Using an oscilloscope is probably the best way you can eliminate distortion out of your system. This article assumes you are already familiar with your oscilloscope and will not go into setting it up. But basically you'll be plugging pre-amp outputs into vertical input of an oscilloscope.
You'll need a test disc with a variety of test tones. I used AutoSound 2000 cd#104 to maximize my systems S/N ratio. AutoSound 2000 cd#101 has a signal, which is unclipped for 20 seconds, clipped for 5, and then unclipped for the last 5 seconds. Viewing this track on your 'scope's display could be useful if you've never seen clipping on an oscilloscope display before.
Moreover, you'll need a "device" to dropp the signal by 40dB. This can be found at Radio Shack. It's a set of red plugs that go in-line of your RCA cables. You need this because if you'll be hooking up the scope at the speaker outputs of the amp or if your decks RCA autput is 5 V or higher, you may damage your scope! So, if you have a crappy deck that has an RCA output of 2.0V, you would plug these just before the amp's RCA input, but if your deck or Line Driver puts out 5V or higher, you'd plug it in just after that componet having high RCA output.
You start by finding the maximum useable (unclipped) signal level from your head unit. Many of the better head units will not clip the pre-amp outputs, even at full volume, but it's always better safe than sorry. Disconnect the RCA's from your head unit. Play the 1 KHz tone (sine wave tone) at 0 dBFS; you'll be playing the loudest signal possible with the 16 bit digital format. If your CD player has a repeat function, set it to repeat just this track. Set your bass, treble, fader, and balance all to center. Loud function can be on or off. It's up to you. Turn the volume all the way up. Probe your right and left front (and rear if you have them) one at a time. Your scope should show a wave, either a sine wave or a clipped sine wave. If you have a standard pure sine wave then all is good, and you're ready to proceed. If you have a clipped wave then you need to turn down the volume, until you see a perfect sine wave on your scope's display. Remember that this point, is the highest you can ever turn up your head unit (maximum useable signal level for your deck). After you set the level for one of your outputs the rest should be the same, but check them all just to be sure. The results will be the same if you leave the RCA's plugged into the head unit and disconnect them at the other end of other pre-amp components like line drivers, crossovers, EQs, ect. The idea is to adjust the signal level of your pre-amp level components so that they all clip at the same time your deck's maximum signal level is reached giving it a maximum signal level from all of the components.
When working on the EQ, it is a good idea to keep all the passbands on your EQ flat at +6dB. But if you have a high quality EQ, bring them up to maximum boost level since later when you'll be adjusting it, your system will never clipp. But you probably think that there is a problem with this setup if you know something about sound since passband filters don't combine well at maximum boost (so if you boost them all to +12dB, you won't get a flat frequency response but a ripple). But if you have a high quality EQ, this isn't really a problem since EQs only cut and never boost!There is one problem though with setting the passbands to the max. ie. you will be cheating yourself in gain.
Consequently, you could leave your EQ's passband filters set the way you usually use it and get the maximum signal out of your EQ. To do this, probe all the outputs of your EQ in the same method as you did your head unit. You should probe each of the outputs using tones that match the bands of your EQ. For example, if you have a 9 band EQ with bands at 50/100/200/400/800/1.5k/3k/6k/12kHz you would probe your EQ 9 times, once with a 50 Hz test tone, once with a 100 Hz test tone, and so on. If your EQ also includes a crossover you'll need to follow the crossover procedure. If any of these processors are clipping you will probably need to turn down your headunit's volume control or make any adjustments on that unit that you can. For example, if you are testing an EQ and you have any bands excessively boosted, try bringing down that band first. That may be causing your clipping.
To test your crossover you need to probe each output using a test tone that is midway between the high and low pass. For example, a channel, which is crossed over, between 100 Hz and 20 KHz (like a front channel) would be tested at 9950 Hz. Since you'll be hard pressed to find a 9975 Hz test tone on your CD use the 10Khz tone. But if you don't have 10kHz, use 1kHz. For a rear channel crossed over with a lowpass of 3500 Hz you would use a 1750 Hz tone. As you again would have problems finding a 1750 Hz tone on a CD use a 2 KHz tone. For a subwoofer channel lowpassed at 70 Hz you would use a 35 Hz tone. This one you may find on your CD, if not use 30 Hz or 40 Hz.
Assuming your crossover or EQ has level input settings and level output settings, begin with the factory pre-set sensitivity settings for both the inputs and outputs. Those are usually in the middle. First, slowly adjust the input gain untill the output just begins to clipp. Now adjust the output level for the largest output possible without clipping. If the output is always clipped with the maximum unclipped signal level from your deck, then drop the output level and start the procedure all over. You'll usually end up repeating this level setting procedure several times.
As far as the amps go, most people can take 3% of distortion (signal clipping) in amplified-speaker combination. Thus, for example, when adjusting the amp running your subs, which are crossed at 80Hz you would play the 40Hz @ -5dBFS and bring the gain up to the point where it begins to clipp on the scope. Consequently, your system will play much louder with distortion, which is not audible!
Once these levels are set, I always do the software check with my ears. I listen to many different types of music and if there is something that doesn't sound right, stop and figure out what it is! Different types of music can show a buzzing tweeter, raspy mid or a flabby sub. But it can also show the strong points of the system. Finally, remember that in the long run, serious tweeking isn't about turning a couple of knobs and calling it done. You will have your good days and your bad once. Be prepared for those days when everything sounds like tihs. ;)
Hope this helps. Good luck.
The above column expresses the ideas of the writer. Many thanks go to David N. and Andy which have contributed some help. If you wish to reply directly to the writer, or if you would like to share your questions exploring ideas for future www columns, please do so at my E-Mail
Created in March of 1998
Updated in August of 1999
