I decided to recap and install new binder posts in a pair of vintage Rectilinear speakers. I discovered one of the tweeter control potentiometers is shot (corrosion and evidence it overheated some time in the past).
I need help identifying appropriate replacements. While there is a lot of information on capacitor replacement, I am finding very little on this topic.
The current version appears to be 32 Ohm version. It's marked AM-3352 32Ω 137 7207 on the back. No indication or wattage. I assume it's an audio taper but, again, nothing obvious.
Finding no options available at 32 Ohms. Most start at 1K and progress to 5K, 10K, 100K, etc. 250K 500K seem typical for guitar tone controls. Closest I seem to be able to find new are either 25 or 100 Ohm in the 3 to 5 Watt range. I also have some old pots out of other speakers that measure at 16Ω, but no idea re watt limit.
I'm assuming I need audio taper and not linear taper, but don't seem to find any. How close do I need to be to the 32Ω of the original? How much impact will higher (100Ω) or lower (16Ω or 25Ω) values have on sound? Do I need to be concerned about Watts in this application. Are their other options/suggestions? Any recommended sources?
I need help identifying appropriate replacements. While there is a lot of information on capacitor replacement, I am finding very little on this topic.
The current version appears to be 32 Ohm version. It's marked AM-3352 32Ω 137 7207 on the back. No indication or wattage. I assume it's an audio taper but, again, nothing obvious.
Finding no options available at 32 Ohms. Most start at 1K and progress to 5K, 10K, 100K, etc. 250K 500K seem typical for guitar tone controls. Closest I seem to be able to find new are either 25 or 100 Ohm in the 3 to 5 Watt range. I also have some old pots out of other speakers that measure at 16Ω, but no idea re watt limit.
I'm assuming I need audio taper and not linear taper, but don't seem to find any. How close do I need to be to the 32Ω of the original? How much impact will higher (100Ω) or lower (16Ω or 25Ω) values have on sound? Do I need to be concerned about Watts in this application. Are their other options/suggestions? Any recommended sources?
Hi Craig,
Does it have 3 legs? If so it's a L-Pad rather than a pot. Show us a picture or two please.
Does it have 3 legs? If so it's a L-Pad rather than a pot. Show us a picture or two please.
It does have 3 legs.
Here's a photo of the offending item, as well as the entire crossover.
Thanks!
Here's a photo of the offending item, as well as the entire crossover.
Thanks!
(How to attach images to your posts.).
4 tweeters per side? You might get a clue measuring the L-Pad at a few different points of rotation as it should tend toward the specified value, even when measured with a meter.
It should work if all you can find are four 8 ohm L-Pads.
4 tweeters per side? You might get a clue measuring the L-Pad at a few different points of rotation as it should tend toward the specified value, even when measured with a meter.
It should work if all you can find are four 8 ohm L-Pads.
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Doesn't a pot have three legs as well, Cal?
I would replace the level control (as well as the one in the other speaker) with an L pad.
The L-pad should match the tweeter load impedance.
In the UK at least, L pads are available in 4, 8 and 16 ohm versions.
L-Pad Attenuators, 4 Ohm, 8 Ohm and 16 Ohm 15 -100 watts - Willys-Hifi Ltd
I would replace the level control (as well as the one in the other speaker) with an L pad.
The L-pad should match the tweeter load impedance.
In the UK at least, L pads are available in 4, 8 and 16 ohm versions.
L-Pad Attenuators, 4 Ohm, 8 Ohm and 16 Ohm 15 -100 watts - Willys-Hifi Ltd
Just for clarification. The Rectilinears at issue are just the lowly XIs. Simple 2 ways, but I love them and they are far simpler for me to tackle than their top shelf offerings.
If you measure resistance all the way across a potentiometer, it won't change as you move the wiper but an L-Pad will.
Tweeter pots were commonly 50 ohm back in the old days before constant impedance L pads.
Like this: Wire Wound Potentiometer - 15 Watt 50 Ohm
Note the linear taper.
What is your country of location for source recommendation purposes?
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Hi Galu,
Yes. I was thinking that when using pots as a tweeter level control in a speaker I am used to seeing only two legs connected. Bad wording on my end.
Cheers.
Yes. I was thinking that when using pots as a tweeter level control in a speaker I am used to seeing only two legs connected. Bad wording on my end.
Cheers.
@Cal: I'd like to see more clearly how the 32 ohm control is wired in conjunction with that ceramic resistor.
@Craig: If the tweeter capacitors are bipolar electrolytic types, this would be an opportune moment to replace them with fresh ones. Doing so could possibly improve the quality of the treble.
@Craig: If the tweeter capacitors are bipolar electrolytic types, this would be an opportune moment to replace them with fresh ones. Doing so could possibly improve the quality of the treble.
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I'm attaching more photos in hopes that one of them clarifies things I am unable to express properly.
The fixed resistor in the tweeter circuit is 8 Ohm 10% 5Watt.
The capacitor is a 16mF Temple, which are reputedly quite bad.
The link to the item available in the UK is very helpful. I'm in the US and found one vendor that has something comparable, but it only handles 5 Watts. Would that lower wattage rating cause an issue?
I ask because the existing pot appears to have overheated at some juncture. If you look at the picture of the label, you may see the crack and some distortion in the plastic. This is not present in the other speaker. Don't know what caused the overheating, but its certainly something I want to avoid.
Also, I have some old pots out of a pair of AR2a speakers I converted to 2ax configuration. They are 16 Ohm as opposed to 32 Ohm and have no indication of wattage rating. Would they work as a substitute?
The fixed resistor in the tweeter circuit is 8 Ohm 10% 5Watt.
The capacitor is a 16mF Temple, which are reputedly quite bad.
The link to the item available in the UK is very helpful. I'm in the US and found one vendor that has something comparable, but it only handles 5 Watts. Would that lower wattage rating cause an issue?
I ask because the existing pot appears to have overheated at some juncture. If you look at the picture of the label, you may see the crack and some distortion in the plastic. This is not present in the other speaker. Don't know what caused the overheating, but its certainly something I want to avoid.
Also, I have some old pots out of a pair of AR2a speakers I converted to 2ax configuration. They are 16 Ohm as opposed to 32 Ohm and have no indication of wattage rating. Would they work as a substitute?
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Galu-
Those are precisely the crossovers I am working on.
I originally intended to replace the capacitors, binding posts and tweeters. Speaking of the capacitors, I originally ordered 8uF 400V versions because I read that was the correct value. After opening the speaker up, I learned they were actually 16uF, so I had to order those. Naturally, that was before I realized the offended pot was shot, which lead me here. Will Solen 16uF 400V Polypropylene Capacitors fit the bill?
As for the tweeters, I was going to install new CTS phenolic ring reproductions, but now I'm concerned that that will introduce a new variable. Should I just reinstall the the Peerless paper cones?
While I'd like to stick to the original as close as possible, would converting to L-Pads offer any advantages (aside from being more readily available)? If so, would the circuit need to be revised?
Apologies for this snowballing.
Those are precisely the crossovers I am working on.
I originally intended to replace the capacitors, binding posts and tweeters. Speaking of the capacitors, I originally ordered 8uF 400V versions because I read that was the correct value. After opening the speaker up, I learned they were actually 16uF, so I had to order those. Naturally, that was before I realized the offended pot was shot, which lead me here. Will Solen 16uF 400V Polypropylene Capacitors fit the bill?
As for the tweeters, I was going to install new CTS phenolic ring reproductions, but now I'm concerned that that will introduce a new variable. Should I just reinstall the the Peerless paper cones?
While I'd like to stick to the original as close as possible, would converting to L-Pads offer any advantages (aside from being more readily available)? If so, would the circuit need to be revised?
Apologies for this snowballing.
If you are not experienced in circuit tracing/modification, then the best course of action would be to replace the damaged 32 ohm control with your 16 ohm one and see how it performs. You won't damage anything with the substitution. I don't know why the fixed resistor is there and I've seen an XI crossover without it!
The 16 ohm AR-2a control is a potentiometer and not an L pad (see attached AR-2a crossover schematic).
I don't know what would have caused the original control to burn out so spectacularly, but a replacement control of 5W power rating is the minimum required to do the job.
If the 16 ohm replacement control works fine, then you may want to do the same to the other speaker cabinet to ensure balanced operation.
The originals were phenolic ring tweeters. There would be no problem in changing back to them.
Your choice of capacitor is fine, although I would have thought 8uF to be the logical value. If you order both 8 and 16uF, I recommend you try each to see which gives the better treble performance.
The 16 ohm AR-2a control is a potentiometer and not an L pad (see attached AR-2a crossover schematic).
I don't know what would have caused the original control to burn out so spectacularly, but a replacement control of 5W power rating is the minimum required to do the job.
If the 16 ohm replacement control works fine, then you may want to do the same to the other speaker cabinet to ensure balanced operation.
The originals were phenolic ring tweeters. There would be no problem in changing back to them.
Your choice of capacitor is fine, although I would have thought 8uF to be the logical value. If you order both 8 and 16uF, I recommend you try each to see which gives the better treble performance.
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Why in the world would one want to turn that tweeter all the way down for zero output? If you have a 16 ohm pot and were replacing a 32, put a 15 ohm resistor in the bottom leg. Then you get the right value, and the upper half of the range adjustment. If you find yourself wanting to turn it down more than it will, move some of the resistance to the top leg.
One could certainly do that, but I don't regard it as absolutely necessary to do so.
What I am more concerned with is the presence of the 8 ohm resistor which is placed between the top and middle terminals of the pot - so ending up in parallel with the tweeter.
I would be inclined to remove this resistor.
Any insight into the function of that resistor, wg_ski?
What I am more concerned with is the presence of the 8 ohm resistor which is placed between the top and middle terminals of the pot - so ending up in parallel with the tweeter.
I would be inclined to remove this resistor.
Any insight into the function of that resistor, wg_ski?
Perhaps an attempt to keep the impedance (and therefore the crossover frequency) “more” constant.
Thanks - I considered that function, but wasn't sure exactly how it would work.
Thing is, I've seen a version of the crossover without that resistor fitted.
Thing is, I've seen a version of the crossover without that resistor fitted.
The cicuit is a series LC circuit with the basic form shown below.
The inductor (L) and capacitor (C) are connected in series across the input terminals. The woofer is connected across the capacitor and the potentiometer outer terminals are connected across the inductor. The tweeter is connected across the middle terminal of the potentiometer and the terminal connected to the LC junction.
A picture would be worth a thousand words if someone has the skill to draw it! 😉
The inductor (L) and capacitor (C) are connected in series across the input terminals. The woofer is connected across the capacitor and the potentiometer outer terminals are connected across the inductor. The tweeter is connected across the middle terminal of the potentiometer and the terminal connected to the LC junction.
A picture would be worth a thousand words if someone has the skill to draw it! 😉
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