Swapping out OEM tweeters with lower inductance replacements. - diyAudio
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Old 9th January 2013, 08:50 PM   #1
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Default Swapping out OEM tweeters with lower inductance replacements.

Hi, folks,
I am replacing the fried tweets in my Realistic T-200's. I am experimenting with a couple of choices and I need to pick the brains of the more educated than I.
The OEM's were 6 Ohm, 20W paper cone tweeters. The VC's are shot as I had pulled the cones off to check them so they are done. I ordered a pair of HiVi tex-dome tweeters that I will try first. They are 6 Ohms with an Spl of 92Db. I do not know any specifics of the originals so I am kind of flying blind. If these 92Db ones are a bit bright at least I can pad them down a bit with a twist of the knob. These speakers have l-pads and just first order x-overs. I am also getting a pair of paper dome tweets that closely resemble the originals but they are just 4 Ohms. I do not think that I would be able to drop the Db's down enough with the l-pad if I use the 4 Ohm ones so I was thinking of a Dayton non-inductive, 10W/2 Ohm resistor on each tweeter.
Is this okay? I never got a chance to see how it sounded originally as I just got these so I am starting new. I already ordered the parts from Parts Express that includes the HiVi tweeters and new caps for the existing crossovers. One of the original drivers were shot but Ken over at Upland Loudspeaker Service performed his "magic" on it. He also set me up with three more surrounds and dust caps so I can finish the remaining woofers to match what he did.

Thanx!
Bud
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Old 9th January 2013, 09:05 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

L-pads can cope with almost any tweeter, at lower settings they will
be near 8R, at higher settings 4R and 16R tweeters will vary the load.

rgds, sreten.
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Old 10th January 2013, 12:35 PM   #3
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I presume you are "re-foaming" the woofers?? Not sure why you need or want 3 surrounds for one woofer, but maybe there are 2 woofies per box and one got fixed by the pro?

Anyhow, you didn't say what the xover is/was?

Is it *just* a single cap??

The ideal circumstance would be to make a determination as to how many dB of cut you need/want, then to put in a discrete L-Pad that preserves the impedance that the xover sees. This will keep the xover point where it is supposed to be - assuming that where it is supposed to be is even good/right/correct/or sensible.

Otoh, it's a Radio Shack product, so not a whole heck of a lot of design work went into it in all probability. So, what you might want to do is to download some FFT freeware and use whatever mic you have on hand (non-critical actually) and have a visual look at the response of the LF and the tweeter and see what they are doing as far as roll off and frequency response across the several octaves either side of the crossover point.

Then you can make a decision as to how to set up the xovers properly for best response and result. Otherwise you'll be making a big compromise as to level setting, unless ur lucky and both LF and HF are very well behaved in the xover region - which would be nice if it happened this way.

_-_-bear

For example:

You might expect to be making the xover at - for example - 2kHz. So you set by the value of a cap that makes the tweeter down 3dB on the high side of 2kHz, the woofer already being set by the mfr. However this presumes that the response of the LF and the HF sections are *flat* for at least an octave before and after the xover point. IF the woofer is *rising* in response and the tweeter is flat then it may be necessary to move the tweeter xover higher in frequency in order for the two to sum close to flat. Otherwise you may get a hump in the frequency response near or at the xover point. So you may have to select your frequency points and/or change the type/slope of the xover itself, and/or put in a "zobel" and/or "notch filter" into a driver's xover.
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Last edited by bear; 10th January 2013 at 12:53 PM.
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Old 10th January 2013, 04:23 PM   #4
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Hello, Bear, I'll elaborate a bit further. These speakers are 8 Ohm cabs. There are two, 10" 16 Ohm drivers in parallel wired directly to the posts. The mid and tweets both run up from a single cap through the l-pads so it is a pretty basic first order XO. Seems pretty lame since these were touted as the TOTL speaker for that time period. Ironically, my Mach III's have a pretty well thought out XO that actually have quite an array of inductors, resistors and caps but I must digress. Anyway, there is a 3.3uF/200V poly cap for the 4 ohm tweeter and a 33uF/50V N.P. cap for the 8 Ohm mid squakker as well as a parallel buffer resistor on it's l-pad. The l-pads are supposed to trim the levels from 1-5kHz for the mid and 5kHz & up for the tweets so I figure that these are the crossover points for the cabs. There is no baffling inside just fiberglass insulation to knock down the bass. Aside from that these sounded remarkably good considering I was just hearing the two ranges. It was hard to figure the roll off or the presence of the timbre since I was lacking the tweets. The tweeters that are on these have a dispersion horn screwed onto them which, I figure, also adds to the aesthetics more than the overall functionality but I could be wrong.
I have some really good speakers in my collection that rival these Rat Shack pieces but yet I have a soft spot in my heart for all things "Realistic" and besides these cabinets are gorgeous pieces! In fact, I am completely rebuilding an STA-2100D on my bench right now which will pair up to these with the Lab-400 TT.

This is the scoop..
Thanx!

Bud
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Old 10th January 2013, 05:44 PM   #5
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visaton FRS5X may be worth a try at 6 each. Their Fs is low and they may handle 1st order depending at what frequency the crossover occurs. The fly in the ointment may be their efficiency, sound-wise theyre far better than the pricetag and one of the few cone tweeters i could find. Alternatively their TW70 may also be suitable, especially as it appears to be suited to a 1st order filter.
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Old 11th January 2013, 03:51 AM   #6
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69sixpackbee,

I suspect there isn't much that you can do to make them get worse!
Sorry to say it that way. I think the only way is up...

You might want to consider adding a simple inductor to roll off the midrange??

But, I'll stick with the basic suggestion - download some FFT software, use any mic, and any soundcard (including internal) and see what the speakers do one at a time and collectively. Then make some decisions.

That's the best way to go. Otherwise ur just feeling around in the dark, pretty much.
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Old 11th January 2013, 08:21 PM   #7
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I'll try it.

Thanx Bear!
Bud
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