SDA 1B Crossover Parts

bigaltx24

I'm looking to redo the crossovers in my 1B's. I downloaded the schematic for these but it's very hard to read. This list is my best guess as to what I need to do two speakers.

2) 2.0 Ohm Resistor

6) 2.7 Ohm Resistor

4) 750pf Capacitor

2) 12uf Capacitor

4) 34uf Capacitor

2) 65uf Capacitor

2) 130uf Capacitor

2) 4.4uf Capacitor

Help with any corrections to this list would be appreciated. I am looking at using Solen Fastcaps, but there are a few capacitors on the list that they don't offer. I couldn't find anything with a 750pf value, but did find ceramic disc capacitors at 680 or 820pf in Mousers' Catalog. Likewise for the 34uf and 65uf, Solen offers either 33 or 36uf and 62 or 68uf. Are these values close enough to the original values? If they are should I use the higher or lower value?

F1nut

Forget the 750pF, you don't need it when using better caps.

I would not recommend using Solen's in the high frequency circuit, use something better.

For the 34 use two 17's or whatever adds up closest, same for the 65.

thsmith

ARe you keeping or loosing the polyswitch ? If you loose it you might consider a .5 ohm 12 watt resister to take a little brightness out.

F1nut

Good point Tracy, although I'd recommend he keep the poly's considering what's being used for power and that's why I didn't bring it up.

bigaltx24

What would you recommend for the high freq circuit?

kawizx9r

Sonicaps from soniccraft.com is what I've been recommended when I was thinking about redoing my SDA 1C's x-overs or Clarity caps (PX series).

jimbo1421

Be sure to check the actual crossovers before you order. I have read that they sometimes vary from the schematic.

Jim

bigaltx24

Thanks for the info. I'm going to take a look at the Clarity caps. Any opinions on Auricaps?

F1nut

I've no experience with Clarity caps, but on the word of someone that does and who's knowledge I respect, I believe they would be an excellent choice.

Schurkey

bigaltx24 wrote: »

I'm looking to redo the crossovers in my 1B's. I downloaded the schematic for these but it's very hard to read. This list is my best guess as to what I need to do two speakers.

2) 2.0 Ohm Resistor

6) 2.7 Ohm Resistor

4) 750pf Capacitor

2) 12uf Capacitor

4) 34uf Capacitor

2) 65uf Capacitor

2) 130uf Capacitor

2) 4.4uf Capacitor

...there are a few capacitors on the list that they don't offer... Likewise for the 34uf and 65uf, Solen offers either 33 or 36uf and 62 or 68uf. Are these values close enough to the original values? If they are should I use the higher or lower value?

I'm about to take this on for my 1Bs. I'm going to eliminate the polyswitch; and I've read that the 750pf caps aren't needed. I'll order caps in proper values to provide 4.4uf and also 5.8uf so I've got parts on hand to use either tweeter (stock or "TL"-style) although I don't foresee buying the upgraded tweeters anytime immediately.

1. Is there a preference for using one big/one little cap to make up the total value; or should I be looking for two similar value caps? For example, I can get a 33uf and a 1uf to make 34uf; or I can get a pair of 17uf caps and get 34uf. In other words, does it matter how the capacitance is divided? I'm leaning toward one big/one little for no particular reason.

2. What wattage rating do the resistors need to be? I don't see a spec on the schematic. I've seen folks recommend 12w; my supplier has them in 10w or 25w. Any preference for the 10w vs. the 25w? Any (useful) advantage or disadvantage in the larger wattage rating resistors other than a few cents in price and a larger physical size?

heiney9

Be sure to look at your crossovers and verify all the values are the same as the schematic. There have been times when values varied a little as sometimes tweaks were made on the fly and not added to an "official" schematic.

Sonicaps are a great choice. If you have a smaller budget I recommend Clarity Caps. I have been extremely happy with them and they perform well beyond their cost. I agree with F1 don't use Solens on the high pass.

A nice budget friendly recommendation

Clarity Caps on the high pass
Solens on the low pass
Mills resistors.

Good luck............you will completely transform your SDA's

H9

F1nut

1. Try to keep the split values as close as possible.
2. The stock resistors are 5 watts, so you can use either 5 or 12 watt Mills Wire-Wound resistors. Do not use the typical cement block resistors like the ones your supplier sells.

Face

F1nut wrote: »

1. Try to keep the split values as close as possible.

This is especially important for capacitors in series with a midrange or tweeter, unequal values can cause image blurring.

Schurkey

F1nut wrote: »

1. Try to keep the split values as close as possible.

Face wrote: »

This is especially important for capacitors in series with a midrange or tweeter, unequal values can cause image blurring.

The high-frequency boards are done; I will be ordering the bigger caps for the low frequency board next. (Already purchased, but have not installed the 33uf caps as a substitute for the 34uf caps specified.)

Any "special" reason Polk would have specified a 130uf and a 55uf capacitor rather than a single 185uf; or two 93uf? My first guess is: that was the cheapest way to achieve the specified 185uf using readily-available caps purchased in bulk from whatever their chosen source was, and not having an equal split wasn't critical for the intended usage--but maybe there's more to it than that. Aside from cost and availability in the mid-1980's, could there be any actual benefit to having the unequal split in this circuit?

More to the point, I have several options for the specified 185uf; depending on what I order I can get "close" using "in-stock" capacitors as a single 180uf, paired caps of 90uf; or of 91uf; or triple caps as a pair of 60s plus a 68.

I'm leaning toward the double-91s or double 90s depending on supplier, based on both price and the suggestion of symmetry.

It seems that being a few uf away from 185 exactly is no big deal. I've seen on other threads that anywhere from 180 to 190 is acceptable.

Schurkey

I measured the resistance of my take-out resistors; two of them are reasonably close to the rated value; the others all have considerably higher value than what is printed on them. Typically, the 2.7 ohm resistors are showing 3.2--3.4 ohms. One of the 2-ohm resistors is showing 2.4 ohms.

I now wish I'd measured the resistors that I put into the crossovers--but they're 1% or 2% rated tolerance; and I guess I'm willing to trust that. So, the resistors NOW in the crossovers should be the same or LOWER resistance that what was in there before. I also bypassed the polyswitches; which after de-soldering now measure .6 and .7 ohms.

The DC resistance of the speaker system before I started work on them, with the original polyswitches, capacitors and resistors was 3.3 ohms on the right speaker, and 3.4 ohms on the left. Polk spec is 4.3 ohms according to the "Quick Tests To Determine Proper Functioning of SDA Loudspeakers". Despite bypassing the polyswitches and installing resistors of lower actual resistance (closer to spec), my system resistance increased to 3.8 ohms on both speakers.

I guess that means the original capacitors were toast, too.

I'm getting anxious to replace the caps on the LF crossover board. I've always liked the bass these speakers produce; I've NEVER been happy with the treble--there was not enough of it, as the primary complaint. Things seem improved; but I'm not sure I'm satisfied...

bigaltx24

What brand of cap did you use?

Schurkey

bigaltx24 wrote: »

What brand of cap did you use?

All the caps on the HF board are Dayton 1% (PMPC-series) tolerance. The LF board will be a combo of Dayton 5% (DMPC-series 33uf units) and--probably--ERSE 3% (PulseX 91uf).

I recognize that these are not the "Favored Brands"; but what the heck. They're clearly tighter-tolerance and better-performing than what was in there.

bigaltx24

I finally took the time to pull the passive on mine to check that the values were the same as the schematic. I did have the 55uf that is parallel with the 130uf as 65uf, very hard to read the schematic. I talked to a friend who has done a bunch of crossovers, mostly in old Altecs and JBLs. He recommends the Jantzen CrossCaps. I think I'm going to take his advice and give them a try. They aren't that expensive, if I don't like how they sound I can try something else.

Face

I wouldn't use CrossCaps on the tweeters, pick something better.

F1nut

I wouldn't use them anywhere.

Face

F1nut wrote: »

I wouldn't use them anywhere.

Personally, I wouldn't either.

dcmeigs

Face wrote: »

I wouldn't use CrossCaps on the tweeters, pick something better.

+1

The better caps in those small values don't cost that much more and are definitely worth it. I've tried crosscaps in a set of monitor 10s and later replaced them with sonicaps to a noticable benefit. Since you have the work done, listen to them for a hundred hours and then swap out to a better cap. You'll learn for yourself the difference a good cap can make AND whether your polk brothers are full of shite.

Schurkey

I received the last of the caps to do the LF crossover. The crossovers are done and re-installed. Hot-glued the stupid nut to the inside of the big inductor; that way the LF crossover board can be popped onto the four little pegs BEFORE shoving it into the cabinet and tightening the screw.





I remember that inspiredsports had problems with the physical size of his new LF caps; they interfered with the passive radiator on his SRS-2s and had to be mounted off-board. No such problem on the 1B; at least when using a pair of 33uf caps instead of four 17uf caps as he did. I put everything on the circuit board--well, so much as that is possible: I had to lengthen the leads for the 91uf caps--and there's plenty of room for the passive. Photo below taken through one of the lower MW driver holes.



Question: Any harm in dressing the wires inside the cabinet as twisted pairs? They were all just randomly thrown into the cabinet before, looked like a rat nest when I first pulled the passive radiator. Twisted pairs tidy it up considerably.

Second question: What is the DC resistance of a MW6509 driver supposed to be?

I'm at the point where I don't trust my ohmmeter any more, so it's probably best to disregard the ohm readings specified in my post above. It's a well-regarded unit (Fluke 88 automotive multimeter) but it's old enough to vote; and getting a stable ohm reading has become an issue. I've gone to testing each device several times and looking for the most "popular" reading. Shameful. A replacement meter is in order. In the meantime:

All 8 of my cone drivers are clearly labeled as MW6509; they've got double date stamps on them from early 1987, and they say "Engineered for SDA SRS 2". I've checked the DC resistance several times; the drivers aren't all the same--the lowest one is 8.0, the highest is 9.5, with the others tending to be over 9 or under 8.5; and to make it even goofier, there were two 9+ ohm drivers and two 8.something drivers in each speaker cabinet. Coincidence, perhaps. I set 'em up with one lower-ohm and one higher-ohm driver in each vertical pair; figured it "evened-out" the load. I have NO idea how the difference in driver resistance is going to affect the AC impedance. None of 'em "sound bad" or have any binding/scraping when I gently push on the cone.

Oh, yeah. Somewhere along the line one of the SL2000 units decided it wanted to make 2/3-music, 1/3-crackling sounds. Moving it from it's position as left lower tweeter to left upper and right lower changes nothing; the other tweets sound fine, and this one is muted and static-y. ****. There is no budget for four new tweeters; so I'm thinking I'll add the capacitance to the HF board and buy a pair of 198s and see what happens.