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Cheap Test Generator

Discussion in 'Amps and Cabs [BG]' started by LeDuck, May 30, 2012.

  1. LeDuck


    May 26, 2012
    Rather than buy an expensive tone generator from HP or Tektronix or Leader, I solved my problem with Stereophile Test CD's and an old Technics CD player and a Radio Shack sound level meter.

    The Radio Shack sound level meter was tested at work against a calibrated sound level meter reference source and found to be well within 1/2 dB. Quite good for the price.

    I have used Stereophile Test CD's for many years in setting up my home audiophile systems. My main one is Test CD2. The main tracks are Signals for Testing Loudspeakers and Rooms. There are also tracks for testing CD players and tape recorders.

    Pink noise can be used as you walk around a room and listen for changes in the sound. Good for spotting where you get the best overall sound and for standing waves. Standing waves can be attenuated by placing low frequency drivers in different places. Which is why bass should be played through at least two separated cabinets in a room. A combo bass amp or any single subwoofer is a room resonance nightmare as any experienced audiophile knows.

    There is a bass decade track (200Hz-20Hz) that will be of most interest to bassists. It uses 1/3 octave warble tones at -20dBFS for checking system output at 200, 160, 125, 100, 80, 63, 50, 40, 31, 25, and 20 Hz. The voltage output from a 2V CD player will thus be 200 mV, or about the same as a hot guitar pickup. It works very well on the CD input of my Marshall practice amp.

    There is also a midrange decade (250Hz-2kHz) and a treble decade (2.5kHz-20kHz) for testing midrange and treble output of speaker cabinets. Very good for spotting problems with midranges, tweeters and crossovers.

    This is a very good way on the cheap to set up for best sound in a room and for testing the performance of sound systems and speaker cabinets. You could literally take it to a equipment showroom to see if bass cab extension meets manufacturer specs, or what the actual output is when tone controls are set to "flat". You may be in for some big surprises.

    Today, I tested the performance of a Marshall AVT20 combo amp with the stock G10-30AVT speaker with claimed "extended bass response". It was strong down to 125 Hz, OK at 100Hz, and fell on its face at 80Hz. Hardly what I would call "extended bass response". Hardly adequate for detuned metal guitar. The amp is due for a new Eminence Beta 10A to be installed, then we shall see.

    Back when I bought them, the CD and sound level meter set me back about $50, and it's probably the best deal in test equipment that I ever got. Very useful if you want to measure things accurately, and not just "by ear". It goes without saying that many musicians have very bad ears when it comes to getting things balanced out. What you hear on stage is not what the audience hears. With a CD remote control in one hand and the sound level meter in the other, you can walk around a room and measure what your system is doing to a fairly precise level at a chump change price.

    John Atkinson, editor of Stereophile, is a bass guitarist, which is why the test CD begins with a Fender bass guitar track. Drum solos and guitar solos come next. There are several high quality acoustic instrument tracks as well, for checking overall sound of a system. This is a great test CD for your PA system as well.

    Most of you will probably discover (if you use the CD and interpret the test results correctly) that when playing low on the E string, most of your perceived loudness will be coming from the second harmonic which is 82 Hz and higher. This is an old trick used by bass cab makers for years to up the efficiency and reduce room effects from bass amps.

    Test CD2 comes with a booklet that tells you how to use it and what it all means. It would be a good idea to study and pay close attention to it. Too much stuff to go into detail about it here. If you don't buy the CD, then it doesn't matter. I know that John Atkinson has some kind of electrical engineering degree, and he is qualified to make this Test CD and has sat in front of more high end audiophile systems than you are likely to ever see. MusicDirect lists the Test CD2 at $12.99, so it should still be easily available, and probably for less on eBay.

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