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I got good advice here about a tweeter replacement for my R A Labs Mini Reference speaker with blown tweeter. I got and installed the new tweeter, but now needs to be padded down a couple of db. I do not want to cut into the enclosure for an externally accessible adjustable L-pad. So what value resistor should I put on the wire to the tweeter? Thanks for any forthcoming advice.
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I will have to finish the project when I get back from my safari in Kenya. My son, who lives in Nairobi working for the Clinton Foundation HIV/AIDS Initiative, is flying his mother and I there for our 40th wedding anniversary.
I have not read any bad advice here. All good points.
There is a simpler way to go about this, however, and not all will agree. Many crossover designers insist that padding be AFTER the crossover and the resulting impedance is used to calculate crossover values. When you insert padding then you've upset the balance of the crossover. If you keep changing the padding resistor(s) after the crossover, you have to make sure that the values you use keep the resulting impedance the same. That takes a lot of calculating if you don't have a program to do it for you. If you have access to David Weems' original book on speaker building which he did for Radio Shack, he has the necessary formulas listed to calculate resulting impedance when using a pad with a series and a parallel resistor. That's help me many times.
Here's what I would do: If you can isolate the lead going to the tweeter BEFORE the cross over take two 1 Ohm Ten Watt resistors and stack them in parallel. That gives you a 2 Ohm, Twenty Watt resistor to put in line before the crossover. (I already hear the disagreements.) If you need more padding, add another 1 Ohm resistor. Then you'll have a 3 Ohm Thirty Watt resistor. An even better way is to do what was suggested and buy an assortment of values. I like to stack resistors for this application because the higher Wattage value is better when you're tapping off the speaker input and going to the tweeter before the crossover.
The next best thing is a variable L-Pad and if you get a quality L-Pad you'll get many years of good service out of this.
...
When connecting resistors in Parallel, THEN combined resistance does not sum but instead REDUCES in value (two same R-value divide by 2 & three same R-value divide by 3, etc). Parallel resistors Do sum their Wattage rating, but not their resistance values, their combined R value shall Always be Less than the smallest-value resistor of the combo. OTOH Series-connected resistors Will sum their R-values but Not their wattage ratings (which will only be what individual resistors are rated). He needs to re-visit the combined resistance formulas also present in Weems' Little Red Book.
Edits: 12/22/11 12/22/11 01/01/12 01/01/12
I still think that its going to be very difficult to get this right w/o a lot of removal and reinstalling of the tweeter if you tryu and use fixed value resistors. At most, an LPad will require a single 3/16-1/4 inch hole in the rear of the speaker. Getting the resistance spot on for padding requires listening to lots of different recordings of various linds of music, and even there, a slightly higher output from the tweeter than flat could restore lost highs especially on older recordings. Not only that, you have room acoustoics to deal with. A speaker in a room with lots of carpet and drapes will require a brighter tweeter setting than a room which is mainly sheetrock and wood floors, for example. Even a slight repositioning of speakers could drastically alter the amount of output that sounds right from the speaker. There are a host of room acoustics variables affecting exactly how much output a tweeter should ideally produce relative to a woofer, and you can fine tune with just a simple L-pad at the cost of a single drill hole for the L-pad shaft. That way you can fine tune the tweeter output till the cows come home.Given the fixed resistor route, I can almost guarantee that you will be second guessing what you did on your first, second and third try, not to mention your fourth or fifth try and you are only going to end up frustrated.
I suppose that if you kept the one tweeter that was working ok in the one speaker, you could attempt to make the speaker that you repaired sound as close as possible to the original factory version, but if you try that approach, it would be far easier using an L-pad than installing and reinstalling the tweeter each time with slightly different resistor values. And who is to say that the speaker came from the manufacturer with the tweeter set up to interact perfectly with your particular room..chances are that too is just a tad too bright or dull. You see where this is going.
My speaker is a sealed enclosure. How do you maintain the air-tightness with the L-pad shaft hole? How do you ensure that the L-pad I buy can allow an air-tight seal with the cabinet?
if you drill the hole the exact size there will be little if any air leak.But you can use speaker or roll caulk to seal the L-pad from the inside. How are you sealing the tweeter to make certain it is air tight? Use the same stuff. You can also use closed cell door and window foam to make a gasket for either the tweeter or the pad which fits between the tweeter or the pad and the wood of the cabinet.
You have a good point. But this is a speaker for a tertiary system , so convenience and keeping expense to a minimum are important factors. I do still have the one good tweeter in the other speaker for comparison. As of now, the new tweeter is slightly brighter than the original--hardly noticeable when the two are working together in stereo. Since the new tweeter is rated at 92 db and the original speaker rating is 90 db, I figure a 2 db attenuation of the new tweeter will be close enough.
Hi Tadlo, Scroll down to "Circuit & Box Design", then 9th one down, "Driver Attenuation Circuit", put in your parameters and it'll calculate what resistors you'll need based on how far you'd like to "mute" your tweeters. 3 db is "supposedly" the smallest difference you're able to hear. Buy cheap resistors until you dial in the match you want, then if you feel the urge, buy some higher quality ones. In a few years, you'll be here with the rest of us wondering what you can improve now.....
Thanks for the line. It was helpful.
THEN you can select the proper resistor pair to pad down the tweeter.
From the data shown in an earlier thread, the impedance is around 5 ohms at 3 kHz and the impedance curve is very flat in that region. So a 1.5 ohm series R and 20 ohm parallel will give about 2 dB attenuation and keep the impedance the crossover sees fairly constant.
Jerry
What should the rating in watts of the resistors be?
I'd use Dayton non-inductive 10W resistors. They are available from Parts Express, and are inexpensive; but quite good.
Jerry
OK, thank you very much for the help. Finishing the project will have to wait til the first of the year, however. I leave tomorrow for Kenya to have Christmas with my son who works for the Clinton Foundation battling AIDS in Africa.
nt
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If you need 2 dB and don't want to change the crossover frequency, consider using a 2 resistor L-pad. 2 ohms in series, and 20 ohms in parallel with the tweeter will give about 2 dB of attenuation, and keep the impedance more or less constant. Just be sure the parallel resistor is closest to the tweeter, so the 2 ohm is in series with both tweeter and 20 ohm.
Jerry
Thanks, I think I get it now. I don't have to buy an L-Pad do I? I just get two resistors--a 2 ohm and a 20 ohm--and connect them to the wires to the tweeter in the way you described--right?
Nope, just a couple of resistors. Nice part is that you can hang it all on the back of the tweeter, so you don't need to open the cabinet or trace out the crossover. By the way, it's called an L-pad because the two resistors form an L. There are also T pads; but you don't need one of those.
The rotary control we call an L-pad is really a variable L-pad and as you rotate the shaft, it changes the series and parallel resistance to maintain a constant input impedance.
Jerry
OK, thanks, Jerry, now I get it.
Thanks a lot, but is there an easier way? Couldn't I just put a single resistor of the right value on one of the wires between the crossover and the tweeter. Please take into account that I know very little about electronics.
One resistor will work, but it will shift the crossover frequency a little lower. Two resistor pad will keep the same impedance, same frequency.
Thanks. Do you know how much lower or how I can calculate how much?
I like to keep 1,2, 3 and 4 ohm resistors in stock in pairs two of ech value. With those you can make any combination 1,2,3,4,5(3+2),6 (3+2+1), 7 (3+4), 8 (3+1+4), 9 (3+4+2)and 10 (1+2+3+4)
just 1, 2 and 3 can be used to get up to 6.
try just 1 ohm first, then 2 and so on.
Another option..wire in a pad in the circuit but just leave it inside the cabinet adjusted to where you want it. Some trial and error installing and reinstalling the tweeter will be involved but so also will the experiments with the individual series resistors.
For that matter you dont need to cut a big hole in the cabinet to install a pad--just a drilled hole 1/4" or so for the shaft long enough so the knob can be placed over...the rest of it is inside the speaker.
David
Thanks, David. To narrow the field a little, do you have any idea how ohms of resistance correlate with db of attenuation?
The amount of attenuation for a given resistor depends on the impedance of the tweeter. For example, a 1 ohm resistor will give twice the attenuation with a 4 ohm tweeter as it will with an 8 ohm. With an 8 ohm tweeter, 1 ohm will give about 1 dB of attenuation.
Jerry
thats an empirical experiment that uses the ears.
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