Help! Newb here - rebuilding my first crossovers from Monitor 75T's

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  • xschop
    xschop Posts: 5,000
    edited January 2023
    I'd try a set of these if anything, since they're 4 ohm and easy to swap in....

    https://www.parts-express.com/Dayton-Audio-ND25FN-4-1-Silk-Dome-Neodymium-Tweeter-Element-4-Ohm-275-053?quantity=1
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  • txcoastal1
    txcoastal1 Posts: 13,270
    zqxb4eoh6s6u.png
    2-channel: Modwright KWI-200 Integrated, Dynaudio C1-II Signatures
    Desktop rig: LSi7, Polk 110sub, Dayens Ampino amp, W4S DAC/pre, Sonos, JRiver
    Gear on standby: Melody 101 tube pre, Unison Research Simply Italy Integrated
    Gone to new homes: (Matt Polk's)Threshold Stasis SA12e monoblocks, Pass XA30.5 amp, Usher MD2 speakers, Dynaudio C4 platinum speakers, Modwright LS100 (voltz), Simaudio 780D DAC

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  • xschop
    xschop Posts: 5,000
    I'm sure they'd sound much better than the soundbar Polk would suggest.
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  • machine
    machine Posts: 36
    xschop wrote: »
    I'd try a set of these if anything, since they're 4 ohm and easy to swap in....

    https://www.parts-express.com/Dayton-Audio-ND25FN-4-1-Silk-Dome-Neodymium-Tweeter-Element-4-Ohm-275-053?quantity=1

    Thanks for the suggestion, but I didnt see it until I already ordered a pair of Eminence SD28's. If the Daytons are a higher quality speaker, I can always return them. Only ordered the SD28's because I read that it was "A common replacement for the Polk 75T tweeters with slight faceplate modification."





    Plus, I remember someone saying Dayton was
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  • machine
    machine Posts: 36
    txcoastal1 wrote: »
    zqxb4eoh6s6u.png


    Uh oh, what did I do now?
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  • Gardenstater
    Gardenstater Posts: 4,457
    Meme(ing) is a fine art here.

    That Dayton is 90dB sensitivity, 3.7+ Ohm min. impedance and the Eminence is 92.5dB, 3.5 Ohm min. impedance. You haven't been able to find any data for the OEM tweeter?
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Onkyo A-8017 integrated
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
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  • machine
    machine Posts: 36
    Specs:
    Impedance: 4 ohms
    Re: 3.0 ohms
    Voice Coil Diameter: 1" (25.4mm)

    This is about all i can find
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  • xschop
    xschop Posts: 5,000
    Gardenstater wrote: »
    Meme(ing) is a fine art here.

    That Dayton is 90dB sensitivity, 3.7+ Ohm min. impedance and the Eminence is 92.5dB, 3.5 Ohm min. impedance. You haven't been able to find any data for the OEM tweeter?

    Soundbar United won't have any details on anything they've designed via planned obsolescence.

    Good find on the Emminence driver. Another member here suggested a Dayton driver for RtiA and possibly TSi models if the tweeter takes a dump. Looks like a quick epoxy install too.
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  • machine
    machine Posts: 36
    edited February 2023
    Just a little quick 3d printed bushing and the Eminence tweeter fits perfectly. Little superglue to leakproof and the tweeter snaps right in. I then go behind the tweeter and melt the bushing over the edge of the tweeter in a few spots to lock it in. dtu4xghnfpkc.jpeg
    gn3kx1ctqpq6.jpeg
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  • machine
    machine Posts: 36
    edited February 2023
    eydi57v8txx8.jpeg
    5vfyif1e1j90.jpeg
    tu3ux9w07ra9.jpeg
    And here’s the larger installed upgraded parts on a small factory board. Had to do some vertical integration and design and print a bracket for the gigantic 9.1uf cap and neighboring resistor.
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  • xschop
    xschop Posts: 5,000
    Looks good. Might want to move the resistor away from the larger Clarity cap.
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  • machine
    machine Posts: 36
    edited February 2023
    I’m assuming that’s because of heat? How hot do they get? They are about 3/16” to 1/4” inch apart.
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  • xschop
    xschop Posts: 5,000
    That's the consensus. I've pulled my x-overs from my 5Bs so many times that I can confirm they do get warm after long playback periods.
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  • Gardenstater
    Gardenstater Posts: 4,457
    Mundorf resistors? What wattage did you go with? Not trying to be contradictory and maybe it is a photographic illusion but they appear to be closer than 3/16" gap between them and the ClarityCap. It would just be wise at this point to give a little more space for airflow. Keeping the temp. down reduces thermal noise.
    George / NJ

    Polk 7B main speakers, std. mods+ (1979, orig owner)
    Martin Logan Dynamo sub w/6ft 14awg Power Cord
    Onkyo A-8017 integrated
    Logitech Squeezebox Touch Streamer w/EDO applet
    iFi nano iDSD DAC
    iPurifier3
    iDefender w/ iPower PS
    Custom Steve Wilson 1m UPOCC Interconnect
    iFi Mercury 0.5m OFHC continuous cast copper USB cable
    Custom Ribbon Speaker Cables, 5ft long, 4N Copper, 14awg, ultra low inductance
    Custom Vibration Isolation Speaker Stands and Sub Platform
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  • machine
    machine Posts: 36
    They are Dayton resistors. The other resistor is close to the inductor, about 3/8” above the board. Do the inductors get hot and/have issues? There’s just not a whole lot I can do with these small boards. Those eminence tweets do sound great though. I did try one with the Solen cap, and it did sound better than OEM, but it was pretty sharp. Glad I was shamed into buying the clarity’s. I usually use this system for mostly watching movies and music when playing the drums, so they aren’t pushed too hard. I have the AVR LPF cutoff bottom out at 60 Hz, and the powered sub goes up to 110, so there’s a slight overlap. I still can’t believe how good these sound. Also found out my AVR was getting hot and cutting off because of the bad crossovers, thought the reciever was dying.
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  • machine
    machine Posts: 36
    And a big thanks to EVERYONE that chimed in and helped me learn more about these. I might design my own crossovers and build some cabinets as a winter hobby once I brush up on 2nd order systems and Bode plots. Makes sense now!
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  • xschop
    xschop Posts: 5,000
    Will you rebuild/make new cabinets for those?

    Also, what is the major diameter of the tweeter faceplate?
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  • machine
    machine Posts: 36
    xschop wrote: »
    Will you rebuild/make new cabinets for those?

    Also, what is the major diameter of the tweeter faceplate?

    As in the OEM plastic piece with the flat cut into it? 04be13q3whj3.jpeg
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  • xschop
    xschop Posts: 5,000
    edited February 2023
    machine wrote: »
    xschop wrote: »
    Will you rebuild/make new cabinets for those?

    Also, what is the major diameter of the tweeter faceplate?

    As in the OEM plastic piece with the flat cut into it? 04be13q3whj3.jpeg

    Yep, I was thinking Polk had 2 versions of these plates, judging by pics. Here are my near-field modded TSi100s. I plan on making a set of billet plates that close the gap better between them and the MW trim rings...
    What does yours measure from c2c (green) on the screw mounts?

    6t79hne29q6n.jpg
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  • machine
    machine Posts: 36
    Using calipers and referring to the above colors, Blue= 4.405, Red=3.700, Green= 3.250
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  • machine
    machine Posts: 36
    edited February 2023
    What drivers are those? The OEM ones? Just wondering because I have a set of the TSi200’s, and they look different. If the tweeters are going, the Eminence ones sound great and are an easy install if you have a 3d printer.I can send you the file I created if needed.
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  • xschop
    xschop Posts: 5,000
    They are oem with the laminate removed and phase-plugged. From pics of your 75T's they are the same originally.

    Ill machine alloy faceplates for mine, but thank you for offer.
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  • plastic_avatar
    plastic_avatar Posts: 708
    You can remove the plastic layer??!

    Huh.

    Nice 3D print fixes by the OP. Well done.
    The Thrifty Setups in Mah House Big thrifty stereo in the basement w/ my custom SDA-1C (built with help from kind forum members) * Beautiful 1966 MCM GE console upgraded w/ Bluetooth, Dual turntable, and Paradigm speakers in family room * Swanky 1980 Realistic system and great TEAC eq with dancing colored lights in the living room * custom 5B on a system for my Dad * Ye Olde college stereo in the garage
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  • machine
    machine Posts: 36
    More pics for anyone that comes across this in the future tkujysaybaye.jpeg
    w0840iwptq6k.jpeg
    12jw6wq4l158.jpeg
    toqkg40degq3.jpeg
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  • plastic_avatar
    plastic_avatar Posts: 708
    That's a great idea. I wish' I'd thought of that when making my bracket recently.
    The Thrifty Setups in Mah House Big thrifty stereo in the basement w/ my custom SDA-1C (built with help from kind forum members) * Beautiful 1966 MCM GE console upgraded w/ Bluetooth, Dual turntable, and Paradigm speakers in family room * Swanky 1980 Realistic system and great TEAC eq with dancing colored lights in the living room * custom 5B on a system for my Dad * Ye Olde college stereo in the garage
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  • cowbot
    cowbot Posts: 4
    Any update or progress on this? I faced with repairing a couple of 75T's with terrible sounding tweeters.

    Thanks!
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  • xschop
    xschop Posts: 5,000
    edited April 2023
    I got around to fixing the deforming bezels.
    I used high density polyurethane and let dry.
    Almost doubled the weight and now the top edge sits flush with the cabinet...

    0vsdd9iz6b8l.jpg

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    m1sb79xlrtyw.jpg





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  • cowbot
    cowbot Posts: 4
    So just to confirm, the resistor marked "5WR75J" is a 7.5 ohm, not a 0.75 ohm? I thought the R indicates the location of the decimal point (and 5W = 5 watt, J= 5% tolerance)
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  • F1nut
    F1nut Posts: 50,494
    That is a 0.75 ohm resistor.
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  • xschop
    xschop Posts: 5,000
    machine wrote: »
    More pics for anyone that comes across this in the future tkujysaybaye.jpeg
    w0840iwptq6k.jpeg
    12jw6wq4l158.jpeg
    toqkg40degq3.jpeg

    Any updates on your Xover burn in?

    Curious how the upper midrange melds with the new tweets.
    Don't take experimental gene therapies from known eugenicists.
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