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AR pots & L-pads AR-2?


dynaco_dan
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Hi there;

AR pots and L-pads used in an AR-2 speaker system?

I am choosing the, AR-2, as an example only, I am certain this was not the only model affected by this topic.

I am writing this because, with all of the write-ups I have seen, I find it overwelming, I cannot read between the lines, as to which is or not correct to use.

I have read, that some listeners do not like the sound with the L-pads.

Why not?

What does a pot do, that an L-pad doesn't?

What does an L-pad do, that a pot doesn't?

This is a simple question, yeh sure, regarding the past and present purposes of both items.

I have come across what appears to be L-pads, rather than pots, in old AR-2 speakers, which I have not opened, yet.

They turn with considerable ease, smoothness, lack of resistance, and quietness compared to the Aetna-Pollock pots.

This was done without any audio connections.

Am I correct in my assumption?

Did AR start out using L-pads?

When and why, did they switch to the 16 ohm pots at some point?

What does the pot do to the AR-2 tweeter output, if plotted on a graph?

What effect does the potentiometer have with the tweeter output, with the pot at maximum to start with?

A. Does decreasing the tweeter output, cause the total tweeter output to drop horizontally equally, from the crossover to the maximum output?

B. Or does the tweeter output start at the extreme top end and slowly start reducing the topend highs of the tweeter, such as an AR amplifier treble control?

I am certain that A. is correct, but, I would appreciate a techies input please.

Also if an L-pad was used, what would happen to a graph profile when the L-pad was slowly turned down?

C. Does turning an L-pad down, cause the entire horizontal tweeter output to lower equally?

D. Or does an L-pad, cause the tweeter output to lower itself starting from the extreme highend tweeter output, such as an AR amplifier treble control?

If an L-pad was used, is replacing it with a pot a simple and worthwhile operation?

Looking at an AR-LST literature profile, I can see the switched steps of the tweeter, with no difficulty.

I realize this is also a stepped switch via the autotransformer.

I hope I made this simple question clear?

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Hi again;

I added to the topic, but lost everything last night.

Here goes again.

I have a sample of, I suspect, 2 AR-2, speaker terminal boards complete.

There is an L-pad on both boards, rather than a pot, with only 2 of the 3 terminals connected to anything.

One L-pad has embossed on the rear, CM24067 10 ohm 140911, the shaft turns with a definite smoothness, needing very little effort to turn, as if brand new.

Not anywhere near the screechy, hair raising screech of the typical 16 ohm Aetna-Pollock pot, nor the dragging feel of the windings, as the rotor passes over the wire.

There is a copper wire coil, approximately 2" in diameter, no markings on it, wound on a short piece of wooden dowel.

There is 2 cute little metal body caps, about 1" x 1" x 1 1/2", with 3 terminals each, taped together with what appears to be 1/4" masking tape covering most of the manufacturers embossed markings.

One cap has 200VDC - 50'C 30Z embossed on it's top.

The other cap is covered with the 1/4" tape.

The other board is identical, except the L-pad has 140918 embossed instead and this L-pad has an original shaft knob.

I would need to remove tape from one or the other to see the cap rating.

The 3 terminal screws are in a triangular configuration.

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>Hi again;

>

>I added to the topic, but lost everything last night.

>

>Here goes again.

>

>I have a sample of, I suspect, 2 AR-2, speaker terminal boards

>complete.

>

>There is an L-pad on both boards, rather than a pot, with only

>2 of the 3 terminals connected to anything.

>

>One L-pad has embossed on the rear, CM24067 10 ohm 140911,

>the shaft turns with a definite smoothness, needing very

>little effort to turn, as if brand new.

>

>Not anywhere near the screechy, hair raising screech of the

>typical 16 ohm Aetna-Pollock pot, nor the dragging feel of the

>windings, as the rotor passes over the wire.

>

>There is a copper wire coil, approximately 2" in

>diameter, no markings on it, wound on a short piece of wooden

>dowel.

>

>There is 2 cute little metal body caps, about 1" x

>1" x 1 1/2", with 3 terminals each, taped together

>with what appears to be 1/4" masking tape covering most

>of the manufacturers embossed markings.

>

>One cap has 200VDC - 50'C 30Z embossed on it's top.

>

>The other cap is covered with the 1/4" tape.

>

>The other board is identical, except the L-pad has 140918

>embossed instead and this L-pad has an original shaft knob.

>

>I would need to remove tape from one or the other to see the

>cap rating.

>

>The 3 terminal screws are in a triangular configuration.

Vern:

What I suspect you have is someone's Rube Goldberg attempt at upgrading the crossover of an AR2a. I'm not an EE and can't answer all the detailed questions in your first post. However, AR must have purposfully used 15 ohm rheostats instead of the typical 8 ohm L pads in their design. 15 ohm L pads are darn near impossible to find today. So, again, your predicessor chose to cobble up something.

Yes, the original rheostats are a bit scratchy when adjusted. However, that should be done infrequently once the speakers and room acoustics are settled in and you've adjusted them to your satisfction.

It's evident, at least to me, that speaker development in the 60's and 70's was a bit of a seat-of-the-pants process. There were no T/S parameters or computer models back then. Tweeters were in a rapid state of development and I'm sure their performance varied quite a bit out of the manufacturing plant and AR didn't want to spend a lot or resources testing all of them to find suitable matches.

IMO AR developed a way to compensate for these variances by adding adjusting mechanisms to help balance each driver's output to the listener's listening satisfaction. Just look at all the speakers Kloss was involved with in the early days. Almost all of them had some means of adjustment. Without this novel flexibility, I wonder how sucessful AR would have been?

Contrast that era with today's. How many loudspeakers do you see sold today have similar adjustments? I suspect VERY few. Driver to driver T/S parameters are relatively more consistent and thus, designers can tune a crossover design to specific components during the development process that maintain sonic output within a very narrow range.

Numerous modern speakers claim + - 2 dB response over a very broad frequency band. I doubt AR could have done that back then with a fixed crossover design that had no adjustments.

What are you plans for these terminal boards?

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

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This is easy to understand, you don't have to be an engineer. A potentiometer is nothing more than a variable resistor. It has three terminals. Two are the connections to each end of a fixed resistor, often a piece of coiled resistance wire. The third is connected to a moving contact which makes contact with the wire at one point along its length depending on where you position it. This can be used to control the volume of a speaker in either of two ways. You can use just two of the terminals so that the potentiometer is a variable resistor in series with the speaker, the crossover elements, and the amplifier. As the resistance increases, the current in the circuit is reduced, as it decreases the current increases. This is how the volume is changed. This was the case in the midrange circuit of AR2a (and it didn't work very well at all.) The other way is to apply the output voltage of the crossover network across the fixed resistor and apply one fixed leg and the variable terminal to the speaker. In this way the potentiometer works as a voltage divider. As the potentiometer is adjusted, the voltage to the speaker at its maximum is the full volatage of the crossover output applied to the speaker which is in parallel with the fixed resistor. At the other extreme, the volatage applied to the speaker is zero because it is the voltage between one fixed terminal and the adjustable one which is at the same position as that terminal. The problem with this scheme is that as you adjust it, it changes the resistance the crossover network sees and this not only adjusts the speaker volume, it changes the crossover frequency because its exact value is an element in the way this filter circuit works.

An L-pad is designed to change the volume of the speaker without changing the resistance the crossover network is connected to. It consists of two potentiometers ganged together mechanically and wired together electrically internally. One is in series and one is in shunt. As you decrease the resistance of one, you are increasing the resistance of the other so that the total remains the same from the crossover network's point of view. This allows you to change the volume of the midrange and tweeter without changing the crossover frequency or slope so the adjustment only "shelves" the output of the driver, it doesn't change its frequency response. This is the scheme most manufacturers who incorporated midrange and tweeter level control used. L-pads are practically universally designed and manufactured to be used with 4 ohm and 8 ohm speakers. You'd be hard pressed to find one for a different load although they may exist.

Here's a link for a pictorial view and another explanation.

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

BTW, you will never see one used to control the volume of a woofer. This is because putting any resistance in series with the woofer compromises its performance by lowering the amplifier's damping factor, that is its ability to control the woofer's inherent tendency to spurious resonances. It's not a problem with midrange or tweeter drivers however because of their low moving mass. Volume controls for subwoofers are invariably in the amplifier's earlier gain stage where they have no such effect on the amplifier output/woofer circuit.

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Exerpt from Soundminded's post:

"The problem with this scheme is that as you adjust it, it changes the resistance the crossover network sees and this not only adjusts the speaker volume, it changes the crossover frequency because its exact value is an element in the way this filter circuit works.

An L-pad is designed to change the volume of the speaker without changing the resistance the crossover network is connected to."

Your explanation of the differences between pots and L-pads didn't answer the vital question aluded to in Vern's original post:

WHAT TRULY WAS AR'S INTENT IN USING POTS?

1) To adjust both volume AND frequency?

2) To adjust volume only and in the process acepted the fact that frequency would also change.

The former would seem to me to be an undesirable design objective.

AR's design was rather novel in any case. I wonder if they had it patented at the time. What say you Tom T.?

p.s. my response to Vern was solely my opinion on why things happened the way they did. I never worked for AR. However, I've worked on a number of different classic speakers and my experience and observations led to these opinions.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

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Hi again;

I may have made an error in my writing.

The L-pad? I mentioned only has 3 terminals, of which, only the center and outer contacts are connected.

They do say 10 ohms, but, as I was reading both your write-ups now, I looked at a 5 watt 20 ohm wirewound pot, that I bought just a few years ago.

In a blind test, I cannot tell which is which, by turning each back and forth, the old L-pad? or the new 5 watt pot.

There is almost no resistance to turning them to speak of, with either one, almost lite-duty or a cheap quality feeling.

Would not an L-pad require a second deck, with more contacts, for the shunt section?

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Look at the top drawing in the link I gave carefully. You will see that despite a little confusion, there are only three terminals to the outside world. These are the white, red, and blue lines. Amplifier (or in the case of a midrange or tweeter level control the crossover section output) is wired across the combined shunt and series resistance, the white and red wires. The speaker is wired between the fixed common white and varible blue wires. So the white wire is common to both input and output. The L-pad is wired internally so that as one resistance value increases, the other decreases keeping the combined circuit the same total resistance from the amplifier's or crossover's point of view. BTW, in order to correctly substitute the proper fixed resistors for the potentiometer to replace it and make it non adjustable, you could use the table or calculator built into the web site but in order to get the one setting which will give the factory recommended "flat" response, you'd have to know at least one of those values, either the shunt or series resistance.

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Carl wrote:

>Contrast that era with today's. How many loudspeakers do you

>see sold today have similar adjustments? I suspect VERY few.

>Driver to driver T/S parameters are relatively more consistent

>and thus, designers can tune a crossover design to specific

>components during the development process that maintain sonic

>output within a very narrow range.

>Numerous modern speakers claim + - 2 dB response over a very

>broad frequency band. I doubt AR could have done that back

>then with a fixed crossover design that had no adjustments.

I suspect that the AR-3A (and other AR speakers), when set to the factory settings, were pretty repeatable. I think there were 2 reasons for the pots. First (and most important) to adjust for the room and second to adjust for user preference. The use of graphics equalizers and similar advanced tone controls probably makes up for the lack of speaker adjustability nowadays. Even so, I like the idea of a speaker that sounds correct when hooked up to a dead flat preamp/amp combo. The only way to get that in various rooms is to have the adjustnments on the speakers, just as AR did.

Joe

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Hi there;

I am still referencing an AR-2 speaker system, a single woofer and dual tweeters.

I am in the process of changing my mind, a man's perogative, that I do not, have L-pads, my reasoning follows.

There is 2 connections, rather than the usual 3 connections with a typical pot.

I feel that it now is a lower cheaper quality, maybe, pot with a lower power wattage rating.

It would just add resistance to the tweeter's input only.

Now my question is, does the tweeter's output level start drooping, as in hinging, from it's crossover point, as the shaft is turned?

Or does the tweeters top end start to droop as the shaft is turned?

Maybe this is now a simpler question?

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>Hi there;

>

>I am still referencing an AR-2 speaker system, a single woofer

>and dual tweeters.

>

>I am in the process of changing my mind, a man's perogative,

>that I do not, have L-pads, my reasoning follows.

>

>There is 2 connections, rather than the usual 3 connections

>with a typical pot.

>

>I feel that it now is a lower cheaper quality, maybe, pot with

>a lower power wattage rating.

>

>It would just add resistance to the tweeter's input only.

>

>Now my question is, does the tweeter's output level start

>drooping, as in hinging, from it's crossover point, as the

>shaft is turned?

>

>Or does the tweeters top end start to droop as the shaft is

>turned?

>

>Maybe this is now a simpler question?

If I understand your question correctly you are asking what the effect of using an L-Pad with only 2 connections would be. This would be equivalent to using an Aetna-Pollak pot with pin '2' disconnected. By increasing the impedance of the pot/tweeter this combo would probably move the crossover frequency slightly lower. Connecting the L-Pad as an L-pad would, by keeping the overall impedance lower, move the crossover slightly higher. The crossover was designed for the Pollak pot and will work as it was designed to only with the pot or resistors that have the identical electrical effect. There are probably many 'improvements' that can be made to the speakers and crossovers but each 'improvement' or change will change (albeit slightly) the characteristics of the sound. I still like the idea of keeping the speaker/crossover electrically identical to the original since the sound of the original is what I want. Better caps make sense, but the better caps should have the same capacitance value as the originals. A replacement 15 Ohm pot is close enough to the 16 Ohm pot to make no difference and even that tiny difference can be eliminated with a 1 Ohm resistor in series with pin '2' (the low side) of the pot. A stepped attenuator makes sense, but it should have the same electrical charcteristics as the original pot. My point is if you liked the stock speaker, keep it that way.

Joe

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  • 2 weeks later...

Hi there;

I have taken the steel rear cap off of an, "L-pad", identical to the one I was asking about at the beginning of this topic.

I received another sample of the, "L-pad", this one would not turn at all.

As I mentioned in an earlier finger turn test, there was almost a no feel resistance with the new, 20 ohm 5 watt Chinese pot, I have here, also the same as with this first, "L-pad".

There is a very good reason for this similarity, upon opening my stuck, "L-pad", I saw a major brown coating splattered all over the inside of the steel cap.

There is a small similarity to the inside of the Aetna-Pollock pots.

There is more of the windings with the, "L-pad", so many I cannot count them, if they were all there.

These are real copper insulated wires of about, human hair size.

The "L-pad", my mistaken identity due to the smoothness of feel, is actually a Clarostat 10 ohm pot.

Without an old Clarostat catalog for information, I will guess that this is about a 5 watt rated pot.

The new Chinese 5 watt pot uses, Resista wire, or nich-chrome wire, with pencil thin line spaces between the windings and are countable.

So now I will call my sample, L-pad", by it's true name, "pot".

The material splattered on to the rear of the cap is in fact, melted wire insulation, something dramatic, gross overload perhaps, happened.

About 1/4 section of the pot windings is almost totally missing, as in burnt off.

Needless to say, the dual tweeters that also came with this pots crossover, are also DOA, gone forever, sadly.

Having read a little bit here and there regarding the dual tweeters, being heavily modified after buying them from CTS.

There is an amount of fibreglass sandwiched between the cone and the tweeters frame.

Now that I have never seen an L-pad, I think I will buy one.

Where is the woofer that did not come with the, dual tweeters and crossover?

I do not know.

Would a fuse have helped save these classic parts?

We will never know.

It certainly would not have hurt to have had them fused, though.

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  • 3 weeks later...

Hi there;

I just did a google for Clarostat, nothing else to do at 3 in the morning.

I came up with Honeywell, the owners of Clarostat's brand name.

Under Series 58, part number 58C110, a sample of a 10 ohm pot is rated at only 4 watts.

Even their Milspec version has a 4 watt rating.

To sum up my progress, I had assumed, that because of the very smooth silky feel, I had L-pads.

Now I know and you know, AR used 10 ohm 4 watt rated pots in their very early speaker versions, at least in the AR-2 8 ohm series speakers.

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  • 14 years later...
On 9/14/2006 at 6:39 AM, dynaco_dan said:

Hi again;

I added to the topic, but lost everything last night.

Here goes again.

I have a sample of, I suspect, 2 AR-2, speaker terminal boards complete.

There is an L-pad on both boards, rather than a pot, with only 2 of the 3 terminals connected to anything.

One L-pad has embossed on the rear, CM24067 10 ohm 140911, the shaft turns with a definite smoothness, needing very little effort to turn, as if brand new.

Not anywhere near the screechy, hair raising screech of the typical 16 ohm Aetna-Pollock pot, nor the dragging feel of the windings, as the rotor passes over the wire.

There is a copper wire coil, approximately 2" in diameter, no markings on it, wound on a short piece of wooden dowel.

There is 2 cute little metal body caps, about 1" x 1" x 1 1/2", with 3 terminals each, taped together with what appears to be 1/4" masking tape covering most of the manufacturers embossed markings.

One cap has 200VDC - 50'C 30Z embossed on it's top.

The other cap is covered with the 1/4" tape.

The other board is identical, except the L-pad has 140918 embossed instead and this L-pad has an original shaft knob.

I would need to remove tape from one or the other to see the cap rating.

The 3 terminal screws are in a triangular configuration.

I know this is an old thread, but I am glad I found it as I was a bit confused about the pots in my AR-2 speakers. Everywhere I searched on the web, all that was ever mentioned was the ceramic pots in the early AR speakers would fry themselves. I guess I'm lucky because the pots in my AR-2 speakers are good and smooth to turn.

Anyway just uploading some pictures for future reference to this thread.

Jay

867390711_AR-2Crossover.thumb.jpg.0609168d89c2bb56ac4d148843b98faa.jpg

1858487298_AR-210ohmpot.thumb.jpg.e302d1469721799de85208ba42f781a7.jpg

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