JoeB Posted August 31, 2006 Report Share Posted August 31, 2006 As I previously posted I have designed a stepped attenuator for replacing the 16 Ohm pots in AR speakers. The prototype boards are square 1.6" X 1.6" printed circuits. My original intention was to have the production boards be round and as small as possible (i.e. a 1.6" diameter circle. Some of the posts in the previous thread have me second guessing myself a little. In particular Dynaco Dan (Vern) posted:>A non-combustible heat shield will be needed as well, to keep>the insulation clear of the switch and resistors.>>Carl uses a 15 ohm 25 watt wire-wound pots and fabricates a>wooden shield to keep insulation away from it's open rear>housing, to lessen the chance of fire.>>I have attempted to contact two pot manufactures with some>suggestions, but as of today, nothing has been done, and I>don't think they are interested as well.I don't agree a heat shield is needed since the power levels needed to overheat the resistors would also, in my opinion, destroy the drivers But, maybe I'm wrong. Also, anyone who has replaced the fiberglass with a flammable material (polyfill?) should consider using a shield (really they should restuff with fiberglass). So I am seriously considering having the production boards built in the same 1.6" X 1.6" square shape as the prototypes and adding 4 1/8" holes near the corners of the board. These holes would not be electrically connected to anything but could be used to anchor a cover which might be manufactured out of steel window screen or some such material allowing air flow but separating the resistors from the fiberglass. So, what do you think? I have attached the board outline (top layer and silkscreen) for comment. The round boards would be very nice looking but the square boards would work just as well and allow the secure mounting of a shield if desired. I await opinions.1540.pdf Link to comment Share on other sites More sharing options...
onplane Posted September 1, 2006 Report Share Posted September 1, 2006 JoeB, I cannot give you a definitive answer, but I can share some observations. Since I’ve had an intermittent problem with the pots and since I’m bi-amping, I completely removed the pots from the xover circuit. From the experiencing of opening the boxes and by-passing the pots, I learned the following:1. existing pots look as though the resistor wire is encapsulated in ceramic?? 2. will all of the fiberglass fill, I can’t imagine there is much air circulating inside the speakers3. without circulation, there will be heat build up IF any power is being dissipated in the pots4. WITHOUT the pots the mid/tweeter requires significantly LESS power (and I mean a lot less)The mid/tweeter without pots became so sensitive that I downsized to a very lower power amp (22 watts). JoeB, my point here is that clearly energy IS being dissipated in the pots. Now the problem is how much energy and how hot will your resistors become. I’m trying to think of how you could test this. I think the worst case would be with your selector switch set to minimum volume on both the mid and tweeter. What you might try:1. remove the strap on one speaker and connect speaker wires on just that speaker to T and 1 (just mid/tweeter will be driven)2. remove the woofer, so that you can access the pots3. leave fiberglass stuffed completely around your board and resistors4. adjust selector switch for minimum volume on BOTH drivers5. setup your amp up to play music at a decent volume (you can judge volume by the other untouched speaker)6. let it run for 30 minutes7. turn off and check to see how hot the resistors have become, if warm turn back on and check again 30 minutes laterJoeB, I think what you want to do is to "stress" your design under worst case to see what happens.Regards,Jerry Link to comment Share on other sites More sharing options...
JoeB Posted September 2, 2006 Author Report Share Posted September 2, 2006 >JoeB, I cannot give you a definitive answer, but I can share>some observations. Since I’ve had an intermittent problem>with the pots and since I’m bi-amping, I completely removed>the pots from the xover circuit. From the experiencing of>opening the boxes and by-passing the pots, I learned the>following:>>1. existing pots look as though the resistor wire is>encapsulated in ceramic?? >2. will all of the fiberglass fill, I can’t imagine there is>much air circulating inside the speakers>3. without circulation, there will be heat build up IF any>power is being dissipated in the pots>4. WITHOUT the pots the mid/tweeter requires significantly>LESS power (and I mean a lot less)>>The mid/tweeter without pots became so sensitive that I>downsized to a very lower power amp (22 watts). >>JoeB, my point here is that clearly energy IS being dissipated>in the pots. Now the problem is how much energy and how hot>will your resistors become. >>I’m trying to think of how you could test this. I think the>worst case would be with your selector switch set to minimum>volume on both the mid and tweeter. >>What you might try:>>1. remove the strap on one speaker and connect speaker wires>on just that speaker to T and 1 (just mid/tweeter will be>driven)>2. remove the woofer, so that you can access the pots>3. leave fiberglass stuffed completely around your board and>resistors>4. adjust selector switch for minimum volume on BOTH drivers>5. setup your amp up to play music at a decent volume (you>can judge volume by the other untouched speaker)>6. let it run for 30 minutes>7. turn off and check to see how hot the resistors have>become, if warm turn back on and check again 30 minutes later>>>JoeB, I think what you want to do is to "stress">your design under worst case to see what happens.>>Regards,>Jerry>Jerry,I have something like that in mind. The 'worst case' in this case is far worse than can be achieved playing music with a real speaker as the control since the 'worst case' would melt the voice coils in the drivers before the resistors could overheat. Playing music at any loudness barely safe for the speakers (say with a 250W RMS amp just below clipping) would just barely warm up the resistors (remember the average RMS power in the music is many dB below the peak music power and most of that is in the bass). Instead I will connect a power resistor in place of the driver and feed a voltage sufficient to stress the resistors and see what happens. You did pick up on an important point, that is that the worst case for the resistors is if the drivers are turned all the way down (i.e. off).Joe Link to comment Share on other sites More sharing options...
ar_pro Posted September 2, 2006 Report Share Posted September 2, 2006 Excellent work - this could be of exceptional value to AR restorers & enthusiasts.Have you considered placing all of the crossover components & controls in an outboard enclosure? Link to comment Share on other sites More sharing options...
JoeB Posted September 2, 2006 Author Report Share Posted September 2, 2006 >Excellent work - this could be of exceptional value to AR>restorers & enthusiasts.>>Have you considered placing all of the crossover components>& controls in an outboard enclosure?That could be done but I suspect the addition of an out outboard unit would not be well received. My goal in the design is for the stepped attenuator to directly replace the pots with no change to the electrical, acoustical, or aesthetic properties of the speakers. Link to comment Share on other sites More sharing options...
dynaco_dan Posted September 2, 2006 Report Share Posted September 2, 2006 >>Excellent work - this could be of exceptional value to>AR>>restorers & enthusiasts.>>>>Have you considered placing all of the crossover>components>>& controls in an outboard enclosure?>>That could be done but I suspect the addition of an out>outboard unit would not be well received. My goal in the>design is for the stepped attenuator to directly replace the>pots with no change to the electrical, acoustical, or>aesthetic properties of the speakers. Hi there;A direct physical replacement, into the existing pot holes, is a very good idea.When I wrote my original concerns, I did write something that in hind site, I should not have written.It was not fair of me to compare apples and oranges.I should not have mentioned the, toasted voice coils, this was the woofers and not the tweeters.The melted pot shafts was appropriate, for they are only in the mids and tweeters circuit.As was the burnt fiberglass insulation at the trear of the pots.If someone wishes to experiment on the melting point of those plastic shafts, then we can see what kind temperatures we are dealing with, in extreme cases of course.Please also remember, these cases were when Dynaco SCA-35/SCA-80/SCA-80Q/ST-120 type amps were popular, not super amps, yet.Fusing was only primarily recommended by Dynaco, AR and Advent.Other than myself, I don't know of anyone local that has or does fuse their speakers.I certainly have read lot's about members blowing speakers though.My thought for today. OH NO.Why did AR, use a 25 watt pot, rather than a lower wattage rating?Also, heavy wire-wound sandcast resistors in their networks, when I am sure that carbon composition (CC) resistors would have quite likely been cheaper, and more compact.Likewise, with the Dynaco classic speakers, using heavy duty sandcast resistors, rather that the less expensive (CC), but, still using a low wattage open MBB switch.The ventilation shield you mentioned, is a must IMO, and should be not prone to shorting out on the resistor leads. Link to comment Share on other sites More sharing options...
onplane Posted September 2, 2006 Report Share Posted September 2, 2006 Hi, Vern!You are 100% correct few of us have fuses on our speakers. Fortunately, many of us have fuses on our amps ... seehttp://cgi.ebay.com/TECHNICS-RECEIVER-110-...1QQcmdZViewItemMy old HH Scott receiver has those real old fashioned spring loaded fuse holders. Then I just bought a used Kenwood amp that has those cheap fuse holders (like in the picture above) under the case.Actually, I think it's a good idea to have the fuses on the amps rather than on the speakers. This way the fuses serve “double duty”. That is:1. they protect the speakers from “blown” amps2. they protect the amps from shorted speaker wires and “blown” driversRegards,Jerry Link to comment Share on other sites More sharing options...
dynaco_dan Posted September 2, 2006 Report Share Posted September 2, 2006 >Hi, Vern!>>You are 100% correct few of us have fuses on our speakers. >Fortunately, many of us have fuses on our amps ... see>>http://cgi.ebay.com/TECHNICS-RECEIVER-110-...1QQcmdZViewItem>>>My old HH Scott receiver has those real old fashioned spring>loaded fuse holders. Then I just bought a used Kenwood amp>that has those cheap fuse holders (like in the picture above)>under the case.>>Actually, I think it's a good idea to have the fuses on the>amps rather than on the speakers. This way the fuses serve>“double duty”. That is:>>1. they protect the speakers from “blown” amps>2. they protect the amps from shorted speaker wires and>“blown” drivers>>>Regards,>Jerry>Hi Jerry;It is a great idea for the manufacturers to include at least a chassis mounted fuseholder, rather than none.It also protects against shorted speaker leads, whether the leads are shorted at the speakers or under the carpet type short.The only shortcoming of this type of fuse is, a fuse of a value undetermined is needed to protect the woofer, mids and tweeters.Only Dynaco and Heathkit recommended a fast blow fuse, so we don't have to research those values.Advent and AR recommended slow blow and we don't have a conversion to fast blow with one excepetion.The Heathkit AS-103 speaker system used a 3 amp fast blow fuse in a chassis style fuseholder with an o-ring even.That is an AR-3A equivalent speaker, for those that did not know.In the, "Other", forum there is a long topic trail for fast fuses and general fusing advice.Generally speaking, there should be a slow blow woofer fuse, a fast blow fuse each for mids and tweeters.Difficult without a little electrical background and a lot of labour of love and a few dollars in materials and experimenting for the correct values.I can say that, a 1 amp fast blow fuse will offer a lot of protection for the typical, but not all, classic speaker systems. Link to comment Share on other sites More sharing options...
JoeB Posted September 3, 2006 Author Report Share Posted September 3, 2006 >My thought for today. OH NO.>>Why did AR, use a 25 watt pot, rather than a lower wattage>rating?>>Also, heavy wire-wound sandcast resistors in their networks,>when I am sure that carbon composition (CC) resistors would>have quite likely been cheaper, and more compact.I think the phrase “2 Watt resistors” has somehow conjured up the image of the antique fat brown carbon composition 2 Watt resistors common in the 1950’s. The resistors used in the stepper attenuator ARE NOT carbon comp resistors. They are modern Metal Oxide resistors built on a ceramic core. The spec sheet is at http://www.mouser.com/catalog/specsheets/XC-600044.pdf . >Likewise, with the Dynaco classic speakers, using heavy duty>sandcast resistors, rather that the less expensive (CC), but,>still using a low wattage open MBB switch.>>The ventilation shield you mentioned, is a must IMO, and>should be not prone to shorting out on the resistor leads. My guess is AR used a 25 Watt potentiometer because it was available. The Aetna-Pollak pot appears to be a very well made potentiometer. The nichrome wire is partially embedded in the ceramic body of the pot allowing some heat transfer. Modern pots generally have 2 ratings. The first is the power rating (25 Watts in this case) and the second is a maximum current rating which is the current that causes the power dissipation to be equal to the power rating when the entire resistance of the pot is in use. Thus, for a 16 Ohm 25 Watt potentiometer the maximum current is 1.25 Amps (the square root of 25 divided by 16). See http://www.ohmite.com/catalog/pdf/rheostats203.pdf for more information on the ratings and uses of potentiometers.This maximum current in the pot used in the AR-3a (and other speakers) is the current from 1 to B (i.e. the sum of the ‘bleeder’ current carried by the whole winding and the tweeter or midrange current). Using the original 10 VRMS design criteria for the stepped attenuator and assuming the attenuator or pot is set to 1.75 Ohms (the factory setting for the tweeter) we find there is a current of 2.05 amps flowing from 1 to B on the pot or attenuator. The power dissipation contributed by this is 7.4 Watts. In the stepped attenuator this 7.4 Watts is being dissipated in seven separate 2 Watt resistors as follows: 3 resistors at 0.7 W each, 2 resistors at 1.05 W each and 2 resistors at 1.6 W each. All well below the maximum rating of 2 W. The story is different for the pot. The same 7.4 Watts is being dissipated in 11% (1.75/16 percent) of the winding. The 16 Ohm pot would need to have a 67 Watt rating to allow this level of current based on the Ohmite tech note. I could easily see how that small section of resistance wire could get very hot under these circumstances. If you examine the inside of the Aetna-Pollak pot you will see the wiper is mounted directly on the end of the plastic shaft and will conduct heat from the hot winding. Melting of the plastic shaft would be an expectable consequence at some power level. This is not to say that the Aetna-Pollak pot is not perfectly suited for use in the speaker, all it says is that 10 VRMS is much more than we would expect to see applied. BTW, I have released the boards to fab with the 4 holes to mount a shield, if desired. I still don’t think a shield is needed and since fiberglass is not flammable and can withstand temperatures much much higher than either a pot or resistors should ever reach, but allowing for a shield may make sense, particularly if someone has replaced the fiberglass with polyfill or some other fill material with undesirable characteristics . Fiberglass is nasty stuff to work with (gloves and breathing mask mandatory) but it is still the right choice for the AR speakers that originally used it.Joe Link to comment Share on other sites More sharing options...
administrator Posted December 12, 2007 Report Share Posted December 12, 2007 >My thought for today. OH NO.>>Why did AR, use a 25 watt pot, rather than a lower wattage>rating?>>Also, heavy wire-wound sandcast resistors in their networks,>when I am sure that carbon composition (CC) resistors would>have quite likely been cheaper, and more compact.I think the phrase “2 Watt resistors” has somehow conjured up the image of the antique fat brown carbon composition 2 Watt resistors common in the 1950’s. The resistors used in the stepper attenuator ARE NOT carbon comp resistors. They are modern Metal Oxide resistors built on a ceramic core. The spec sheet is at http://www.mouser.com/catalog/specsheets/XC-600044.pdf . >Likewise, with the Dynaco classic speakers, using heavy duty>sandcast resistors, rather that the less expensive (CC), but,>still using a low wattage open MBB switch.>>The ventilation shield you mentioned, is a must IMO, and>should be not prone to shorting out on the resistor leads. My guess is AR used a 25 Watt potentiometer because it was available. The Aetna-Pollak pot appears to be a very well made potentiometer. The nichrome wire is partially embedded in the ceramic body of the pot allowing some heat transfer. Modern pots generally have 2 ratings. The first is the power rating (25 Watts in this case) and the second is a maximum current rating which is the current that causes the power dissipation to be equal to the power rating when the entire resistance of the pot is in use. Thus, for a 16 Ohm 25 Watt potentiometer the maximum current is 1.25 Amps (the square root of 25 divided by 16). See http://www.ohmite.com/catalog/pdf/rheostats203.pdf for more information on the ratings and uses of potentiometers.This maximum current in the pot used in the AR-3a (and other speakers) is the current from 1 to B (i.e. the sum of the ‘bleeder’ current carried by the whole winding and the tweeter or midrange current). Using the original 10 VRMS design criteria for the stepped attenuator and assuming the attenuator or pot is set to 1.75 Ohms (the factory setting for the tweeter) we find there is a current of 2.05 amps flowing from 1 to B on the pot or attenuator. The power dissipation contributed by this is 7.4 Watts. In the stepped attenuator this 7.4 Watts is being dissipated in seven separate 2 Watt resistors as follows: 3 resistors at 0.7 W each, 2 resistors at 1.05 W each and 2 resistors at 1.6 W each. All well below the maximum rating of 2 W. The story is different for the pot. The same 7.4 Watts is being dissipated in 11% (1.75/16 percent) of the winding. The 16 Ohm pot would need to have a 67 Watt rating to allow this level of current based on the Ohmite tech note. I could easily see how that small section of resistance wire could get very hot under these circumstances. If you examine the inside of the Aetna-Pollak pot you will see the wiper is mounted directly on the end of the plastic shaft and will conduct heat from the hot winding. Melting of the plastic shaft would be an expectable consequence at some power level. This is not to say that the Aetna-Pollak pot is not perfectly suited for use in the speaker, all it says is that 10 VRMS is much more than we would expect to see applied. BTW, I have released the boards to fab with the 4 holes to mount a shield, if desired. I still don’t think a shield is needed and since fiberglass is not flammable and can withstand temperatures much much higher than either a pot or resistors should ever reach, but allowing for a shield may make sense, particularly if someone has replaced the fiberglass with polyfill or some other fill material with undesirable characteristics . Fiberglass is nasty stuff to work with (gloves and breathing mask mandatory) but it is still the right choice for the AR speakers that originally used it.JoeHi Joe,Are you still out there? I think it may be worthwhile to revisit this subject now that the new version of the website is coming together.Mark Link to comment Share on other sites More sharing options...
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