AR 2ax new L-pad installation

ozmoid said:

Pot vs L-pad comparison

The original AR part is a 15.7 ohm (fixed resistance) potentiometer/rheostat.

In the case of the PE L-pads, the outer resistor (3) is 8.8 ohms, the inner resistor (1) is 40.0 ohms. That's how I got the crossover leveled out - I metered the L-pad across 1 and 3, then adjusted it until I got 15.7 ohms to match the fixed resistance of the AR pot. Since the lowest possible Ohm setting on the L-pad is 8.8, it will never show the amp a lower load than that. I don't think the amp sees a constant 8 ohm load, just 8 ohm as a maximum load.

For the L-pad to have a constant resistance from terminals 1 to 3, the two resistors would have to be identical. If you do that, you just need the one resistor anyway, and you now have a potentiometer! (or rheostat, if you like.)

Click to expand...

An 8 ohm L-pad should measure a constant, or near constant, resistance when an 8 ohm load is connected. If the load is not exactly 8 ohms then the load will vary a bit. Did you try to measure the L-pads and the AR level pots with a speaker or an 8 ohm resistor attached, and compare those resistances? That is the load the crossover will 'see', not the L-pad load on its own.

Good point! I'll do it tonight and post back here.

<EDIT- Measurements deleted due to inaccuracy of metering. I'm working on it!>

I attached an AR midrange from 2a for the tests. Same load for all tests. Load follows tested pins, and that is where I missed - I need to test the pots/pads with the speaker in its default hook-up, and not move it around with the leads. even at that, the results here are very interesting. The 8 ohm L-pad is performing very close to the AR pot, but I can hear the "bloom" shacky has discussed at the lower levels of the L-pad.

psychoacoustics, anyone?

Those measurement make more sense now, although I'm not sure which setup is the stock one. But these measurements go along way to clearing up a lot of the confusion on L-pads vs the pots - thanks for doing this and posting about it!

Yeah, under load the L-pad does act a lot more like the AR pot than I expected, given the difference in design. I'm a mechanical guy, still trying to wrap my brain around the electronics! Now I wonder just why the "sweet spot" is so different between the two?

More tests coming... if anybody wants to suggest something for me to try here, go ahead. Just struggling to understand through hands-on trials...

ozmoid said:

......Load follows tested pins, and that is where I missed - I need to test the pots/pads with the speaker in its default hook-up, and not move it around with the leads. ....

Click to expand...

OK, reading it again I now I see you did....the speaker you used is about 6.6 ohms?

Yes, I think you need to attach the speaker load to terminals 1 and 2 in both cases, and measure the resistances across 1 to 3 and also 1 to 2 for various locations through the range of travel.

db drop = 20 * log (R12/R13) (question: is it 20 or 10? I think it's 20 because we're dealing with voltage here....)

A plot of db-drop vs. crossover load (R13) will show what might be the audible difference between the original 16 ohm pot and the L-pad.

I've been trying to model this with an excel spreadsheet - and the l-pad and pot look remarkably close up to about 10db drop in level. A 16 ohm l-pad might be even better - as was suggested earlier!

I'm also learning that when a book on pro-sound says "an L-pad maintains a constant load to the cross-over" they actually mean "we don't really know what the hell they do, but we were told to say this..."

Finally got back to it...

OK, here's the skinny on the PE L-pad vs. AR pot. Under load, the values at the extremes (full increase/decrease) look like this:

AR pot (looking at the BACK of the AR pot, 1=left terminal, 2=center post, 3=right terminal)
Terminals 1-2: full increase=3.6 ohms, full decrease=6.0 ohms
Terminals 1-3: full increase=22 ohms, full decrease=9 ohms
Terminals 2-3: full increase=19 ohms, full decrease=5.7 ohms

PE L-pad (using the terminals as they are numbered on the contacts)
Terminals 1-2: full increase=7 ohms, full decrease=3.9 ohms
Terminals 1-3: full increase=11.5 ohms, full decrease=6.6 ohms
Terminals 2-3: full increase=11.3 ohms, full decrease=3.6 ohms

The speaker load is attached postive=terminal 1, negative=terminal 2.

There is no correlation between the two, and there is no setting on the L-pad that mimics the AR values anywhere in the range.

As I was handling the two controls, I realized that the PE terminals are reversed L to R in the way they are labeled from the AR pot. When I installed them in my speakers, I just transferred the wires to the terminals in the SAME LOCATION on the pot, not paying attention to the numbers. What I had done is wire the positive through terminal 3, and passed the negative in on terminal 1 and out on terminal 2. So I hooked the load up that way and measured again:

L-pad reversed:
Terminals 3-2: full increase=5.3 ohms, full decrease=3.2 ohms
Terminals 3-1: full increase=5.4 ohms, full decrease=39.5 ohms
Terminals 1-2: full increase=2.8 ohms, full decrease=38 ohms

In this configuration, there is a way to mimic the AR pot! At about 1/4 turn, the reverse-wired L-pad reads:
Terminals 3-2: 4.8 ohms
Terminals 3-1: 15.9 ohms
Terminals 1-2: 13.9 ohms

If you run the AR pot just shy of fully "on" (as most people recommend) it reads:
Terminals 1-2: 4.3 ohms
Terminals 1-3: 15.8 ohms
Terminals 3-2: 14.2 ohms

I think this is why I couldn't hear the "hole" Shacky was describing - because I screwed up and wired the L-pad backwards! The terminal 1 wire on the AR pot (positive pass-thru) is attached to terminal 3 on the L-pad. The center wire remains on the center terminal (2), and the feed from the capacitor goes to terminal 1, instead of terminal 3 as on the AR pot.

If you adjust the L-pad too far off the "sweet spot", it is no longer yielding values this close to the AR, so I would say if you want to restore the AR's to full original operation you'd need to purchase a 16 ohm, 20 watt wirewound ceramic potentiometer or rheostat.

Hacking An L-pad

For the 8 ohm PE L-pad:


Once you've done that, trim the center wiper from the L-pad control:


Reassemble the L-pad and you're done. This method will allow you to adjust the control from 8 ohms to 15.7 ohms, but you will NOT be able to "0" the control. It will retain the proper fixed resistance across the pot.

If you want to use the 16 ohm L-pad, it's even easier:

Remove and discard the inner coil completely. As above, trim the center wiper from the control, you will no longer need it.

I have NOT tried to fit the 16 ohm PE L-pad into the AR, I do not know if it fits without adjusting the mount holes.

YMMV.

This is the schematic to the modified 16 ohm L-pad.

The connection from the inner coil to terminal 1 is broken. Terminal 1 is then connected to the other end of the outer coil.

Leave or remove the inner coil and wiper, it does not matter.

I'm confused. If I get ahold of the 16 ohm L-pad, I can mod it into a direct replacement for the old pot? Do I have to reverse the wiring? Has anyone actually tried this for cabinet clearances?

winters860 said:

I'm confused. If I get ahold of the 16 ohm L-pad, I can mod it into a direct replacement for the old pot? Do I have to reverse the wiring? Has anyone actually tried this for cabinet clearances?

Click to expand...

I have not tried the 16 ohm version for clearances. The mods are above, post #28, you want the second mod. If they fit, post back here and let us know! (Once you mod the pot, you can use either end as "1", won't matter. Just make sure the wires are organized the same as the original. The mod makes the pot symmetrical.)

Before junking the original controls, try to take them apart [non-destructivly]
and see if they are simply corroded from heat and condensation. If so clean em up with scotch brite, 600 sand paper, comet cleanser, or simichome. Wash
well in detergent and rinse in distilled water, dry with hairdryer. Lube with Pam. YES PAM for cooking. My jensens and Infinity controls act up every few years after the torture I give them and it works. The new tin controls don't impress me in the heat dissapation area compared to the ceramic body ones.

I hope this helps.

Tronhead said:

Before junking the original controls, try to take them apart [non-destructivly]
and see if they are simply corroded from heat and condensation. If so clean em up with scotch brite, 600 sand paper, comet cleanser, or simichome. Wash
well in detergent and rinse in distilled water, dry with hairdryer. Lube with Pam. YES PAM for cooking. My jensens and Infinity controls act up every few years after the torture I give them and it works. The new tin controls don't impress me in the heat dissapation area compared to the ceramic body ones.

I hope this helps.

Click to expand...

I second the idea of retaining the original pots as they are really designed to dissipate power. However, I don’t believe in cleaning (more about that it a minute).

As for PAM, many of the contact cleaners include a lubricant in the formulation. It’s a “secret” ingredient, but guess what? Most often it’s just plain old mineral oil and … it works!

As for those existing pots, the low cost, permanent solution is:

1. Totally eliminate the tweeter pot from the xover circuit
2. Solder the wiper lead on the mid pot to the "top of the pot"

These changes will bring your AR’s very, very close to a flat frequency response. Further, no matter how much energy you spend cleaning and plating the pot wipers, once those dissimilar metals are back in the presence of current and heat, … they’ll just start corroding all over again.

People just don’t realize how just a little bit of corrosion impacts those sensitive drivers (mid & tweeter). While the power in music exists in the lowest frequencies, all of the information is contained in the mid and high frequencies. This is where music gets its very life and just trace amounts of corrosion … “dulls” everything. :tears:


Regards,
Jerry

Another thing to keep in mind is the stuffing material has a lot to do with pot deterioration. I've yet to open my 1965 pair of 2AX tp replace the original rockwool. Sulfur content degrades the metal. See this:

http://www.classicspeakerpages.net/...ic_id=11218&mesg_id=11218&listing_type=search

I have a pair of 2AX's from the 70's using fiberglass and the pots were near pristine.

Tronhead said:

...try to take them apart [non-destructivly]

Click to expand...

The AR rheostats have a wire clip that holds the cover on. No destruction involved! Just pop the clip, and it comes right apart.

I might leave off the PAM, and try some fader lube or something similar.

onplane said:

...the low cost, permanent solution...

Click to expand...

By-passing the control by using onplane's method is a simple, low-cost solution - IF you like the way the speakers sound in your listening room with the pots run at the highest setting.

If you don't like the "full-on" sound, and you can't get your controls to clean (and some of them just won't), and you don't want to go the L-pad route, the only real solution is to find 16 ohm ceramic wire-wound rheostats to replace the originals. Expect to spend in the neighborhood of $100 for 4 controls.

Ozmoid:

That was one of the BEST "how-to-do" presentations I have ever seen....great pics too!!

Steve

Absolutely, a great thread. This has opened up a whole new range of options for my newly acquired AR-2ax speakers. Thanks OZ!

I've just been pointed to this tutorial, and can't thank you enough..

The pair of AR2ax speakers I am using have got replacement L-pad(s?) with longer control shafts on one speaker only. The speaker with original controls always sounds fantastic. The other speaker with replacement controls sounds really scratchy (upper range distortion) on most of the turn settings. I gather that this is because the replacement pad (pads?) are no where near the same build as required. Setting it by ear, I seem to find a setting that is better than most. Is this the expected result of the repair work? Or should I have the job done over again?