L pad question

[DeanP]

Okay,
After using my audax set up for a while, I am finding them a bit "bright".
I was wondering if installing a L-pad on the tweeter circuit would help with this? I would use a variable one like this L Pad

Help?

[ThomasW]

Isn't there a fixed L-pad built into the crossover?

Do you have a copy of the schematic?

[DeanP]

Isn't there a fixed L-pad built into the crossover?

Yes, I guess so...Here are the tweeter schematics for both the center and mains.

[servicetech]

Have you double checked your amp settings to make sure everything is set flat? When I replaced the tweeters my speakers sounded bright then I realized had the treble turned up. With todays amps it's easy to forget the electronic settings.

[DeanP]

Have you double checked your amp settings to make sure everything is set flat?

That, I think is where the problem lies, as it is a Yamaha and they are known to be "bright". I have the treble down already...
Maybe on a different reciever they would be okay but for now, I need to pad them down some

ThomasW:
If you need the values, I'll have to look for the table...

[DeanP]

I found the schematics with the values...

[augerpro]

Change R3 in the center and R4 in the L&R

[DeanP]

Change R3 in the center and R4 in the L&R

What value would be a good start? And if I don't find it enough, in what increments should be used? IE: start with R3 @ 10 Ohms, then proceed with 1 ohm increments.

[servicetech]

What are the current values of R3 and R4?

[DeanP]

Post #6 shows the schematics with the values.
R3(center)=8.2 ohms
R4(mains)=4.7 ohms

[servicetech]

A small bypass cap across the "R3" part of the L-pad would be worth trying. The only way to know for sure is to try it out. I've used variable L-pads in the past and I don't care for them, they can get "scratchy" over time like an old school volume control. No telling how accurate the resistance is on these once they get a little dust in them. I think of them as an 70's/80's thing.

[DeanP]

I've used variable L-pads

Though maybe if I use one and then find the sweet spot, I could measure the resistance and then replace R3 & R4 with the appropriate resistors?
Would this work?
Also, what does the bypass cap do?

[servicetech]

I don't see why it wouldn't work, just be sure to take accurate resistance readings. A bypass cap will allow some of the highs to go around the L-pad. Since they are normally wire wound they have a little inductance.

[Undefinition]

Though maybe if I use one and then find the sweet spot, I could measure the resistance and then replace R3 & R4 with the appropriate resistors?
Would this work?
Also, what does the bypass cap do?

Yes, that is a very valid way of voicing a speaker. If it's good enough for Curt C and Lynn Olson, it's good enough for me. ;x(

Also, the TMF025 has always been a "bright"-sounding tweeter, to my ears anyway--but I don't mean that in a bad way. I've always liked that tweeter, and I'll miss Audax. :cry:

Hope that helps,
Paul

[DeanP]

Well if nobody else has any other ideas, maybe I'll order two variable l-pads and give it a try...
EDIT: we posted at the same time! Ha! Ha!

I guess I'll order two l-pads...

[ThomasW]

If you buy one L-pad you can put it in the circuit and adjust the tweeter level. Then remove it from the circuit and measure the values for the fixed resistors you'll need.

Don't change the level when removing the L-pad from the speaker, otherwise you'll need to set the level all over again.

[DeanP]

Could I wire it into the circuit and have it hang out of the port?
This way I could adjust it on the fly and measure it at the same time.

[ThomasW]

Yes you can hang it out the port, but it has to be physically removed from the circuit for an accurate measurement.

[DeanP]

it has to be physically removed from the circuit for an accurate measurement

Gotcha! Now I wait for the delivery of the parts...

[bluewizard]

I'm pretty sure this idea of measuring an L-pad won't work. What you probably need is a normal potentiometer (volume control), even then you will have a hard time finding a 15 ohms potentiometer.

Keep in mind that an L-Pad is a constant impedance device. In other words, it always measures 8 ohms when in the circuit regardless of where it is set.

A regular potentiometer on the other hand, would make a straight forward voltage divider which is what the exiting 'l-pad' (R3/R4) circuit is doing.

I would suggest removing the resistors, and putting in an appropriate L-Pad and just leaving it there. That way you can always tailor the sound to suit your mood in the moment.

Attached is a drawing of an L-Pad in a Piexo attenuation circuit. It is comprise of the two potentiometers marked Rp1 and Rp2.

Hey...it's just a thought.

Steve/bluewizard

[servicetech]

I don't see why you can't just measure each section separately, we're just talking about 2 potentiometers here. Using a regular 15 ohm potentiometer won't work either, the crossover will see a different total impedance depending on the setting.

[bluewizard]

Well, you can try, but I still don't think it is going to work.

At half setting, you have 4 ohms in series with the tweeter, and 300 ohms to ground. That is not a voltage divider in the sense that the R3/R4 circuit is. Essentially the 600 ohm POT is out of the circuit. That is, it's impedance is so high as to have no effect on the circuit. Again, the goal here is to maintain a constant impedance while still being able to vary the volume.

The R3/R4 is a simple voltage divider which is what a normal potentiometer is. I believe some one here said the resistors were -

R3=8.2 ohms
R4=4.7 ohms

That is simply the equivalent of a linear 12.9 ohm potentiometer set at 64%.

The impedance seen by the crossover is 4.7 ohms in series with (8.2 ohms in parallel with 8 ohms) [assuming an 8 ohm tweeter].

Which means the assumed overall impedance is 4.7 ohms in series with 4.05 ohms for a total impedance of 8.75 ohms (variations over frequency ignored for the moment).

Now for the crossover to work accurately, we are going to have to come up with resistor combinations that maintain that approximate load. Rather than mess around with resistors (or potentiometers for that matter), why not just put the L-Pad in and leave it there? That's what L-Pads are for.

Hey...I'm just saying...
Steve/bluewizard

[Dennis H]

Rather than mess around with resistors, why not just put the L-Pad in and leave it there? That's what L-Pads are for.

I don't get it. You write several paragraphs explaining why you don't think an L-pad will work and then you recommend using one?

[servicetech]

An L-pad doesn't use linear tapers resistors. They are audio taper resistors which will not read 1/2 resistance when put in the 1/2 position. L-pads are designed to maintain an 8 ohm impedance to the crossover regardless of settings, that's why people use them instead of regular POTs.

[bluewizard]

Dennis H,

Don't think so. I explain how the resistors worked and I explained out the L-Pad worked, but in both cases I said -

1st Post = #20

"I would suggest removing the resistors, and putting in an appropriate L-Pad and just leaving it there. That way you can always tailor the sound to suit your mood in the moment."

2ndn Post = #22

"why not just put the L-Pad in and leave it there? That's what L-Pads are for. "

The guy is already buying the L-Pads, he is already going to put them in the circuit (hopefully by-passing the resistors), so, why not just leave it there? Like I said, that's what L-Pad are for.

I'm not saying USING an L-Pad won't work, I'm saying MEASURING an L-pad won't work, or I don't think it will.

Servicetech,

Right, on the taper, you might have me on a technicality, but I still stand by what I said.

And for the record, I calculated the perceived impedance of an L-Pad connected to an 8 ohm load across it's assumed linear range and it consistently came out 8 ohms. It would seem that if it wasn't linear, my calculations would have been off.

Still, despite all this talking, all I'm saying is 'use the L-Pad'.

Steve/bluewizard

[servicetech]

Bluewizard you're probably right, the L-pad probably won't make an audible difference. At least until it gets some dust in it anyways, how many old L-pads have you seen that drop the sound unless you find a "sweet spot" in it's position?

[Dennis H]

I'm saying MEASURING an L-pad won't work, or I don't think it will.

Sure it will.

Series = speaker lug to amp lug
Shunt = speaker lug to ground lug

http://www.bcae1.com/lpad.htm

However your concerns about mucking up the XO frequency by changing the impedance the XO sees are valid.

[Dennis H]

I found an impedance curve for that tweeter on the Wayback Machine (Audax doesn't sell to DIYers anymore.) I don't know if it's to scale or not. If it is, you could figure a 7 ohm impedance for the tweeter at the XO frequency.

[DeanP]

Okay...now what am I to do?
BTW The L-pads are on their way...

[bluewizard]

Well, short version -

Still, despite all this talking, all I'm saying is 'use the L-Pad'.

As a side note:

My L-pads were installed in 1984, and still sound fine, and they were pretty much bog-standard off-the-shelf L-Pads purchased from MCM Electronic. Very similar to the L-Pad selected by the original poster.

7 ohm...8 ohm...it doesn't matter because the impedance seen by the crossover, even if we adjust for a 7 ohms tweeter, is still 8.48 ohms vs 8.75 ohms for an 8 ohms tweeter (R3+R4//T). So, use 8 ohm L-Pads and you will be fine. Just remember to remove the R3/R4 resistors from the circuit.

As far as setting the L-Pad, and measuring the resistance on each side of the wiper, well, maybe it will work. But the L-Pad linked to shows that it contains two 8 ohm potentiometers (Edited: my mistake, they claim 8 ohm + 120 ohms), and I'm pretty sure that is wrong. It's 8 ohms and 600 ohms. Still, it can't hurt to try; if it works it works and better for everyone, and if it doesn't, nothing is really lost.

The R3/R4 resistor calculator on this site look pretty handy and seems to do a pretty complete analysis.

Still, despite all this talking, all I'm saying is keep the L-Pad.

Steve/bluewizard

[stangbat]

FWIW, I have four Radio Shack L-pads in a pair of speakers I built in 1986 (one for mid, one for tweet on each speaker). They are not scratchy nor do they cause any problems.

[ThomasW]

Okay...now what am I to do?
BTW The L-pads are on their way...

Dean,

Do what you were told to do many posts ago.

Jon and I have been using the 'measure the L-pad' method of determining fixed resistor L-Pad values for more than 40 yrs and it works fine.

Most carbon based L-pads are clearly audible in the system when compared to good quality non-inductive fixed resistors, if the playback electronics are adequate to the task. L-pads do deteriorate over time, and again this is audible if the playback electronics are adequate to the task.

BTW, I'm not sure why this thread has turned into a pissing contest, but it's going to stop or people will be banned.

[DeanP]

Do what you were told to do many posts ago.

Just as I thought Thomas; I trusted your opinion from the start but one never knows sometimes if someone else knows more than you guys. :wink:

[bluewizard]

Thomas, if you've done this and it's works, then great; problem solved - solution found. But I hardly think this was necessary -

BTW, I'm not sure why this thread has turned into a pissing contest, but it's going to stop or people will be banned.

I certainly don't see a 'pissing contest'. If you read what I said, I expressed doubt of this method, but I also expressed doubt in my doubt. I never absolutely said it wouldn't work, I was careful to preface with 'I think...', 'I'm pretty sure...', and it one point said '...measuring an L-pad won't work, or I don't think it will' which was in response to someone misinterpreting what I said. Again, I very carefully NEVER stated my opinion on this with absolute certainty, and I assumed people saw that.

[edited] I'll also add that i said, Still, it can't hurt to try; if it works it works and better for everyone, and if it doesn't, nothing is really lost.

I'm more that willing to be proven wrong. Your statement of having done this effectively for years essentially established the fact, and I'm happy to concede.

My central point was never about whether measuring an L-Pad would or wouldn't work; that was more or less a side note. My central point was, as long as you have the L-Pads and as long as you are putting them in the circuit, why not just leave them there?

My point now is that, I saw no hostile argument or belligerent discussion, only people calmly discussing a matter of opinion. A matter of opinion which could easily be resolved by someone's statement of having done this effectively and accurately. Since you've done this many times and it has worked, that seems to have easily resolved the debate.

Hey...I'm just saying...

Steve/bluewizard

[DeanP]

Okay, I have the L-pad; now there was adiagram on the box and I think I figured out how to implement it.
Could somebody check out my diagram?

[technimac]

Yes, I guess so...Here are the tweeter schematics for both the center and mains.

Dean,
I built four Audax HT MTM's and a WMTW center using the TMO25F7 titanium dome in each. With this tweeter, HF is crisp and detailed, but not harsh. :T

When using the F7 in place of the TMO25F1 (textile dome) the C5 (2mfd) cap is omitted in the MTM's. With the TMO25F1, that cap is supposed to roll-off a high-frequency rise in the tweeter's response to produce an overall flat response, according to the build instructions from Audax. There's no adjustment needed with the WMTW crossover when swapping these tweeters.

Did you check the crossover wiring to ensure that it's correctly done and functioning properly? :W

Which Yammie receiver do you have?

When I first ran Yamaha's YPAO setup, it found one of the MTM mid-woofers was wired out-of-phase. 8O
Although I thought I'd checked all the connections several times before closing everything up, somehow I missed that one! ops:

HTH, Bruce

[augerpro]

You have it backwards, which is easy to do since the diagram is upside down relative the words.

C4 (amp+) should go to 3, then 2 to speaker + terminal. 1 goes to speaker - terminal or ground.

If you turn the diagram upside down as if you were looking at the real Lpad, the + terminal for both amp and speaker are on top of the word "amp" and the speaker icon. And of course - terminal is under the word "amp" and the speaker icon.

[Dennis H]

It goes like this, after you remove the fixed resistors.

[DeanP]

Okay Great! Thanks for the help! :T They should mark + & - on the box also. :M
Tomorrow night I'll get started.

When I first ran Yamaha's YPAO setup, it found one of the MTM mid-woofers was wired out-of-phase.

Well actually my x-over diagram here has a couple of mistakes also. It shows L1 to be a S16.18 instead of a S161.8 and it was noted elsewhere that the main's mids are supposed to be wired out of phase. I'll have to dig that up somewhere to show. I got these diagrams from Solen.ca when they had them, but actually used an another to build the x-overs.
EDIT: I was wrong, it was the center that was to be wired out of phase. ops:
Here is the link I used to build my x-overs.

http://www.avsforum.com/avs-vb/showthread.php?postid=3099956#post3099956

[bluewizard]

Sorry but I don't think either of these wiring diagrams is right.

Try this one -

3 = Input from Amp
2 = Output to Speaker
1 = ground

You are going to need to remove both R4 and R5, then replace R4 with a straight piece of wire.

See attached diagram.

Steve/bluewizard

[augerpro]

When I first ran Yamaha's YPAO setup, it found one of the MTM mid-woofers was wired out-of-phase. 8O

That's pretty normal for 3 way, and it DOES NOT mean you had the mid incorrectly wired. Well, you might have, but that would be a coincidence.

[augerpro]

Sorry but I don't think either of these wiring diagrams is right.

Try this one -

3 = Input from Amp
2 = Output to Speaker
1 = ground

You are going to need to remove both R4 and R4, then replace R4 with a straight piece of wire.

See attached diagram.

Steve/bluewizard

That's exactly what me and Dennis just said :W Er, well, I said, and Dennis drew.

[bluewizard]

In addition, this is how the L-Pad should go in the circuit.

See attached drawing.

The section of the L-Pad that has the lowest resistance (8 ohms) should be between the crossover pin 2 and the center pin of the L-pad. The high impedance section of the L-pad should be between the center pin and ground.

See my original drawing in post number 20 in this thread.

Steve/bluewizard

[bluewizard]

Dennis has drawn Pin 3 in the wrong place. It doesn't connect to the right side of C4; it connects to the far left side of C3. Pins 1 and Pins 2 are OK.

Steve/bluewizard

[augerpro]

Hmmm you'll have to explain why you would connect pin 3 before the filters. If it simply takes the place of the two resistors, which it does, Dennis has it correct. You're own drawing even shows this way. :huh: