How I got started
As a hardware development engineer without any experience in using microphones, I first had to educate myself on what makes a good microphone. I had some hobbyist knowledge level on audio, acoustics, and microphones and learned quite a lot from my son, but I felt I still had a large knowledge gap. So I first started reading some relevant books (see the Links page). After several years of reading many threads on the MicBuilders and GroupDIY forums, having many discussions with my son about what he considers good microphones, and studying many microphone designs, I now think I have a good understanding of what requirements I should set for the circuits I wanted to design. In the beginning, I assumed, or hoped, that it would suffice to just design one universal circuit that would cover all use cases and requirements. How wrong I was… I still think that you want to design a circuit in such a way that finds broad usage, but I’ve abandoned the idea of developing a Swiss Army knife that meets all needs. I think I will end up with two or three basic topologies.
Transparent versus “colored” microphones
The main distinction I’ve learned to make is that, when you look at it very black and white, there are two groups of microphones and therefore two different types of electronic design directions. The first group is what you could call transparent microphones, which are characterized by inaudible linear and nonlinear distortions. The second group is the type of microphones that give a certain coloration to the sound; call it “Mojo,” whatever that means. Transparent mics are often used to record classical music or Jazz, where you want to record the timbre and character of the instrument or artist with the best possible fidelity. The “colored” group of microphones is used to give a certain artistic character to the recorded sound. Not as extreme as a guitar distortion pedal or guitar cab would do, but you get the idea.
General requirements
Whether you’re building a transparent or “colored” microphone or microphone circuit, there are some requirements that both of them should meet. They may not be equally important to everybody or in all circumstances, but I think fulfilling these requirements could make the very difference between an ordinary microphone and an exceptionally good one. And isn’t that exactly what we as DIY mic builders strive for? The general requirements I can think of are:
- Lowest possible noise (hiss), though it does not make sense to design a circuit having much lower self-noise specs than the capsule self-noise. The equivalent self-noise of the circuit should remain below 10-15 dBA SPL to stay below the typical self-noise of SDC capsules.
- It should be capable of handling 130 dB SPL without a pad or 150 dB SPL with a -20 dB pad engaged. AFAIK, 150 dB SPL is about the maximum a microphone will ever have to deal with. From this, and with some assumptions being made, we can calculate the minimum RMS and peak-peak capsule voltage the circuit should be able to handle. Assuming a typical SDC capsule with a 20mV/Pa (i.e. 20mV @ 94 dB SPL) sensitivity when polarized at 60V, then we can expect a maximum input voltage of 1.26V RMS, or say 3.5 Vpp @ 130 dB SPL. More headroom would be welcome if the circuit is also going to be used for LDC capsules or at higher polarization voltages than 60V for improved SNR. Let’s strive for at least 2.5 V RMS (7 Vpp), but more is better. Many commercial designs cannot handle such high voltages, so this might prove challenging.
- Now suppose you can build a circuit that has a much higher maximum undistorted output level than 2.5V, maybe even more than 10V. Would that make sense? I have no clear requirements for the maximum undistorted output level that the mic should be able to produce. Ideally, it should be somewhat more than the maximum input level the mic preamp can handle. Then the first circuit to clip is the mic preamp, which will clearly show that through the clipping LED. Then you know it’s time to engage the pad switch on the microphone. Clipping of the microphone is not always immediately noticed, I think. My Mackie ProFX12 clips at 5V RMS input and I assume many other mixers will be in the same range. So my best guess is that between 5 and 10V RMS would do the job. More is not needed, but due to the limitations of the P48 Phantom supply, it is yet to be seen if this is feasible at all. More than 5V RMS into 2kOhm will be very tough to achieve, as 7.6V RMS is the absolute theoretical limit into such a load.
- The pad circuit should reduce the signal level from the capsule to reduce distortion from the impedance converter (JFET). So I consider only a capacitive pad or reduction of the polarization voltage as acceptable solutions.
- I’ve seen several posts on the internet where people reported noise and dropout problems that may have been moisture-related. These were mainly inexpensive microphones such as the Behringer B-5 or the t.bone SC-140. Such microphones have little or no protective measures against moisture. So the circuit shall have adequate protection measures against moisture.
- Last but not least: The microphone must have good RFI rejection, especially for cell phone frequencies in the UHF band. I have been to several live events where there was audible cell phone interference. In 2024 that seems sooo amateurish…
This seems all quite reasonable, doesn’t it? Yet, many microphones that I tested fail on one or more of these requirements.
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