Polk SDA-1A Crossover Rebuild Question

Joshwest90

Hey all!

I’ve looked around the forums, called Polk ect. I have a question. I know many of you recommend removing the polyswitch. I have 350 WPC @4ohm, Carver TFM35x power these 1A’s. I’m not worried about clipping so I was going to remove this. My question follows:
As far as replacement components I would like recommendations on replacement for each component.
Do these values look correct? Some of the components on the crossovers I cannot see the writing so I’m going off the schematics.

On the Stereo Module:
(2)- 750 microfarad 1000v 50% Silver Mica Capacitor
(1)- 0.5 microfarad Mylar Capacitor (not sure of voltage)
(1)- 3.5 ohm 5 watt Resistor
(1)- 12 microfarad 100v Mylar Capacitor
(1)- 27 microfarad 50v Electrolytic Capacitor

On the Dimensional Module:
(1)- 3.5 ohm 5 watt resistor
(1)- 750 microfarad 1000v 50% Silver Mica Capacitor
(1)- 12 microfarad 250v Mylar Capacitor
(1)- 2.7 ohm 5 watt resistor
(1)- 27 microfarad 50v Electrolytic Capacitor


If someone has the replacement parts recommended or values ect I would appreciate it.
These are the components I’m thinking about Mills MRA Resitors
Clarity Px Metallized Paper Capacitor


This is my first cross over rebuild so any suggestions would be appreciated

pitdogg2

https://us.v-cdn.net/5021930/uploads/attachments/1/8/5/1/1/72432.pdf

get rid of the polyswitch and replace with a .5 ohm 12 watt vishay-mills resistor.
the silver mica's go away you no longer need them with better caps.
cap voltages should all be 200v as I personally have not seen any modern poly caps below 200 volt. anything bigger will be a major pain to get all on the boards.

all resistors should be bumped up to 10 or 12 watt cheap insurance.

and yes that amp will still clip ...they all will if pushed

Joshwest90

@pitdogg2 ok perfect! Thank you!
So the Resistors go with same nominal ohm rating, but higher wattage. Using a mills wire wound resistor will be sufficient? Polyswitch replace with .5 ohm Vishal-mills 12 watt resistor, silver mica caps can be removed completely, Mylar and electrolytic caps can be replaced with 200v clarity metallized paper capacitors?

Are all resistors Mills MRA wire wound sufficient
Are all caps Clarity Px Metallized Paper caps sufficient?

Joshwest90

Joshwest90

For the 0.5 mfd Capacitor they are calling for if you use this Capacitor will that be an issue? I cannot seem to find a 0.5mfd at 200-250v

pitdogg2

Joshwest90 wrote: »

For the 0.5 mfd Capacitor they are calling for if you use this Capacitor will that be an issue? I cannot seem to find a 0.5mfd at 200-250v

will be fine


yes keep ohm value the same but increase wattage to 10 or 12 watt.

Joshwest90

@pitdogg2 I cannot fins any Mundorf MoX resistors in 3.5 ohm. Any help?

pitdogg2

http://www.soniccraft.com/index.php/vishay-mills-12-watt-c-29_419_421

or if you are wanting to stay with mundorf maybe jump the Polyswitch with wire and use a 4ohm since 3.5ohm and .5 ohm seem to be in the same circuit.

just a thought maybe someone else will have more info.

Schurkey

When it was me and my 1Bs, I just jumpered the polyswitch location on the circuit board, and did NOT install an equivalent resistance. The treble was still not loud enough compared to the low-frequency drivers.

If you measure the resistance of the polyswitch out of the circuit, I bet you'd find that the ones in good condition (not tripped often) would measure substantially less than 1/2 ohm; while those that were tripped fairly often might register higher due to burned contacts. Of course, you'd need a properly-zeroed, accurate, sensitive, and repeatable ohmmeter to measure tenths of an ohm reliably. I used an "original" Fluke 88 (not the 88V) which has a low-ohms option by pushing a certain button as the unit is turned on. The 88V may also have this option.

Joshwest90

@Schurkey @F1nut @pitdogg2

What is the ohm rating of the MW 6501 and the SL2000 tweeter do you know off the top of your heads?

Schurkey

Joshwest90 wrote: »

@Schurkey @F1nut @pitdogg2

What is the ohm rating of the MW 6501 and the SL2000 tweeter do you know off the top of your heads?

God Bless the "sticky" posts at the top of the list.

dorokusai wrote: »

These are the only drivers, aside from the RD tweeters, being manufactured to support vintage products, such as, Monitor, RTA and SDA. You should print this as a reference and keep it handy for the values it contains. There are no grill parts, cables, crossovers, plates or anything else available for the older loudspeakers, unless you know the Polk G O D.

1 Driver Type Q BL Compliance DC Resistance Fs

3 MW 6501 1.47 5.96 N 3.25 E - 3N/M 7.75 Ohms 31 Hertz
4 MW 6502 1.97 5.16 N 1.47 E - 3N/M 3.52 Ohms 44 Hertz
5 MW 6503 1.82 5.24 N 3.55 E - 3N/M 6.54 Ohms 29 Hertz
6 MW 6509 2.1 4.57 N 3.54 E - 3N/M 8.97 Ohms 30 Hertz
7 MW 6510 1.39 6.32 N 3.41 E - 3N/M 6.57 Ohms 31 Hertz
8 MW 6511 1.405 4.34 N 3.496 E - 3N/M 3.13 Ohms 29 Hertz
9 MW 6512 1.905 4.99 N 1.588 E - 3N/M 3.466 Ohms 40 Hertz

Joshwest90

You sir are a gentleman and a scholar thank you!

Schurkey wrote: »

Joshwest90 wrote: »

@Schurkey @F1nut @pitdogg2

What is the ohm rating of the MW 6501 and the SL2000 tweeter do you know off the top of your heads?

God Bless the "sticky" posts at the top of the list.

dorokusai wrote: »

These are the only drivers, aside from the RD tweeters, being manufactured to support vintage products, such as, Monitor, RTA and SDA. You should print this as a reference and keep it handy for the values it contains. There are no grill parts, cables, crossovers, plates or anything else available for the older loudspeakers, unless you know the Polk G O D.

1 Driver Type Q BL Compliance DC Resistance Fs

3 MW 6501 1.47 5.96 N 3.25 E - 3N/M 7.75 Ohms 31 Hertz
4 MW 6502 1.97 5.16 N 1.47 E - 3N/M 3.52 Ohms 44 Hertz
5 MW 6503 1.82 5.24 N 3.55 E - 3N/M 6.54 Ohms 29 Hertz
6 MW 6509 2.1 4.57 N 3.54 E - 3N/M 8.97 Ohms 30 Hertz
7 MW 6510 1.39 6.32 N 3.41 E - 3N/M 6.57 Ohms 31 Hertz
8 MW 6511 1.405 4.34 N 3.496 E - 3N/M 3.13 Ohms 29 Hertz
9 MW 6512 1.905 4.99 N 1.588 E - 3N/M 3.466 Ohms 40 Hertz

pitdogg2

We don't know squat we pull it out of our britches

6501's are super rare and very hard to get if you see some on the auction site better get to asking questions and buying. Or just buy then ask questions.

The mw6600 is also suitable for your speakers and also difficult to obtain.

Joshwest90

Well all of my 6501 check out at 7.8 ohm and the SL2000 were at 7.2 ohm. So this speaker rebuild is going smoothly then! Haha

pitdogg2 wrote: »

We don't know squat we pull it out of our britches

6501's are super rare and very hard to get if you see some on the auction site better get to asking questions and buying. Or just buy then ask questions.

The mw6600 is also suitable for your speakers and also difficult to obtain.

Joshwest90

westmassguy

Josh, consider disconnecting the dimensional tweeters. They were eliminated starting with the 3rd generation SDAs.

Joshwest90

westmassguy wrote: »

Josh, consider disconnecting the dimensional tweeters. They were eliminated starting with the 3rd generation SDAs.

I’ll have to search what the dimensional tweeter is and why it’s a problem. Any info would be appreciated.
On a side note
@pitdogg2, @Schurkey in rebuilding the crossovers a 2.7 ohm resistor is needed. I am struggling to find the Mills MRA 2.7 ohm resistor. Parts Express has a 2.5 and a 3.0, but the only place I’ve found it was parts connexion. I’d rather buy Everything from the same plus but parts connexion is back ordered on the 3.5 ohm. So I’m left with ordering from parts express. Should I just buy the 2.7 ohm from parts connexion or grab a 2.5 ohm from parts express?

westmassguy

The dimensional tweeters are the outer tweeters. If you're facing the left speaker, it's the left tweeter, right speaker, right tweeter. Polk found that the bulk of the interaural crosstalk canceling signal was located below 2KHz. The higher frequencies can cause odd, phasy artifacts.

Schurkey

Joshwest90 wrote: »

You sir are a gentleman and a scholar thank you!

I am not. Anyone who knows me knows that you're wrong on both counts.

I do, however, stand on the shoulders of giants. In this case, Dorokusai, and He Who May Someday Become More Committed To Audio Than He Is Now (DarqueKnight) whom I entirely failed to properly credit in the original post.




The 2.7ohm resistor is a shunt to ground via an inductor, reducing the signal to the tweeter. There's two per speaker cabinet, one for each tweeter. (unless you're disabling the SDA tweeter.) I don't see that part listed on the "Stereo module" section of your parts list, post #1. I'm basing this on the published SDA 1A schematic.
1. The proper 2.7 ohm resistor is likely the best choice.
2. Using a resistor of less "ohms" will slightly reduce the impedance of the speaker, and make the tweeter less loud as more energy is siphoned-off, lost as heat.
3. Using a resistor of higher "ohms" would force more power to the tweeter, making it a hint louder. Again, changing the resistor value will have some effect on speaker impedance, in this case it would increase the impedance, in the frequency band covered by the tweeter..
4. If this was me, and I absolutely couldn't source 2.7 ohm resistors, I'd go with 3 ohm rather than 2.5 ohm in order to add a hint of volume to the tweeter. I've wanted more treble from my 1Bs since I bought them in '87 or so; but I don't know if the 1A is similarly treble-shy.

Joshwest90

Ok, I went ahead and ordered all of the same values. Thank you again!