Use a Battery Box With Plug In Power Microphones

Better Recordings from PiP Mics On Loud Sounds With External Power

Aug 19, 2009

Recorder plug-in-power is poorly specified and often too low to power the microphone properly on loud sounds leading to distortion on sounds like rock concerts.

Plug-in-power was introduced by Sony with their Walkman recorder range more than twenty years ago. Unlike professional phantom microphone powering, manufacturers can implement it in various ways. Plug-in-power is rarely high enough in modern equipment to allow for a lot of headroom.

The result is clipping on loud sounds, which is bad enough, but where those loud sounds are sustained, like at a rock concert, the whole recording becomes muddy and unclear. Concert tapers can get better recordings with a battery box.

Why Electret Microphones Need a Battery Box to Record Loud Sounds Clearly

According to Shure (makers of the popular SM58 vocal microphone) a rock concert can reach 120dB SPL at the back of the hall. Using the values for a typical electret microphone capsule such as the Panasonic WM61A, the output signal is in the order of 1Vp-p at this SPL. There just is not enough headroom since there should always be at least 1.5V DC across the capsule. Modern recorders often use a low voltage of 2-3V to power the microphone, and often 1V of that is lost in the series resistance of the PiP circuit. (see diagram)

The microphone output can be estimated from the sensitivity of -35dB relative to 1V/Pa. Since 1Pa is 96dBSPL (at 1kHz) the rock concert can reach 120-96-35 = -11dBV or 0.28Vrms or 0.8Vp-p

Schematic of a Battery Box to Power Electret Plug-in-Power Microphones

A battery box contains a battery and a replacement powering circuit for the microphone. These typically use a 6 to 9V battery to power the microphone via a resistor, in exactly the same way as the recorder's original plug in power circuit did. The key difference is that the battery voltage is 2 or 3 times higher. Battery boxes are available from several companies such as Soundman, Sound Professionals and Core Sound.

Building a battery box is easy enough for someone who is reasonably handy with a soldering iron. Most electret microphone capsules have a current drain of between 0.5mA and 1.5mA and require a voltage across them of 1.5V up to a maximum of at least 8V. If a 9V battery is used with a 4.7k resistor, then the capsule will have between 1.95V and 6.7V across it depending on current drain. Purists would adjust the value to give about 5V at the capsule, optimising headroom. However, the values indicated will give a great improvement in headroom for most electret plug-in-power microphones. The schematic is shown below.

Use a Battery Box for Better Sound with Plug-In-Power Inputs

Using a battery box improves the large signal handling capability of PiP microphones, particularly when compared to some later minidisc recorder inputs, which sourced PiP from a low voltage and a relatively high resistance. The result is clearer gig or garage band recordings, and the difference is also noticeable on urban street recording which can have a lot of low-frequency noise from traffic. Even the recording of quieter outdoor sounds like birdsong can benefit, because wind noise will not push the microphone into overload so easily, giving some more chance against wind rumble.

The copyright of the article Use a Battery Box With Plug In Power Microphones in Recording Engineering is owned by Richard Mudhar. Permission to republish Use a Battery Box With Plug In Power Microphones in print or online must be granted by the author in writing.