I wish to attenuate a tweeter. Assuming a single capacitor for the crossover for the tweeter, there are 2 ways to attenuate that I am looking into.
1) to use an Lpad of 8ohm.
2) to use a series resistor (with obviously a different value capacitor.)
Please could you tell me what is the practical disadvantage of using option 2?
1) to use an Lpad of 8ohm.
2) to use a series resistor (with obviously a different value capacitor.)
Please could you tell me what is the practical disadvantage of using option 2?
A 2-resistor Lpad between the crossover and driver has the advantage of allowing you to juggle both resistor values to get the desired attenuation while pretty much maintaining a constant impedance as seen by that crossover, thus not shifting the crossover corner, etc.
Paul
Paul
If you tweak a series resistor you have to adjust the crossover as well anyhow. It is less complex buying pairs of series and parallell resisitors keeping the impedance constant. One additional advantage is that it also flattens the resistance, if you have a high Q tweeter that helps a lot. Especially a first order filter
http://www.diyaudio.com/forums/multi-way/234605-philips-ad-0160-tweeter-replacement.html
http://www.diyaudio.com/forums/multi-way/234605-philips-ad-0160-tweeter-replacement.html
I think the thing to keep in mind is that the impedance of the tweeter is not fixed at its rated 8 ohms. Much like a woofer, a tweeter has a peak followed by a deep valley then a gradually increasing impedance as the frequency rises.
So, with only a series resistor, the ratio of the resistor and the driver is constantly changing. Now, the result of that could be good or bad depending on the circumstances and what you are attempting to achieve. Many people use a series resistor to great effect.
However, if you use a series resistor then the impedance load, even the average load, seen by the crossover is no longer 8 ohms. It is 8 ohms plus the series resistor. You would need to design your crossover accordingly.
The Series/Parallel L-Pad provides a constant impedance to the crossover. More so in that the changing impedance of the tweeter is now in parallel with a fixed resistor meaning the shifting impedance as seen by the tweeter is moderated.
Using a common low cost Dayton Tweeter as an example, and assuming 4dB of attenuation.
The L-Pad Rs and Rp values are Rs = 3 ohms and Rp = 14 ohms.
Using this Dayton Tweeter DC28F-8 -
http://www.parts-express.com/pdf/275-070s.pdf
We see that the impedance is 10.6 ohms at 650hz, 6.2 ohms at 3khz, and 10.8 ohms at 20khz.
If those impedance are put in parallel with a 14 ohm resistor then in series with a 3 ohm resistor, the resulting impedances are -
650hz = 6.03 ohm plus 3 ohm = 9.03 ohms
3khz = 4.30 ohms plus 3 ohms = 7.30 ohms
20khz = 6.03 ohms plus 3 ohms = 9.03 ohms
As you can see the result stays closer to 8 ohms. The shifting impedance of the tweeter has less effect on the crossover.
This is a tweeter with moderate shifts in the impedance. Other tweeters can be substantially more. The 8 ohm rated Morel MDT-29 impedance ranges from about 6 ohms up to 15 ohms. A substantially wider swing than the Dayton.
Again, some people use a Series only resistor to great effect. But you have to understand the effect to use it effectively, and you have to design the crossover accordingly.
Just a few thoughts.
Steve/bluewizard
So, with only a series resistor, the ratio of the resistor and the driver is constantly changing. Now, the result of that could be good or bad depending on the circumstances and what you are attempting to achieve. Many people use a series resistor to great effect.
However, if you use a series resistor then the impedance load, even the average load, seen by the crossover is no longer 8 ohms. It is 8 ohms plus the series resistor. You would need to design your crossover accordingly.
The Series/Parallel L-Pad provides a constant impedance to the crossover. More so in that the changing impedance of the tweeter is now in parallel with a fixed resistor meaning the shifting impedance as seen by the tweeter is moderated.
Using a common low cost Dayton Tweeter as an example, and assuming 4dB of attenuation.
The L-Pad Rs and Rp values are Rs = 3 ohms and Rp = 14 ohms.
Using this Dayton Tweeter DC28F-8 -
http://www.parts-express.com/pdf/275-070s.pdf
We see that the impedance is 10.6 ohms at 650hz, 6.2 ohms at 3khz, and 10.8 ohms at 20khz.
If those impedance are put in parallel with a 14 ohm resistor then in series with a 3 ohm resistor, the resulting impedances are -
650hz = 6.03 ohm plus 3 ohm = 9.03 ohms
3khz = 4.30 ohms plus 3 ohms = 7.30 ohms
20khz = 6.03 ohms plus 3 ohms = 9.03 ohms
As you can see the result stays closer to 8 ohms. The shifting impedance of the tweeter has less effect on the crossover.
This is a tweeter with moderate shifts in the impedance. Other tweeters can be substantially more. The 8 ohm rated Morel MDT-29 impedance ranges from about 6 ohms up to 15 ohms. A substantially wider swing than the Dayton.
Again, some people use a Series only resistor to great effect. But you have to understand the effect to use it effectively, and you have to design the crossover accordingly.
Just a few thoughts.
Steve/bluewizard
Bluewizard, good point. I should add that the calculator I referenced would provide you with a good starting point, but some measurements made at that point would help you see the response with that pad. Adjustments and further refinement (including listening evaluations) could be effected.
I took a several hour tour of the old Glendale JBL works right at the time the L-166 was entering the sales phase. An engineer candidly told us that they were working lots of state of the art science and measurements, but always in conjunction with listening and some empirical wisdom.
I think it would be safe to say that the results were successful.
I took a several hour tour of the old Glendale JBL works right at the time the L-166 was entering the sales phase. An engineer candidly told us that they were working lots of state of the art science and measurements, but always in conjunction with listening and some empirical wisdom.
I think it would be safe to say that the results were successful.
The result will be response errors. The L-pad has some immunity from this compared to the single resistor.
AllenB,
When is an error not an error?
Answer: When it works.
But in the general sense you are right, and that is what I tired to convey. Since the crossover doesn't have a constant (or near constant) load, it has be designed accordingly. It has to be design to the series resistor PLUS the nominal impedance of the Tweeter.
It will give you uneven attenuation because the resistor ratio will keep shifting. But again, I've seen designs that seem to produce good results, both DIY and commercial, that use a single resistor.
I personally wouldn't do it when the impedance stable L-Pad option is available. But it must work for some people some of the time, or people wouldn't use a single series resistor.
So, while I slightly bristle at your use of the word ERROR, I can't actually say you are wrong in the general sense.
If we use the 4dB attenuation in my example above. The average ratio of the Series section to the Parallel section is 5.2 to 3, or 1.73:1. If we use the nominal impedance of the Dayton Tweeter (8 ohms), then the series resistor needs to be 1.73 times that or 13.9 ohms. That makes a combined impedance of (13.8 + 8) 21.8 ohm.
I would assume you would have to design the tweeter section of the crossover to accommodate a 21.8 ohm load. Also, if you find you need a different attenuation, and therefore a different Series Resistor, then you need to consider how much that change is going to throw off your Crossover, and you might need to redesign the crossover. Seems like a lot of unnecessary complexity, when you have a impedance stable option available.
Personally, I would just go with the impedance stable L-Pad.
But then, that's just me.
Steve/bluewizard
When is an error not an error?
Answer: When it works.
But in the general sense you are right, and that is what I tired to convey. Since the crossover doesn't have a constant (or near constant) load, it has be designed accordingly. It has to be design to the series resistor PLUS the nominal impedance of the Tweeter.
It will give you uneven attenuation because the resistor ratio will keep shifting. But again, I've seen designs that seem to produce good results, both DIY and commercial, that use a single resistor.
I personally wouldn't do it when the impedance stable L-Pad option is available. But it must work for some people some of the time, or people wouldn't use a single series resistor.
So, while I slightly bristle at your use of the word ERROR, I can't actually say you are wrong in the general sense.
If we use the 4dB attenuation in my example above. The average ratio of the Series section to the Parallel section is 5.2 to 3, or 1.73:1. If we use the nominal impedance of the Dayton Tweeter (8 ohms), then the series resistor needs to be 1.73 times that or 13.9 ohms. That makes a combined impedance of (13.8 + 8) 21.8 ohm.
I would assume you would have to design the tweeter section of the crossover to accommodate a 21.8 ohm load. Also, if you find you need a different attenuation, and therefore a different Series Resistor, then you need to consider how much that change is going to throw off your Crossover, and you might need to redesign the crossover. Seems like a lot of unnecessary complexity, when you have a impedance stable option available.
Personally, I would just go with the impedance stable L-Pad.
But then, that's just me.
Steve/bluewizard
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I used 'error' to simply mean that it is causing something extraneous. It can help to see it this way.. some errors can be fixed by way of a compromise and some will need to be corrected independently.
This particular problem comes down to convenience and component count. Errors are not a bad thing, but they still need identifying. There is also the element of finding an efficient way to succeed.
In what way would you say that an L-pad is stable?
This particular problem comes down to convenience and component count. Errors are not a bad thing, but they still need identifying. There is also the element of finding an efficient way to succeed.
In what way would you say that an L-pad is stable?
Just by theory it would seem that higher frequencies would be attenuated more than lower ones. As F increases the tweeter becomes higher in impedance and shift the emf balance towards the resistor. It could be used as a tool if you wish to attenuate AND darken the tweeter, maybe take some of the piercing effect out.
Actual component configuration can alter basic theory.
Actual component configuration can alter basic theory.
I am building K&T CT235 loudspeaker (Eminence Beta 10CX woofer + Eminence APT50 supertweeter). The crossover uses series resistor for the tweeter. It has one advantage over L-pad that the combined impedance is much higher than with L-pad. The compression horn drivers have some peculiarities and prefer high damping factor amplifiers. With valve amps the new higher impedance of the tweeter will produce the same effect as having higher damping factor amp. Besides, it seems that with higher impedance driver lower value crossover elements can be used that are cheaper and less bulky.
In some cases series resistor is a good trick.
In some cases series resistor is a good trick.
Allen, keep in mind I actually didn't disagree with you, I just thought the statement needed a little more context, which you have now explained very nicely.
An L-Pad (by itself) provides a stable purely resistive load to the crossover and to the amp within the bounds of normal realistic operating temperatures.
The only unstable, or inconsistent, element is the tweeter itself which is not fixed at a nominal 8 ohms but changes from say about 6 ohms up to about 12 ohms.
However, the presence of the L-Pad and more specifically the parallel resistor will soften those impedance swings. So to the degree that it is possible, the L-Pad helps present a consistent impedance to the crossover and amp.
The whole purpose of the L-Pad is to give you the attenuation you need while at the same time, to the extent possible, maintaining a consistent impedance as seen by the crossover.
So the combination is not absolutely stable, but it is more stable, or perhaps more consistent.
Seems a small point though. Again, I never said you were wrong.
Steve/bluewizard
Steve, we're on the same page 
With regards to L-pads, the tweeter will begin to appear as a pure resistance at some point but this will be at a high level of attenuation. At typical levels the impedance will be somewhere between that of the tweeter itself, and the resistance.
If you imagine an L-pad and tweeter connected directly to an amp it can be shown that the series resistor and the parallel resistor are both in parallel with the tweeter. In order for the impedance to appear closer to resistive, at least one of these resistors needs to be small compared to the tweeter. So if that is the intention, the L-pad is preferred.
The higher the tweeter impedance at some point, the lower the attenuation will be. Series resistance causes the frequency response to take on a shape that resembles the impedance curve itself.Wavewhipper said:
With regards to L-pads, the tweeter will begin to appear as a pure resistance at some point but this will be at a high level of attenuation. At typical levels the impedance will be somewhere between that of the tweeter itself, and the resistance.
If you imagine an L-pad and tweeter connected directly to an amp it can be shown that the series resistor and the parallel resistor are both in parallel with the tweeter. In order for the impedance to appear closer to resistive, at least one of these resistors needs to be small compared to the tweeter. So if that is the intention, the L-pad is preferred.
I came back and caught that. I was in a big fuzz for a few days over a 3 way crossover and got it backwards. After 30 years ? Yep, I am so ashamed I somehow did not see the correct EMF, Q, and bandwidth.
Well, at least with an L pad, the bandwidth stays more constant over L-pad settings.
Well, at least with an L pad, the bandwidth stays more constant over L-pad settings.
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My friends and I had a simple cap and inexpensive lpad that we used for 2-ways for years, they sounded great, many of them still going (made approx 50).
As far as "series" resistor, I thought these sounded better than others, The Madisound Speaker Store
The eagle brand was also good, hard to find them now.
As far as "series" resistor, I thought these sounded better than others, The Madisound Speaker Store
The eagle brand was also good, hard to find them now.
I use L-pad only when the attenuation needed is too much. This is in combination with a series resistance but at the input of the crossover, not next to the tweeter.
All of this approach is based on listening. Technically, it is very simple. But when listening is concerned, that is what I want.
All of this approach is based on listening. Technically, it is very simple. But when listening is concerned, that is what I want.
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