Built in mic preamps

[biffjunior]

Hi,

So I’ve just learned about electret condenser mics, and how they have built in preamps. So my question is, do non-electret condensers also have built in preamps? And what are some examples of well known electret and non-electret condensers?

[Arpangel]

Just asking, but why is this important to you?

[Hugh Robjohns]

So I’ve just learned about electret condenser mics, and how they have built in preamps.

Sadly, this is a common misunderstanding.

All capacitor (condenser) mics contain some electronics. However, this is NOT a preamp—it does not boost the signal level.

Instead, the electronics are there to convert from the very high-impedance circuit environment of the capacitive capsule to a low-impedance environment necessary to drive a mic cable.

For that reason the electronics are actually known as an IMPEDANCE-CONVERTER, not a preamp!

So my question is, do non-electret condensers also have built in preamps?

No, but they do all have impedance-converters.

And what are some examples of well known electret and non-electret condensers?

Non-electret capacitor mics are normally described either as true-capacitor mics or a DC-biased capacitor mics. I'm sure you can do your own research for examples... but to get you started, the AKG C3000 is an electret, while the AKG C414XLS is a DC-bias capacitor mic.

I'm intrigued why you want to know, and where you learned your dodgy information?

[biffjunior]

https://www.sweetwater.com/insync/under ... t%20signal.

It was in this article:

"The reason for this is that condenser mics have amplifiers built right into the mics (sometimes called head amps) that provide the voltage for the mic’s output."

I then went on the wikipedia article for microphones, scrolled down to condenser mics and read about electret mics:

"Unlike other capacitor microphones, they require no polarizing voltage, but often contain an integrated preamplifier that does require power (often incorrectly called polarizing power or bias)."

[Hugh Robjohns]

It was in this article

Ah. Okay... Lynn has simplified his explanation, resulting in some confusion.

The reason for this is that condenser mics have amplifiers built right into the mics (sometimes called head amps) that provide the voltage for the mic’s output."

This is not exactly true. The reason capacitor (condenser) mics typically have a high output level (high sensitivity) is simply because the DC bias voltage applied to the capacitor capsule is very high (typically 60V or so), and so the voltage variation when the diaphragm moves is quite large too — usually in the 15-30mV range (for 1 Pascal air pressure, or 94dB-SPL).

In contrast, the voltage generated by a moving-coil (dynamic) capsule is typically 1-5 mV at 94dB-SPL.

As I said the 'head-amp' Lynn mentions is a buffer-amp and it provides a very high-impedance buffer to the capsule and a low impedance output to the connector. It doesn't usually introduce gain, so it's not, strictly, a preamplifier.

I then went on the wikipedia article for microphones, scrolled down to condenser mics and read about electret mics:

"Unlike other capacitor microphones, they require no polarizing voltage, but often contain an integrated preamplifier that does require power (often incorrectly called polarizing power or bias)."

It's true that electret mics don't require an external DC bias voltage. The capsule is instead 'biased' by the static charge carried on the electret material on the back-plate.

But, just as with a DC-biased capacitor mic, a very high-impedance buffer (head-amp) is still required to prevent the stored electret charge from leaking away, and to provide a low-impedance output.

This impedance-converting head-amp in an electret mic is usually powered by an internal battery, or by external power (plug-in power for consumer mics, or phantom power).

The important point to take away here is that capacitor (condenser) 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.

And the electronics inside the mic primarily convert the high capsule impedance to drive the output cable from a low impedance.

Hope that helps.

[biffjunior]

Yes very informative thanks! Appreciate the help

[Hugh Robjohns]

[ConcertinaChap]

This is not exactly true. The reason capacitor (condenser) mics typically have a high output level (high sensitivity) is simply because the DC bias voltage applied to the capacitor capsule is very high (typically 60V or so), and so the voltage variation when the diaphragm moves is quite large too — usually in the 15-30mV range (for 1 Pascal air pressure, or 94dB-SPL).

This is something that I'd never wondered about about (capacitor mics have higher output than dynamic mics - end of story) but reading this now I realise I'm missing out. Fascinating stuff. Thanks, Hugh.

CC

[ef37a]

This is not exactly true. The reason capacitor (condenser) mics typically have a high output level (high sensitivity) is simply because the DC bias voltage applied to the capacitor capsule is very high (typically 60V or so), and so the voltage variation when the diaphragm moves is quite large too — usually in the 15-30mV range (for 1 Pascal air pressure, or 94dB-SPL).

This is something that I'd never wondered about about (capacitor mics have higher output than dynamic mics - end of story) but reading this now I realise I'm missing out. Fascinating stuff. Thanks, Hugh.

CC

Indeed and the term "pre amplifier" to capacitor mic electronics instead of "impedance converter" is probably another of many areas of audio 'lore' that is not as scrupulously accurate as it might be.

The impedance converter is however still an 'amplifier'! It just amplifies current instead of voltage taking the almost infinitesimal current from the capsule and boosting it many times to drive a mic input stage.

Dave.

[sonics]

I always think of the "pre-amplifier" as the electronics before an amplifier (or now, another processor, mixer etc.). That may include impedance and/or level matching, level control, switching and EQ. We now use preamps without amps, more than with them!

[Hugh Robjohns]

There deep semantic rabbit holes here.

Traditionally, a pre-amplifier was/is the control unit connected to a power-amplifier. It typically allows the volume to be adjusted, sources to be selected, and provides gain for low-level sources like mics or turntable pick-ups.

A mixer also serves as a pre-amp as it typically does all those things too.... but its raison d'etre is to mix signals together, so we prefer to call it a mixer. The channel section that accepts low-level input sources is called the pre-amp (or mic-amp).

A mic pre-amp provides gain for the mic and can be connected directly to a power-amp if desired, so it's still reasonable to call it a pre-amp... although mic-amp is also common.

But the over-riding condition associated with a pre-amp is that it provides voltage gain to raise the level of a quiet signal to something more suitable for a power-amplifier (or to standard line level).

In contrast, the electronics in a capacitor mic don't normally provide voltage gain*, and don't normally make the signal bigger. They are there to serve a different — but critically important — function: that of impedance conversion.

Calling the mic circuitry a 'preamp' conceals its true function and prevents understanding of the operating principles involved.

If you need more level from a mic, mic boosters re available that provide that facility. I wonder if they should be called pre-pre-amps?

*The inevitable exception, is in an active ribbon mic which really does have a pre-amp to raise the output level... although the voltage gain is often provided by a passive transformer and the circuitry may just provide impedance conversion again!