Polk SDA-1a Woofer ohm value oddities

Benjisan

Hi all.

I inherited a pair of Polk SDA-1a speakers from my recently deceased brother. He was the original purchaser of these when he was in his early twenties.

I remember him having these in his living room right after he came home from being stationed in Germany in the late eighties and upon the purchase of his first house with my sister in-law.

These were in storage in his basement for quite sometime (at least 20 years) and my sister law was in the process of selling the house and asked me if I wanted them. I said sure and they came home with me untested.

Well I hooked them up and they sounded terrible.

So I decided to pull the drivers and test each of them individually and disconnected from the wire leads and to my surprise three of the 6.5" drivers in the left cabinet were indicating 18.2 ohms, 18.1 ohms, 18.4 ohms and the fourth reading 7.3 ohms.

The two Sl2000 tweeters were showing 8.1 and 8.3 ohms each.

On the right speaker 3 of the 6.5" woofers were testing 9.4, 8.8, and 9.1 with the 4th woofer reading 0.0 ohms indicating it was open. The two tweeters were showing 8.7 and 8.2 ohms.

1. Does anybody here have an explanation as to why the 6.5" woofers in the left speaker are reading such high ohm values?

2. Does Polk sell replacement drivers or know of a place besides eBay where I can source 4 new drivers for these speakers.

TIA

RandyCroissant

Hi Benjisan and welcome to the forum. Did you disconnect the drivers from the crossovers? Also did you check to see if they move freely with no scraping sounds? Let us know, also pictures would help as you move along with the diagnosis

Gardenstater

Welcome! Speakers, as a rough guide, should read DC resistance of around 2/3 the nominal impedance. Those ones that are reading consistently around 18 ohms are really pretty bizarre. You have disconnected the wires from the crossovers which is correct. A high resistance can only be due to a bad connection or partially broken connection inside the driver. Those other resistance readings are higher than normal suggesting that your meter may be a bit *off* or not calibrated correctly.

eBay is your best bet to get the original drivers in good condition. Just ask the seller to pack carefully with lots of cushioning and to make sure that the voice coils are not scraping at all prior to shipment.

PS: 0 ohms reading indicates a short, not open. Infinite ohms is an open.

PPS: Your drivers should be MW6501 (you should look at the labels to verify) but I am pretty sure the MW6600X is interchangeable with those, so set up eBay saved searches for both.

Driver: MW 6501 Driver's model #.
Brand: Polk Driver's brand name.
Fs: 31.0000 Hz (Free air resonance)
Vas: 2.7545 ft^3 (Compliance volume)
Qts: 0.2790 Total Q
Qms: 1.4700 Mechanical Q
Qes: 0.3440 Electrical Q
Sd: 0.0130 M^2 (Piston area)
Xmax: 3.1800 mm (Cone excursion, linear)
Revc: 7.7500 ohms (DC resistance)
Znom: 8.0000 ohms (Nominal impedance)
Pmax: 35.0000 watts maximum rated power

Benjisan

RandyCroissant wrote: »

Hi Benjisan and welcome to the forum. Did you disconnect the drivers from the crossovers? Also did you check to see if they move freely with no scraping sounds? Let us know, also pictures would help as you move along with the diagnosis

Hi Randy,

Yes, I disconnected the internal feed wires and removed each driver from the cabinet. All the 6.5" drivers move freely in each cabinet.



===================================================================

Gardenstater wrote: »

Welcome! Speakers, as a rough guide, should read DC resistance of around 2/3 the nominal impedance. Those ones that are reading consistently around 18 ohms are really pretty bizarre. You have disconnected the wires from the crossovers which is correct. A high resistance can only be due to a bad connection or partially broken connection inside the driver. Those other resistance readings are higher than normal suggesting that your meter may be a bit *off* or not calibrated correctly.

eBay is your best bet to get the original drivers in good condition. Just ask the seller to pack carefully with lots of cushioning and to make sure that the voice coils are not scraping at all prior to shipment.

PS: 0 ohms reading indicates a short, not open. Infinite ohms is an open.

PPS: Your drivers should be MW6501 (you should look at the labels to verify) but I am pretty sure the MW6600X is interchangeable with those, so set up eBay saved searches for both.

Driver: MW 6501 Driver's model #.
Brand: Polk Driver's brand name.
Fs: 31.0000 Hz (Free air resonance)
Vas: 2.7545 ft^3 (Compliance volume)
Qts: 0.2790 Total Q
Qms: 1.4700 Mechanical Q
Qes: 0.3440 Electrical Q
Sd: 0.0130 M^2 (Piston area)
Xmax: 3.1800 mm (Cone excursion, linear)
Revc: 7.7500 ohms (DC resistance)
Znom: 8.0000 ohms (Nominal impedance)
Pmax: 35.0000 watts maximum rated power

Hi, Gardenstater.

I'm a Garden-Stater too. Exit 132 off the GSP.

I'm pretty sure my DVM isn't finely calibrated, but I'm confident that the readings of 18+ ohms on the 3 drivers are correct in showing very high resistance in those 3 speakers. I did see white labels indicating that six of the 6.5" drivers were 6051 and two were different in appearance with regards to the dust caps and were marked with SDA-ll labels. It's one of those two SDA-ll drivers that read 0.0 ohms.

Thanks for the spec sheet for the tweeters, but I'm not hearing any problems with the four tweeters.

I'm going to pull a driver and take a short video of the ohm reading and post a link to my Youtube channel a little later.

Benjisan

RandyCroissant wrote: »

Hi Benjisan and welcome to the forum. Did you disconnect the drivers from the crossovers? Also did you check to see if they move freely with no scraping sounds? Let us know, also pictures would help as you move along with the diagnosis

Gardenstater wrote: »

Welcome! Speakers, as a rough guide, should read DC resistance of around 2/3 the nominal impedance. Those ones that are reading consistently around 18 ohms are really pretty bizarre. You have disconnected the wires from the crossovers which is correct. A high resistance can only be due to a bad connection or partially broken connection inside the driver. Those other resistance readings are higher than normal suggesting that your meter may be a bit *off* or not calibrated correctly.

eBay is your best bet to get the original drivers in good condition. Just ask the seller to pack carefully with lots of cushioning and to make sure that the voice coils are not scraping at all prior to shipment.

PS: 0 ohms reading indicates a short, not open. Infinite ohms is an open.

PPS: Your drivers should be MW6501 (you should look at the labels to verify) but I am pretty sure the MW6600X is interchangeable with those, so set up eBay saved searches for both.

Driver: MW 6501 Driver's model #.
Brand: Polk Driver's brand name.
Fs: 31.0000 Hz (Free air resonance)
Vas: 2.7545 ft^3 (Compliance volume)
Qts: 0.2790 Total Q
Qms: 1.4700 Mechanical Q
Qes: 0.3440 Electrical Q
Sd: 0.0130 M^2 (Piston area)
Xmax: 3.1800 mm (Cone excursion, linear)
Revc: 7.7500 ohms (DC resistance)
Znom: 8.0000 ohms (Nominal impedance)
Pmax: 35.0000 watts maximum rated power

RandyCroissant wrote: »

Hi Benjisan and welcome to the forum. Did you disconnect the drivers from the crossovers? Also did you check to see if they move freely with no scraping sounds? Let us know, also pictures would help as you move along with the diagnosis

Hi Randy and George,

Okay, it was late last night when I was measuring the drivers and I made the error of testing some of the drivers (3 of them) resting with the magnet side up and the surround side facing down. Apparently my placing the driver upside down for three of the eight drivers gave the errant ohm values. Must have been enough movement of the cone in the voice coil to increase the ohm values.

I retested the drivers again and all 3 now measure between 7.8 and 8.4 ohms.

So it looks like I need just one replacement driver now and I just purchased two 6600x drivers from someone off ebay for $60.00

That's for the pair with free shipping.

Hey George?

I see you're from NJ.

So am I.

I'm right off GSP exit 132.

Here's a picture of my inherited SDA's

Gardenstater

Huh! Well that's a new one on me regarding the cone/voice coils being displaced effecting the resistance reading! Interesting.

Those look very nice and it's a nice thing they stayed in the family! Sorry about your brother. Let the "what exit" jokes ensue (lol) but I am closest to 139B, so only several miles north of you, in Springfield.

Getting those drivers for $30 apiece is really quite a score. Make sure you check all of them, the new ones and existing ones, for voice coil scraping. In case you don't know it is a common problem with these speakers that the magnet/pole piece assemblies can slip due to deteriorated adhesive. Most people put a ring of adhesive on the top and bottom of the magnet to prevent that. The 5min epoxy with the mixing nozzles is probably a good bet. I used Gorilla Maximum Strength Clear Construction Adhesive which worked well but it does not come in a small tube so I had to transfer it to a syringe.

Benjisan

Gardenstater wrote: »

Huh! Well that's a new one on me regarding the cone/voice coils being displaced effecting the resistance reading! Interesting.

Those look very nice and it's a nice thing they stayed in the family! Sorry about your brother. Let the "what exit" jokes ensue (lol) but I am closest to 139B, so only several miles north of you, in Springfield.

Getting those drivers for $30 apiece is really quite a score. Make sure you check all of them, the new ones and existing ones, for voice coil scraping. In case you don't know it is a common problem with these speakers that the magnet/pole piece assemblies can slip due to deteriorated adhesive. Most people put a ring of adhesive on the top and bottom of the magnet to prevent that. The 5min epoxy with the mixing nozzles is probably a good bet. I used Gorilla Maximum Strength Clear Construction Adhesive which worked well but it does not come in a small tube so I had to transfer it to a syringe.

Hi George,

I think that moving the voice coil ever so slightly inside the magnetic field of the speakers magnet induced voltage and throwing off the ohm readings. I guess when you move the speaker cone/voice coil, this induced voltage adds to (or subtracts from) the meters measurement, thus the confusing ohm readings.

It's the only explanation I have since retesting with those three speakers sitting upright provided a more realistic measurement.

And yes, these speakers are in excellent shape as they were in his finished basement recreation room along with his 1960's vintage Brunswick regulation size pool table which I didn't want to be moving lol

I want to get these working out of curiosity and because it's also nice to have something of his passed down as a nice family keepsake.

Now what's this about the drivers pole pieces slipping and bonding them with adhesive?

Gardenstater

Benjisan wrote: »

I think that moving the voice coil ever so slightly inside the magnetic field of the speakers magnet induced voltage and throwing off the ohm readings. I guess when you move the speaker cone/voice coil, this induced voltage adds to (or subtracts from) the meters measurement, thus the confusing ohm readings.

It's the only explanation I have since retesting with those three speakers sitting upright provided a more realistic measurement.

And yes, these speakers are in excellent shape as they were in his finished basement recreation room along with his 1960's vintage Brunswick regulation size pool table which I didn't want to be moving lol

I want to get these working out of curiosity and because it's also nice to have something of his passed down as a nice family keepsake.

Now what's this about the drivers pole pieces slipping and bonding them with adhesive?

Yes that's the only thing I can figure as well. With the drivers being supported by the surrounds, kinda trampoline-like, the voice coil must've been oscillating somewhat and creating a voltage. Cool.

Yes it is a very common problem to have the shifting, resulting in a scraping voice coil. I don't know for sure what kind of adhesive they used but it dried up or lost its adhesion. Sometimes it may be pretty much the magnetic attraction that is preventing shifting and any small jarring can result in the dreaded voice coil scrape. The test is to carefully place 3 fingers and a thumb spread out evenly where the dust cap meets the cone and move the cone back and forth and listen/feel for any scraping. Another test is that is good to do as a backup is to play low frequency test tones on each driver and listen for any problems. Usually is worse with low frequencies due to resonance. Sometimes the magnet will visually be off center on the pole piece and you still won't get a scraping if you are lucky. This is because the ID of the magnet is much larger than the voice coil OD.

pitdogg2

Huh! Well that's a new one on me regarding the cone/voice coils being displaced effecting the resistance reading! Interesting.

I'll be testing this tonight.

I've got MW6600, MW6501, MW6502 and MW6510.
I've always put them face down on the table to check all previous time because it is just easier to get at the input tabs. I'll clip my leads and move the cone to see just what gives. Then recheck face up sitting on the magnet to see difference.

Benjisan

Gardenstater wrote: »

Benjisan wrote: »

I think that moving the voice coil ever so slightly inside the magnetic field of the speakers magnet induced voltage and throwing off the ohm readings. I guess when you move the speaker cone/voice coil, this induced voltage adds to (or subtracts from) the meters measurement, thus the confusing ohm readings.

It's the only explanation I have since retesting with those three speakers sitting upright provided a more realistic measurement.

And yes, these speakers are in excellent shape as they were in his finished basement recreation room along with his 1960's vintage Brunswick regulation size pool table which I didn't want to be moving lol

I want to get these working out of curiosity and because it's also nice to have something of his passed down as a nice family keepsake.

Now what's this about the drivers pole pieces slipping and bonding them with adhesive?

Yes that's the only thing I can figure as well. With the drivers being supported by the surrounds, kinda trampoline-like, the voice coil must've been oscillating somewhat and creating a voltage. Cool.

Yes it is a very common problem to have the shifting, resulting in a scraping voice coil. I don't know for sure what kind of adhesive they used but it dried up or lost its adhesion. Sometimes it may be pretty much the magnetic attraction that is preventing shifting and any small jarring can result in the dreaded voice coil scrape. The test is to carefully place 3 fingers and a thumb spread out evenly where the dust cap meets the cone and move the cone back and forth and listen/feel for any scraping. Another test is that is good to do as a backup is to play low frequency test tones on each driver and listen for any problems. Usually is worse with low frequencies due to resonance. Sometimes the magnet will visually be off center on the pole piece and you still won't get a scraping if you are lucky. This is because the ID of the magnet is much larger than the voice coil OD.

Okay both of what you mentioned above seems easy enough to do since I have a Sheffield Acoustic test disc that I converted to a Flac file in my music files portable SSD that has a multitude of different frequency test tones to test the 8 mid woofer drivers.

And re-cementing the magnets on all the drivers that test good is simple enough too.

I see the mention of an SDA Handbook.

Where can I obtain a copy of this book?

Gardenstater

This is the link to the SDA Handbook:

https://us.v-cdn.net/5021930/uploads/editor/qx/d5a14j8apinc.pdf

I haven't been able to find the manual for the SDA 1A online. I can find the SDA 1, SDA 1B, and SDA 1C but no A. Here's the link to the SDA 1 that came before yours and the one that came after the 1B:

https://polksda.com/pdfs/SDA1.pdf
http://akdatabase.org/AKview/albums/userpics/10006/polk SDA 1B OM.pdf or
https://www.manualslib.com/manual/362833/Polk-Audio-Sda-1b.html?page=10#manual

Benjisan

Gardenstater wrote: »

This is the link to the SDA Handbook:

https://us.v-cdn.net/5021930/uploads/editor/qx/d5a14j8apinc.pdf

I haven't been able to find the manual for the SDA 1A online. I can find the SDA 1, SDA 1B, and SDA 1C but no A. Here's the link to the SDA 1 that came before yours and the one that came after the 1B:

https://polksda.com/pdfs/SDA1.pdf
http://akdatabase.org/AKview/albums/userpics/10006/polk SDA 1B OM.pdf or
https://www.manualslib.com/manual/362833/Polk-Audio-Sda-1b.html?page=10#manual

Hi George,

First let me say thank you for all the above links....

Aesthetically the SDA, SDA-1 and SDA-1a look to be the same except for the tweeters in the SDA.

What could be accounting for the prefix variations between the models with regards to component hardware?

Is it the internal crossovers and mid woofer speakers specifically?

pitdogg2

http://gimpod.com/sda-id.html

Pretty accurate SDA descriptions.
Get your read on.

Benjisan

pitdogg2 wrote: »

http://gimpod.com/sda-id.html

Pretty accurate SDA descriptions.
Get your read on.

Fascinating Pitdog.

I had no idea that there were so many iterations of the SDA design which even includes bookshelf types.

I'm guessing a lot of these Polk's SDA's and SRS speakers are pretty rare and hard to find.

I'm hoping that my replacement driver improves the sound of my set.

I also checked my interconnect cable for continuity this evening and discovered it has an issue which appears to have been damaged from being caught in a vacuum cleaner or something.