DIY Mic Mods & Builds LIST OF “MICS”

A JFET Take on the CCDA Dual Triode Mic Circuit

Wed, 12 Nov 2025 21:57:09 -0600

While working on the Weird Audio “Little Red Monster” tube mic, it occurred to me that the same idea of a common cathode input stage direct coupled to a cathode follower output stage with a transformer is easily done in solid state. Take a more or less standard KM-84 style circuit and tack on a source follower between the FET and the output capacitor, and voila! Only two additional parts, an FET, and a resistor.

There’s no change in voltage gain, but greater available current and a lower driving impedance make the transformer’s job easier. The circuit has greater control over the transformer’s frequency and transient response. To test the idea, I modified my KM-84 LDC test mic, described HERE.

USB Mic Teardown

Sun, 30 Jun 2024 21:46:43 -0500

I bought this microphone because it was cheap and it looked like it might make a good donor body for a mic with a couple of switches on the front panel. But before ripping it apart, let’s see if it works, what the circuit looks like, and whether there’s anything mod-able.

In the box is the mic, a USB A to B cable, and a manual which reads like it was translated to English by Google. Notably missing is any sort of stand, shock mount, or mounting clip. You’ll have to supply an AKG shock mount or a universal type. The dark blue metallic paint looks nice.

Plugging the mic into my Mac laptop, a blue LED comes on inside the mic, and it appears as a mono input source and a stereo output in the Audio MIDI Setup control panel. It works at 16 bits and 48KHz only. “CD Quality” in other words. Pressing the knob mutes the mic, and the LED changes to red. The LED also changes color if you mute and unmute the mic from an audio app or the computer keyboard.

The mic requires a USB host, a computer, iPad, or smart phone to work. Just giving it power from a USB power bank doesn’t work. No sound even in headphones plugged directly into the mic. When connected to a host, the built-in headphone amplifier has a direct connection without latency to the mic at all times, mixed with audio from the computer. Monitoring from the computer can produce weird echoes due to driver and app latency, and is best avoided. Fortunately, that’s the default in most apps.

Voice pickup is crisp and easy to understand. There is an effective foam pop filter inside the grille. The pattern is directional, and noise pickup from the rear is minimal. As supplied, it’s a better mic than the one built into most computers for Zoom conferences and the like because you can place it where it sounds best, close to your mouth, assuming you have a suitable stand.

I was afraid it might sound like some of the BM800 mics with cheap USB sound cards. It doesn’t. Digital noise is well filtered, there is a proper headphone driver circuit, and a low noise op-amp mic preamp built in. It does hum when your hand comes near the volume control or plastic front panel if your body isn’t grounded to the mic or computer. So let’s look inside.

OPA Alice Measurements

Mon, 15 Apr 2024 22:14:25 -0500

How do the op-amp Alice mics measure? In short, they are the current standard for low harmonic and IM distortion. These modern TI audio ICs have gotten extremely good. Quiet, with vanishingly low distortion, they can be expected to outperform most preamps and test gear. Still, it’s a good idea to run tests to make sure there are no artifacts or printed circuit anomalies which impact the sound.

Homero Leal sent me some OPA Alice PCBs to check out. The electret version is the same as the JLI circuit. The next has a Hartley LC oscillator 60V polarizing DC-DC converter on board for “true condenser” capsules. The third has the DC-DC and a Sallen-Key 2nd order HPF at 40 Hz.

Tests are difficult because the OPA mics have distortion and response that rivals my MOTU M4 mic preamp, which is the cleanest I have. The setup is M4 and Mac Mini as signal gen & analyzer, running FuzzMeasure 3.3 or 4.0 or FEAT. The M4’s output is DC coupled. Mic pre is 1dB down @ 10 Hz, 0.25dB down @ 40K, so audibly flat 20-20K. The graphs are compensated for the preamp’s response.

Op-Amp Mics

Mon, 15 Apr 2024 11:10:16 -0500

The idea of op-amps as mic impedance converters isn’t new. A number of popular mic circuits are op-amps made of discrete transistors, such as the AKG C-414 and mics from Røde and others. What has changed recently is the availability of integrated circuit op-amps with low enough power consumption for phantom power, high enough input impedance for condenser capsules, and low enough noise and distortion to use in microphones. In fact, because of the large amount of negative feedback employed in op-amp designs, distortion is at least an order of magnitude less than a typical Schoeps or JFET & transformer circuit.

For an introduction to the OPA Alice mics, check out Jules Ryckebusch’s ‘Sound Sleuth’ videos on YouTube:
https://www.youtube.com/watch?v=uuTaU78Td2Y
https://www.youtube.com/@SoundSleuth/videos
There is also a lot of discussion on the micbuilders forum.

If you are interested in building an op-amp mic, pre-assembled circuit boards are available from JLI. Considering that many of the components are only available as surface mount packages, this is much handier than getting your own printed circuits made and soldering tiny parts by hand. See:
https://www.jlielectronics.com/diy-accessories/?page=1

Homero Leal has been experimenting with these new low power FET input op-amps from TI and has laid out several printed circuit boards also. He sent me samples of three different boards to run tests on. One is a basic board for an electret, like the JLI single OPA Alice board. A second board adds a transistor oscillator and voltage doubler to polarize a ‘true condenser’ capsule. A third PCB is based on a tube emulator circuit using a JFET and op-amps.

Red Monster Wrap Up

Sat, 17 Feb 2024 18:44:52 -0600

The original plan for the Monster was to swap the OEM capsule for a Mic-Parts RK-12 and go for a C-12 sound profile. After electronic mods and some testing and loaning the mic to friends for their opinions, we found we like the sound of Alctron’s ’67 capsule, at least this particular one. It’s on the airy, bright side of neutral, but the high frequency peak is broader and not as tall as most Chinese ‘67s. It can be EQ’d if need be since it is a curve rather than a spike.

I did try an RK-12, but the tube circuit didn’t do anything super special for it, so the original capsule was reinstalled and measurements were made to confirm our impressions from listening.