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Crossover Potentiometers revisited


Dchristie
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Hi Guys, this may have been answered somewhere already and if so, I apologize for bringing it up again. I am restoring a pair of AR 2ax speakers and of course the potentiometers are corroded beyond use ( even after cleaning) so I am going to purchase new ones. I know that new 16 ohm potentiometers are  out there but not plentiful and rather expensive so I am thinking of following Roy's suggestion to just use 8 ohm level controls shunted with a 25 ohm resistor. However , just noticed Larry at the Vintage AR store ( eBay) also lists pots that are listed as direct replacements for the originals and also listed as better than the originals. These look like a good alternative to the 8 ohm level/resistor shunt control but I am just looking to see if any one has any experience with Larry's level controls? All the parts that I have purchased from the AR store have always been good quality so these may be a real option and the price is certainly reasonable all things considered. Just have not seen them mentioned in any of the Forum discussions.

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So, I guess they are not really direct replacements of the originals ( true 16 ohm potentiometers) but are L-Pads so the 25 ohm shunt would still be needed. Just a copy of Roy's recommendation as a 2nd best option to the original potentiometer.

Too bad because if they were direct drop in replacements, would be nice to have since they are so difficult to find.

 

Thanks

 

Dean

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As an added comment, here is the message  I received from vintage -ar regarding the part in question. 

 

The AR-3a Restoration Guide is an excellent document. However, we don't agree on all points and the need for the 25 ohm shunt resistor is one area. We do not supply it with this resistor. "These are not 16 ohm pots. but L Pads and we have sold over 1,000 with complete satisfaction.
You are welcome to add the shunt resistor but I don't feel it is necessary and you will lose a bit of the slightly higher tweeter volume capability of this control. The AR-2ax can certainly benefit from a little more volume as they are all a bit subdued by now."

 

 

 

 

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The original pots were used in a variety of AR models which employed various high and mid frequency drivers. Replacement of original pots with new L-pads has generally proven to be a satisfactory method for keeping these old speakers alive and still retaining variable control of frequencies above the woofer's range. The use of the additional resistor is fully optional, and vintage-AR's comment is consistent with many user's opinions, particularly for the 2ax tweeter which, after 40 or 50 years, probably has a quieter voice than it did when it was new. Omitting the added resistor from this circuit will allow you to get a bit more "oomph" out of the aging tweeter, and most restorers are looking for every bit of tweet they can grab.  

The L-pads should work perfectly fine as replacements for the original pots, and you may or may not wish to include a couple of inexpensive resistors with your parts order for experimentation to determine what you like best as a final assembly.  

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Thanks, I guess I would be interested in some of the AR3a restoration manual authors opinions as well (Roy , and /or Tom) as  I thought the 16 ohm pot was somewhat important to maintaining the correct crossover values . I thought this is why it is important to leave it in the circuit even when the original pots are bypassed.  Kind of confusing since the original pot is a potentiometer and L-pads are not quite the same so little difficult to understand how one could be just substituted for the other without the shunt resistor on the L-pad.

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14 hours ago, Dchristie said:

Thanks, I guess I would be interested in some of the AR3a restoration manual authors opinions as well (Roy , and /or Tom) as  I thought the 16 ohm pot was somewhat important to maintaining the correct crossover values . I thought this is why it is important to leave it in the circuit even when the original pots are bypassed.  Kind of confusing since the original pot is a potentiometer and L-pads are not quite the same so little difficult to understand how one could be just substituted for the other without the shunt resistor on the L-pad.

To clarify a bit:

- 15 watt 8 ohm L-pads purchased from Vintage AR, Parts Express, or Madisound are the same. Shaft lengths vary, but they are otherwise identical. The "8 ohm" designation of the L-pad does not mean the same thing as the "15 or 16 ohm" designation of the potentiometer, so disregard this spec. Vintage AR removes the "8 ohm" stamp from the back of his controls to avoid confusion.

-Adding the 25 ohm parallel resistor makes the L-pad electrically the same as the original pot for most of the rotation (off maximum). It lowers parallel resistance to that of the pot at the most often used settings.

-The additional 25 ohm parallel resistor is often of limited benefit when used with original 3a/2ax type tweeters, and more output can be achieved from degraded tweeters without it (though the crossover point is lowered and power handling is reduced). I strongly recommend its use, however, with the midrange L-pad. The level control circuit's power handling and the mid's crossover point are adversely affected without it. The mid's response will not be as smooth as it should be.

Roy

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Ok- thanks, Roy.

So, I have the older, cloth surround bass drivers and the original, phenolic ( AR 3-type) tweeters in my AR 2ax speakers. Sounds like I should definitely use the L-pad/resistor shunt combo for the mid/tweeter cone but what do you recommend for the older , phenolic dome tweeters ? From your response above, using the resistor shunt on the L-pad is of limited value  with these older tweeters but is there any concern that the tweeters may be damaged when using the L-pad  without the resistor from the added output power and lowered crossover point? 

So, I am thinking it might be best to use the tweeter dome with the original 16 ohm potentiometer still in circuit but bypassed as per your instructions in the other threads, and then use the L-pad  with shunt resistor on the mid/tweeter cone?  I guess what I am wanting to do is have maximum output from the tweeter ( but still make sure it is properly protected ) and still have some flexibility with the mid/tweeter cone as it seems to have the most effect on the sound of the upper mids and highs. 

Or comparatively, would it be  better to use L-Pads on both the mid cone and phenolic dome tweeter but  apply resistor shunt only  to the cone - leave it off the dome.

 I certainly want to be able to adjust the levels of the upper mid and highs to some extent as needed, but also want to make sure I don't overpower those tweeters since they are so difficult (and expensive) to replace. 

So, I guess what I am really asking is how much more risk is my tweeter at I  if I use the L-pad without the resistor shunt compared to it with the shunt  compared to just bypassing ?  

Thanks

Dean

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OK, after numerous searches on this topic, it looks like ALL of my questions/concerns have been addressed several times before. So, I am good to go . I did not take into account that these older phenolic tweeters are most likely no longer in spec due to aging parts so even using the L-pad without a resistor is not an issue. I believe it was Roy who indicated these old tweeters have less variability as a result of using the resistor-less L-pad than they do as a result of aging. And it appears that vintage-ar is also in agreement. 

The good thing about the Classic Speaker Pages is most of the questions one asks are probably already addressed somewhere, the fun part is just finding them in the right Forum. 

It IS easy to get neurotic about the minutia  in this hobby and forget we are dealing with 50 year old loudspeakers which also have 50 year old parts and adhesives, foams, etc which are most likely causing more variation in the drivers than anything else (maybe with the exception of bad capacitors). It is truly amazing to me that these speaker designs still sound as good as they do considering their age.

Thanks for everyone's assistance!

Dean

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