AR-3 Schematic Question

[Guest leopoldstotch]

I have a question about the AR-3 schematic. In looking at the schematic with recent notes I have noticed that at one point the AR-3 midrange was changed to an AR-3a midrange and a 50uf cap was added to the mid circuit. I have also taken notice to an addition of a 150 uf cap to the woofer circuit but I do not understand how exactly it is wired in the woofer circuit. The reason I ask is I am installing AR-3a midranges in my AR-3's because the 3's mids are toast. If someone can kindly explain to me in easy terms how to wire the 150uf cap I would greatly appreciate it! Thanks!

Bruce

[RoyC]

Bruce,

That AR-3 schematic you are referring to appears to pertain to converting an AR-3 to an AR-3a. To do so would entail a number of changes. Along with the midrange, the AR-3 and AR-3a tweeters are also different, making the woofer the only driver shared by the 2 models.

Adding the 150uf woofer cap and changing the mid cap to 50uf *without*:

-changing the woofer inductor to #9,

-switching to the #1 (series) and #4 (parallel) midrange coils,

-adding .5 ohms in series with the mid,

-and swapping out the tweeters,

will result in something other than a 3 or a 3a.

Attached is the Library schematic you are referring to, as well as a drawing of the AR-3a crossover.

Roy

[dynaco_dan]

Hi there;

When they ran out of the original AR-3 midrange drivers, during production, they used, "modified", AR-3A mid drivers.

Maybe someone here knows what, "modified", means in this instance.

There was an inner dispersion ring fastened to the innner screen and on the later AR-3A screens was a center round bandaid, about 1/2" in diameter.

Both are quite likely for dispersion characteristics, or not.

[soundminded]

>I have a question about the AR-3 schematic. In looking at the

>schematic with recent notes I have noticed that at one point

>the AR-3 midrange was changed to an AR-3a midrange and a 50uf

>cap was added to the mid circuit. I have also taken notice to

>an addition of a 150 uf cap to the woofer circuit but I do not

>understand how exactly it is wired in the woofer circuit. The

>reason I ask is I am installing AR-3a midranges in my AR-3's

>because the 3's mids are toast. If someone can kindly explain

>to me in easy terms how to wire the 150uf cap I would greatly

>appreciate it! Thanks!

> Bruce

Roy of course is correct, you need to make all of these changes to convert an AR3 into an AR3a. Partway measures will result in a speaker system which is neither one nor the other. That being said, the role of the 150 mfd capacitor which is to be wired directly across the woofer terminals is to limit the woofer's high end response to 575hz or 525hz (I can't remember which) instead of around 1khz which I think is the crossover frequency for AR3. It works optimally in conjunction with the replacement coil to form a second order low pass filter. It was felt that the upper end of the woofer response had a somewhat rough frequency response and the system would benefit from the new improved midrange dome driver handling the range above this lower crossover frequency right up to the tweeter crossover (5khz?)

The change in the series capacitor from 24mfd to 50 mfd lowers the lowest frequencies in the midrange driver's circuit to match the new upper crossover frequency of the woofer. Along with the new coils they form a bandpass filter which restricts signals at frequencies both above and below the midrange driver's optimal range from reaching that driver. Unless all of the specified changes are made to this circuit, neither the crossover frequencies nor the filter slopes the midrange driver operates within will be as intended in the AR3a design.

Obtaining a properly functioning AR3a replacement tweeter is apparently very difficult. Unfortunately, there is no suitable modern substitute drop in replacement tweeter available at this time. Design philosophies among speaker manufacturers to meet current market tastes dictate designs with substantially poorer high frequency dispersion. Since the materials inside this tweeter have usually deteriorated with time (most notably the foam damping material) it is unlikely that any AR3as exist which sound exactly as they did when they were in production.

[RoyC]

It would be interesting to know the story behind the schematic. It lists the AR-3a inductor values at the bottom left hand corner, but the #4 inductor is mislabeled as .35 mh instead of .88 mh.

Roy

[soundminded]

>It would be interesting to know the story behind the

>schematic. It lists the AR-3a inductor values at the bottom

>left hand corner, but the #4 inductor is mislabeled as .35 mh

>instead of .88 mh.

>

>Roy

Unless you know someone who was there, all we can do is speculate. However, I noticed that there are two values for the series coil in the woofer circuit. L#4 in conjunction with the 50mfd cap forms a second order high pass filter for the midrange driver. Lowering its value will lower its frequency dependent shunt impedence at any given frequency and therefore raises the midrange's lower crossover frequency. Similarly coil #7/#9 operates in conjunction with the 150 mfd cap across the woofer to create a second order low pass filter. Lowering its value lowers the frequency dependent series impedence at any given frequency and therefore raises the woofer's high end cutoff crossover frequency. Is it possible that these two changes were made in conjunction somewhere along the way to simultaneously effect a crossover frequency design change? I really wish they'd give it a new model designation when they do these kind of things, it would make life a lot simpler. Anyway it's just a guess. BTW, while I'm wishing for things, I wish they'd have drawn the schematic upside down with T at the bottom and 1 at the top. It's easier for me to look at it that way...maybe I'm just dislexic. It's also common practice to place a dot where wires cross on a schematic if they are meant to be connected together. Isn't it funny how some people are just nitpickers :-)

[tysontom]

>Hi there;

>

>When they ran out of the original AR-3 midrange drivers,

>during production, they used, "modified", AR-3A mid

>drivers.

>

>Maybe someone here knows what, "modified", means in

>this instance.

>

>There was an inner dispersion ring fastened to the innner

>screen and on the later AR-3A screens was a center round

>bandaid, about 1/2" in diameter.

>

>Both are quite likely for dispersion characteristics, or not.

>

http://www.classicspeakerpages.net/dc/user_files/2783.jpg

AR-3a Special Midrange Replacement for AR-3

--Tom Tyson

[RoyC]

Tom,

Did the modified mid require a crossover modification? Any idea what the schematic "with recent notes" is about?

Roy

[tysontom]

>Tom,

>

>Did the modified mid require a crossover modification? Any

>idea what the schematic "with recent notes" is

>about?

>

>Roy

Roy,

Yes, the crossover had to be modified to limit the output of the AR-3a-style driver to match the original unit. On AR-3s after s/n C19467, it was necessary to add a 143-turn 0.4 mH choke and a 6 MFD capacitor. Earlier units had a 51-turn choke, which has to be removed. Towards the end of production of the AR-3, with s/n 70228 and higher, these crossover mods were already in place insofar as the new-style midrange superceded the no-longer-available 2-inch dome unit. I'm not sure what you are referring to concerning "with recent notes."

--Tom Tyson

[dynaco_dan]

Hi Tom;

Certainly is a beautiful photo of the AR-3 driver, thank you.

The large yellow-orange ring is the one I was referring to.

I believe the AR-3A dome had fiberglass between that ring and the apex round bandaid.

[RoyC]

>I'm not

>sure what you are referring to concerning "with recent

>notes."

It is the one referred to earlier in this thread. It is labeled as such in the forum library, but I think you answered the question. It appears to show the changes you described for the modified 3a midrange.

Thanks,

Roy

http://www.classicspeakerpages.net/dc/user_files/2822.jpg