AR-3a Potentiometers: Factory Setting versus “Bypassing”

[johnieo]

Recently there have been discussions on a number of threads relating to the difficulty in replacing Polack 16-Ohm, 25-W potentiometers used in many early AR crossovers (AR2-ax, -4x, -3, -3a, -5 and such). Several solutions have been offered. One was to purchase new pots, but they are not manufactured in large quantities or widely stocked. Replacements are listed in the Allied Electronics (Richardson TX) catalog for about $23. Several months ago, they were in stock. another suggestion, which we address here, is the recently discussed practice of "bypassing" the tweeter pot—moving the rotor lead to the hi side of the pot, setting it at maximum output.

To see the effect of pot rotation we plotted the tweeter crossover transfer function (V tweeter/V input) for the AR-3a. See attached jpg file. At the factory "flat" setting, the response is -3 dB at 5 kHz as it should be. At maximum output, the tweeter sees an increase of +3.5 dB from just below 5 kHz to ~8 kHz. Bypassing the wiper thus gives a permanent +3.5 dB output increase. Some increase may be needed in highly absorbing rooms, but not that much or in every situation.

When a 0.2-Ohm series resistor is added to the 6-uF capacitor to simulate ESR, the curves look similar except for a slightly reduced Q. The peak level for the "bypass" condition is reduced by 0.5 dB from its value without the "ESR." The peak of the "flat" curve is reduced by 0.25 dB.

The output curve will be qualitatively the same for the other early speakers using the same x-o circuit, but with different x-o and peak frequencies.

If bypassing "sounds better," or has increased detail, then it is likely a result of the tweeter output being magnified in the 5–8 kHz range. An optical analogy is the telescope; it allows one to see a portion of the visual field in detail not observable with the unaided eye.

2007.txt

[dynaco_dan]

Hi John;

An excellent write-up, thank you very much.

[onplane]

Hi, John!!

Interesting analysis of the tweeter's xover.

John, I assume the peak is caused by the series resonance of the cap and inductance in the voice coil ... correct??

I think you are saying that the resistance in the voice coil + esr of the cap aren't sufficient to dampen the resonant circuit ... right?

John I calculated the resonant frequency just slightly above 6000Hz. At this point the impedance of the cap and voice coil should cancel each other out and the amp should see just the resistance of the voice coil + small amount of esr. When I measured my tweeter, I got a DCR reading of 3.2 ohms, but I have no way to measure the cap.

Now, if the 16 ohms were across the tweeter, won't that drop the resistance even more causing even more current to flow?? I think the answer is it does, but the pot grabs a fair amount of that current and the net flow through the tweeter is reduced.

John, what have you measured?

With a scope, oscillator and spl meter, we should be able to detect a 3 db spike with ease. I mean that should be a piece of cake!

Regards,

Jerry

[RoyC]

>When I

>measured my tweeter, I got a DCR reading of 3.2 ohms, but I

>have no way to measure the cap.

Jerry,

The 3a tweeter typically has a DCR around 2.5ohms or less. The older 3/4" tweeters run closer to 2.3 ohms. Maybe your tweeters had some issues prompting you to seek increased output.

Roy

[johnieo]

Jerry:

One cannot analyze a circuit in the manner you are describing. One must analyze an entire circuit, not two elements alone and then mentally add others. The driver is modeled as a series R-L, so the voltage across L is not acessible. The peak is not 6 kHz; it changes with resistance. This level of RLC circuit analysis would have been a homework problem in a sophomore circuits course fify years ago, a hundred years after Kirchoff's Laws were proven! But if you are curious, ensure that your scope probes do not load the circuit.

[soundminded]

>Jerry:

>

>One cannot analyze a circuit in the manner you are describing.

>One must analyze an entire circuit, not two elements alone and

>then mentally add others. The driver is modeled as a series

>R-L, so the voltage across L is not acessible. The peak is not

>6 kHz; it changes with resistance. This level of RLC circuit

>analysis would have been a homework problem in a sophomore

>circuits course fify years ago, a hundred years after

>Kirchoff's Laws were proven! But if you are curious, ensure

>that your scope probes do not load the circuit.

Thank you for the right answer. This is one disadvantage of using a potentiometer instead of an L-pad for adjusting the level of the driver output, as the control is adjusted, the changing value of R becomes an element not only in the voltage drop to the driver but in the characteristic FR of the filter itself. One possibility is to substitute fixed resistors for the potentionmeter values at its indicated "flat" setting. This should give the factory measured optimal system performance. Any further adjustments can be made with a graphic equalizer. The advantage of the equalizer is that it alters the FR predictably and controllably. This can include a shelved increase or decrease for a particular driver as well. During the years of production of AR3a, such equipment was not available to the consumer audiophile but only to audio professionals at very high cost. Today, they can be had for very little cost. BTW, removing the potentiometer completely has the same disadvantage.

[Guest matty g]

One possibility is to substitute fixed resistors for

>the potentionmeter values at its indicated "flat"

>setting. This should give the factory measured optimal system

>performance. Any further adjustments can be made with a

>graphic equalizer.

This is the method which I was describing in another thread, with which I have had repeated success. By leaving the pot wired in place, the crossover is not electrically altered in any way. By removing the wire from the wiper terminal of the pot and soldering a fixed resistor of a given value between the tweeter lead wire and the terminal on the pot which would contact the wiper arm at the max setting, the tweeter is fixed to the position of your choice. Typically a 2 or 3 ohm resistor will put you just above norm but slightly below the max setting. This of course applies to the midrange as well, although I usually use a 5 ohm resistor to dampen the midrange.

Matt

[onplane]

>Jerry:

>

>But if you are curious, ensure that your scope probes do not load >the circuit.

>One cannot analyze a circuit in the manner you are describing.

John, I am very curious. I'll try this week to measure the voltage drop across the tweeter while scaning from 3000Hz to 10000Hz.

There is little danger my scope will load anything. It's 1M ohm with 30 pf across. Here are the specs:

http://www.testequipmentdepot.com/usedequi...3&2215specs.htm

Not a fantastic scope, but plenty good enough for audio.

As for my analysis, John, it was for MY xover and it is accurate. All I have in MY xover is a cap and the tweeter (resistance + inductance). Resonant frequency is NOT determined by the resistance. It's Q will be impacted, however.

As for what voltage peak I measure, that will be a function of the actual value of the cap and the actual value of the inductance in the driver.

Roy, you are right! I measured again, but this time scraped more black paint off and got a reading of 2.7 ohms. Not sure how accurate my DMM is though. My guess is we are in the right ball park now. I needed to scape off more black paint anyhow so that I can get the scope probes hooked to the tweeter.

soundminded, just for the record, if you set an L-pad (doesn't matter whether it's a 4, 8 or 16 ohm) to max output, it is EXACTLY the same as eliminating the tweeter pot. That is, when you select max output on an L-pad, the series resistance is set to zero and the shut resistance is open.

John, observed before that eliminating the pots LOWERS slightly the xover frequency. Let's not forget that I also removed the mid pot, so we have more at play here.

Voltage on the tweeter, consequently, only tells a part of the story. I think I have some decent mics around so I might watch the voltage on those as well while sweeping through the high frequencies. For all we know, this little "bump" John is talking about may fill a void I've created in the mid driver.

Regards,

Jerry

[RoyC]

>This is the method which I was describing in another thread,

>with which I have had repeated success. By leaving the pot

>wired in place, the crossover is not electrically altered in

>any way.

Matt,

Your method actually does alter it somewhat, but certainly not as much as completely removing the pot.

Leaving the pot's 16 ohm parallel resistance in place is much better than not, but to truly "fix" the attenuation at an original pot setting you would have to subtract your series resistance from the 16 ohm parallel resistance.

The "norm" (dot) tweeter setting is 1.75 ohms in series and 14.25 in parallel. The "norm" setting for the mid is 3.25 ohms in series and 12.75 in parallel.

Jerry,

I recently tried your bi-amping arrangement on a pair of "AR-3a Limited" speakers I have been working on, using two old (identical) Technics receivers. These speakers use the AR-3a replacement tweeter (with an associated crossover modification), and l-pads instead of pots.

First the good news: I didn't fry anything, and the two volume controls and 4 tone controls were fun to play with.

The bad news: With the tweeter and mid level controls set to maximum, the sound was rather harsh and "in your face" until I backed off the speakers' midrange l-pads. No adjustment of tone controls could remedy this...the warm character of the 3a was compromised.

It may work better for you, as the new tweeter has more output than the old, but I cannot imagine NOT having the ability to pad the midrange relative to the tweeter. With all of those controls, I could not find an acceptable balance relative to the traditional 3a sound.

Roy

[onplane]

Roy, I want to hear much more about your bi-amp experiment! There are a whole bunch of things bundled in my scheme.

Before we get to that, I saw your comment to Matt on the "dot" setting at 1.75 ohms series and 14.25 parallel. Now, I have never even attempted to measure the pot. I just assumed that with the corrosion in my pots all measurements would be meaningless.

Now, Roy, how did you arrive at this 1.75 ohms?? When I look at those pots, they appear to be linear and the dot appears to be about midway in the total travel.

Roy, please don’t misunderstand. I’m NOT challenging your assertion … just trying to understand how this works. From my perspective, 1.75 ohms with 14.25 in parallel is pretty close to my tweeter all by itself! I thought the “dot” would be more like 8 ohms in series and 8 ohms parallel, which would be a country mile from a pot totally by-passed.

Returning to your bi-amp, Roy, you mentioned 4 tone controls, yet I only have two working controls (treble on one amp and bass on the other). (Glad you didn’t fry anything! I’ve done this on 7 different maps now and have had zero problems.) Sooooo, how did you get 4 controls???

Roy, did you vertical bi-amp??? That is, one Technics receiver for each speaker (i.e. LEFT channel for the woofer, RIGHT for the mids/tweeters)? This is NOT my scheme. My interest is EXCLUSIVELY horizontal bi-amping, where one receiver controls the woofer and the other controls the mids/tweeters. Balance between the woofer and mid/tweeters is achieved via VOLUME controls. This is absolutely necessary, because with the pots out, the mid and tweeter are more sensitive. We simply MUST throttle back the voltage sent to those drivers via the volume controls to bring everything into balance. (Now in my case, I use a much LARGER amp on the woofers 170 wpc vs 30 wpc on the mids/tweeters)

Now for another tidbit! When you turn an L-pad to maximum output, it is IDENTICAL to eliminating the pots. That is, at max output the L-pad sets the series resistance to zero and the shunt to OPEN … leaving the driver all by itself! This means you can simulate my setup on your speakers simply by turning the L-pad to max. Roy, that is a very nice feature!

Lastly, if you found a certain “harshness – in your face” was resolved by turning DOWN the L-pad on the mid driver, I would say … YES exactly! Those drivers are way more sensitive with the pots removed. I’d work the volume control, however, to bring them down, NOT the L-pad! To me the whole idea is to move control away from the speakers and place control on the independent amps. So in a bi-amp arrangement, Roy, I'd respectfully suggest that the one thing you can't control via the amps is the balance between the mid and the tweeter. So, it's the tweeter L-pad that I'd be adjusting.

Roy, does this make any sense … or have I made a hash of this explanation?

Regards,

Jerry

[RoyC]

>Now, Roy, how did you arrive at this 1.75 ohms?? When I look

>at those pots, they appear to be linear and the dot appears to

>be about midway in the total travel.

The setting values are in a forum library schematic.

I (as well as other forum members) have confirmed it a number of times in working 3a's.

>Sooooo, how did you get 4 controls???

:-) Other than the two receivers' volume controls, I was counting the bass and treble controls on both receivers. Obviously the treble control of the "bass receiver" was pretty useless, though I could hear the effects of all others.

>interest is EXCLUSIVELY horizontal bi-amping, where one

>receiver controls the woofer and the other controls the

>mids/tweeters. Balance between the woofer and mid/tweeters is

>achieved via VOLUME controls.

That is what I did, per your instructions in other threads.

>This

>means you can simulate my setup on your speakers simply by

>turning the L-pad to max.

That is why I maxed the speakers' controls. I wanted to see what it would be like with the controls "removed".

>Lastly, if you found a certain “harshness – in your face” was

>resolved by turning DOWN the L-pad on the mid driver, I would

>say … YES exactly! Those drivers are way more sensitive with

>the pots removed. I’d work the volume control, however, to

>bring them down, NOT the L-pad!

I tried all receiver controls at my disposal:-). My point was, that in spite of the various types of controls, the l-pad/speaker level controls were necessary to achieve something close to the traditional AR-3a sound. Adjusting both the tweeter and midrange in tandem with just the receiver controls was insufficient. That mid needs to be pulled back for a 3a to sound right to me.

Don't get me wrong, Jerry. The experiment went smoothly and provided an entertaining afternoon. Regardless of the number of amps in the mix, however, I just think the level control circuits are very necessary in the 3a. If I didn't have the speaker level controls, I might have described the result as sounding something like PA speakers...but, then again, I was using a more aggressive tweeter than the original.

Roy

[johnieo]

The issue of L-Pads never fails to ceep (sideways) into any discussion of potentiometers. Here are some things to consider.

1. L-Pads of a certain "characteristic resistance," say 8 Ohms, will only maintain that characteristic if the load (or source) is 8 Ohms purely resistive. That is (8 +j0) Ohms. They do not function as advertised when connected to a reactive load such as a loudspeaker driver.

2. L-Pads do not present a constant resistance to a crossover circuit when connected to a driver, therefore it is impossible for them to hold the x-o frequency constant, as the attenuation is varied. This is an urban legend!

3. The broad definition of an L-Pad is: "Two ganged sections wired in the shape of a backward L. The ganged sections work to provide either a constant input or a constant output impedance regardless of the attenuation setting. Since modern analog audio electronic circuits consist of stages characterized by very high input and very low output impedances, the term is now broaden to include all L-shaped networks without the requirement of providing constant impedance to the source or load."

4. The "constant input" design would be constructed so that the resistance looking into the pad’s output terminals would remain constant with rotation, if the source resistance were the characteristic value. A "constant output" design would keep the input resistance to the pad constant when rotated, if the load resistance were the characteristic value. (See first attached jpg file.)

5. Until I made measurements for another purpose, it was always my understanding when plunking down our Pesos, that we purchased the "constant output" design (right-hand image in the first jpg file). Nada! Guess what, I just found out that’s also an urban legend. It is costly to make the proper parallel winding profile, so commercial versions use a linearly 40-Ohm parallel winding. The two original Rp profiles, along with everything you buy at any Supermarche are shown in the second jpg file. In this figure the value of Rp was calculated for the two original designs, and measured on a number of pads from different sources, vintages and power ratings. The values of Rp of interest to AR-3a restorers (corresponding to Rs = 3.25 Ohms - midrange flat, and Rs = 1.75 Ohms - tweeter flat), are marked on the “commercial” curve.

6. An L-Pad, can’t provide the same Rp – Rs combination as a potentiometer—even if it did, it’s not keeping x-o constant, as you may have been lead to believe.

7. Setting the potentiometers at maximum; that produces a + 3.5 dB increase in signal above flat--all or part of which may be needed in certain environments.

8. Removing the potentiometers altogether; well that transports the discussion out of my world of AR restoration into the realm of DIY.

[soundminded]

Again I agree with you. Anything which alters either the function or the performance goals of the original design intent is no longer a restoration, it is a DIY re-engineering project, even just replacing the potentiometers with fixed resistors.

I didn't mean to suggest that you could simply replace the potentiometers with L-pads, clearly you couldn't. You'd still need the fixed resistors and if the L-pads weren't exactly matched to the drivers, performed perfectly, and the driver's impedence not constant, they would introduce their own alteration of the filter response curve from the idealized intent. Which is worse as a level control for a passive crossover network, a potentionmeter or an L-pad? I don't know, it would have to be measured and compared. If I had to take a wild guess, I'd say the potentiomter which may explain why so few speaker designers who did provide level contols used simple pots. Here's a link which at least explains the theory if not the real world facts of L-pads;

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

You could of course engineer a multiposition switch which approximates the performance of an L-pad, or for that matter a potentiometer. I can't remember who it was but someone here posted photos of some he built and it was clear it took a great deal of time and effort. He seemed to have done an excellent job of it from the photos.

Another solution which probably offers the least change of filter characteristics with gain is to remove the passive crossover network altogether and use an active crossover network with multiple amplifiers. This is far more elaborate than probably any other solution but given that it is comparatively low in cost compared to some loudspeaker systems, I'm surprised it isn't more widely used. After all, a subwoofer with a built in plate amplifier almost gets you there already. And some speaker systems did use them such as the Infinity ServoStatik and Phillips Little David. Personally, I'd still install a series capacitor for each driver just to be on the safe side so that a LF transient such as from a turn on thump doesn't damage it. And of course at that point you are presented with a very wide range of options for slope and alignment type, often easily switchable. Plenty of opportunity to get into lots of trouble. :-)

[onplane]

>

>Don't get me wrong, Jerry. The experiment went smoothly and

>provided an entertaining afternoon. Regardless of the number

>of amps in the mix, however, I just think the level control

>circuits are very necessary in the 3a. If I didn't have the

>speaker level controls, I might have described the result as

>sounding something like PA speakers...but, then again, I was

>using a more aggressive tweeter than the original.

>

>Roy

>

Roy, to me this is the real "fun" in our hobby. That is trying different things and then evaluating as best we can ... what we hear.

When I first setup the bi-amp, I had some issues with the two volume controls. That is, I wasn't sure how to make the two work together.

Now, however, I have a very simple routine. When changing sources, I set both controls full OFF. Then, 1st I bring up the woofer amp to achieve the overall volume I desire (that will depend upon the time of day, other things I have to concentrate upon, other people and what they are doing watching TV, sleeping, etc.). Then I slowly bring up the mid/tweeter amp to achieve the clarity I desire given the source and the music.

To be honest, I rarely use the tone controls – I leave them at 1 o’clock.

Roy, if you ever try bi-amping again there is another little trick I’ve found that helps with vintage amps (see pic). The lamp cord patch just provides more current return options. I’ve often been critical of AR because of the corrosion on those pots (and probably a tad unfairly). In any event, vintage electronics can suffer from corrosion as well; especially in those speaker selection switches. After all, we once again have current and often dissimilar metals in contact. Then on some vintage gear (mine for example), the speaker selection switches put the A and B speakers in series.

Anyhow, Roy, I noticed a slight improvement in the very deepest bass with this little trick.

In closing, Roy, I have one question for you. You mentioned that regardless of the number of amps, you felt the level controls on the speakers were necessary. Supposing … just supposing you had three amps; that is, one amp for each type of driver. Would you still feel that you’d require controls on the speakers?

Regards,

Jerry

http://www.classicspeakerpages.net/dc/user_files/2011.jpg

[onplane]

>

>Personally, I'd still install a series capacitor for each

>driver just to be on the safe side so that a LF transient such

>as from a turn on thump doesn't damage it.

soundminded, you are the 2nd person who recommended the cap when converting to ACTIVE operation.

My question is, aren't we going in a circle?? For me, anyhow, the entire xover network for the tweeter is ... a single cap!

Regards,

Jerry

[onplane]

>

>The values of Rp of interest to AR-3a restorers

>(corresponding to Rs = 3.25 Ohms - midrange flat, and Rs =

>1.75 Ohms - tweeter flat), are marked on the “commercial”

>curve.

>

>7. Setting the potentiometers at maximum; that produces a +

>3.5 dB increase in signal above flat--all or part of which may

>be needed in certain environments.

>

John, both you and Roy mentioned these values for the series resistance. Roy said it corresponds to the "dot" position, yet you referenced it as the "flat" position.

Are the AR pots tapered? I've never measured them so have no idea.

Also, if this is "flat", John, do you know the source of this and how it relates to another memo in our library (see below)? To be honest any series resistance seems to be a long ways from "levels controls must be turned all the way up".

Regards,

Jerry

http://www.classicspeakerpages.net/dc/user_files/2012.jpg

[soundminded]

>>

>>Personally, I'd still install a series capacitor for each

>>driver just to be on the safe side so that a LF transient

>such

>>as from a turn on thump doesn't damage it.

>

>

>soundminded, you are the 2nd person who recommended the cap

>when converting to ACTIVE operation.

>

>My question is, aren't we going in a circle?? For me, anyhow,

>the entire xover network for the tweeter is ... a single cap!

>

>Regards,

>Jerry

>

>

I can't speak for anyone else but myself. I'd try to make the cap small enough to protect the driver from damage but large enough so that it is NOT the controlling element in the FR. The goal which I am suggesting is to get the FR shaping circuit for each driver at the preamplifier signal level, not the power amplifier output level. This allows the driver's volume to be controlled without altering the FR inadvertently at the same time except of course by shelving the output of the particular driver under control. And as I suggested, it also coincidently allows the tinkerer to select from a wide number of options in FR shaping such as different alignments (Butterworth or Linkwitz Riley), orders of filtering, etc. I guess the first approximation would be to choose a circuit comparable to the design intent of the manufacturer. Aren't some of these crossovers in AR designs second order using for example an inductor in prarallel with the AR3a tweeter? Haven't checked the schematic lately. Anyway, it's just one more alternative for someone who is ambitious and has some time and money on his hands he doesn't know what to do with. :-)

[RoyC]

Jerry,

The pot setting values I cited are for the "norm"/dot positions.

The max/flat position (full "increase") would be 16 ohms in parallel with no series resistance.

No matter how you read that AR document you posted (at least three times :-)), a properly restored pair of 3a's will give the typical user the flexibility to adjust for recordings, room acoustics, and personal preference...as originally intended.

BEFORE we get into a "flat vs reality" or "bi-amping" debate again :-) I would like to share a recent observation that may be an answer to the corroded pot issue.

When a 24 to 25 ohm resistor is placed across the #1 and #2 tabs of a common $3+ dollar Parts Express "8 ohm" l-pad, the meter reads very nearly the same as an old 15-16 ohm AR pot for series and corresponding parallel resistance. It appears to hold together for MOST of the range typically used, including our "norm" settings.

If this works out, brand new inexpensive level controls can be installed with similar characteristics to the original pots, with minimal effect on the electrical characteristics of the original crossover (*when set within the typical range of use*).

I tried it on a 3a today, and it seems to work quite well!

This almost seems too simple...I'll let the engineers jump in here.

Roy

[Shacky]

>>

>>Personally, I'd still install a series capacitor for each

>>driver just to be on the safe side so that a LF transient

>such

>>as from a turn on thump doesn't damage it.

>

>

>soundminded, you are the 2nd person who recommended the cap

>when converting to ACTIVE operation.

>

>My question is, aren't we going in a circle?? For me, anyhow,

>the entire xover network for the tweeter is ... a single cap!

>

>Regards,

>Jerry

>

>

Jerry,

Isn't the crossover the 16 ohm pot, cap, and resistance of the tweeter driver combined?

I tried L-Pads for my 2AX project and was not happy with the results. Very bright with a hole in the midrange. Having never listened to 2AX's before, I had nothing to compare except my knowledge of how the music "should" sound and having experience with set of original Polk separates that had a hole in the midrange.

I learned from many here and at AK, that the 8 OHM L-Pad significantly raised the crossover for both mid and tweet - maybe double the crossover of the original 16 OHM pot. I cleaned up a good set of AR pots, replaced the L-Pads and my 2AX's sound incredible. Rich midrange and highs that are in no way piercing. The idea above of using resistor in combo with the L-Pad is very interesting indeed. Not many people want to plunk down $60 for set of Leeds pots for 2AX's.

I plan to restore my 3a's with Leed's 15 OHM new pots. Even bought a second set of the Leed's pots to use in my 2AX's when I get up the energy to take them apart again. But I'm not rushing as the music I'm listening to now with Sansui 890DB and the correctly restored 2AX's is the best I ever had in my home!!! I know you highly recomend bi-amping but I have enough of a WAF problem with one old Sansui in the living room.

Jim

[onplane]

>Jerry,

>

>Isn't the crossover the 16 ohm pot, cap, and resistance of the

>tweeter driver combined?

>

>I tried L-Pads for my 2AX project and was not happy with the

>results. Very bright with a hole in the midrange. Having

>never listened to 2AX's before, I had nothing to compare

>except my knowledge of how the music "should" sound

>and having experience with set of original Polk separates that

>had a hole in the midrange.

>

>I learned from many here and at AK, that the 8 OHM L-Pad

>significantly raised the crossover for both mid and tweet -

>maybe double the crossover of the original 16 OHM pot. I

>cleaned up a good set of AR pots, replaced the L-Pads and my

>2AX's sound incredible. Rich midrange and highs that are in

>no way piercing. The idea above of using resistor in combo

>with the L-Pad is very interesting indeed. Not many people

>want to plunk down $60 for set of Leeds pots for 2AX's.

>

>I plan to restore my 3a's with Leed's 15 OHM new pots. Even

>bought a second set of the Leed's pots to use in my 2AX's when

>I get up the energy to take them apart again. But I'm not

>rushing as the music I'm listening to now with Sansui 890DB

>and the correctly restored 2AX's is the best I ever had in my

>home!!! I know you highly recomend bi-amping but I have

>enough of a WAF problem with one old Sansui in the living

>room.

>

>Jim

>

Hi, Jim!

In my AR's I have by-passed COMPLETELY both pots. Now, I don't recommend by-passing the mid pot UNLESS you plan to bi-amp as well.

L-pads clearly have a different impact upon the xover frequency than the 16 ohm pots. I think it depends upon where you set the L-pad and where you set the pot. As John explains above, you simply cannot duplicate the Rs/Rp of a pot with an L-pad.

As for the complete by-pass, again it's a little confusing. This much I know. If you compare the tweeter xover network without the pot to a network with the pot and the pot at MAX, the xover frequency WITHOUT pot is slightly LOWER.

Again, Jim, it's a little confusing, because when we talk about xover frequency it's the point at which each driver is providing half of the sound. So a change in the network for one driver, really only impacts that driver.

Removing the tweeter pot completely (like I do):

1. RAISES the impedance seen by the cap (compared to pot at MAX)

2. LESS current flows in the tweeter network

3. MORE current flows in the tweeter itself (pot is no longer dissipating power)

4. This increase in current flow is NOT uniform over the frequency range. More current flows at lower frequencies. Thus there is a slight tendency to LOWER the xover frequency.

Regards,

Jerry

[johnieo]

>>>Personally, I'd still install a series capacitor for

>each driver just to be on the safe side so that a LF

>transient such as from a turn on thump doesn't damage it.

>>soundminded, you are the 2nd person who recommended the

>cap when converting to ACTIVE operation.

>>For me, anyhow, the entire xover network for the tweeter is

>>... a single cap!

>Jerry,

>

>Isn't the crossover the 16 ohm pot, cap, and resistance of the

>tweeter driver combined?

>

>Jim

Correct, Jim, the pot, cap and resistance of the tweeter driver together form the crossover network.

If one removes the pot completely, then one only has the capacitor and driver. OK, so we think this would be the greatest for biamping, but as it was noted, a transient could zap the tweeter. So, we decide to put a small capacitor in the circuit to prevent that. Yes, a good idea. But how small? Dunno ... let's try one a tenth the size of the intended capacitor-- say 0.6 uF. Hmmmm, doesn't seem to have any audible output. Well, let's try a capacitor ten times the size intended, say 60 uF. Wow! the response goes through most of the midrange!! can't biamp with that--even with three terminals. (see jpg attached.)

So, what do we do? What size capacitor do we use? Need data. We could obtain a calibrated microphone and try to measure the sound power accurately. As Tom Tyson noted, the AR-3a is a "far field" radiator, so measurements taken at 1 meter have no validity. We would then need to build an anechoic chamber. One large enough to obtain correct far field measuremnts. Experimental physicists know the first several measurements in a new test facility are usually wrong. Good data never comes cheaply.

When all is said and done, we would likely come to our senses and conclude that the AR engineers working under Roy Allison's direction were a pretty smart team. The crossover they designed works well and furthermore someone carefully marked flat potentiometer set points--not on front office literature, but on a "drawn by Dilbert" circuit drawing.

When compared to the cost of purchasing and shipping AR-3as, the cost of new pots becomes small. One of Parkinson's Laws states that the cheaper the item, the more time is spent arguing about the expenditure.

[dynaco_dan]

Hi John;

What I have noticed over a period of time is, the normal, or dot position, may be what AR considered to be, just that.

If you check a lot of different classic AR speaker crossovers, that is not the 50% turning radius point of the pots though.

I had always assumed that the dot, was half way around the pot's perimeter.

In otherwords, the pots were not set at midstroke and that was the dot, but about another 45 degree angle or more past that point.

This would make them slightly brighter than their true midpoint.

I'm just splitting hairs here.

[Shacky]

>Hi, Jim!

>

>In my AR's I have by-passed COMPLETELY both pots. Now, I

>don't recommend by-passing the mid pot UNLESS you plan to

>bi-amp as well.

>

>L-pads clearly have a different impact upon the xover

>frequency than the 16 ohm pots. I think it depends upon where

>you set the L-pad and where you set the pot. As John explains

>above, you simply cannot duplicate the Rs/Rp of a pot with an

>L-pad.

>

>As for the complete by-pass, again it's a little confusing.

>This much I know. If you compare the tweeter xover network

>without the pot to a network with the pot and the pot at MAX,

>the xover frequency WITHOUT pot is slightly LOWER.

>

>Again, Jim, it's a little confusing, because when we talk

>about xover frequency it's the point at which each driver is

>providing half of the sound. So a change in the network for

>one driver, really only impacts that driver.

>

>Removing the tweeter pot completely (like I do):

>

>1. RAISES the impedance seen by the cap (compared to pot at

>MAX)

>2. LESS current flows in the tweeter network

>3. MORE current flows in the tweeter itself (pot is no longer

>dissipating power)

>4. This increase in current flow is NOT uniform over the

>frequency range. More current flows at lower frequencies.

>Thus there is a slight tendency to LOWER the xover frequency.

>

>

>Regards,

>Jerry

>

Jerry,

Not to belabor this point but I recall going through my L-Pad experiance learning the following:

Going from 16 OHM pot tp 8 OHM L-Pad (assuming 8 OHMS for the L-Pad which I know is not absolute) raises the crossover frequency - it about doubles it. Can't find the reference I had to the equation but going down in OHMs increased crossover. It's why my 2AX's didn't sound right with the L-Pads in place. I was missing big portion of the frequency range.

Now I may be looking at this too simply - but if you remove the pot completely, wouldn't you in effect be "lowering" the OHMs in the circuit and thereby increasing (not lowering) the crossover? Could be that I just don't know enough about electronics to understand.

Jim

[onplane]

>

>Jerry,

>

>Not to belabor this point but I recall going through my L-Pad

>experiance learning the following:

>

>Going from 16 OHM pot tp 8 OHM L-Pad (assuming 8 OHMS for the

>L-Pad which I know is not absolute) raises the crossover

>frequency - it about doubles it. Can't find the reference I

>had to the equation but going down in OHMs increased

>crossover. It's why my 2AX's didn't sound right with the

>L-Pads in place. I was missing big portion of the frequency

>range.

>

>Now I may be looking at this too simply - but if you remove

>the pot completely, wouldn't you in effect be

>"lowering" the OHMs in the circuit and thereby

>increasing (not lowering) the crossover? Could be that I just

>don't know enough about electronics to understand.

>

>Jim

>

Jim, I haven't really analyzed L-pads, as I have little interest in them. I can tell you, however, about one specific position in an L-pad; that is, the full ON or MAX sound position.

If you have installed L-pads and set them to their MAX position (makes no difference if you use 4, 8 or 16 ohm L-pad), it is IDENTICAL to completely removing the pots. In this one position the Rs = zero and the Rp = OPEN, leaving the driver alone with the cap.

Now, when the pots are removed this raises the impedance over a system with the pots at MAX, because the pots are in parallel with the driver. For example, let's assume the driver impedance is 4 ohms. Then the net impedance with the full 16 ohms in parallel is 3.2 ohms. Removing the pot brings us back to 4 ohms.

Raising the impedance this small amount has the tendency to slightly LOWER the xover frequency.

Hope this helps ...

Regards,

Jerry

[onplane]

>Hi John;

>

>What I have noticed over a period of time is, the normal, or

>dot position, may be what AR considered to be, just that.

>

>If you check a lot of different classic AR speaker crossovers,

>that is not the 50% turning radius point of the pots though.

>

>I had always assumed that the dot, was half way around the

>pot's perimeter.

>

>In otherwords, the pots were not set at midstroke and that was

>the dot, but about another 45 degree angle or more past that

>point.

>

>This would make them slightly brighter than their true

>midpoint.

>

>I'm just splitting hairs here.

Hi, Vern!

Like you I'm a little confused. Roy says the 'dot' position is at 1.75 ohms series and 14.25 ohms parallel. John, claims the "factory 'flat' setting" is 2.5 ohms. (these refer to the tweeter pot settings)

So, today I just tried my non-working pots to see if the dot is in the middle of the total turning radius and it appears to be!

I can't believe those pots are tapered, but ... they might be! Now, if they are tapered, this would explain everything.

Has anyone measured the pots at the center of their rotation?

Regards,

Jerry