Resistor error in thinking...

[Wolf]

https://www.youtube.com/watch?v=zoLBnEdVP6c

I know this is 2 years old, but I just saw it, and figured I'd relay the info. I could not post a comment on the youtube page for some reason. Also of note that there are MANY other ways to attenuate a tweeter, and that only a 2-resistor after is known as the conventional L-pad. And I also know that a lot of newer builders follow Toid's advice.

My main concern is the tweeter resistor before the network will NOT see loads more power than those after. It is still under the effect of the highpass to where its frequency response is limited. Connect the negative line and run the circuit. You get a series LCR across the amplifier terminals with regards to the xover and resistor. The tweeter is really a resistor across the highpass coil. The fact that the cap is there eliminates the lower frequencies from passing the resistor.

Using the same method but adding the shunt resistor before the xover is where a lot of newer builders make the error and heat resistors. This places a pair of resistors across the amplifier in series, and then they get everything the amplifier is sending.

There are many methods of contour filtering with various arrangements, but anything that is seen by the amplifier without a capacitor in the string is where power and heat may become an issue.

Please, be aware of this as fires can result by being careless if driving under louder levels. Trace your circuit, and see if a low bass signal can get from + to - without being impeded..

Best regards,
Wolf

[Geoff Millar]

Thank you, I've often wondered about that; apart from two experiments, all my crossover designs have been other peoples' but it's good to know some do's and don'ts.

Geoff

[tktran]

Hi Wolf,

not sure if I understand the wording entirely.

here is a picture of a my crossover





is my 2.7 ohm resistor at risk of absorbing too much, because it is the first resistor in series with the music signal?


regards,
Thanh

[Wolf]

You are using my preferred method of Lpad, and there should be no problem with it.
Wolf

[DeZZar]

is my 2.7 ohm resistor at risk of absorbing too much, because it is the first resistor in series with the music signal

Yes. It's directly after the amp receiving the full signal. If it was after the 15uf it would only see the high pass frequencies and dissipate less energy.

If it needs to be this way, perhaps for impedance reasons, I would always put several resistors in parallel to share the power dissipation. Just remember that the resistance is shared so to achieve 2.7ohm with two resistors in parallel you need two 5.4ohm or three 8.1...and so on.

[DeZZar]

ok....maybe I read the question wrong?

According to simulations of this xover this resistor is at risk of dissipating more than 5W from 20W+ input, 10W from 35W+ input and so on. So depending on the power rating of the resistor - it could be at risk of over extending itself. Sorry if that's not what was being asked?

[rpb]

How loud is the tweeter playing at those power levels?

[Wolf]

I have a sneaking suspicion that the power dissipation plot in Xsim on the fore resistor is exaggerated. I've not sat down and tried to test my theory, but something tells me this is the case. The one after is in series with the tweeter, so they share the load together. But, the fore resistor is still highpassed and should not have a problem in reality. I've never had a fore resistor get warm if using sandcast or ceramic resistors. The one resistor that gets a little warm at any rate is the metal oxide variety, but that's not due to position in the circuit.

[a4eaudio]

Here is my interpretation, Wolf , let us know if I am interpreting correctly...Toid123 calls all of these an "L-pad"

A is fine, but is simply a resistor in series after the crossover and is not called an "L-pad"
B is fine and is called an L-Pad
C is a resistor in series before the crossover and is not called an "L-pad"

DeZZar and Toid123's video both are saying that scenario C can be bad because the resistor may see a lot of power. Wolf is saying just the resistor in series (C) is okay, but adding the resistor in parallel (D)(i.e., putting a full L-pad in front of the crossover which is not the case in C that Toid123 discusses) is bad because it then may see a lot of power.

[rpb]

Hi Wolf,

not sure if I understand the wording entirely.

here is a picture of a my crossover





is my 2.7 ohm resistor at risk of absorbing too much, because it is the first resistor in series with the music signal?


regards,
Thanh

The 12uf cap will limit the current, even though a full range signal is applied.. If the speaker is going to be played at 90dB or so, I don't think there's any need for concern. If you are pushing it to 100dB, the resistor might get warm. If this cap were a 30uf, the resistor might get warm with lower SPL levels since more mid-range would be passed by the resistor. I like to touch all of the resistors after a few loud songs. I seldom have a significant amount of heat that I can feel in a tweeter circuit. I always check for heat from resistors in the mid-range portion of a x-over. I have wired stuff wrong before, and found the error later. I shake my head, and say "I can't believe I did that." This is usually with the parts spread out on the floor connected by jumpers.

It is wise to question things, and be as safe as possible.

[DeZZar]

I like to touch all of the resistors after a few loud songs. I seldom have a significant amount of heat that I can feel in a tweeter circuit.

Likewise. I have a current circuit with a resistor placed here and after playing music at an uncomfortable level I can barely feel any temp change. That being said I still doubled up the resistors just for peace of mind.

[Wolf]

The 12uf cap will limit the current, even though a full range signal is applied.. If the speaker is going to be played at 90dB or so, I don't think there's any need for concern. If you are pushing it to 100dB, the resistor might get warm. If this cap were a 30uf, the resistor might get warm with lower SPL levels since more mid-range would be passed by the resistor. I like to touch all of the resistors after a few loud songs. I seldom have a significant amount of heat that I can feel in a tweeter circuit. I always check for heat from resistors in the mid-range portion of a x-over. I have wired stuff wrong before, and found the error later. I shake my head, and say "I can't believe I did that." This is usually with the parts spread out on the floor connected by jumpers.

It is wise to question things, and be as safe as possible.

If the cap is a 30uF, then even a series resistor after will get equally as warm as the first, and for the same reason.
David is correct in his assessment of my wording.

Another place resistors get hot after highpassing is the parallel resistor spot having too low a magnitude. This can cause too much current to flow.

Never place resistor in series with a woofer, and if singly across the woofer, use higher value and wattage ratings.

Later,
Wolf

[billfitzmaurice]

I agree with Wolf. I can't think of any good reason to place a resistor before the high pass filter, and there's a few why it's a bad idea. I also can't think of a good reason to use anything but an LPad after the filter. Since you can't know exactly how things will balance out without audio testing I use an adjustable LPad temporarily wired in place, test the system, adjust the LPad to the degree of desired attenuation, then measure the LPad itself to determine the resistor values to use in a fixed value LPad.

[AEIOU]

I've attempted to do that, the problem being is that the usual variable (adjustable) wire wound L-pad is horrible, doesn't maintain a very constant impedance at all. The ones I've measured vary from 7-11 Ohms and this was with an 8 Ohm non-inductive resistor in place of a tweeter, so there was very little reactive component.

[Steve Lee]

AEIOU - The suggested technique is still valid even if the quality of your L-Pad (Potentiometer) is not.

[tktran]

Can you recommend an adjustable L pad product/model?

[Wolf]

Bill, I'm okay with a resistor before the xover. In fact, I normally use a split Lpad with the series before, and the shunt after. I'm trying to debunk the myth that seems to be popular thought.

There is no bad reason to use a resistor in series before, and it's actually an improvement in impedance phase as a result. This also does not tilt the response like the one after can. The 2 best ways to attenuate and not change the spectral shift are as my split example.

Later,
Wolf

[rpb]

Bill, I'm okay with a resistor before the xover. In fact, I normally use a split Lpad with the series before, and the shunt after. I'm trying to debunk the myth that seems to be popular thought.

There is no bad reason to use a resistor in series before, and it's actually an improvement in impedance phase as a result. This also does not tilt the response like the one after can. The 2 best ways to attenuate and not change the spectral shift are as my split example.

Later,
Wolf

I spent an hour trying to figure out how to word my post, because I was unsure of what you were saying. This clears it up. We are in complete agreement as far as I can tell.

[Billet]

I also spent about an hour working on a post, only to delete it. This is a complex topic for sure!

[rpb]

I have some old Radio Shack L-pads if anyone wants them.

[DeZZar]

I have a sneaking suspicion that the power dissipation plot in Xsim on the fore resistor is exaggerated. I've not sat down and tried to test my theory, but something tells me this is the case. The one after is in series with the tweeter, so they share the load together. But, the fore resistor is still highpassed and should not have a problem in reality. I've never had a fore resistor get warm if using sandcast or ceramic resistors. The one resistor that gets a little warm at any rate is the metal oxide variety, but that's not due to position in the circuit.

So I setup a quick test this morning using the above mentioned crossover schematic as a basis.



With an 8Khz test tone and the volume knob arbitrarily set the input saw 7.43Volts before the first resistor. I didn't have exactly 2.7 so I used the closest I had @ 2.97.

After the resistor the voltage dropped to 4.86v. This is with 1.97amps across the 8ohm resistor pretending to be a tweeter. After a couple of min the resistor did warm up quite a lot (about a hot cup of coffee temp).

With a 500hz tone the voltage only drops by around 0.07v with virtually no amp draw at the "tweeter". So I stand corrected the resistor wont see the "full range" as such positioned here due to the amp draw.

As mentioned earlier, under real world music conditions and reasonable listening levels I've never had resistors get hot and certainly cant see how it would become so hot that your speakers would...burst into flames!

[davidB]

Try putting a low bass note through a low-to-mid impedance shunt before the network.