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If I want to use an 8 ohm driver with a 16 ohm driver in a speaker design, is there a way to make the amp see the 8 ohm driver as 16 ohms? Is it as simple as adding 8 ohms of resistor somewhere? (somehow I doubt it :-)
I apologize for this possibly obvious question, but I'd really like to use the drivers I already have, and don't know how to effectively do this
Thank you,
Rob
Follow Ups:
The Speltz ZERO autotransformers can make your 8-ohm drivers appear as 16 ohms to the amps. There are also cheaper units, but I don't recall the brand names.
Whatever you do is going to reduce the voltage seen by the 8-ohm drivers compared to the voltage seen by the 16-ohm drivers. You will have to match the output levels somehow.
Actually it is just as simple as adding a series 8 ohm resistor. The problem is that when you add that 8 ohm resistor, it eats up half the applied power, and there's no way around that. Now if you are turning an 8 ohm midrange horn or driver or even some tweeters into a 16 ohm driver you probably can get away with the conversion. However, trying to use an 8 ohm woofer as a 16 ohm speaker with a series connected 8 ohm resistor is a bad idea; even a dangerous idea. Such a connection became dangerous for me once when trying to 'adjust' the impedance of a woofer. I added the series resistor to the crossover, put the crossover back in the speaker enclosure, reconnected and installed the woofer and forgot about it. Several weeks later I kept smelling a pine fragrance coming from that corner of the room. Eventually, I reopened the speaker enclosure to discover that the series resistor had gotten so hot it had blackened the interior wood of the box. I was lucky I didn't start a fire. Well, at least I didn't put my eye out with one of those things.......whatever the hell that means.:-)
Is this an 8 ohm tweeter with a 16 ohm woofer? Two woofers, one 8 and the other 16 ohm?
Good point. I have a vintage pair of R&A full range (no whizzer), 8 ohm, 8" drivers that seem to be VERY efficient, in an open baffle. I have a pair of vintage, efficient, 8 ohm, 12" woofers, each in a sealed cabinet, but they are still not loud enough to match. I like these old woofers (from an EV 4), and can get another identical pair, so I thought I'd build bass cabinets with two woofers in each, but that means either a 4 ohm or 16 ohm bass box. So I'm trying to figure how to successfully mate the 8" full rangers with the bass box, hence my desire to increas the impedence of the 8".
Also, Id like to keep the complete system as efficient as possible so I can experiment using them with low power SET amps in the future.
Thanks again - Rob
Consider adding a tweeter with a lowish (700hz or so) resonant frequency to the system. There's no way your 8" woofers can be considered full range. You'll get serious beaming of the high frequencies around 4kHz without a tweeter. If you have the means to measure the frequency response of the 8" at about 15 degrees off-axis, I think you'll find a significant drop in output above the 4kHz range.
Here is what you can try before you go the electronic crossover and second amplifier route.
Wire the two 12" speakers in parallel, that makes a 4 ohm load.
Wire an 8 ohm 25W resistor in parallel with the 8", that makes a 4 ohm load.
Wire the two 4 ohm loads in series with each other, that makes an 8 ohm load.
Because of the voice-coil inductance of the 12s, the high end feeding the 8 will roll off. You also want to keep the HF out of the 12s and feed it to the 8. All you need to do is put a cap in PARALLEL with the 12s. I would start out with about 50µF and go from there.
This will raise the volume of the 12s almost 6dB with respect to the volume of the 8. Adjusting the cap and the value of the resistor will fine tune the levels and tonal balance.
If you want to take some of the deep bass out of the 8 and into the 12s, put an inductor in PARALLEL with the resistor/8 combo. I would start with about 2.5mH and go from there.
Well, there are several ways to do this but in my opinion, the best way would be to bi-amp the system using an external adjustable crossover and solid state amp for the 12" woofers. The crossover will have to have high and low level controls for both the 12" and 8" drivers so you can adjust the output relative to each other. It's too bad the 12" drivers aren't the efficient half of the equation because that way you could use plate amps that would incorporate level controls for the 12" drivers as well as the necessary crossovers.
You could also use an external passive crossover with level controls or fixed resistors in an "L" pad arrangement to bring down the level of the 8" to match the 12" woofers. However this would require large inductors for the woofer crossover that tend to be power robbers.
I would not add resistors in series to the 8" drivers to increase their impedance. The resistors would rob power from the 8 inchers and also generate significant amounts of heat.
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