Electronics > Beginners
Changing the impedence of a speaker driver
_Wim_:
I would suggest to first measure the actual impedance. You can do this with a normal sound card, a reference resistor (100 ohm for example) and free software:
https://www.roomeqwizard.com/help/help_en-GB/html/impedancemeasurement.html
You can use the same software to actually measure your frequency response (you will need a mike for that) and crossover.
SoundTech-LG:
Autoformers...
EXCLUSIVE MCINTOSH OUTPUT AUTOFORMERS.
An impedance mismatch between an
amplifier and loudspeaker can cause distortion and a reduction in power. The legendary
McIntosh autoformer is a hand-crafted transformer with output connections for 2, 4, and 8 ohms,
allowing an ideal impedance match. A McIntosh amplifier with an autoformer can also safely
drive multiple speakers connected in parallel without shortening the life expectancy of the output
stage. There is absolutely no performance penalty with an autoformer. In fact, its frequency
response
exceeds
that of the output circuit itself, and extends well beyond the audible range.
Distortion is so low it is virtually immeasurable.
The “Bifilar” winding technique
used in the making of autoformers
earned McIntosh one of its first
patents. The design is so advanced
it is still used today.
https://www.audioclassics.com/brochure/MC602.pdf
james_s:
You don't need a transformer, you need a crossover. That will route the low frequencies to the woofer and the high frequencies to the tweeter and you'll end up with something near 4 ohms nominal. This is how speakers work, I've never seen one with a built in matching transformer for parallel drivers.
SoundTech-LG:
Right...
The OP meant well, but his question was "how to change the impedance of a speaker". the real question is how to use High Range, and Low Range drivers together, in a system, without causing an impedance mismatch seen by the amplifier. Because, he is right... without a crossover at some range, the impedance will be too low. Obviously in a properly designed crossover there will be a distributed impedance across the entire frequency range that will provide corrected impedance over that range, and at the very important crossover region of the two drivers, seen by the amplifier. Determining the correct Xover frequency for whatever those two chosen drivers are, becomes critical. -3dB there, for each driver. Then the frequency response of the individual drivers is left. There is always the possibility that the chosen drivers do not even cross at a desirable frequency, or level. Driver mis-match.
In a pinch, manufacturing specs could be used, but safer to measure each driver, and both drivers together, properly crossed over in the chosen cabinet, with pink noise, using an RTA with calibrated Mic, to see the acoustic response over the range. Your handy anechoic chamber will help as well.
Using an electronic Xover is another easy way to manually adjust the Xover region, and levels (using two amplifiers) as you watch the response. Then, with that information, you can design the right passive crossover for the job, or just set your electronic one, and be done.
james_s:
When I was a kid I pieced together quite a few speakers using whatever drivers and other parts I could get my hands on for free. Some sounded much better than others, I didn't really know what I was doing so it was a crapshoot. They all worked to some degree though and none ever damaged the amplifier or caused it to misbehave. In practice the details are not *that* critical, unless you are going for audiophile sound quality. Something as simple as a capacitor in series with the tweeter will probably work just fine.
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