Inductor question

[Guest radkrisdoc]

Hi everyone!

I have a question for all those with a LCR meter: this is about the rivets (the nail-like pins that hold the coils to the crossover board) that pass through the center of each mounted coil. Are the coils wound taking these pins into account, or what happens to inductance if these pins are removed?

I just removed all the components from the upper board of one AR-90 enclosure. My plan is to make the crossover external and to use MDF instead of hardboard for the speaker terminals; that way MDF will also reinforce the cabinet, decreasing resonances (like an external brace).

The reason for my question about the rivets is that anything in the core of the coil will introduce distortion and also affect the system's power handling due to saturation of the core. Introducing a steel pin in the middle of an air core coil is not good. I would like to remove all the pins if possible, keeping the coils pure air core, thats why the question.

I have been following the AR 12" woofer experiment here with interest. That is the absolute one way to to find out which refoam/recone job is closest to the original. Way to go Bret, Pete, Ken and everyone here!

[roundhome]

hello

As far as I know they are made out of a non farise metal they have no ability to interfer with the coils.

JIm

[r_laski]

Kris,

>> …this is about the rivets (the nail-like pins that hold the coils to the crossover board) that pass through the center of each mounted coil. Are the coils wound taking these pins into account, or what happens to inductance if these pins are removed?…

The reason for my question about the rivets is that anything in the core of the coil will introduce distortion and also affect the system's power handling due to saturation of the core. Introducing a steel pin in the middle of an air core coil is not good. I would like to remove all the pins if possible, keeping the coils pure air core, that’s why the question.<<

I’ve rebuilt a pair of AR90 crossovers and just finished my 5th and 6th AR11 crossovers. Jim (Roundsound) is correct about the pins. As you stated, these are “air core” chokes (as opposed to iron core chokes). The pins are made of a non-ferrous (non-magnetic) metal (my guess is Aluminum) and do not affect the inductance of the coil any more than the plastic spool does. A more important consideration is the magnetic field interaction between inductors which is a function of the distance between inductors and their orientation to one another.

When I built the AR90 crossovers, I build them on 1/8” pegboard / hardboard. I used 1 1/2” flat head brass screws, washers, and nuts along with clear silicone adhesive to secure the inductors to the crossover boards. I checked to ensure the brass screws do not contain a ferrous (magnetic) core. This is just my preferred method.

A very popular method is to use plastic cable ties and silicone adhesive to secure inductors (and capacitors) to the crossover board. You may prefer this method.

>> My plan is to make the crossover external and to use MDF instead of hardboard for the speaker terminals; that way MDF will also reinforce the cabinet, decreasing resonances (like an external brace).<<

If I understand you to mean you plan to build your crossovers on MDF and mount these boards to the backs of your AR90s I don’t see how that would effectively add bracing to the cabinet. At least not as effective as adding internal bracing.

I removed the metal pins and mounted my boards to the original boards using Liquid Nails. I designed the layout of the back boards so that the original speaker binding post holes are used. On the AR90s I removed the equalization switch boards and plugged the holes in the back board with plastic plugs.

IMO, adhering the additional layer of hardboard to the original board gives you plenty of additional stiffness to the backboard. I encourage you to consider keeping your crossover internal and if you feel the need, add additional internal bracing.

Rich

[Guest radkrisdoc]

Jim and Rich, you are both right about the non ferrous material, I didnt think of it when I started this thread. I will be taking them out anyway, like Rich suggested and gluing them to the new board.

Rich,

Though the idea of mounting the board behind the speakers did cross my mind, that was not the bracing I was talking about. I am planning to drill holes on the back of the speaker in a symmetrical fashion and install a 3/4th inch MDF board. I will also be applying glue to hold the two boards together. The screws will help clamp the two boards together as a real clamp cannot be used in this situation. I will be finishing the MDF with gloss black lacquer; I also plan to repaint all the black surfaces as they have sustained damage from the staples on the grills. I've already carefully uninstalled the acoustic blanket and have totally removed the adhesive. The next step would be to sand all surfaces. I plan to do that whenever I have the time.

I also noticed that the 8" lower midrange driver's enclosure is made of rolled paper/cardboard. I am planning to remove it and install a seamless aluminum tube of the same dimension. An aluminum plate will complete and seal the enclosure for the 8". Only time is going to tell if I am going to do it.

[r_laski]

Kris,

>>Though the idea of mounting the board behind the speakers did cross my mind, that was not the bracing I was talking about. I am planning to drill holes on the back of the speaker in a symmetrical fashion and install a 3/4th inch MDF board. I will also be applying glue to hold the two boards together. The screws will help clamp the two boards together as a real clamp cannot be used in this situation.<<

I think I understand your plan. You are going to laminate 3/4” thick MDF to the entire surface area of the back of your AR90s. Your crossover will be completely external and not affixed to the speaker – connected by wire to speaker terminals mounted to the MDF/back of speaker. To do this would require speaker terminals for each “driver” -- four pairs of binding posts -- mounted on the back of the speaker (I’m counting the twin 10” woofers as one “driver” in this situation and assuming you will be installing them).

>>I also noticed that the 8" lower midrange driver's enclosure is made of rolled paper/cardboard. I am planning to remove it and install a seamless aluminum tube of the same dimension. An aluminum plate will complete and seal the enclosure for the 8". Only time is going to tell if I am going to do it.<<

That enclosure looks like a small cylindrical cardboard concrete form. My concern with your proposal to replace this with aluminum is that you might create a more resonant chamber. Even with the polyester stuffing I would think the aluminum would have a tendency to “ring like a bell” at certain frequencies unless you coated it with acoustic damping material. You would of course have to drill hole(s) in your new chamber to run wires to the 8” lower midrange, and use rubber grommets to seal the hole(s) and avoid wire insulation chaffing.

Rich

[Guest radkrisdoc]

>Kris,

>

>>>Though the idea of mounting the board behind the speakers

>did cross my mind, that was not the bracing I was talking

>about. I am planning to drill holes on the back of the speaker

>in a symmetrical fashion and install a 3/4th inch MDF board. I

>will also be applying glue to hold the two boards together.

>The screws will help clamp the two boards together as a real

>clamp cannot be used in this situation.<<

>

>I think I understand your plan. You are going to laminate

>3/4” thick MDF to the entire surface area of the back of your

>AR90s. Your crossover will be completely external and not

>affixed to the speaker – connected by wire to speaker

>terminals mounted to the MDF/back of speaker. To do this would

>require speaker terminals for each “driver” -- four pairs of

>binding posts -- mounted on the back of the speaker (I’m

>counting the twin 10” woofers as one “driver” in this

>situation and assuming you will be installing them).

Yup that is exactly what I had in mind! As for the 10" woofers, I plan to provide only one pair of binding posts for both together, connecting them parallel.

>That enclosure looks like a small cylindrical cardboard

>concrete form. My concern with your proposal to replace this

>with aluminum is that you might create a more resonant

>chamber. Even with the polyester stuffing I would think the

>aluminum would have a tendency to “ring like a bell” at

>certain frequencies unless you coated it with acoustic damping

>material. You would of course have to drill hole(s) in your

>new chamber to run wires to the 8” lower midrange, and use

>rubber grommets to seal the hole(s) and avoid wire insulation

>chaffing.

The aluminum will by a solid 3/4th inch thick. I dont think it will ever ring if used in the 8" enclosure configuration as it will be only 3.5" deep. As for the wire, I dont know why AR skimped that part. I plan on using two nickel plated screws and nuts, with wires soldered at both ends of the screws after they are mounted, so there is no problem of sealing a hole. That hole (pic attached) has no right to be there anyway.