AR2AX Crossover Schematic

[Guest baumgrenze]

I am not an electrical engineer, nor do I pretend to be one.

I started a new thread because I found some historic posts commenting on the crossover schematic for the AR2AX in the library. (If you are looking for it, it is accessed via the Library>AR>Documents (which is nearly the last item in the long list, and easily overlooked.

I am rebuilding a pair of AR2AX's and have found much useful information here. I find that my crossover came from the factory wired differently than the schematic shows. My S/N's are 48757 and 46094. I bought my speakers new from AR, mail order I believe, in 1967.

Here is what I find in the crossover circuit.

The capacitors are from Industrial Condenser Corp. of Chicago, part # 5203. They are in a waxed box.

The tweeter has black and yellow leads. The black lead goes to the "T post" and, along the way is attached to one end of the HI pot winding. The other end of the HI pot winding is attached to the 4 μF capacitor carrying a black lead. The yellow lead is attached to the pot wiper. This is consistent with the library diagram.

The midrange has green and yellow leads. The green lead goes to the "T post" and along the way is attached to one end of the MID pot winding. The other end of the MID pot winding is attached to the 6 μF capacitor carrying a green lead. The yellow lead is attached to the pot wiper. The connection to the "T post" is NOT consistent with the library diagram.

The woofer has red and blue leads. The red lead is connected to the "2 post." (The one in the center of the three posts.) The blue lead is connected to the inductor coil. It is pinned to the case above the coil. The other end of the inductor coil is attached to the blue or common lead on the capacitor package. This is consistent with the library diagram.

Engineers, what is the impact of this wiring change? Do I follow what I found or do I use the library diagram? Does anyone know the source of the library diagram?

For what it is worth, I found that the black putty that was used by AR as an acoustic seal was still flexible enough to make into a ball. I was disappointed to see that it had been applied quite unevenly, so that there is a good chance I did not get a well sealed speaker set from the factory. I noticed that the flange on the speaker is ~ 1/16" wider than the mounting flange on the box. The putty had piled up on this extra flange, which is towards the center of the speaker.

The woofers are the cloth suspension type. One spider failed when my son insisted that I watch "X-Men." I had both spiders rebuilt by:

Millersound

1422 Taylor RD

Landsdale PA 19446

I'll probably raise a lot of hackles, but I have decided to move all of the active components out of the speaker box, to make them more accessible. I plan to mount another 6 terminal screws, using the same approach that AR used for the speaker wire terminal posts. Once I'm satisfied that I've got everything right, I make some nice 1.5" walnut molding that sits flush with the sides of the speakers that I can screw into the particle board back. This will allow me to put a screwed down panel across the back for easy access.

I redrew the library schematic using Autosketch. It is a bit higher contrast than the original. I also drew a copy of the schematic of the crossover wiring I found. I have a photo of the crossover wiring as I found it, too. I think it is a bit clearer than others I have found here. I can post any or all of them if people think it is worthwhile.

[onplane]

>

>The midrange has green and yellow leads. The green lead goes

>to the "T post" and along the way is attached to one

>end of the MID pot winding. The other end of the MID pot

>winding is attached to the 6 μF capacitor carrying a green

>lead. The yellow lead is attached to the pot wiper. The

>connection to the "T post" is NOT consistent with

>the library diagram.

Yes, it is consistent with the schematic. Please remember this is NOT a wiring diagram, but a schematic. Both the mid driver and the "bottom" of the pot are connected to terminal "T". This is correct!

Actually the wiring for both the tweeter and the mid driver are symmetrical up to this point.

>Engineers, what is the impact of this wiring change? Do I

>follow what I found or do I use the library diagram? Does

>anyone know the source of the library diagram?

There is no impact as your speakers are wired consistent with the schematic.

>

>I'll probably raise a lot of hackles, but I have decided to

>move all of the active components out of the speaker box, to

>make them more accessible. I plan to mount another 6 terminal

>screws, using the same approach that AR used for the speaker

>wire terminal posts. Once I'm satisfied that I've got

>everything right, I make some nice 1.5" walnut molding

>that sits flush with the sides of the speakers that I can

>screw into the particle board back. This will allow me to put

>a screwed down panel across the back for easy access.

>

>I redrew the library schematic using Autosketch. It is a bit

>higher contrast than the original. I also drew a copy of the

>schematic of the crossover wiring I found. I have a photo of

>the crossover wiring as I found it, too. I think it is a bit

>clearer than others I have found here. I can post any or all

>of them if people think it is worthwhile.

Sure post whatever pics you have!!

Just so you know, those old pots can cause your AR-2ax's to sound quite "dull". Corrosion (see pic), that developed over all of these years, causes resistance where NONE was ever intended.

I recommend a simple "mod" (see attached), that will make your speakers sound similar to 40 years ago. In addition, if most of your music source is digital (CD, MP3's, FM, ect), this "mod" will get you closer to a flat frequency response.

Regards,

Jerry

http://www.classicspeakerpages.net/dc/user_files/1991.jpg

http://www.classicspeakerpages.net/dc/user_files/1990.jpg

[charger3834]

>

>Just so you know, those old pots can cause your AR-2ax's to

>sound quite "dull". Corrosion (see pic), that

>developed over all of these years, causes resistance where

>NONE was ever intended.

>

>I recommend a simple "mod" (see attached), that will

>make your speakers sound similar to 40 years ago. In

>addition, if most of your music source is digital (CD, MP3's,

>FM, ect), this "mod" will get you closer to a flat

>frequency response.

>

>Regards,

>Jerry

>OK, I may not be the brightest guy here, and I'm not going to get in to a philosophical discussion over "flat" response and the recording industry today verses 30 years ago etc. etc, however just from a common sense standpoint, you should not bypass the mid/tweeter in any 2x or 2ax system! That driver is highly efficient and is way too "hot" wide open. In the case of the old pre 1970 AR-2ax, this is a very "unnatural" sound.

I am also confident that you did not get an poorly sealed cabinet from the factory, as AR had incredibly high quality control during that period of time.

Good Luck

>

[onplane]

>OK, I may not be the brightest guy here, and I'm not going

>to get in to a philosophical discussion over "flat"

>response and the recording industry today verses 30 years ago

>etc. etc, however just from a common sense standpoint, you

>should not bypass the mid/tweeter in any 2x or 2ax system!

>That driver is highly efficient and is way too "hot"

>wide open. In the case of the old pre 1970 AR-2ax, this is a

>very "unnatural" sound.

>

Hi, Brad!

I agree, we don't need any more philosophy here. Let's be practical and speak the language of engineers.

First off, if you look at my diagram above, I do NOT recommend by-passing the mid driver pot. My "mod" just nails the mid to it's max position, but eliminates any and ALL corrosion in the mid pot.

I do, on the other hand, recommend by-passing the tweeter pot. The net result of this is approx. 14% (on the average) MORE current flowing through the tweeter than would flow at it's maximum pot position ASSUMING ZERO CORROSION IN THE POT.

14% more current equates to approx 1.15db MORE sound from the tweeter. Below is the chart for the 2ax with the pots at max. As you can see, this "mod" will NOT in any way, shape or form significantly retard the tweeter roll off.

No philosophy ... just math!

Regards,

Jerry

http://www.classicspeakerpages.net/dc/user_files/1998.jpg

[charger3834]

Hi Jerry,

Interesting indeed. One little thing to clarify is that you have referenced the response curves of the "new" 2ax, note the crossover frequencies. This system is a bit different than the 1964-69 version. I don't know if that really matters for the purpose of this discussion, but I tend to nit pick. The 1 3/8" tweeter is a tad inefficient, and when I listen to those I run them on the max position.

I am really quite confident that the 3.5" CTS mid is still really too hot on the max position. This is the case with both the pre and post 1970 versions. As I mentioned, I am not the most academic forum member, however I feel I must through caution to the wind on this subject.

[onplane]

Brad, you could very well be correct on the different versions of the 2ax. I really couldn't say as I do NOT have the frequency charts for the other versions.

In any event, just wanted to say that I did NOT do the mod on my AR-3a's that I recommeded above. On my AR-3a's I by-passed both pots!

That's not all I did, however, as I passively bi-amped as well. With two independent amps powering the AR-3a's, I have effectively "moved" the function of the pots back to the volume controls on the amps.

So on any given day, I can decide "how I want it" without having to adjust the controls on the speakers (which for me is NOT easy given their location).

This change to passive bi-amping made a huge difference in the resulting sound; far, far greater than the pot mod.

Passive bi-amping does require an additional mid to low power amp.

That was NOT a problem for me as I have amps ... ALL OVER THE PLACE! I picked up 4 more just two weeks ago.

Bi-amping gives me a chance to use amps that were previously collecting dust. It further gives me a chance to put to good use HIGH quality mid and low power amps; amps that are of zero value driving AR's by themselves.

Regards,

Jerry

[Guest matty g]

>>OK, I may not be the brightest guy here, and I'm not going

>to get in to a philosophical discussion over "flat"

>response and the recording industry today verses 30 years ago

>etc. etc, however just from a common sense standpoint, you

>should not bypass the mid/tweeter in any 2x or 2ax system!

>That driver is highly efficient and is way too "hot"

>wide open. In the case of the old pre 1970 AR-2ax, this is a

>very "unnatural" sound.

I have found that this has a tendency to lean towards an "overly bright" tweeter and a rather harsh midrange response. I have had great success with leaving the pot in place and soldering a 3 ohm 5 watt ceramic resistor to the top of the pot between the tweeter and the pot. This sets the driver at the fixed position just below "max" and above "norm". I have found the mid is best dampened in the same way but with a 5 ohm resistor. Depending on the room accoustics the values can be changed, but using the preamp controls would be easier.

By leaving the pot wired in series the impedance stays the same, and by wiring in a resistor to soften the driver output nothing is being changed electrically. All that has been done is setting the control just shy of "max" for the tweeter and about at "norm" for the mid.

Matt

[onplane]

Matt,

Did you cut the lead from the pot wipers going to the drivers?

If so, then you soldered those resistors to the top of the pot and attached the other end of the resistor to that now "free" lead from the drivers?

If this is what you did, Matt, you have 'fixed' the output (pot wiper no longer does anything) and have eliminated the resistance caused by corrosion on the pot wipers.

Being curious, Matt, I computed the difference with your resistor in the tweeter xover vs. the tweeter pot at max. The difference is:

-2.0 db @ 6000Hz

-2.2 db @ 10000Hz

In short, this "mod" makes the tweeter "roll off" happen a little sooner and a little faster.

Now, if we compare your mod against the pot completely removed from the circuit, we get:

-3.4 db @ 6000Hz

-3.2 db @ 10000Hz

This difference will clearly be noticeable, but … we’re into the area of personal preference. I mean, that’s why we have tone controls on our amps.

My problem is I need all the boost in high frequencies I can get to counter balance the gradual decline in HF sensitivity that age brings on.

Regards,

Jerry

[Guest matty g]

Hi Jerry -

In short, I removed the wire from the wiper terminal of the pot and soldered the resistor between that wire's end and the terminal on the pot to which it would have made contact in the "max" position.

On the AR4, a 3 ohm resistor knocks back the harshness of the fully powered cone tweeter. On the AR3a, a slightly lower resistance may be satisfactory for the tweeter, however a 5 ohm seems just about right for the mid driver. I have experimented with many values and this combo works well for me. I agree that the tweeter should not be overly softened, however I do not agree with eliminating the pot altogether.

This rig basically leaves everything electrically intact, and simply "fixes" the pot position between "norm" and "max". To what degree is entirely up to the owner, determined by resistance values.

Matt

[onplane]

>

>however I do not agree with eliminating the pot altogether.

>This rig basically leaves everything electrically intact, and

>simply "fixes" the pot position between

>"norm" and "max". To what degree is

>entirely up to the owner, determined by resistance values.

>

>Matt

Matt, if this works for you, I think that is great. I'd still recommend that folks by-pass the pots to get as close to a flat response as possible. Then, only if the individual feels the sound is "over bright", would your solution be a nice compromise.

Now, Matt, let me tell you about my solution ... passive bi-amping with both mid and tweeter pots eliminated!

Beauty of this solution, Matt, is via the volume controls on the amps I have COMPLETE control over the balance. The "line level" volume controls on amps are much, much easier to maintain than the power dissipating pots inside the speakers.

To be honest, I don't find myself adjusting for "brightness". Instead, I seem to strive for clarity. That is, I seek to be able to hear the individual instruments. Matt, I never realized how the balance between the woofer and the other drivers impacts "clarity".

The other thing I've learned with the bi-amp experiment is that there is no "perfect" balance. That is, the correct balance will vary by source and ... sometimes by the recordings themselves.

For example, cassette tape and MP3's need more mid and tweeter to achieve clarity than CD's. FM is very strange in that I often find differences between stations!?

All the people I've met, who've bi-amped, seem to like very much the control they've achieved over the resulting sound. I know ... I do!

Regards,

Jerry

[Carlspeak]

>>

>>however I do not agree with eliminating the pot

>altogether.

>>This rig basically leaves everything electrically intact,

>and

>>simply "fixes" the pot position between

>>"norm" and "max". To what degree is

>>entirely up to the owner, determined by resistance

>values.

>>

>>Matt

>

>

>Matt, if this works for you, I think that is great. I'd still

>recommend that folks by-pass the pots to get as close to a

>flat response as possible. Then, only if the individual feels

>the sound is "over bright", would your solution be a

>nice compromise.

>

>Now, Matt, let me tell you about my solution ... passive

>bi-amping with both mid and tweeter pots eliminated!

>

>Beauty of this solution, Matt, is via the volume controls on

>the amps I have COMPLETE control over the balance. The

>"line level" volume controls on amps are much, much

>easier to maintain than the power dissipating pots inside the

>speakers.

>

>To be honest, I don't find myself adjusting for

>"brightness". Instead, I seem to strive for

>clarity. That is, I seek to be able to hear the individual

>instruments. Matt, I never realized how the balance between

>the woofer and the other drivers impacts "clarity".

>

>The other thing I've learned with the bi-amp experiment is

>that there is no "perfect" balance. That is, the

>correct balance will vary by source and ... sometimes by the

>recordings themselves.

>

>For example, cassette tape and MP3's need more mid and tweeter

>to achieve clarity than CD's. FM is very strange in that I

>often find differences between stations!?

>

>All the people I've met, who've bi-amped, seem to like very

>much the control they've achieved over the resulting sound. I

>know ... I do!

>

>Regards,

>Jerry

>

>

>

I'll second that motion! I do the same with my system.

It's all about the music

Carl

Carl's Custom Loudspeakers

[joelongwood]

I'll third that! Everytime I turn on my Jerry-rigged AR3's I'm floored by the sound.

[kkantor]

A brief comment:

In the past, I have (rather casually) condoned removing the pots altogether. However, it has been demonstrated to me (eg- by JO), that some resistance is critical to damping the circuit. Removing the pot entirely will yield a large, ugly resonant peak at the low end of the tweeter's response. Even running the pot wide will show this.

I'm not against bi-amping, and there are good reasons to replace the pot with fixed resistors. (Power handling, corrosion, etc.) However, it would seem that simply taking the pot out of the circuit will yield a response that is difficult to correct, and might even put the tweeter in some jeopardy at higher levels.

I'm not passing any judgement on the sonics here, maybe I would like the sound better without the pot? I dunno. But, I do know that the crossover circuit won't be working properly.