pots vs. L-pads in 2ax

[Guest grassulo]

I know there are a million posts about pot cleaning and replacement, but in reading them over I've become confused. Someone here said impedance changes would affect sound quality, and endanger the tweeters, aren't L-pads supposed to offer constant impedance due to the fact that their 2 ganged resistors? Also, someone warned against replacing the original pollock rehostats with L-pads, but with the rehostats dosen't the impedance change as their adjusted? You can all probably see why I'm now quite confused. What brought all this up, is I've been planning on replacing the rehostats in my 2ax's with L-pads, and swapping the 1" brown tweeters in them with some .75" black dome tweeters from a pair with a blown woofer, since the other pair sounded better than my current ones, till a cheap onkyo amp decided to fry and take one woofer with it. The level controls have become intermittent on this pair even after cleaning them, and I'm quite fed up with them, however if replacing with L-pads really does mess the design up this badly I'll clean them with a dremel and use dielectric grease such as someone here described.

About tweeters frying, I would figure this would be because of leaky crossover capacitors, so mine are going to be replaced with Dayton audio poly caps from parts express. I would think why they would fry is because of capacitance increasing till they became shorted, you see this all the time in power supply capacitors in older tube equipment, so why not in speakers too?

Please respond about why replacing the rehostats with L-pads would cause tweeters to fry, I'd just swear It'd be the old capacitor's fault and not the L-pad, also why would changing them affect the sound?

-Garret

[soundminded]

I've often wondered about this myself. An L pad has two ganged potentiometers, one whose resistance is in shunt and the other in series with the load. They are arranged in such a way that as you turn it, current through the load changes but the impedence the source circuit sees doesn't. It seems the ideal solution for adjusting the volume of a loudspeaker driver at the power amplifier level without changing the crossover network interaction. A single potentiometer on the other hand seems like a poor choice. I'm also always puzzled as to why the L pad isn't connected to the speaker driver after the crossover components. That's what I would try. Of course, if you have a six ohm driver, getting the right L pad would be just about impossible. If you added a two ohm resistor in series to the driver to get to 8 ohms, the crossover components would be all wrong. I'm curious to see what others have to say about this. I'll be they would say that the change in resistance in the AR design doesn't result in much change in the crossover frequency.

BTW, here's a nice site to review L-pads.

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

[Guest Bret]

Looks to me like altering the resistance in line with the speaker doesn't change the crossover point, but changes where that point ought to be in relation to the rest of the components. Maybe pots or L-pads make it possible to mitigate the damage with their variability and I haven't tried it. This is all just theory.

Take the 10pi example where to get the more efficeint woofer to "shut up" and stop over-shadowing the rest of the cabinet, UniBox says that putting a resistor in series with the driver load will quieten the woofer nicely, and drop the Fc of the cabinet considerably (along with half a dozen other parameters).

The more I fool around with trying to repair these old speakers the more I see that changing the circuit or drivers at all just throws the entire system out-of-whack (which is not the conclusion I wanted to come to, by the way). Some of that, as you know, isn't theoretical. I've tried it and nothing good has come of any of it. Sure, we could redesign the crossover and put in new, better, more efficient drivers in place of the ones that burned-out, and change the stuffing. . . now all we have to do is build another box and we've invented our own brand.

For me, from now on, I'm replacing things that are broken for which a nearly identical part is available, fixing the broken old parts that can be fixed, and "giving up" on anything requiring a change to the drivers or a crossover component. I'll have to start buying mutliple sets of speakers and cabinalizing them to get one good set; if I bother at all.

I'm with you, I don't understand why those controls are where they are in the circuit. Somebody must have had a reason.

Bret

[Guest Bret]

Something about the whole "changing tweeters" idea was bugging me and I couldn't put my finger on it, so I went looking.

You are going to want to read Mesg #2768 before you go-ahead with this project. It looks to me like you will need to bring the "black tweeter" crossover into the "brown tweeter" pair if you plan to switch tweeters.

It might be easier to replace the woofers and controls somehow.

Bret

[Guest grassulo]

the thing is, in both these speakers, the crossover values are EXACTLY the same, 6mfd and 4mfd caps and 15ohm pots, there is no difference, this is why I feel ok swapping them between the speakers, and the black smaller tweeter has much better dispersion and is more efficient than the brown one, theres an article on why the switched them somewhere on this forum.

No one has answered my question though about why not to use L-pads, the whole reason for the swap in the fist place is 2 of my speaker cabinets have 4 bolt woofers and the other 2 have 4 bolts on 6 bolt adapter rings, if anyone has any cloth surround 6 bolt woofers for sale and isnt going to charge me an arm and a leg for them let me know.

-Garret

[Guest Bret]

>No one has answered my question though about why not to use L-pads, <

I can't imagine any reason. I could see not using a pot where an L-pad was, but I don't know why not do it the other way 'round.

Are the chokes the same in both sets, too? That would make the message I pointed you to highly suspect, wouldn't it? This might be a case of "true, but not always true."

I know what you mean about the woofers. I currently have two sets of 2ax's and long-ago had a set of the black-tweetered, weird woofered ones.

When I first saw the old (brown tweetered) pair I thought someone had been goofing with them and put an old tweeter in them from a pair of 2a's, or had updated a 2a to a 2ax.

What color are the shafts on your controls?

Bret

[RoyC]

The 2ax is rated as an 8 ohm system and may not suffer greatly from conventional 8 ohm l-pads. It doesn't take much in the way of impedance changes however to alter the sound of a system. Anybody who has tried to implement "fixed" l-pads in speaker building projects can attest to this.

To answer your question, the action of the old 15-16 ohm pots keeps the circuit impedance lower than an 8 ohm l-pad. The higher impedance of the l-pad can lower the crossover point and make the old AR tweeter (typically protected by nothing but a single cap) more vulnerable to low frequency damage. This is a greater potential problem when using an 8 ohm l-pad in a <4 ohm system like an AR-3a. It also alters the original sound due to lower frequencies reaching drivers with the new l-pads.

More tweeters (of any era) have been fried by clipping amps than by bad caps. The old AR tweeters are vulnerable by the nature of the crossover design.