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Old 18th August 2011, 02:45 PM   #1
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Question Want to install different impedance tweeter, how to modify crossover properly?

Hello!

I have a pair of speakers with some rather unusual (and somewhat defective) tweeters. They are 25 ohm and replacements are not easily found.

I would like to modify the crossover so that I can install new tweeters. I have a pair of Audax HD 12X9D25 4 ohm tweeters that I would like to use, if possible. Will replacing the tweeters, regardless of crossover modifications, change the overall impedance of the speaker (which is currently 8)?

Overall specs of the entire speaker:
Woofer - 8 ohm
Squawker - 16 ohm
Tweeter - 25 ohm
Overall - 8 ohm

I have the schematic for the crossover but I am unable to figure out what I am supposed to do. All I know that lines 1 and 4 are for the tweeter, and that the lower right part of the schematic is just for an overload warning -circuit.

If anyone would be willing to give me a hand in this, I would really appreciate it. I can also provide photos of the crossover if that is needed.

The schematic (click)

Click the image to open in full size.

Last edited by Coconuts 500; 18th August 2011 at 02:50 PM.
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Old 18th August 2011, 03:46 PM   #2
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The tweeter is connected to the upper most xover part - the cap, inductor to ground, then cap... looks like pins 1 &4. Looks like a series resistor, adjustable before that for level control.

Assuming the amp can drive the load, you only have to take this filter, a "3rd order" or "18dB/octave" and dump the present values into an online crossover calculator widget, and see where the xover frequency comes out.

Next, you put a 4 ohm load in place of the 25 ohm load in the calaculator and dork it until the values are good for the same xover freq and the 4 ohm load.

The indicator lights may not work properly with the changed impedance tweeter, but who cares...

This procedure will get you "in the ballpark".

To do it most properly, you need to run a freq response curve looking at the xover region and try to optimize the values of the HP filter going to the tweeter you are putting in, in light of the acoustic rolloff that you find.

Also, the new tweeter has to go LOW enough to meet the driver below, matching the original tweeter's abilities in that regard. Kinda depends on what that xover freq works out to for the original one.

What type of tweeter is it? Dome? Cone?

How about a jpeg of the speaker?

Hope this helps...

_-_-bear


PS. no matter what, film caps will sound better than electrolytics (look at what is in there now...)
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Old 18th August 2011, 03:53 PM   #3
tinitus is offline tinitus  Europe
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sometimes 8ohm tweeters could be more like 5-6ohm

and the 4ohm, maybe 3.5ohm, or even 5ohm, where it matters most

so you can't say that its half impedance, thus needs twice as big series cap
and obendrin there's the different acoustic rolloff to consider
it's not really that simple

but you could mount it as is, and give it a listen
might not need that much adjustment
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Old 18th August 2011, 04:53 PM   #4
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Thank you for your replies. This seems much more complex than I had hoped, I don't fully understand what I am supposed to do. I was hoping that there would be an easier solution.

I guess it would be easiest of all to just dump the current mid range and tweeters (and the entire crossover) and replace those with 8 ohm equivalents, and then make a new filter altogether?

There are no electrolytics in the filter, it's all PIO caps. Here's a picture (not mine) of a filter from a slightly different speaker in the same series:

Click the image to open in full size.

And here's a picture of the speaker itself (also not mine) -

Click the image to open in full size.
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Old 18th August 2011, 11:45 PM   #5
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Ok, first step is to just put an ohmeter across the tweeter's terminals, or the wires that run to the tweeter from the xover.

Figure on the DCResistance being a slight bit lower than the ACR, which is the impedance of the tweeter... if it is truly a 25ohm tweeter (very very unusual) then the DCR will be high, over 16 ohms, maybe above 20 ohms...

You do not need to REMOVE the old caps, you can just ADD capacitance across the old caps to arrive at a value suitable for the new tweeters...

BTW, what is wrong with the old tweeters??

The crossover is really simple - caps pass high frequencies, and coils pass low frequencies. So, you have a cap, then a coil to ground, then a cap... each one is a "6dB/octave rolloff" three together = 18dB/octave rolloff.

The caps electrical size sets the LF point that they work, and this depends on the impedance of the load they see - the frequency does. As the frequency goes LOWER the cap looks more and more like a higher impedance (resistance at some frequency), this works to rolloff the driver as the freq goes down.

Similarly the coil *shunts* the LF to ground, as you go lower it looks more and more like just a piece of wire (almost a dead short).

So, when you change from 25 ohms to 4 ohms you need to scale the impedances that the parts in the xover present.

You can see intuitively that if you have a 25 ohm resistance in series with a 25 ohm resistance the power is split equally between the two resistors. So now if you want the same effect at 4 ohms, you then need a 4 ohm in series with a 4 ohm, NOT a 25 ohms and a 4 ohms...

That's where the online calculator comes in - you just figure out the frequency that the existing xover worked at and then plug in the new impedance (4 ohms) and the old frequency and out comes the new values!

The new caps go in parallel to the old and the new inductor goes either in series with the old or else take the place of the old (just unsolder one end, leave it there and solder in the new part...).

That's it...
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Old 19th August 2011, 12:33 AM   #6
tinitus is offline tinitus  Europe
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25ohm tweeter ?
obviously I missed that

btw, interesting speaker components
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Old 19th August 2011, 12:56 AM   #7
godfrey is offline godfrey  South Africa
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How does the sensitivity of the new tweeters compare to that of the old. If the new ones are more sensitive, then you can pad them down with a series resistor, which will also get the impedance closer to the original.

e.g. A 10 ohm resistor drops the level about 10dB, and gets the impedance close enough that you don't have to change anything in the crossover. It won't be perfect, but will get you in the ball-park. Then you can tweak as needed.
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Old 19th August 2011, 02:17 AM   #8
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re:' you only have to take this filter, a "3rd order" or "18dB/octave" and dump the present values into an online crossover calculator widget, and see where the xover frequency comes out..' tried that, unfortunately the schem shows both caps to be equal value, a 3rd order xover has different values. The xover freq is ~ 6KHz, but that's just a WAG. Maybe it's better simply to scale the component values according to their relative DCRs? (Big caveat, without impedance plots, it's all guesswork...)
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Old 19th August 2011, 11:04 AM   #9
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Common mistake:
"I want to substitute an old tweeter (no references) with this new tweeter (no references), and speaker (no references besides the xover layout)". No brand names/specs, plots, nothing. Hi, hi
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Old 19th August 2011, 11:49 AM   #10
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Idea: If I disconnect the tweeter, and run its wires out of the cabinet... And then connect that to a new cabinet that I make, which contains three 8 ohm tweeters connected in series. Will this work without any modification to the crossover necessary?

The problem is the range of one tweeter, which only goes up to about 9000 Hz. The other one is better but still not great. There's very little overall treble in these speakers, and I like treble. The impedance is indeed 25 ohm. It is stamped on the magnets and I have checked with a multimeter.

Specs: (use Google Translate if you need it)
Tweeter 6???-6-25
Mid range: 20???-1-16
Woofer: 75???-1-8
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