“OPA Alice” Op-amp Mics
Op-Amp Mics
Monday, April 15, 2024
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.
I hooked the electret board to a pair of 25mm capsules via a DP3T ON-OFF-ON switch to give three pickup patterns, Cardioid, Omni, and Fig-8 (see articles on 3-pattern electrets for how-to).
The two boards with polarizing oscillators were given SPST switches and 34mm K-12 double-side capsules, for Cardioid and Omnidirectional patterns.
Since theoretically a non-polar electrolytic capacitor exhibits less harmonic distortion than a polarized one, I measured the ‘plain’ circuit with Homero’s polarized output caps, then changed them to non-polar audiophile types and re-measured. Those are the green ones in the photo above. It made a barely measurable difference. These op-amp mic circuits are super clean and distortion free regardless.
The tube emulator works; that is, it adds harmonic distortion like a tube or JFET, because the signal is run through a carefully biased JFET. I am a bit perplexed at the idea. If you want that kind of distortion, why not build a Schoeps or KM-84 circuit which ‘naturally’ offers that kind of harmonics?
I want to do further research and testing of op-amp circuitry, and it’s been added to the list of things to try. The op-amps have been in short supply recently. Hopefully they will come back in stock at distributors soon.
Regardless, thanks to Homero and Jules for the chance to try out some of their circuits. I wouldn’t have gotten around to building OPA mics this soon. Test results in the next article.
