Replacement For 16 Ohm Pots in AR-3a (and others)

[JoeB]

I had mentioned in a previous reply to SamS on his thread "New pair of AR-3a's, looking for advice on restore" that I had designed a switched attenuator (actually a stepped 16 Ohm potentiometer) to replace the Polak 16 Ohm pots in the AR-3a (and other AR) speakers. I have finally installed the pot replacements in one speaker and they are working well. No more scratchy adjustments and mid/high drop outs!

Here are 2 attenuators installed in an AR-3a:

http://www.classicspeakerpages.net/dc/user_files/1505.jpg

The attenuator uses 20 2-Watt resistors to mimic the function of the 25 Watt pot. The steps are:

Switch Position 1(Full CW) Tap at 0 Ohms (Full Volume)

Switch Position 2 Tap at 0.5 Ohms

Switch Position 3 Tap at 1.0 Ohms

Switch Position 4 Tap at 1.75 Ohms

(Factory Tweeter setting for AR-3A)

Switch Position 5 Tap at 2.50 Ohms

Switch Position 6 Tap at 3.25 Ohms

(Factory Midrange setting for AR-3A)

Switch Position 7 Tap at 4.0 Ohms

Switch Position 8 Tap at 5.0 Ohms

Switch Position 9 Tap at 7.0 Ohms

Switch Position 10 Tap at 9.4 Ohms

Switch Position 11 Tap at 12.4 Ohms

Switch Position 12 (Full CCW) Tap at 16 Ohms (Off)

Here is a schematic of the circuit:

http://www.classicspeakerpages.net/dc/user_files/1507.jpg

I'll be installing two more this weekend in my other AR-3a.

1501.txt

1502.txt

1503.txt

[stan461]

Bravissimo!

Can you please post a parts list with supplier and prices?

[rrcrain]

I am impressed! Very well done to say the least.

[onplane]

Richard,

Very, very impressive!!!

It looks like you had 3 attachments, but we can only see 2. Is the missing attachment the parts list?

Regards,

Jerry

[Guest Eunomians]

Wow, this is great stuff. I only wish that I had the skills to make these pots from your schematics/parts list. My only alternative would be to purchase fully-assembled pots.

I would like to hear if you notice any sound difference between OEM and your fantastic creation.

I've been wanting to change my 3a pots since they cut out a bit despite cleaning them, but I have not wanted to alter their sublime stock sound in any way.

Cheers

[dynaco_dan]

>I had mentioned in a previous reply to SamS on his thread

>"New pair of AR-3a's, looking for advice on restore"

> that I had designed a switched attenuator (actually a stepped

>16 Ohm potentiometer) to replace the Polak 16 Ohm pots in the

>AR-3a (and other AR) speakers.

I have finally installed the

>pot replacements in one speaker and they are working well. No

>more scratchy adjustments and mid/high drop outs!

>Here are 2 attenuators installed in an AR-3a:

>

>The attenuator uses 20 2-Watt resistors to mimic the function

>of the 25 Watt pot.

Hi Joe;

I really admire people who do what no one else has done before, a pioneer.

Thank you for posting your very positive idea.

I have never seen such a modification in a speaker before.

Except in replacing a volume control, in a pre-amp with a 23 position MBB switch and suitable resistors.

This, of course, would be handling milliwatts, not 25 watts or more.

Dynaco did use a 5 position MBB switch with wire-wound resistors in their Dynaco A-25, A-35 and A-50 speaker system.

I do have concern of the low power ratings of the resistors that you have used.

I was privileged to have have worked at our local AR warantee depot many years ago.

During that brief 2 or so years, I saw many unusual sights, to be sure.

Melted AR pot shafts from the high current placed upon the pots to handle.

Fiberglass insulation charred around the pots as well.

Speaker voice coils burnt just like toast.

My thoughts are that the resistors should be at least 25 watt rated and the MBB switch rated accordingly.

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.

In all fairness to the manufacturers, there is not a great big market for pots today.

I have been on the prowl for several months now, looking for alternate safety protection shields, but I haven't found exactly what I want yet, I am getting closer though.

James has been a big help motivating me, thank you very much, James.

Ohmite and a Chinese brand are the only reasonable and readily available pots that I see around now, there may be others.

Consider that it is not out of place for the air coming out of the dome of a woofer to be as high as 150 degrees celcius.

One of my many hats that I have worn has been, as an electrical contractor and I always erred to the side of safety.

This certainly is on a great path to another solution to fouled up pots, thank you, Joe.

[JoeB]

>Bravissimo!

>

>Can you please post a parts list with supplier and prices?

>

>

Parts List For Attenuator

All Part Numbers are Mouser Electronics

2 0.5 Ohm 2W R14 R15

Mouser P/N 262-0.5-RC

4 1.0 Ohm 2W R4 R5 R16 R17

Mouser P/N 262-1.0-RC

11 1.5 Ohm 2W R1 R10 R11 R12 R13 R2 R3 R6 R7 R8 R9

Mouser P/N 262-1.5-RC

1 2.4 Ohm 3W R18

Mouser P/N 283-2.4-RC

1 3.0 Ohm 2W R19

Mouser P/N 262-3.0-RC

1 3.6 Ohm 2W R20

Mouser P/N 262-3.6-RC

1 CK1054 SW1

Lorlin Nylon Rotary Switch Mouser P/N 10WA344

12 PST Make Before Break Rotary Sw

1 Custom Printed Circuit Board

Note the 3W resistors were used because the 2W were

Out of stock. They are identical in dimensions.

Prices are based on quantities and are all available at mouser.com

If anybody wants to make their own PC boards I can post the layout. My thinking right now is to maybe make up and sell kits including the parts and PC boards but anyone who wants to build their own from scratch (for their own use) is welcome to do so.

[JoeB]

>Richard,

>

>Very, very impressive!!!

>

>It looks like you had 3 attachments, but we can only see 2.

>Is the missing attachment the parts list?

>

>Regards,

>Jerry

There were only 2 attachments.

[JoeB]

>Wow, this is great stuff. I only wish that I had the skills

>to make these pots from your schematics/parts list. My only

>alternative would be to purchase fully-assembled pots.

>

>I would like to hear if you notice any sound difference

>between OEM and your fantastic creation.

>

>

>I've been wanting to change my 3a pots since they cut out a

>bit despite cleaning them, but I have not wanted to alter

>their sublime stock sound in any way.

>

>Cheers

My main motivation was to not alter the stock sound at all. I have been using the attenuators for a couple of days now and I think they sound great - just exactly like the AR-3a with good pots.

Joe

[Guest Eunomians]

Wow! The simpe fact that they retain the OEM sound is great news.

Making kits to sell would be fantastic for inept folks like me! Seriously though, you have an excellent idea. I am surprised that there has not been a drop-in replacelement made for AR3/3a yet. Some original pots are beyond repair/cleaning to get them working again - and the choices for replacements such as L-Pads, etc... Are not at all enticing.

You could fair well if you sold ready-to-go kits on the HiFi boards as well as eBay.

Cheers

[Carlspeak]

>Wow! The simpe fact that they retain the OEM sound is great

>news.

>

>

>Making kits to sell would be fantastic for inept folks like

>me! Seriously though, you have an excellent idea. I am

>surprised that there has not been a drop-in replacelement made

>for AR3/3a yet. Some original pots are beyond repair/cleaning

>to get them working again - and the choices for replacements

>such as L-Pads, etc... Are not at all enticing.

>

>You could fair well if you sold ready-to-go kits on the HiFi

>boards as well as eBay.

>

>Cheers

Wow, so much work. I guess if electronics are a hobby, this is an excellent project for an enthusiast interested in audio and electronics.

BTW, I offer brand new, high quality 15 ohm rheostats the function nicely as replacements for the originals. They are $15 ea. plus shipping. I have posted earlier details regarding these rheostats. Do a history search for more info.

Carl

Carl's Custom Loudspeakers

[JoeB]

>>I had mentioned in a previous reply to SamS on his

>thread

>>"New pair of AR-3a's, looking for advice on

>restore"

>> that I had designed a switched attenuator (actually a

>stepped

>>16 Ohm potentiometer) to replace the Polak 16 Ohm pots in

>the

>>AR-3a (and other AR) speakers.

>

> I have finally installed the

>>pot replacements in one speaker and they are working well.

> No

>>more scratchy adjustments and mid/high drop outs!

>>Here are 2 attenuators installed in an AR-3a:

>>

>

>>The attenuator uses 20 2-Watt resistors to mimic the

>function

>>of the 25 Watt pot.

>

>

>

>Hi Joe;

>

>I really admire people who do what no one else has done

>before, a pioneer.

>

>Thank you for posting your very positive idea.

>

>I have never seen such a modification in a speaker before.

>

>Except in replacing a volume control, in a pre-amp with a 23

>position MBB switch and suitable resistors.

>

>This, of course, would be handling milliwatts, not 25 watts or

>more.

>

>Dynaco did use a 5 position MBB switch with wire-wound

>resistors in their Dynaco A-25, A-35 and A-50 speaker system.

>

>I do have concern of the low power ratings of the resistors

>that you have used.

>

>I was privileged to have have worked at our local AR warantee

>depot many years ago.

>

>During that brief 2 or so years, I saw many unusual sights, to

>be sure.

>

>Melted AR pot shafts from the high current placed upon the

>pots to handle.

>

>Fiberglass insulation charred around the pots as well.

>

>Speaker voice coils burnt just like toast.

>

>My thoughts are that the resistors should be at least 25 watt

>rated and the MBB switch rated accordingly.

>

>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.

>

>In all fairness to the manufacturers, there is not a great big

>market for pots today.

>

>I have been on the prowl for several months now, looking for

>alternate safety protection shields, but I haven't found

>exactly what I want yet, I am getting closer though.

>

>James has been a big help motivating me, thank you very much,

>James.

>

>Ohmite and a Chinese brand are the only reasonable and readily

>available pots that I see around now, there may be others.

>

>Consider that it is not out of place for the air coming out of

>the dome of a woofer to be as high as 150 degrees celcius.

>

>One of my many hats that I have worn has been, as an

>electrical contractor and I always erred to the side of

>safety.

>

>This certainly is on a great path to another solution to

>fouled up pots, thank you, Joe.

>

Vern,

You have made some very good points here. I will be replying later with a more complete technical description of the circuit but for now I'll make the following points.

1.) A 25 W Potentiometer has a current rating of 1.25 Amps (P=I squared R) as rated by Ohmite. Pollak may have used a different method allowing higher currents when part of the pot is used - there is no data on that available.

2.) The highest current occurs at the first attenuating position. The attenuator at that position can handle about 3.4 Amps continuous without exceeding the 6 W (3 1.5 Ohm 2W in parallel) rating of the first step. This is 2.7 times the 1.25 Amps rating of a modern 25W 16 Ohm pot.

3.) I used an assumed value of 10 VRMS (note that is 25 Watts continuous in the frequency range of the driver only) and calculated the dissipation on each resistor in every switch position. The highest dissipation on any single resistor is 1.7 Watts at position 10 (-13.5dB).

Your idea for a cover to keep the fiberglass away from the resistors sounds interesting.

I'll be posting more about this later.

Joe

[dynaco_dan]

>>>I had mentioned in a previous reply to SamS on his

>>thread

>>>"New pair of AR-3a's, looking for advice on

>>restore"

>>> that I had designed a switched attenuator (actually

>a

>>stepped

>>>16 Ohm potentiometer) to replace the Polak 16 Ohm pots

>in

>>the

>>>AR-3a (and other AR) speakers.

>>

>> I have finally installed the

>>>pot replacements in one speaker and they are working

>well.

>> No

>>>more scratchy adjustments and mid/high drop outs!

>>>Here are 2 attenuators installed in an AR-3a:

>>>

>>

>>>The attenuator uses 20 2-Watt resistors to mimic the

>>function

>>>of the 25 Watt pot.

>>

>>

>>

>>Hi Joe;

>>

>>I really admire people who do what no one else has done

>>before, a pioneer.

>>

>>Thank you for posting your very positive idea.

>>

>>I have never seen such a modification in a speaker

>before.

>>

>>Except in replacing a volume control, in a pre-amp with a

>23

>>position MBB switch and suitable resistors.

>>

>>This, of course, would be handling milliwatts, not 25

>watts or

>>more.

>>

>>Dynaco did use a 5 position MBB switch with wire-wound

>>resistors in their Dynaco A-25, A-35 and A-50 speaker

>system.

>>

>>I do have concern of the low power ratings of the

>resistors

>>that you have used.

>>

>>I was privileged to have have worked at our local AR

>warantee

>>depot many years ago.

>>

>>During that brief 2 or so years, I saw many unusual

>sights, to

>>be sure.

>>

>>Melted AR pot shafts from the high current placed upon

>the

>>pots to handle.

>>

>>Fiberglass insulation charred around the pots as well.

>>

>>Speaker voice coils burnt just like toast.

>>

>>My thoughts are that the resistors should be at least 25

>watt

>>rated and the MBB switch rated accordingly.

>>

>>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.

>>

>>In all fairness to the manufacturers, there is not a great

>big

>>market for pots today.

>>

>>I have been on the prowl for several months now, looking

>for

>>alternate safety protection shields, but I haven't found

>>exactly what I want yet, I am getting closer though.

>>

>>James has been a big help motivating me, thank you very

>much,

>>James.

>>

>>Ohmite and a Chinese brand are the only reasonable and

>readily

>>available pots that I see around now, there may be

>others.

>>

>>Consider that it is not out of place for the air coming

>out of

>>the dome of a woofer to be as high as 150 degrees

>celcius.

>>

>>One of my many hats that I have worn has been, as an

>>electrical contractor and I always erred to the side of

>>safety.

>>

>>This certainly is on a great path to another solution to

>>fouled up pots, thank you, Joe.

>>

>

>Vern,

>You have made some very good points here. I will be replying

>later with a more complete technical description of the

>circuit but for now I'll make the following points.

>

>1.) A 25 W Potentiometer has a current rating of 1.25 Amps

>(P=I squared R) as rated by Ohmite. Pollak may have used a

>different method allowing higher currents when part of the pot

>is used - there is no data on that available.

Hi Joe;

You are a techie, good, because, I am not.

I do not have any technical information regarding the Aetna-Pollock pots.

They did have the advantage of an integral rear steel heat shield, which the two other previously mentioned pots, do not.

This lack of, a rear shield, was an issue that, Carl, has discussed.

I didn't expand enough on the melted pot shafts comment.

There was black and red shafts, which one came first I do not know, and whether there was a difference, I do not know.

Possibly, there was a melting point difference.

A later version and I believe it was the last, had the aluminum shaft.

I don't have a disassembled version of each to look at, Carl, may offer his expertise in this issue.

I will ask this question, Joe, how do your 2 watt resistors stand up, when the original pot shafts melted and the fiberglass insulation got charred from the heat from the pots, even with a steel heat shield on?

As I already mentioned previously, Dynaco, used sandcast wire-wound 5 watt 2.2 ohm resistors.

I have never seen any burn marks, in or around a Dynaco pot or the resistors, which are glued directly to the masonite backing board, and they are also without a non-combustible heat shield, as well.

As a side note, the goop that they used to seal the woofers, tweeters, grille cloths and also to glue the resistors to the board as well.

I am enjoying this topic, Joe, thank you for starting it.

>

>2.) The highest current occurs at the first attenuating

>position. The attenuator at that position can handle about

>3.4 Amps continuous without exceeding the 6 W (3 1.5 Ohm 2W in

>parallel) rating of the first step. This is 2.7 times the

>1.25 Amps rating of a modern 25W 16 Ohm pot.

>

>3.) I used an assumed value of 10 VRMS (note that is 25 Watts

>continuous in the frequency range of the driver only) and

>calculated the dissipation on each resistor in every switch

>position. The highest dissipation on any single resistor is

>1.7 Watts at position 10 (-13.5dB).

>

>Your idea for a cover to keep the fiberglass away from the

>resistors sounds interesting.

>

>I'll be posting more about this later.

>

>Joe

[JoeB]

>I will ask this question, Joe, how do your 2 watt resistors

>stand up, when the original pot shafts melted and the

>fiberglass insulation got charred from the heat from the pots,

>even with a steel heat shield on?

>

>As I already mentioned previously, Dynaco, used sandcast

>wire-wound 5 watt 2.2 ohm resistors.

>

>I have never seen any burn marks, in or around a Dynaco pot or

>the resistors, which are glued directly to the masonite

>backing board, and they are also without a non-combustible

>heat shield, as well.

>

>

Hi Vern,

There are 20 2 Watt resistors in the circuit. They are wired as a series parallel tapped array. Think of the Aetna-Pollak potentiometer as a series of small identical resistors tapped by the wiper. The attenuator is the same except the resistors are larger and, very importantly, not identical. The first step down from full on is only one-half ohm (that is one-thirty-second of the pot travel) but that first step is 3 resistors in parallel (6 Watts total). With 10 VRMS applied (more on that later) each of the 3 resistors is dissipating 1.2 Watts for a total of 3.6 Watts (well within the 6 Watt rating). The Aetna-Pollak pot would, under the same conditions, be dissipating the same 3.6 Watts in one-32nd of its winding length. The very short section of resistance wire would be getting pretty hot.

Attached is a PDF of the expected dissipation in each resistor in each switch position. The assumption made here is that the driver is 4 Ohms. The assumed applied voltage is 10 VRMS. That 10 Volts is equivalent to 25 Watts at 4 ohms and is a continuous rating. The heating in resistors should be similar to heating in a voice coil and I can’t imagine either a tweeter or midrange surviving those power levels for long.

The distribution of power between the woofer, midrange, and tweeter is an interesting problem. I suspect there is no single solution but my first guess would be that the woofer requires much more power than the mid or tweeter for reproducing music. The total power supplied is somewhat limited by the amplifier, but driving an undersized solid-state amp deeply into clipping can increase the RMS power as well as increase the high frequency content due to clipping distortion. This may have contributed to the destruction of the drivers you mentioned earlier.

So how much is enough? My thinking on the subject is that the attenuators should have sufficient power dissipation so that the drivers would be destroyed before the attenuator is damaged. If the speaker is properly fused, this should never be an issue. In addition, using an undersized amplifier and overdriving it is a very bad idea. The AR3-a speakers, in my opinion, need an amplifier that is good for at least 200W at clipping. I used to use a Dynaco Stero 400 (350 Watts per channel at 4 Ohms at clipping) with good results. I have recently used both the Hafler P3000 (200 W/ch at 4 Ohms) and P9000 (500 W/ch at 4 Ohms) with good results.

Joe

1508.pdf

[Carlspeak]

>>Wow! The simpe fact that they retain the OEM sound is

>great

>>news.

>>

>>

>>Making kits to sell would be fantastic for inept folks

>like

>>me! Seriously though, you have an excellent idea. I am

>>surprised that there has not been a drop-in replacelement

>made

>>for AR3/3a yet. Some original pots are beyond

>repair/cleaning

>>to get them working again - and the choices for

>replacements

>>such as L-Pads, etc... Are not at all enticing.

>>

>>You could fair well if you sold ready-to-go kits on the

>HiFi

>>boards as well as eBay.

>>

>>Cheers

>

>Wow, so much work. I guess if electronics are a hobby, this is

>an excellent project for an enthusiast interested in audio and

>electronics.

I just remembered this rotary switch concept is similar in principle to the KLH Model 5's. There are differences though. The 5's implemented both caps and resisters in the circuit at different switch positions. Diagrams and photos are available for viewing in the library area.

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers

[JoeB]

>I just remembered this rotary switch concept is similar in

>principle to the KLH Model 5's. There are differences though.

>The 5's implemented both caps and resisters in the circuit at

>different switch positions. Diagrams and photos are available

>for viewing in the library area.

>

>

>Remember, it's all about the music

>

>Carl

>Carl's Custom Loudspeakers

Hi Carl,

I took a look at the KLH Model 5 schematic and I suspect that the method KLH used avoids most of the issue of power dissipation completely. If a designer were willing to change the electrical characteristics of the AR crossover, some variation of this method would probably work pretty good. My goal was to make a replacement that is electrically identical to the pot so that the sound of the speakers is identical to speakers with new perfect pots.

Joe

[Carlspeak]

>>I just remembered this rotary switch concept is similar

>in

>>principle to the KLH Model 5's. There are differences

>though.

>>The 5's implemented both caps and resisters in the circuit

>at

>>different switch positions. Diagrams and photos are

>available

>>for viewing in the library area.

>>

>>

>>Remember, it's all about the music

>>

>>Carl

>>Carl's Custom Loudspeakers

>

>Hi Carl,

>

>I took a look at the KLH Model 5 schematic and I suspect that

>the method KLH used avoids most of the issue of power

>dissipation completely. If a designer were willing to change

>the electrical characteristics of the AR crossover, some

>variation of this method would probably work pretty good. My

>goal was to make a replacement that is electrically identical

>to the pot so that the sound of the speakers is identical to

>speakers with new perfect pots.

>

>Joe

One last postscript comment Joe. Henry Kloss left AR and was one of the founding father's of KLH. Could it be that by then, Henry devised the Model 5's switching concept as an improvement over the AR's in order to solve that power dissipation problem????

Remember, it's all about the music

Carl

Carl's Custom Loudspeakers