide the metal housing sleeve back in place. Note the tab and corresponding little cutout on the sleeve.
Screw on the threaded base and the microphone is complete.
Step 6: Testing, Use, and Further Exploration
Connect your new microphone to either a mixer or mic pre-amp with phantom power and ensure it is functioning. Most problems are due to mis-wiring. Hum or buzz is usually a ground wiring issue.
This microphone stands up there with most large diaphragm condensers. I own a couple really good ones and it delivers. Works great on vocals, acoustic guitar. I am working on getting a couple things recorded with it and will put links up in the Instructable when I do.
I am really thrilled with the performance of this mic. It is all from a $13 mic capsule (less if you buy ten...) I am 90% complete on a project with multiple capsules for recording stereo. That Instructable is coming shortly.
Update October 2015: I have had a chance to record an orchestra with these Soundcloud link. I also ran sound for volunteer Food Truck fest and had the fun of using these on stage with several talented vocalists and a Jazz Trio. Mic sounded great and very transparent.
For more information on DIY microphones in general I highly recommend the microphone builders group on Yahoo.
And if you want to build or modify a non electret microphone check out Microphone Parts. I have built a pair of mics using his CK-12 Capsule.
Step 7: Update January 2016! Pimp That Circuit!
After building few of these, studying the original Schoeps circuit and getting schooled a bit by some of the veterans on the mic builders group I came up with an improved circuit. I call it the “Pimped Alice” There are three main changes:
1. The addition of two more RF and EMI suppression capacitors. The two 470pF ones that tie the base of the two PNP transistors to ground. These help out with anything the FET picks up and limit the bandwidth of the PNP emitter followers.
2. The portion that provides 12V to the FET circuit is changed. We have the 47uF capacitor charging up from the phantom power coming into the mic from XLR pins 2&3 through the 49.9 ohm resistors and the two PNP transistors. The supplies a nice low impedance path for audio frequencies cleaning things up a bit. From there then we go to the 4.7K resistor to the zener diode. This resistor sets and limits the conduction current that the zener diode uses. Zener diodes can produce a small amount of electrical noise just due to how they work. The 330 resistor and 100uF capacitor filter that out and maintain a nice clean DC voltage for the FET and 2.4K resistor phase splitter.
3. The 1Meg pot is new. This adjusts the bias on the FET. This is probably the biggest improvement in the circuit. As the pot is adjusted we are trying to split the voltage that the zener produces so that about half is dropped across the FET and the other half split between the two 2.4K resistors. This is pretty easy to do. Before connecting the actual microphone capsule you need to connect the circuit to a microphone pre amp so we can power the circuit. Measure the voltage on the + pin of the 100uF capacitor referenced to ground. In my “as built” circuits I had about 11.5 to 11.8 volts. Measure the voltage and divide by four. Say the voltage is 12 VDC. Dividing by four gives us 3 VDC. While measuring at point “A” (see the circuit) adjust the pot until you get 3 VDC. Measure the voltage at point “B” you should have 9 VDC. The pot is a ten turn pot so get ready to rotate the little screw a few times. Historically people would do this and substitute fixed resistors for the values of the pot setting. While that might save a few cents, it is time consuming. Using a pot is much easier.
You can see my protoboard build front and back. The two arrows point to the PNP transistor collctors and are where you would connect the 49.9ohm resistors on way to the XLR connector. Once again the 22nF caps are located on the XLR connector.
Another really cool thing is a member
of the Mic Builder group on Yahoo built one of these using the “Pimped” version of the circuit and sent it to another member who tested the microphone. Read about that on Audioimprov here: Homero's Pimped Alice. Synopsis is the circuit is very low distortion and electronic noise is below what the capsule will put out in a quite room. Also, Homero designed a PC board for this and graciously provided all the docs for it. It is single sided and will fit into the Chinese knock of mics BM-700 and BM-800's
I now have four of these in my mic locker and am super happy with them. Closing thoughts on parts. The FET above is a substitute for the J305. Either will work. When buying resistors and capacitors the price drops significantly if you buy in quantity. I highly recommend buying the resistors a hundred at a time and the small capacitors the same. I usually go less for the larger electrolytic ones. If you continue with the wonderful hobby of electronics, you will find at some point you already have what you need to build the next project.
Thanks to Henry and Homero from the Mic Builder group on Yahoo! Talk about a great collaborative effort for the Builders, Makers and DIY'ers out there.
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