04Jul2005

Pages from '88, the early days

Here are a few of the first steps along the design path from around the time of the original HL Compact and HL5, launched in 1988/9. In those days of paper-trace pen-charts, the only convenient way to make permanent records was by gluing the trace sheets into bulging Log Books. Literally "cut and paste". But it had a very personal feel to it. And it never locked-up or crashed!



Here I was working an an impedance-leveling circuit for upper frequencies in the the bass unit operating band. These components would have been incorporated into the internal crossover circuit so that the drive unit behaved as a flat 6 or 8 ohms across its frequency range. That would align the theoretical crossover circuit and the practical one much more accurately.



Turning to the SEAS tweeter, a measurement of its impedance shows a broad bump centered around 650Hz. Without knowing the model number I can tell from this characteristic shape that the tweeter must have a vented chamber on the back, hence the very low resonant frequency. Note that just like the bass unit (previous page) the impedance creeps up with increasing frequency (brown trace). By designing some basic circuitry we can flatten the whole curve to the blue line, discussed on the next pages.



To smooth out the tweeters two impedance characteristics (see previous page) we will need two different but parallel-acting circuits in our crossover. We have to do some simple calculations for the components we will need to address the low-frequency bump, and also the high frequency impedance rise:



... continued. And now we can see what effect this impedance correction will have at both ends of the electrical circuit that drives the tweeter. Note that uncorrected (brown trace) the naturally ring impedance at both ends of the tweeters working band results in more volts being developed across the unit - hence more acoustic output.



And here is the final result. By the careful choice of two resistors, a coil and two capacitors soldered 'across' the tweeter (actually on the crossover pcb) we have completely removed the two impedance rises. We've added complexity to the basic crossover circuit, and of course cost too and there may be long-term reliability issues. I wonder whether it sounds better or not or whether a more intelligent approach would be to work 'with' the tweeter's natural characteristic rather than to try and work against it?