Q: "Why do some microphones have an external power supply - are these tube mics only?"

A: Usually… but not always. And not all tube mics use external power supplies either. The first thing that should probably be made clear is that all condenser microphones require power for two reasons; polarization of the capsule and powering the preamp (which should not be confused with what we commonly refer to as a mic preamp, which should actually probably be called simply a "mic amp"...the preamp in a microphone is what brings the small voltage that comes off of the capsule up to mic level). Some microphones use pre-polarized, or electret, capsules and only need power for the preamp circuitry. The reason most tube microphones use external power supplies is that tube preamps generally require a higher voltage than the 48v (or less) that phantom power supplies. However, there are a few tube microphones that have hit the market in recent years that don't require an external power supply and are capable of running entirely on phantom power.

There are also a few solid-state microphones that require an external power supply. These typically have preamplifiers that run at a higher voltage than phantom power (such as the 130V DPA microphones, which are probably the best-known of this type of microphone). Increasing the supply voltage allows for a wider dynamic range and an increased SPL handling capability, so these microphones are especially popular for recording material with a wide dynamic range such as classical music. There are also advantages to not relying on phantom power; as mentioned earlier, not all preamps put out a full 48 volts, and especially on cheaper mixers there may not be enough "juice" to go around if many preamps are being used at once, especially on high SPL sources.