Expert Advice

Answers to readers' technical questions. XLR Q&A Q: How are the three prongs on microphones such as the Shure SM58 related to the connections on a guitar

Answers to readers' technical questions.XLR Q&AQ: How are the three prongs on microphones such as the Shure SM58 related to the connections on a guitar cable? I'm trying to make an adapter that allows me to plug my mic into my guitar amp, but I'm not sure how the two are related. Any help would be appreciated.

Joe Reinhartvia e-mail

A: Your project is more complicated than just making a 11/44-inch-to-XLR adapter cable. Because the output impedance of an SM58 microphone is less than 600z, its signal level is very low compared with that of a guitar's high-impedance pickups. So you need a microphone-input transformer (such as those made by Shure and Radio Shack) to step up the signal level enough to make the input channel happy. Because the transformer is a passive device, the output impedance is also increased.

The transformer looks like a little tube with an XLR female connector on one side and a male 11/44-inch plug on the other. They cost about $15 and work quite well. Just plug the 11/44-inch plug into your amp and the microphone cable into the female XLR jack. But here's a sound-engineer caveat: a guitar amp can be a poor substitute for a proper P.A. speaker. Guitar amps are intended for the midrange-heavy sound of a guitar and don't offer the full-frequency reproduction of a horn-equipped P.A. cabinet. As a result, you could be in for some serious feedback problems. However, try the adapter out and see how the guitar amp works in your application.

If your application doesn't require a transformer - for example, if you need to connect a mixer's speaker outs to a powered monitor's XLR ins, here's how you would wire the cable (see Fig. 1). With a TRS (Tip/Ring/ Sleeve) stereo plug, the tip goes to pin 2 on the XLR connector, the ring to pin 3, and the sleeve to pin 1. With unbalanced 11/44-inch "mono" plugs, both pin 1 and 3 on the XLR are tied to the phone plug's sleeve, while pin 2 is tied to the phone plug's tip.

Although most audio equipment is wired "pin-2 hot," some foreign gear is wired "pin-3 hot." According to one of my British buddies, this is the U.S. manufacturers' fault: there have been several different official European standards over the past few decades, but U.S. builders would not agree on which pin should be hot. As a result, we sound folks carry around pin-2-to-pin-3 flipper cables for times when we need our gear to interface with gear from the other side of the pond. Not all gear is marked to indicate which pin is hot, so you may have to try your flipper cable both ways.

What Did You Say?Q: I'm starting an internship as part of the music-production degree program at the Hartt school. I'll be working in sound reinforcement, and I'm considering getting custom-molded earplugs. Where should I search?

Mike Shearvia e-mail

A: Start your search at H.E.A.R. (Hearing Education and Awareness for Rockers) online (www.hearnet .com). Some of the best ear protection is for sale at the site. You can get anything from simple foam plugs to custom-molded earplugs with different attenuation filters. The price of custom in-ear hearing protection starts around $200 and really works great - no more ringing ears after a gig. If you take protective steps now, in a few decades you'll still be able to hear normal conversations. Nothing (and I mean nothing) is worse than a deaf audio engineer.

Getting FireWiredQ: Do any Macintosh audio interfaces use FireWire? At 400 Mbps, FireWire should be plenty fast enough to handle multichannel audio. It would be great if something could get eight or more channels of audio in and out of an iMac DV or G3 PowerBook. And what would really be awesome is a standard protocol for transmitting multichannel audio in real time over FireWire. Manufacturers of digital mixers could offer FireWire I/O as an option, and you could just hook up your console to your computer. This seems like such a natural way to go that I can't believe it's not happening - or is it?

David Bakervia e-mail

A: The short answer to your question is not yet, but hopefully soon. FireWire is a high-speed serial-transmission standard that was first developed by Apple Computer in the late 1980s. The company submitted it to the Institute of Electrical and Electronics Engineers (IEEE), which adopted it as international standard 1394. At the moment, IEEE 1394 transmission is limited to cable lengths of 4.5 meters between devices, but an addendum to the specification, called 1394B, will allow cable lengths of up to 100 meters using fiber-optic and CAT5 cable.

The protocols for sending media data over 1394 are defined by the International Electrotechnical Commission (IEC) in a standard called 61883. This standard has several parts: the first five parts govern the transmission of video, and the sixth part (IEC 61883-6) governs the transmission of multichannel digital audio and MIDI. This recently adopted 61883-6 standard was originally designed by Yamaha and called mLAN.

At several recent trade shows, notably the January 2000 Macworld Expo in San Francisco, Yamaha demonstrated three prototype mLAN products: an mLAN interface for the Yamaha 02R digital mixer, an interface for the CS6x and S80 keyboard synthesizers, and an mLAN-retrofit box. At the Macworld demo, these products interfaced with a FireWire-equipped Power Mac running a special version of Steinberg's Cubase VST sequencer.

Apple has not announced a schedule for adding native mLAN support to the Mac OS. However, several other music-technology companies, including Roland, Korg, and KRK (see Fig. 2), have announced that they will support mLAN. At the recent Summer NAMM show in Nashville, Korg showed a prototype Triton Rack module equipped with mLAN, but the mLAN features were not ready to be shown.

Impedance InquiryQ: I have a speaker cabinet with four 10-inch speakers rated for 400W @ 4z. My combo amp has an external speaker output that reads 300W @ 8z. Is it unsafe to plug this amp into the speakers?

Rickvia e-mail

A: I assume this is a bass or guitar rig of some sort. Judging from what you've told me, the setup should work fine. A transistor amplifier rated for 300W at 8z will probably output around 400W to 450W into 4z. Just make absolutely sure that the amp is capable of driving a 4z load. Check the documentation or call the manufacturer.

You may have to disconnect the amplifier's internal speakers to avoid overloading the amp, because an internal 8z load wired in parallel with an external 4z load will end up as less than 3z. You should ask the amplifier's manufacturer about the safety issues. You'll probably be okay with a transistor amp, but tube amps can cook the output tubes if the proper transformer tap isn't selected. From a wattage standpoint, a 450W amplifier should be fine with a 400W speaker cabinet.

Many professional engineers regularly "overpower" speakers with huge amplifiers to gain extra clarity and headroom. But this practice isn't recommended unless you really know what you're doing. Speaker ratings don't apply in all situations, so even amplifiers rated at half a speaker's acceptable wattage can fry the speaker, especially if you send frequencies outside of the driver's frequency cutoff. For this reason, it takes only a few watts of 40 Hz signal to take out a 100W compression driver in a horn. Also, an improperly tuned bass cabinet will let the speaker cone bottom out long before it reaches rated power. The rule is if it sounds bad, turn it down.

Of Mics and GuitarsQ: I need some microphone advice. I'm an accomplished musician, and I'm building a home recording studio centered around an Alesis ADAT LX-20. I am recording a multitrack CD, on which I'm playing numerous guitars, including a Fender Strat and 12- and 6-string acoustics. What microphone should I use to record the soft acoustic guitars, the electric guitars playing through a Peavey amp with a variety of distortion devices, and so on? I'm also looking for a retractable mic boom that can be built in to the ceiling and lowered and adjusted as necessary.

Don Bensonvia e-mail

A: Although a lot of microphone choices are quite personal, some old standards have passed the test of time and are used by practically everyone. They're good guidelines to start with. First, for recording electric guitars played through a distorted amplifier, it's hard to beat the venerable Shure SM57. It's my automatic choice for standard guitar-amp combinations such as a Les Paul and a Marshall or a Strat and a Fender Twin. Some stage engineers put up Sennheiser 421s for this application. If you want a slightly brighter sound to cut through the rest of the mix, an Audio-Technica 4060 will work wonders. It's a side-address condenser microphone with a large diaphragm that has become quite popular for guitar recordings with some of the larger live-sound companies.

When miking any guitar amp, remember that each of the speakers will sound a little different, and moving the mic a few inches in any direction will make a world of difference. Typically, you should start with the mic pointed dead-on at the speaker and about one-third of the way in from the edge of the cone (see Fig. 3). Then get someone to move the mic around a little while you listen on your monitor speakers until you find the best spot. This method is better than equalization, and it won't muck up the phase linearity of the signal.

For acoustic guitars, the best choices are usually good condenser mics. Many of the new side-address condensers are quite affordable, but if you have a bit more money, it's tough to beat a pair of AKG C-414s. My favorite mics for acoustic guitar are a matched pair of Neumann U64s, tube-based, high-voltage, small-capsule condenser mics that I picked up about ten years ago. If you're looking for lower-priced alternatives, check out the Shure KSM32, the Audio-Technica 4047, and any of the excellent GT mics from Alesis.

Finally, remember that among outboard mic preamps the sound varies quite a bit, mostly due to various loading effects and interaction with the microphone. There is also a wide range of prices for mic preamps, with some units costing many thousands of dollars. On the lower end of the scale, a Tube MP from A.R.T. would be a good starter unit, and it costs less than $150. Personally, I like the preamps from SPL and D.W. Fern, and my absolute favorite is the Millennium Media HV-3. It's quite costly, but worth every penny.

As for a ceiling-mounted boom, I just screwed a standard mic-plate mount to the ceiling of my drum booth. When I need the mount for overhead, I screw on an Atlas Baby Boom. I don't know of any company that makes a boom specifically for your application, but with a little blacksmith work you should be able to bang together something out of Atlas parts. How about sending us a picture if you come up with anything interesting?