A Tale of Four L-Pads

[johnieo]

History:

-------

These L-pads were removed from a discarded pair of AR-3 cabinets of unknown parentage where they had served as replacements for the original Pollack potentiometers. They are 8-Ohm, 15-W with the word “JAPAN” impressed in the Bakelite. A connection circuit diagram was printed on a paper label glued to the back cover. Since I was considering using these in a reconstruction of a second pair of Asian AR-3a "Limited," I checked them electrically (not good), so decided to autopsy.

Observations:

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1. **ALL** four pads were beyond use. Three were photographed before the camera batteries needed recharging and displayed in the image below. The fourth is a clone of those shown.

2. In each pad, the parallel (32-Ohm) winding failed, not the 8-Ohm series winding.

3. All the pads were apparently set at a knob rotation corresponding to about 20% rotation above the minimum (counterclockwise) level.

4. At the above knob settings, the resistances of the winding fractions were of order 6-to7 Ohms (series) and 3.8-to-4 Ohms (parallel).

5. The parallel resistor substrate was seemingly made of a thermally inadequate insulation material. Gross melting and twisting of the windings is shown in the photograph.

Conclusions:

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1. Both mid- and hi-level controls were set to a moderately low level at the time their host speakers failed. The listener likely increased the amplifier power until the mid/hi drivers sounded appropriate. At that power level, excessive charge flowed through the (4-Ohm) parallel leg, causing the insulator to melt and the coil to warp catastrophically in *both* the tweeter and midrange pads. (See the black goo on the brass wipers at the top of each coil.)

2. A 15-W L-pad may not be an appropriate substitution for a 25-W potentiometer, even if the insulation were adequate, and it were sonically equivalent. (Note that the windings in 15-W L-pads sold by AB Tech Services use a superior, ceramic-like substrate.)

[dynaco_dan]

Hi John;

Thank you for that information and photos.

[onplane]

Pots?? L-Pads??

Everything is a losing fight, because we are attempting to dissipate power inside the speaker boxes. Any solution (with the possible exception of JoeB's stepped switch) will eventually cause problems due to corrosion that is accelerated by the presence of current and dissimilar metals.

As I see it, there are three long terms solutions:

1. Bi-amp and completely remove the pots – function of pots is transferred to low power volume controls that are easy to maintain

2. Solder both driver leads to the tops of the pots – this is equivalent to setting both drivers (on the AR-3a’s) to max increase. From our library this is the correct position for FLAT response (along with a slight treble boost).

http://www.classicspeakerpages.net/dc/user_files/1660.jpg

Once you solder the leads, you are effectively by-passing the pot wiper, which will corrode. The 16 ohms is still in the circuit and will still perform its function of “padding”. Further, the 16 ohms is encased in ceramic and is capable of handling power without much risk.

3. JoeB’s stepped switch solution also looks promising – Joe owes us the results of his “worse case test” to see how much heat is actually generated.

Regards,

Jerry

1659.txt

[Carlspeak]

The stepped pot project is a great opportunity for the electronics enthusiast as I suggested in an earlier post.

My suggestion is for folks who aren't to bypass the bad pots and invest in an inexpensive equalizer. It would accomplish the same thing.

For the audio purist however, go the more costly route of bi-amping. Then, there are no intermediary devices to cause problems and 'muck up' the signal path.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

[RoyC]

I think John's point that 15 watt l-pads may not be a safe haven for those replacing old 25 watt AR potentiometers is well taken...and l-pads rated for higher power handling are too large to fit where the pots used to be.

Even if the power handling issue and different sound characteristics of the 8 ohm l-pads are overlooked, anything that has moveable "contacts" can degrade over time. (Even Joe's device could face that issue, though switches seem to hold up better.)

In my opinion, the "by-passing the pots/bi-amping" scheme is not an option if one is attempting to maintain the original sound character of the design. For one thing, it compromises the ability to adjust the tweeter independently of the midrange. Carl's equalizer would be preferable.

In my experience simply by-passing the 3a midrange pot makes it blare on many recordings. Also, if one is using the AB Tech 3a replacement tweeter, by-passing both the mid and tweeter pots will make your fillings fall out.

If I were to eliminate the pots it would be with fixed resistors that duplicate the pots at their normal/white dot settings. For the mid it would be about 3.25 in series and 11.75 in parallel, and for the tweeter it would be 1.75 ohms in series and 13.25 in parallel. These can be adjusted to taste as long as the total resistance equals 15 to 16 ohms for each. Once again an equalizer can easily be used for further tweaking if desired.

Having tried the other options, my personal preference is to continue using refurbished 25 watt potentiometers in the AR-3a.

Roy

[dynaco_dan]

Hi Roy;

Excellent write-up.

[onplane]

Hi, Roy!

I well understand your concern about bi-amping and the original sound character. Please don't forget that we have NOT lost all control of the mid-range. In fact, by means of volume controls we have simply "moved" the mid-range pot before the power amp.

You are correct on the tweeter as its relative volume "rides with the mid-range". That is, the amp volume control drives both the mid-range and tweeter together.

This is a little off topic, Roy, but I find that powering the mid-range and tweeter independent of the woofer allows those terrific drivers to really shine. I mean the clarity of sound from the mid and tweeter is simply UNBELIEVABLE ... just crystal clear. I don't think many people appreciate the quality of those drivers as they are simply superb.

Switching subjects, Roy, how do you feel about soldering the driver lead to the top of the pot?

We technically aren’t by-passing the pots. All we are doing is setting the pots to max and by-passing the wiper. The 25 watt 16 ohm wire resistor encased in ceramic is still in the circuit. I seriously doubt that resitor would give any trouble whatsoever.

Regards,

Jerry

[johnieo]

I find the clarity of the drivers to be absolutely superb, if the aging capacitors (especially mid- and tweeter-crossover capacitors)are replaced with extremely high quality polypropylene capacitors, and the (AR-3a) speakers are driven with a power amplifier capable of, say, 200 Watts into 4 Ohms; something like an ADCOM 5400 or larger. If polypros are used in the woofer circuit, a half-ohm 20W resistor should be added in series to mimic the original capacitor's ESR so as not to change the characteristic at crossover. I do not feel that gimicks are needed to enjoy these speakers. Fortunately, I have not encountered any badly corroded 25-Watt Pollack potentiometers, although I realize there are many beyond repair. Were I to need replacements, I would likely spring for 25-W units (IRC?). Although dear, they are no expensive than a good 40-uF capacitor; a small part of the overall cost of restoration.

My interests are in restoration (without changing the crossover design), and reconstruction - with original drivers - of a currently unavailable model for the fun of it. DIY speaker design is not my forte. I am interested in L-pads only because they were used in the (Asian market) AR-3a Limited.

[johnieo]

I find the clarity of the drivers to be absolutely superb, if the aging capacitors (especially mid- and tweeter-crossover capacitors)are replaced with extremely high quality polypropylene capacitors, and the (AR-3a) speakers are driven with a power amplifier capable of, say, 200 Watts into 4 Ohms; something like an ADCOM 5400 or larger. If polypros are used in the woofer circuit, a half-ohm 20W resistor should be added in series to mimic the original capacitor's ESR so as not to change the characteristic at crossover. I do not feel that gimicks are needed to enjoy these speakers. Fortunately, I have not encountered any badly corroded 25-Watt Pollack potentiometers, although I realize there are many beyond repair. Were I to need replacements, I would likely spring for 25-W units (IRC?). Although dear, they are no expensive than a good 40-uF capacitor; a small part of the overall cost of restoration.

My interests are in restoration (without changing the crossover design), and reconstruction - with original drivers - of a currently unavailable model for the fun of it. DIY speaker design is not my forte. I am interested in L-pads only because they were used in the (Asian market) AR-3a Limited.

[johnieo]

sorry for the double post -- router hiccup.

[JKent]

> Were I to need replacements, I would likely spring for

>25-W units (IRC?).

Hi John

If needed, where would you find such an animal? I googled "IRC Potentiometer 25w" but found the search results kind of confusing.

regards

Kent

[RoyC]

Hi Jerry,

>You are correct on the tweeter as its relative volume

>"rides with the mid-range". That is, the amp volume

>control drives both the mid-range and tweeter together.

That is a problem for me. Some of the "fun", and flexibility of the original design has to do with the ability to control 'em independently. Further, if the tweeter is replaced with a modern version, it is not even an option in my opinion.

>Switching subjects, Roy, how do you feel about soldering the

>driver lead to the top of the pot?

It may be possible to get away with this in the case of the tweeter pot if the original tweeter is still installed, and there is no alternative to an impossibly corroded pot. However, I personally do not like the midrange driver set to maximum. I believe that approach results in too much output from the midrange...and to control the combined output of the two drivers using your second amp would then reduce or restrict the tweeter output (which I realize you would then attempt to adjust with tone controls). Regardless of your ability to control the combined output of the mid and tweeter with a preamp control, you are still compromising the original "voicing" of the speaker itself.

>We technically aren’t by-passing the pots. All we are doing

>is setting the pots to max and by-passing the wiper. The 25

>watt 16 ohm wire resistor encased in ceramic is still in the

>circuit. I seriously doubt that resitor would give any

>trouble whatsoever.

Understood...My comments were made assuming the 15 to 16 ohm parallel pot resistance was still present in the circuit. The resistance characteristics of the pots in the crossover circuit contributes greatly to the original sound of the 3a. That is why the fixed resistor approach I mentioned, as well as Joe's device, must emulate the original pots.

I know you feel strongly about your set-up, so we will probably have to agree to disagree on your "bi-amping" approach to the AR-3a. One good amplifier and, possibly, the judicious use of an equalizer is as far as I am willing to go. I'm more inclined to play around with multi-amping, active crossovers, etc with new "non-classic" speaker projects...but thats just me.

Regards,

Roy

[RoyC]

For a number of years I had the opportunity to live with the Asian AR-3a Limited crossover John is referring to. (Information on this early 1990's iteration of the 3a is posted elsewhere in the forum.)

Very slight modifications were made to the original AR-3a crossover to accomodate a higher output textile 3/4" dome tweeter, similar to the one used in the AR-11, as well as conventional 8 ohm l-pads. In my opinion the configuration works quite well, and retains much of the character of the original AR-3a. For folks so inclined, L-pads could be (more) successfully used in the AR-3a if willing to make the necessary crossover adjustments.

Roy

[dynaco_dan]

Hi Kent;

There is a Chinese verion on the market and at least one American made by, Ohmite.

They are both 25 watt wire-wound 15 ohm pots.

Being in Canada, I only know of one local source for the Chinese one and the retail price was $15.00 CDN.

The Ohmite, in a google search, came up with Mouser or Digikey, I think and it runs about $25.00 US or about $40.00 CDN retail.

Un-fortunately, you can see a study that Carl had done in the past year, they both have exposed rear windings.

This leaves a potential for insulation, the brown sheet or a wire to cause a jam-up in the mechanism.

As well as a more serious heat problem.

A non-flammable heat shield would be desirable.

The original Aetna-Pollack pots had a very simple integrated stamped steel and removeable end cap.

I have been working with, James, in arriving at something, off the shelf, that may be suitable.

Lot's of close calls, but, nothing universally available and safe yet.

A problem at hand is also that, these are not big sellers and one or both manufacturers may discontinue manufacturing them at any time.

The, "Magic", answer I have been looking for would be for both pots.

The search continues.

[johnieo]

Hello Kent:

Just found the catalog pages in my files. (See attachment.) The pots (listed under rheostats) are mfg. by Ohmite, not IRC--sorry. I believe Roy C. posted this quite some time ago, but I could not find his link.

Ohmite Model H, p.n. RHS15R, is a 15-Ohm, 25-W pot with an O.D. of 1.560". It will fit in the existing 1.75-in.-spaced mounting holes. I called two of three distributors given to me by Ohmite and both stock the model without the back cover.

Newark Electronics: (800-463-9275) $29.69 (1-9) and ~$26 (10-?).

Allied Electronics: (800-433-5700) $23.50 (1-4); $22.09 (5-14); $21.40 (14-25).

I did not call Richardson Electronics: (800-348-5580)

A small cover that would keep fiberglass insulation away from both pots could be fabricated from aluminum sheet stock with little dificulty. Ohmite does sell these in an enclosed version (14-week special order). Note that all the early AR speakers using these pots were stuffed with fiberglass. Polyester stuffing did not arrive on the scene until later; by then switches had replaced pots.

1667.pdf