Polk SDA2s and dimensional tweeters

westmassguy

machone wrote: »

Westmassguy: Would my SDA-1As benefit using a Erse SuperQ Inductor for the bass drivers like your 2As? From what I have read it seems people are using it with speakers with the sub bass drive circuit and looking at the diagrams I see that the 1As are wired different than the 2As.

There is no sub-bass drive circuit in the 1As

westmassguy

machone wrote: »

My SL-1000 tweeters are flat and then are recessed flush with the face of the enclosure. It looks like the newer tweeters used for upgrades are not flat and will stand proud of the face of the enclosure. Would this interfere with the grill?

You have two choices for mounting the new tweeters, enlarge the existing recess, or grind off the back lip on the new tweeter for surface mounting. Either method will not interfere with the grill.

machone

westmassguy: Thanks for the info about the tweeter. Would the SDA-1As also benefit by changing the bass circuit inductor to a one with a lower DCR? I have a Hafler 500 so I don't think I have to worry about lowering the impedance. Thanks again.

westmassguy

machone wrote: »

westmassguy: Thanks for the info about the tweeter. Would the SDA-1As also benefit by changing the bass circuit inductor to a one with a lower DCR? I have a Hafler 500 so I don't think I have to worry about lowering the impedance. Thanks again.

This is where I would normally say "Don't fix what ain't broken". The 1st and 2nd generations do not have a sub-bass drive circuit or Inductor. That Inductor has been specifically addressed by MP, and is a recommended upgrade.
That being said, the 5.6mh Inductor in series with the two bass drivers could be a candidate for replacement, with a lower resistance Inductor of the same value. I've never done it, nor have I heard of anyone else doing it on that generation. I can't tell you whether the result will be good or bad. I can tell you, the overall impedance of the speaker will drop.
Keep in mind, the reason those bass drivers are there, is to counteract the out of phase low bass frequencies being produced by the dimensional driver.
The Value and DCR of those Inductors were chosen by engineers a lot smarter than me.
In the 3rd through 5 generation, the higher DCR sub bass drive Inductors were a necessary compromise to keep the overall impedance high enough for the amplifiers of the time.
I don't know if that same compromise was made in the 1st and 2nd generation.

machone

Westmassguy: My SDA-1A uses a 4ohm resistor with a fuse for SL-1000 tweeter protection. A revision on the diagram shows a change to a polyswitch with a 3.5ohm resistor for tweeter protection (the diagram does not show a tweeter part#). I plan to upgrade to the RDO-194 tweeter and keep the fuses making sure that the fuse holders are in good shape. When using fuses for protection I assume that I should order replacement 4ohm resistors for the RDO-194 tweeters. Is this correct?

westmassguy

machone wrote: »

Westmassguy: My SDA-1A uses a 4ohm resistor with a fuse for SL-1000 tweeter protection. A revision on the diagram shows a change to a polyswitch with a 3.5ohm resistor for tweeter protection (the diagram does not show a tweeter part#). I plan to upgrade to the RDO-194 tweeter and keep the fuses making sure that the fuse holders are in good shape. When using fuses for protection I assume that I should order replacement 4ohm resistors for the RDO-194 tweeters. Is this correct?

Correct. The Polyswitch adds approximately .5 ohms to the circuit.

machone

westmassguy wrote: »

machone wrote: »

westmassguy: Thanks for the info about the tweeter. Would the SDA-1As also benefit by changing the bass circuit inductor to a one with a lower DCR? I have a Hafler 500 so I don't think I have to worry about lowering the impedance. Thanks again.

This is where I would normally say "Don't fix what ain't broken". The 1st and 2nd generations do not have a sub-bass drive circuit or Inductor. That Inductor has been specifically addressed by MP, and is a recommended upgrade.
That being said, the 5.6mh Inductor in series with the two bass drivers could be a candidate for replacement, with a lower resistance Inductor of the same value. I've never done it, nor have I heard of anyone else doing it on that generation. I can't tell you whether the result will be good or bad. I can tell you, the overall impedance of the speaker will drop.
Keep in mind, the reason those bass drivers are there, is to counteract the out of phase low bass frequencies being produced by the dimensional driver.
The Value and DCR of those Inductors were chosen by engineers a lot smarter than me.
In the 3rd through 5 generation, the higher DCR sub bass drive Inductors were a necessary compromise to keep the overall impedance high enough for the amplifiers of the time.
I don't know if that same compromise was made in the 1st and 2nd generation.

Westmassguy: Thanks for the info about the bass inductor circuit for the SDA-1A. For now I will keep that one in my hip pocket and do the cap upgrade with better binding posts, speaker rings, etc first. One last item. Why is it difficult or not recommended to duplicate later design tweeter circuits into the SDA-1A and use the RDO-198? I see that you did it on a SDA-2A.

westmassguy

My 2A's hi-pass circuit in the crossover is nearly identical to the 4th generation 2Bs. It was an easy conversion. Your 1As are very different than the 1Bs or 1Cs, which use a progressive point source/dual tweeter configuration. Apples and oranges.

machone

Westmassguy: Thanks for the info about why it is best not to use the RDO-198 with a SDA-1A. I plan to use a single RDO-194 for stereo and delete the SDA tweeter. I will not be moving the tweeter to the center of the cabinet! I read in another post where someone changed the SDA-1A tweeter circuit to the SDA-2B circuit because he was concerned about an output loss due to using one tweeter rather than originally using two tweeters. I don't know if I agree with this because the second tweeter was not used for stereo. Is this a valid concern or should I replace the tweeter cap and resistors with the exact original values?

westmassguy

The 1A Tweeter's SPLs were matched to their respective woofer outputs. The Dimensional Tweeter's level was matched to the Dimensional woofer, same for the Stereo Woofers. Upgrade the Stereo Hi-Pass/Tweeter Section just as it is, same values that are there now. If, for whatever reason, you feel the tweeter output is low, you can always change the 4ohm resistor to 3.5ohms. Leave the Dimensional Hi-Pass section as is, since you won't be using it.

BTW, you will not be replacing the 750pf Silver Mica Caps. Those are eliminated.
You will need to replace the .5uf cap that shunts the 4ohm resistor.

machone

Westmassguy:
Thanks for your help.
Over time my upgrades will be as follows:
1. Upgrade the caps and resistors to Sonicap/Mills.
2. Upgrade the stereo tweeter to RDO-194.
3. Delete the SDA tweeter.
4. Dynamat
5. Armacell
6. Binding posts for amp to speaker connection.
7. Larry's rings or hurricane nuts
8. Add internal cabinet stiffeners to large unsupported areas.
9. Upgrade the interconnect cable and connection.
10. Inspect and possibly upgrade the stereo tweeter fuse holder.

michaeljhsda2

machone wrote: »

Westmassguy:
Thanks for your help.
Over time my upgrades will be as follows:
1. Upgrade the caps and resistors to Sonicap/Mills.
2. Upgrade the stereo tweeter to RDO-194.
3. Delete the SDA tweeter.
4. Dynamat
5. Armacell
6. Binding posts for amp to speaker connection.
7. Larry's rings or hurricane nuts
8. Add internal cabinet stiffeners to large unsupported areas.
9. Upgrade the interconnect cable and connection.
10. Inspect and possibly upgrade the stereo tweeter fuse holder.

Add spikes?

westmassguy

Spikes are very good

machone

Yes. Forgot about the spikes.

machone

I noticed that when I pull the fuse for the Dimensional tweeter the impedance for the speaker does not change. I thought that it would? I get 3.7 - 3.8 ohms measured at the binding posts with the speaker wires disconnected and without the interconnect cable connected. Is this the correct way to check speaker impedance?

westmassguy

You're measuring DC Resistance, not AC Resistance (Impedance)

machone

Understood. What is the correct procedure to check the DCR for an SDA speaker?

F1nut

Impedance changes with frequency. To test you would need one of the programs and equipment for said purpose.

machone

Obviously I am in over my head. The reason I was asking is that I really like the way the SDA-1As sound without the Dimensional tweeter. I have removed the fuse and was wondering if the absence of the tweeter in the circuit would affect the total resistance and if so would is be better to substitute a dummy load for the tweeter and reconnect or jump the fuse?

westmassguy

Just leave the fuse out. The tweeters use a relatively small portion of the total wattage delivered by your amplifier. Most of it is converted to sound and heat by the woofers, and additional heat from the passive/resistive components in the crossovers.

machone

I reassembled my SDA-1As this weekend and have begun the burn in process. The sound is much better. Everything is smooth and enveloping without any harshness. Vocals are great. When I close my eyes the speakers disappear. I have noticed changes in the sound quality during the burn in process as you guys have shared. I have been leaving them on with the tuner playing during the night and while at work to accelerate the process (200+hrs).

So far I have completed the following list.
1. Upgraded the capacitors and resistors to Sonicap/Mills.
2. Upgraded the stereo tweeters to RDO-194s.
3. Deleted the dimensional tweeter.
4. Applied Dynamat Extreme to the driver and PR baskets.
5. Glued the driver magnets with Loctite Power Grab.
6. Upgraded to Cardas binding posts.
7. Installed #8-32 hurricane nuts for all drivers, PR and tweeter.
8. Upgraded the interconnect cable to 12 ga.
9. Upgraded the interconnect with Speakon connectors
10. Added 3/4" Sonic barrier behind the tweeters and drivers.

Lots of hours!!

I am down to spikes.
Installing them into the lower skirt looks risky.
I am thinking about gluing a block of wood behind the lower skirt and installing a threaded insert into the block for the spikes.
What are recommended ways to add spikes to this style of cabinet??
Thanks guys

westmassguy

Great work, glad you're enjoying them. The more hours you put on them the sweeter they'll sound. I removed the flimsy bases completely on my 2ATLs, and used taller spikes.

machone

How are the bases attached?

westmassguy

There should be 4 wood screws holding them

machone

I did not notice them. They must be under the four plastic feet?

westmassguy

On my 2As, the wood screws were countersunk, in the middle of each side of the base.
The should be 4 holes visible, with screws in them.
They could also be screwed on internally, but I doubt it.

machone

I did not see them inside during the rebuild. I'll find them tonight. Thanks

Schurkey

machone wrote: »

Obviously I am in over my head. The reason I was asking is that I really like the way the SDA-1As sound without the Dimensional tweeter. I have removed the fuse and was wondering if the absence of the tweeter in the circuit would affect the total resistance and if so would is be better to substitute a dummy load for the tweeter and reconnect or jump the fuse?

How do you have fuses in an SDA 1A? The schematic shows the fuses were deleted and polyswitches installed. Did early-production 1As have fuses?

The dimensional tweeter is not grounded when the SDA interconnect is un-plugged. Therefore, no resistance change when you pull the fuse because it's not in a functioning circuit.

You want to resistance-test that circuit, as best I can tell (and I may be wrong), you'd plug the SDA interconnect cable into both speakers, insert a jumper wire tying the negative posts of both speakers together, test the resistance at one cabinet's + and - terminals, then pull the fuse, re-test and compare.

machone

Yes, early production did have fuses and a 4 ohm resistor. This was later revised to add the polyswitch with a 3.5 ohm resistor.
Before any modifications I listened with and without the dimensional tweeter and liked the sound better without.
Then I upgraded to the RDO-194 tweeter and never looked back.
I never tried to measure the resistance with and without the dimensional fuse.
When I rebuilt the x-over I deleted the dimensional tweeter circuit.

machone

I replaced the deleted dimensional tweeters (SL-1000) with homemade tweeter plates. They are made out of 1/8" thick aluminum with 3D printed letters. I attached the letters with Loctite Power Grab and spray painted everything black. Also see the photo of the passive radiator screw. It is an 8-32 truss head phillips and has a larger diameter head than the standard 8-32 pan or round head phillips screws (see photo). The larger truss head covers the larger hole in the passive radiator allowing the use of 8-32 hurricane nuts or rings. The screws can be purchased through McMaster Carr (pn-94779A595
).