SOS Forum post
SOS Technical Editor Hugh Robjohns replies: The brief answer is “yes, kind of,” but I’m going to pick you up on some terminology first! The scientific community in the UK adopted the term ‘capacitor’ in the mid‑1920s, to avoid confusion with steam condensers. So, while both ‘capacitor’ and ‘condenser’ technically mean the same thing in an electrical context, ‘capacitor’ is the more modern and less ambiguous term.
The electret capacitor mic is a variation derived from the original ‘DC‑biased’ capacitor mic. Both designs share core operating principles, but the way in which the capacitor capsule is electrostatically charged is different. A DC‑biased capsule is charged by an external DC voltage, whereas an electret capsule has its charge installed during manufacture, held on a special dielectric material affixed internally to the capsule’s backplate. When suitably charged, the capacitor capsule generates a voltage between its terminals, and that voltage varies if the capsule’s capacitance varies, which it does as the front diaphragm moves relative to the (static) backplate when sound waves impinge upon it. This varying voltage is thus analogous to the amplitude of the sound waves, and we can use it to generate an electrical signal that can be recorded or reproduced over a loudspeaker.
The capacitor operates within a very high‑impedance environment, but we require a low‑impedance output to drive a small current along the microphone’s output cable...
Although this varying voltage is quite large, the capacitor capsule (unlike a moving‑coil microphone) cannot supply any significant current. Essentially, the capacitor operates within a very high‑impedance environment, but we require a low‑impedance output to drive a small current along the microphone’s output cable. Consequently, all DC‑biased and electret capacitor microphones incorporate some electronics which serve as an impedance converter. Originally, the circuitry was built around a thermionic valve, but today FETs or op‑amps are more commonly used.
Usually, the impedance converter doesn’t boost the signal voltage at all — it just provides sufficient output current to drive the output cable — so ‘preamplifier’ can be misleading here. Most people associate that term with a device capable of boosting the signal level. A better term might be a ‘head‑amp’, but I prefer to call it an impedance converter since that’s what it’s really there for!
Perhaps one reason some people call the impedance‑converter electronics a ‘preamp’ is because capacitor and electret microphones usually produce a much higher output level than, say, moving‑coil microphones, and they assume the electronics inside the capacitor mic are boosting the signal. This is not, generally, the case. The signal level is higher from a capacitor mic simply because the DC bias voltage applied to the capacitor capsule is very high (typically 60V or so), and hence the voltage variation when the diaphragm moves is quite large too — usually in the 15‑30 mV range (for 1 Pascal air pressure, or 94dB SPL). In contrast, the voltage generated by a moving‑coil (dynamic) capsule is typically between 1 and 5 mV (at 94dB SPL), but it’s a low‑impedance source that’s typically capable of driving the output cable directly.
The important points to take away are that (a) the electronics inside the capacitor/electret mic form an impedance converter to translate between the very high capsule impedance and the low impedance necessary to drive the output cable, and (b) capacitor and electret mics have a high output level because of the way the biased capacitor capsule works, not because of gain from an internal preamplifier.