What You May Have Missed at the MECP Webcast...
By Jack Cutts

On Thursday, August 20, MECP held webcast presentation by Honorary Master Installation Technician Todd Ramsey - the man who forgot more about 12 volt than you ever knew! The title of the presentation was "Tools, Techniques, Technology" and covered the tools and practices that any serious 12 volt installation professional should own and know. Topics of discussion included digital multimeter tips and uses, test leads and probes, when to use a hall-effect clamp, testing batteries and charging systems, and much more.

To view the accompanying Powerpoint presentation or listen to the presentation in MP3 format, click here. For a sample of the high-quality technical advice you might've missed during this hour-long webcast, check out this snippet of the post-webcast Q&A:

Question: "How much distortion on the oscilloscope is acceptable on most subwoofers? I’ve been setting them all with no distortion but if I can push it a little past that, I would like to know." Answer (Todd Ramsey): "First off, you should disconnect the speakers before setting levels using the scope with test tones. If not, it’s easy to burn up speakers when using test tones for extended periods and introducing clipping (even easy to burn up tweeters with no clipping if you have enough power). The amplifier is not effectively seeing a load at that point so it’s effectively not doing the same work as it would loaded, but experience doing a lot of level setting has taught me that the good amplifiers vary only a little when setting clip levels loaded or unloaded." "Should you need to get some high power load resistors to see for yourself, Parts Express sells 4 and 8 Ohm dummy loads in 100 and 200w versions that can be connected in a series-parallel format to get to just about any power handling capacity. I recommend having twice as much power handling in the load resistor as you have output power so the resistor does not saturate or give feedback oscillation to the amplifier output. In any case, this is a formality if you want to do it, but for setting levels it’s not mission critical if you are using good amplifiers. You can still get pretty darn close – a lot closer than doing it 'by ear'." On to the question about how much distortion – Introducing a 'soft clip' with a 0dB referenced test tone in the appropriate frequency range is where to start. Then you need to note that measured level of output voltage (let’s say it's 25 VAC). There after the best way to implement a suitable overlap is to use the method of incrementally attenuated test tones found on Autosound 2000 CD 104. Instead of 0dB reference levels, the test tones are at -5dB, -10dB, etc. and then when you play the system at the same volume position, the measurement on the scope reads lower because of the lower signal level. All you need to do to introduce some added (measured) gain is raise the input sensitivity control back to the original output voltage you measured before (in this example it was 25 VAC). Then you will have added that much more gain in a measured way. I recommend 5dB for critical listeners, 10dB for subs or for less critical listeners on mid/high speakers. Over that and you open the door to way more hiss, more frequent distortion that’s audible, and if the amplifiers are powerful enough you could also burn voice coils." "Point is you could not do any of that in a measured, accurate way without your scope."

To view the entire post-webcast Q&A, click here. Stay tuned for announcements of future MECP webcasts featuring Master Installation Technicians answering your tough questions!

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