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I have some 3-prong attinuators and I need help installing them so I can adjust the volume on a pair of compression drivers.
Is the middle prong unnecessary? Is this device connected in a parallel circuit to a single connection wire between the crossover and the compression driver; right? Do you use only the left and right prongs?
Thank you.
http://www.hostboard.com/forums/hbmc.../2012/11/1.gifQuote:
Originally Posted by robertbartsch
A picture is worth 1000 words, isn't it? :)
thank you!
Being naive, I would have guessed that using connections 2-3 would simply act as a variable R?
What is the purpose of connection #1? Is it some sort of negative feedback?
You can use just 2 legs as a variable R, it's an old trick. But that's not the point of the L-Pad. The point is that it presents a more or less constant impedance to the amp and/or crossover. So the crossover always sees 8 ohms, or 16 ohms, depending on the model. A simple pot would vary that impedance and change crossover points and other things. The L-Pad allows you to attenuate and not change impedance. Np matter where you turn the knob, it's approximately 8 ohms.
If you dope out the circuit, there is a series resistance and a parallel to "ground'. As the series resistance goes up, the parallel resistance drops.Quote:
Originally Posted by gortnipper
Think it thru. An 8 ohm speaker with a 4 ohm series resistor is 12 ohms. Referring to Ohm's law...24 Ohms in parallel is needed to bring it back to 8.
So you attenuate more, and have 8 ohms in series (16 total)....16 in parallel gets you back to 8.
Get the idea? As series goes up, the parallel has to go down.
You can calculate and put in fixed resistors...many do.
For the math impaired, a parallel resistance calculator...for the series value just add.
PARALLEL RESISTOR CALCULATOR
As was aptly put "approximately 8 ohms". there are no perfect components. Fixed resistors are closer to ideal than variable, since to be 'perfect' a variable would have to "track' exactly, and none do. Also, if you go fixed, you can hand select components.
On original Model 19 XO's. Altec only used 2 taps on the L-pads on the Mid-freq and High-freq. attenuator. Why ?
Here are my original unmolested 19 XO's.
http://www.hostboard.com/forums/hbmc...2012/11/62.jpg
http://www.hostboard.com/forums/hbmc...2012/11/63.jpg
http://www.hostboard.com/forums/hbmc...2012/11/64.jpg
http://www.hostboard.com/forums/hbmc...2012/11/65.jpg
I suspect the signal was not passing thru that L-pad to the driver. That one likely was wired to "short" (shunt is a more proper term) a bandpass filter to 'ground" rather than reflect a constant impedance. You can't "boost" a frequency in passive, just attenuate one. Or actually a "band" So to get extra highs the driver has to be turned up, then the mids attenuated. Wiring it as an L-pad would have attenuated the whole driver.Quote:
Originally Posted by Altec Best
Ta-da! Here is the HF portion thx to Messrs. Marquardt and Easley. Note wired to ground.
http://www.hostboard.com/forums/hbmc...2012/11/66.jpg
Thanks OG ! :Big smile: From that schematic it looks like tap #3 is unused or goes to the ground. Am I reading it right ?
3 is unused. 2 is wired to ground, thru a series resistor that doesn't allow it to reach zero- ie at least 2 ohms to ground. 3 is just hanging out with nothing to do, sorta like me.
I noticed in the photo that the other lpad (MF?) is wired with tap 1 unused...?
This L-pad seems to be configured as a basic speaker volume control.Quote:
Originally Posted by Earl K
Depending on the value of the L-pad from mostly on to full off. Yes?
Yes, that's how they work.
Correct A.B.. In this use it is for EQ, not overall level, so you do want the impedance of that section to change.. Might not even be an L-Pad, just a power resistor (pot).
[QUOTE=Old Guy;1844179] then the mids attenuated. <snip> Ta-da! Here is the HF portion [QUOTE]
That looks like the MF attentuation, or is that what you meant? And why are they doing that on the HF as well?
[QUOTE=Cal Weldon;1844198][QUOTE=Old Guy;1844179] then the mids attenuated. <snip> Ta-da! Here is the HF portionIt's two filters. Not just one. Each filter has a level control In this circuit they are just variable resistors.Quote:
That looks like the MF attentuation, or is that what you meant? And why are they doing that on the HF as well?
When used as an L-pad it is wired AFTER the filter, not as part of it.
So, why is the HF section wired with tap 3 unused, and the MF wired with tap 1 unused?
Because they are not used as L-pads. I've been trying to say they are part of the complex filter (since it is two combined).Quote:
Originally Posted by gortnipper
http://home.earthlink.net/~jmarkwart...1201-8aRev.pdf
Just look at the schematic- do you see one AFTER the filter? An L-pad goes between the filter and the load. As has been said, you can use just one section, and it is a variable resistor.
See Rontec's post...the crossover normally would be BEFORE the L-pad. In the model 19 the VARIABLE RESISTOR- not being used as an L-Pad- is IN the filter, or complex filer as it is two combined...hope it makes more sense now? An L-Pad only varies VOLUME (theoretically), where as in the Altec they are used to vary bands not the entire out put. I think your mistake is you think there is a separate HF section- there is not. there is only one driver. So you have to vary the response in the crossover, you don't have a mid and high driver you can turn up and down independently. THEN you would use L-pads. I apologize if not clear, but a LOT of engineering by better men than I went into the 19 crossover.
Yes, that makes more sense to me now. Sorry, I can be slow!Quote:
Originally Posted by Old Guy
The reason I am asking this is because I have been mucking about with a L-pad wired onto (but after) my crossover as both a variable resistor and an L-pad in order to tune the GPA-IIIs horn to the cab/room better. I found that by using it as series-R instead of an L-pad, the sound was slightly better - a bit more "clear" is I guess is how I would characterize it, and I am trying to better understand why.
Don't worry. I can be slower! Have had more years to practice!Quote:
Originally Posted by gortnipper
There are so many variables. Could be: L-pads don't track well. Maybe having a higher load flattens the crossover response. maybe it's not flatten the response but instead enhancing freqs you like...see what I mean? Nobody else has a setup exactly the same. That's why testing beats predicting every time. I just resign myself to some things I won't ever figure out...such as Redheads.
I might have to try that.Quote:
Originally Posted by gortnipper
Dude - I am in the same boat. I have my own Redhead as well...Quote:
Originally Posted by Old Guy
I just used an alligator clip on the first leg - it makes it real easy to A/B.Quote:
Originally Posted by cradeldorf
If you think your hard at understanding... If you could regale me as to which of the 3 prongs your using I would be most appreciative. Are you leaving out the ground wire then? Which wire goes where? And if done that way shouldn't it be before the crossover?
Lpads have two wirewound resistors in them. As OG said, one is in series with the input, while the other is parallel to ground.
If you use them as rheostats (as shown in the M19 examples above), the two rheostats act differently, so you can get different functions from them if you use pins 1/2 or 2/3. If you use the 1/2 rheostat, the resistance range is 0 - infinity (well, not really, but the min setting is no impedance, and the max setting is infinity - there's a "usable" maximum resistance in there before it goes to infinite resistance, depending on the wire that's in there, but usually around 3x the nominal rating of the LPad if I remember correctly). If you connect to the 2/3 rheostat, it acts quite differently. As OG said, the parallel resistance is reversed. So at the "min" setting, the connection across 2-3, will see the rated resistance value of the LPad (typically 8 ohms). As you crank up the setting, parallel resistance will move towards zero.
So put another way:
PINS - - - MIN - - - MAX
1/2 - - - zero - - - infinity
2/3 - - - 8 ohms - - zero
Okay, I get it now, That was an awesome description. Thanks Mike. :)Quote:
Originally Posted by Alien_Shore
Also found this,
http://www.bcae1.com/lpad.htm
The jury is still out, Redheads are either God's gift to man, or the female half of the race's revenge for everything males have ever done.Quote:
Originally Posted by gortnipper
It is a paradox that they can be both within nano-seconds of each other...Quote:
Originally Posted by Old Guy
I wired mine 2/3 on the positive leg after the xo.Quote:
Originally Posted by cradeldorf
Who knew that a humble L-Pad could spark such a discussion? :Star: