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Tuesday, December 18, 2012

My first self-built loudspeaker (disaster!)

 Everyone should build a loudspeaker at least once in their life. But for this would-be loudspeaker builder, their first attempt was something of a disaster...

By David Mellor, Course Director of Audio Masterclass

There are some things that you just have to do at least once in your life - for example see a total eclipse, perform a parachute jump, and make your own set of loudspeakers. Actually, I might give that parachute jump a miss. But I've done the total eclipse. I remember the immense sense of collective disappointment of the thousands of would-be viewers gathered together at Parc l'Eclipse (really!) near Cherbourg when a cloud obscured the event from view at the last moment. I can also remember the massive sense of disappointment when I heard what my first self-built loudspeaker sounded like.

It was back in the hazy days of the 1970s, when the rule book of loudspeaker design was still some way from completion. It was then the fashion for guitarists to have a 'stack' comprising an amplifier and two 4 x 12 loudspeaker cabinets. I would have liked to have a Marshall stack, just like Jimi, but I couldn't afford one. So I bought eight 12-inch drive units, the cheapest I could find (far from the quality of the Celestion illustrated!), and several sheets of chipboard. I bought the chipboard because it was the cheapest material I could get, but I later learned that it is actually quite a good material for loudspeaker cabinets.

So I sketched out a design for a cabinet that was rather larger than the Marshall equivalent. I don't know why I did that... yes I do ' I just wanted my stack to be bigger than anyone else's! Then over a couple of evenings I put that first cabinet together, covered it with a cheap Rexine imitation, fitted the drive units and handles. Finially I wired up the drive units to the jack socket and screwed on the back. I have to say that it looked great. Of course I couldn't wait to hear it, so I plugged in my amp and guitar and performed my best Pete Townsend power chord.

Er... there was something wrong. It didn't sound good at all. All the drive units were working but the sound was just wrong. Over the next few days I came to the conclusion that I didn't know as much about loudspeakers as I needed to, so I decided to cut my losses and sell the cabinet. So I advertised it at a price that just about covered the cost of the materials. I soon got an enquiry from a local working mens' club, as they were called in those days. Since I'm an honest trader (with 100% positive feedback on eBay, as of writing!) I needed to give them a demonstration so that they could properly consider what they were buying - a speaker that worked but wasn't all that good. So I took the cabinet round to the club and played some music through it. The committee were satisfied and said they would have the pair. "The pair?", I don't know how that happened, but since I had the parts for the other speaker, I realized that if I wanted to get my money back, I would have to make another one.

So we shook hands, although they did take the opportunity of exploiting my youth and innocence and knocked down the price to about two-thirds of cost. I set to work building the other speaker. Once finished, I had to test it of course. And... it sounded great! It was exactly what I wanted a 4 x 12 cabinet to sound like, and I reckon it would have given the Marshall a run for its money. So I looked at the first speaker again to see what had gone wrong. I spotted the error straight away, when hours of looking hadn't helped just a few days ago. I had wired the four drive units in the conventional series-parallel way, but two of the drivers were wired in reverse phase. So effectively at any instant when two of the drivers were pushing at the air, the other two were pulling. I corrected my error, and I now at last I had my brilliant twin 4 x 12 stack. Except I had agreed to the sale and later that day the club sent a van round. I never did make any more 4 x 12 cabinets, but at least I knew that I could if I wanted to.
Publication date: Friday April 10, 2009
Author: David Mellor, Course Director of Audio Masterclass

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