When I was young, in the early 70s, I stumbled into the audio field. We were a gang of technical youngsters who arranged parties for our buddies. And this happened when rock’n roll took over as the cheeky music style for rebelling youths. Technically, this happened when power transistors took over from life-threatening radio tubes. The arena was set for night-long parties, with power amplifiers and big loudspeakers assembled by unruly hobby technicians.
Now, after 50 years, I renovated my loudspeakers from that golden era. Loudspeakers have certainly been developed a lot over such time period, but to authentically reproduce rock’n roll from that time, these old loudspeakers became first-class. However, from my long career in engineering, I decided to improve the amateur-designed crossover filters used to split the audio frequency to tweeters, mid-range and woofer loudspeakers.
Coils are not often used in low-frequency electronics, but they are more or less unavoidable in crossover filters. Coils can be bought from a well-stocked electronics store, but they are rather big and costly, and you need them in many different inductances and resistances. And design material for a coil is very simple – insulated copper wire of different thicknesses is all you need.
There are a lot of formulas to use for calculating the number of wire turns and inner and outer diameter of the coil. But the most simple is to wrap a coil and measure the inductance when ready. Then you use what wire you have, thicker wire for woofers and thinner for tweeters.
From my measurement classes at the university, I remember that measuring inductance was not a simple thing. But maybe something has happened during 50 years? So I googled for 10 minutes and found a handheld instrument for 15 €. This was quite a shocking discovery; in my classes 50 years ago, such an instrument was a cube of approximately 1 meter on sides, and you needed half a day to learn how to use it. I thought this could not be true, but I could not resist ordering one instrument over the internet.
Nothing happened, and nothing was delivered, so I forgot the whole thing. Then, finally, after several months, I received a notification from my post office. They had a parcel from China for me to pick up. The freight we 18 €, i.e., somewhat larger than the cost for the instrument, but I got curious and fetched it. Imagine my surprise when I had unpacked the instrument – it was only to connect the coil to the instrument, settle the selector to 2 milli Henry and read the coil inductance on the display to be 0.826 milli Henry.
Fifty years earlier, this had taken half a day with an instrument of meter size. The cost of such an instrument you could only dream about.