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Using Microphones

Microphone Basics

A rundown on microphone basics.


Having concentrated over previous months on the intricacies of microphone placement and special recording hints, I think it's about time to cover some fundamental facts about microphones. Facts that, although well known to the veteran home studio owners, are of vital interest to the many newcomers. Facts, also, that are rarely discussed in recording magazines and yet which everybody seems to understand. Actually, most of this article consists of questions people keep asking me through the years.

Can I plug any mic into my Portastudio?

Yes. Cassette-type multitrack equipment is designed to accept a wide range of input levels. Plug any mic into the input and adjust the input gain for optimum level. Though dynamic, low impedance microphones won't deliver too high a voltage when used at great distances from the instrument to be recorded, it should suffice.

With regard to signal-to-noise ratios: I've had my trusty 144 Portastudio measured in a friend's laboratory and he told me that an extra 20dB of gain (required to make low impedance output equal to high impedance output) costs me a 2dB decrease in signal-to-noise, worst case! So, I wouldn't worry too much about impedances and utilise common (low impedance) microphones.

What about Mic Inputs (of tape decks etc.)?

It's okay to use these with any dynamic mic. If you have condenser mics (the ones that need powering from the mixer or batteries) you may overload the input amplifier. An alarm signal should start ringing inside your head as soon as you have to work with faders all the way down. Don't trust your VU meters in this instance, they may be well in the safe area and yet you still get only garbage on tape. There's a sad but true story behind this folks: though using top grade equipment, AKG mics and a Nakamichi tape deck, I once blew a recording this way by trusting the VU display. It's best to use condensers with attenuation pads or (worse) the 'line' input of your equipment. Line inputs of mixing desks will, of course, accept all signal levels.

What length mic cable?

As short as possible, but trust your ears. Long, unbalanced cables (the type usually employed with home recording gear) may pick up stray magnetic fields resulting in 'hum'.

High impedance?

High impedance (high Z) microphones must have short cables to ensure high frequency content won't get lost. High Z mics are rare on this side of the Atlantic. Some Shure models, the 565 for example, are dual impedance types and are usually shipped connected for low impedance operation; so, as long as nobody has changed the internal connections, there's nothing to worry about. If in doubt, check the supplied data sheet or ask a recognised dealer. Rewiring is easy - just change the plug connection for low impedance operation as this best suits recording purposes.

It doesn't work...!

No output from the microphone? Well, a mic breaks down quite seldom, so check your cables first for bad soldering joints and correct wiring (see this column in HSR July/August). Check your mic's on/off switch (it does happen!) and replace the battery if it's an electret condenser type of mic.

Having traced the fault to your mic itself, send it to a dealer for repair, but if you're pushed for time you can try a basic repair job if it's a dynamic type microphone.

Figure 1. Principal internal design of a microphone.


Figure 2. Mic with on/off switch.

Figure 1 shows the basic design of a dynamic mic. The two leads coming from the transducer element (capsule) are directly soldered to pins 2 and 3 of the mic's XLR connector (pins 1 and 3 for a DIN connector).

Figures 2/3/4 show the principal connections if an on/off switch, bass cut switch or an output transformer is employed.

Figure 3. Mic with switchable bass cut switch.


Figure 4. Dual impedance mic with on/off switch. Impedance switch shown in low Z position. Note: phase change and unbalanced wiring for high impedance (used in many models).


How do I check the mic, electrically?

For a first check, an ohmeter is of great help. By connecting it to the hot pins (2 and 3 XLR, 1 and 3 DIN) it must 'see' the capsule's resistance - very roughly corresponding with the mic's own impedance. The ground (earth) pin must not have any connection with the hot pins. Ohmeter readouts:

Pins 2/3 (XLR), Pins 1/3 (DIN) 1-100 Ohms (low Z).
Pins 1/3 (XLR), Pins 1/2 (DIN) Infinite Ohms.
Pins 2/3 (XLR), Pins 2/3 (DIN) Infinite Ohms.

Having discovered any irregularities, open up the mic (most capsules may be unscrewed counter-clockwise). The chances are that you'll find a bad soldering joint or a lead that doesn't have a connection to its soldering lug. You can also check the resistance of the capsule directly at its soldering lugs, but a visual inspection will let you find the fault quickly.

If the capsule is out of order you'll need a replacement and these are generally available as spare parts from the manufacturer/distributor.

Microphone care... what should I do?

Microphones, fortunately, don't need any servicing except for integrated windscreens which are usually made of special foam. Every now and then, these should be pulled out and washed carefully in a mild detergent solution, then leave to dry overnight. A heavily soiled pop screen does affect the sound and feedback characteristics of a microphone so pay attention to the cleaning.



Previous Article in this issue

Aiwa AD-F770 Stereo Cassette Recorder

Next article in this issue

Mainframe: Sound Sampling


Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - Nov 1984

Donated & scanned by: Mike Gorman

Previous article in this issue:

> Aiwa AD-F770 Stereo Cassette...

Next article in this issue:

> Mainframe: Sound Sampling


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