Changing speaker impedance.

[audioguru]

I don't get it. Why should a speaker circuit be so complex?

Because tweeters are very fragile and we are talking about using a few hundred watts of amplifier power.
A simple series capacitor attenuates low frequencies at the rate of only 6dB per octave. Therefore for the 3kHz crossover frequency, the power to the tweeter would be 1/4 at 1500 Hz which would probably blow the tweeter. This speaker uses a 3rd-order filter for the tweeter and if its values are correct, the power to the tweeter at 1500Hz would be only 1/64th.

The design of a speaker crossover that uses a capacitor or two and an inductor is complex. The series capacitor and inductor resonate, and if an incorrect source or load impedance is used, at resonance there would be either a notch or a peak in the tweeter's response.
If the impedance of the tweeter is much too high which happens if it blows, then the series capacitor and inductor to ground resonate with a dead short to ground.

 

[audioguru]

I wonder if the final values were arrived at with acoustic tests, rather than calculations?.

Probably. I hope they didn't just use whatever parts they had in stock. :lol:

 

[Nigel Goodwin]

Just to keep you updated, I've fitted a pair of Celestion Truvox 1225 8 ohm speakers in the cabinets - funnily enough, the one that was REALLY! bad already had an 8 ohm driver, of some unspecified make! - the other was a Celestion 4 ohm, presumably original?.

Having done ONLY that, I gave them a brief test, and they sounded pretty good like that - as I'm struggling getting hold of suitable inductors, I might leave the bass crossovers alone, it will only give a bit more mid-range to the bass units. I've order some resistors for the tweeters, but I wasn't able to locate 6.2 ohm ones, so I've ordered 4.7 and 6.8, I'll try one in each and then compare them - see which I prefer the sound of.

I crudely 'measured' their overall impedance tonight, simply feeding them through an 8 ohm resistor, and monitoring the signal on either end of the resistor with the two beams of my scope. I simply set one beam to twice the gain of the other, and slowly swept a sinewave up the audio band. I was pleased to see it never dropped appreciably under 8 ohm, even without the resistor change - most of the range was just over 8 ohms.

 

[audioguru]

Nice going, Nigel. :lol:
Today I replaced the worn-out flexy wires to the woofer of one of my best speakers. It is about 42 years old!
The cabinet is sealed so the woofer makes long strokes at its resonance
and I liked hearing its good bass. It was a pain to pick away the glue that held the original wres and unwrap the voice coil wires from them, then I repeated the excercise to remove flexy wires from another speaker that I didn't like.
I used 5-minutes epoxy cement to hold the front then a few hours later did the rear. I put it back together and it sounds like new! :lol:

 

[Nigel Goodwin]

Nice one! - I wouldn't have the patience to do that (or steady enough hands!).