MikeRivers

Here are some important details for those who have never done anything like this before.

First, readers may not know what a "signal generator" is. You don't need a piece of calibrated lab equipment for this. Most DAWs and audio editing programs have a function to generate a sine wave. For something more portable (I hate to be tied to my computer workstation when doing shop tests or troubleshooting) there are web resources for downloading WAV or MP3 files of sine waves. You can transfer those to a handheld recorder and that becomes your test generator. Alternatively, there are a few apps for mobile devices to generate test tones. You'll need to be creative with cables but that shouldn't be a problem. I have a few cables with common audio connectors on one end and alligator clips on the other end to make it easy to hack together something temporary.

The article doesn't state where the generator and amplifier or other device under test are to be connected. Since signal flow on a diagram is usually assumed to be left to right, so the generator should be connected to the jack on the left (3). The device under test (DUT - an amplifier or other input device) should be connected to the jack on the right (1).  It really doesn't matter once you understand how a pot works, but it's best to set up a convention.

As drawn, the pot will be at zero ohms (or, technically, as close to zero as it will get), when the shaft is turned fully counterclockwise with  the pot oriented so that the shaft is pointing toward you. Check it with the Ohms setting on the multimeter just to be sure. 

Measure the generator level at the jack to be connected to the DUT, but with nothing but the meter there connected yet. Write down the voltage and then connect the DUT with the meter still connected. Then rotate the pot and stop when the voltage across the DUT input has dropped to half the open circuit voltage.

The drawing show a 1 megohm pot, but some devices have in input impedance greater than that and even with the pot at maximum resistance the voltage at the DUP input won't drop to half the open circuit voltage. You can stop there and be happy that you have an input that's greater than 1 megohm, or you can start tacking 1 megohm resistors in series (between the pot and the right (1) jack) until you can get to half voltage with the pot, and then add the extra resistance to what you read when you measure the pot. You could get a 10 megohm pot, but that reduces the resolution at the low end, should you want to start measuring everything within sight, where line level inputs are typically in the 5 to 20 kilohms.

Before you go off checking you mic preamp's input impedance, understand that nearly all mic inputs are differential. You can use this same technique and test setup, but you'll need to connect the generator between pins 2 and 3 of the XLR input and ignore pin 1. You'll also want to monitor the preamp to be sure it's not clipping. You may find that the input impedance varies over a small but measurable range as you adjust the input gain of the preamp.

If the preamp output is connected to a DAW input, you can skip the voltmeter measurement, just set the generator output low enough so that you aren't clipping the preamp, and monitor the output level using a meter in the DAW. First check and jot down the level with the pot at zero ohms, then turn the pot so that the level drops by 6 dB (that's 1/2 the voltage) and measure the pot's resistance.

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Mike Rivers used to write regularly for a few magazines but they won't let him do it any more because all of his articles are too long. He has retired to the safety of on-line writing since adding another page doesn't require selling another ad.

"Don't ever go over 0 VU when recording digitally."

Digital gear doesn't have VUs or VU meters. The level you don't want to exceed, and actually, that you can never exceed, is 0 dBFS. If you're using a VU meter for reference, for example on an analog mixer or mic preamp, you need to establish a relationship between 0 VU on the analog meter and 0 dBFS on the digital meter. Usually the system is calibrated so that at 0 VU, you'll have a recording level somewhere between -14 and -20 dBFS, depending, if you have a choice, of how much headroom you want to allow.

See my article entitled Meter Madness"

http://mikeriversaudio.files.wordpress.com/2010/12/meter-madness_1-2_revised.pdf

"While I don't see guitar companies doing away with the 3/4†jack anytime soon,"

I think you mean 1/4" jack. There was a Gibson system (the Les Paul Recording) that used a balanced connection between guitar and amplifier on an XLR connector, but they left the 1/4" jack so the guitar could be played with a conventional amplifier.

It's about time somebody wrote an article like this. Guitar switches and knobs are notorious for being unmarked and in the "well, everybody knows what that switch does" category.

The real meat here is in your last paragraph, except that I've never seen a manual for a guitar (maybe because I've never bought a new electric guitar) or much of an explanation on the manager's web site. Ask a clerk? Ummmm . . . maybe they know more about guitars than microphones or mixers.

But thanks for the starter info.

Always good to see Sister Rosetta Tharpe included in such a list. She should be #1, though. The clip here is pretty tame, but she's where Chuck Berry got his licks, and without Chuck Berry, where would real rock and roll be?

"Remember those ground posts on turntables with the screw terminals where you could attach a ground wire to keep hum at bay? Have you ever seen ground posts on anything else? Of course not!"

Craig's memory is a little faulty here. Turntables had a ground _wire_ that was permanently attached. The ground post, where you connected the spade lug on the other end of the turntable's ground wire was on the receiver or preamp. So convenient - you didn't have have to buy a piece of wire to ground your turntable's innards, nor did you have to solder anything or drill any holes.

The problem with fretting one string to get it to the pitch of the next higher string is that frets aren't always that accurate, and even if they're spaced mathematically accurate, the bridge is in the correct place, and the instrument is set up well, you're still at the mercy of the tempered scale - unless of course you have one of those kinky oddly fretted fingerboards that attempt to compensate for what was designed for Mr. Bach's Well Tempered Clavier.

One system that works better most of the time is to compare octaves. It's easy enough to hear if two notes an octave apart are identical by listening to beats and tuning so there aren't any. You'll have to fret a bit, like tuning the D string by fretting it at the 2nd fret and matching this E to the 1st or 6th string, which you've already tuned by octaves to match.

Or better yet, get a clip-on tuner, tune each string to it, then correct it by ear. At a workshop at a Newport Folk Festival many years ago (back when a "festival workshop" consisted of a pole stuck in the ground in the field, with a sign on it and a chair, someone asked Reverend Gary Davis why he always tuned his B string a little sharp. He replied "Well, it's SUPPOSED to be that way.