3.1TL: That unusual 2ohm Resistor

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lawdogg
lawdogg Posts: 450
edited February 2023 in Vintage Speakers
Today's the day I finally took my 3.1TLs down to epoxy the drivers and install new rebuilt crossovers (aquired those of @gwg_97 w/ ClarityCap ESA & Mills throughout).

In examining my original crossovers and comparing to the schematic, I spotted an extra 2ohm resistor.

58ale44jc7qu.jpg

I did a search and found this resistor has only come up twice before ... neither time offering much in the way of useful info.

https://forum.polkaudio.com/discussion/124267/should-i-remove-resistor-3-1tl
https://forum.polkaudio.com/discussion/106099/3-1tls-before-and-after

So, I examined the PCB and noticed that the SDA 18mH inductor lead is routed differently in my crossovers vs. my replacements (which match the official schematic):

5md9no1ybyzk.jpg

The difference is this: the 2ohm resistor runs in series with the big SDA-effect inductor. I added it (in MS Paint lol) to the schematic and will copy to modified schematic below:

i8wl3fzsmfou.jpg

So I guess the effect would be to ... lower the current through the SDA inductor? Would this soften the SDA effect? Some other effect compared to not having it?

There's never been real discussion about this extra resistor, in terms of analyzing its effect on the speakers (and specifically from what we can tell, the SDA effect in particular) ... so I'd be curious if anyone has any additional thoughts.
<3 my 3.1TLs

I will fix your shifted magnets for free. :)
Post edited by lawdogg on
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  • Gardenstater
    Gardenstater Posts: 4,182
    edited February 2023
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    I think that's the sub bass inductor which is allowing the Dimensional Driver to play stereo signal for frequencies below a certain Hz in the bass area. Maybe your 3.1tl speakers had a lower DCR inductor as in maybe it was a heavier gauge wire than 22awg and they needed to add 2 Ohms to make the adjustment? I think if it was 2.5 Ohms it would say 2R5 and the 5 is for 5% tolerance.

    Edit - I just saw where it says "22awg was 20awg", so that could explain why they didn't need the 2 Ohm resistor anymore. Are the dates on your 3.1tl's earlier than the date on that schematic? Hmm but then why doesn't it say Deleted 2 Ohm resistor?
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
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  • xschop
    xschop Posts: 4,684
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    It probably keeps the single 6511 from bottoming out.
    Don't take experimental gene therapies from known eugenicists.
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  • lawdogg
    lawdogg Posts: 450
    edited February 2023
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    D'oh, 2ohm not 2.5ohm. Edited to fix it.

    I wonder then ... should I keep it (transfer it to the replacement crossovers)?

    Edit: Gah! In searching for threads about a 2ohm resistor and not a 2.5ohm resistor, came up with many more ...

    https://forum.polkaudio.com/discussion/186129/3-1tl-crossover-question
    https://forum.polkaudio.com/discussion/166927/sda-srs-3-1tl-driver-phase

    And ... LOL one that I started myself! I totally forgot ... hahah ... wow. I was better at reading resistor values in 2016 apparently.

    https://forum.polkaudio.com/discussion/173295/help-identifying-component-values-on-early-3-1tl-crossover-be0031-a

    ... but anyway looks like I found the answer, just like @Gardenstater said:

    https://forum.polkaudio.com/discussion/188677/srs-3-1tl-crossover-pics-and-questions
    Just be aware these came in two flavors. Early versions, "A", used a 20 gauge - 18mH dimensional inductor and required an additional 2 ohm 25 watt cast resistor to make up the impedance. Later crossovers, Version "B", like the one pictured, used a 22 gauge - 18mH inductor and didn't utilize the resistor.

    I guess that's that: needing the resistor depends on the inductor used.
    <3 my 3.1TLs

    I will fix your shifted magnets for free. :)
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  • xschop
    xschop Posts: 4,684
    edited February 2023
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    I would keep it. Is the extra resistor in a later/newer set than the non-resistor one?

    Nevermind, looks like you've found the answer.
    Don't take experimental gene therapies from known eugenicists.
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  • lawdogg
    lawdogg Posts: 450
    edited February 2023
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    Edit: Yeah, thanks!

    Looks like my 3.1TLs were assembled May 1990. The replacement crossovers I got came with speaker terminals having Nov 1990 date stickers on them. So the change from 20ga w/ 2ohm resistor to 22ga w/o a resistor must have happened in that time, when that drawing rev took effect...
    <3 my 3.1TLs

    I will fix your shifted magnets for free. :)
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  • VR3
    VR3 Posts: 28,020
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    Polk kept the resistance high to make the speaker easier to drive.

    The c coil I use in my mods has virtually no resistance but provides far superior bass response but ultimately makes the speaker a more difficult load .

    For the majority of modern day amplifiers this is ok.
    - Not Tom ::::::: Any system can play Diana Krall. Only the best can play Limp Bizkit.
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  • xschop
    xschop Posts: 4,684
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    VR3 wrote: »
    Polk kept the resistance high to make the speaker easier to drive.

    The c coil I use in my mods has virtually no resistance but provides far superior bass response but ultimately makes the speaker a more difficult load .

    For the majority of modern day amplifiers this is ok.

    If you added a C-coil in place of factory sub-bass coil in the 3.1, wouldn't that translate to alot more excursion on the already taxed 6511 driver?

    It is the only model SDA with a single SDA driver to 4 Stereo drivers.
    Don't take experimental gene therapies from known eugenicists.
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  • VR3
    VR3 Posts: 28,020
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    Never had any issues, Polk had advocated for low dcr sda inductors for a long time
    - Not Tom ::::::: Any system can play Diana Krall. Only the best can play Limp Bizkit.
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