Zobel Network on old school bookshelf Infinity speakers

Discussion in 'Speakers' started by grevock, Aug 27, 2020.

  1. grevock

    grevock New

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    Ok, so I've been upgrading my work from home desk. The shitty BestBuy speakers needed to be put out to pasture. Was researching new speakers when it occurred to me I had some vintage Infinity RS 2000 speakers in my basement crawl space. Pulled them out and was heartbroken. The foam surrounds disintegrated. Did some recon, Simply Speakers sold some kits to replace the foam surrounds. High A ! That worked great but of course while I was doing recon came across a Zobel network DIY, which sounds like voodoo. The foam surround fixed the speakers but the sound was lacking bass so after investigating the Zobel network, I figured it was worth a shot. So I ordered a combo of resistors and capacitors from partsexpress (4 ohm/10v R, 22 mF C) and soldered them in series to the woofer terminals. Fired it up and voila....speechless. Deeper bass, great sound. Literally tears in my eyes. Kinda wonder why Infinity couldn't apply the zobel upon original design but maybe they saved the $5 improvement for the next level up. These speakers were late 80s vintage (purchased in 86 IIRC) for about $200 for the pair back then. If you've got something similar, I highly recommend looking into the Zobel network retrofit. I'm blown away.
     
    Last edited: Aug 27, 2020
  2. scblock

    scblock Friend

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  3. purr1n

    purr1n Desire for betterer is endless.

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    I have doubts. All a zobel on a woofer will do is make the low pass crossover slope more effective past the point where the woofer meets that Polycell tweeter. If zobel wasn't used, all Infinity would have to do is to increase the XO values. It depends on the driver too. If the woofer inductance is sufficiently low, a zobel wouldn't have to be used in the first place. Ultimately it comes down to cost and minimizing parts to achieve the desired result.

    I think it's a stretch to wonder about why Infinity didn't do this or that when we have no FR measurements post and pre, no impedance measurements of the woofer, and no schematic of the LP xover. For all all we know, the deeper and better bass could have been the new foam surrounds breaking in, lowering Fs and increasing Qts of the woofer by a bit more. Or it could have been that by the next time you took a good listen, you happened to put those speakers in a place with more boundary reinforcement.

    The Infinity RS bookshelves were superb value back in the day.
     
  4. grevock

    grevock New

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    That's a fair comment re the measurements. I was thinking of doing the pre and post measurements but frankly didn't know where to start although I now have the software. I had one done and hooked it up and there was a sonic difference between the 2 and I preferred the modded version. I'll try to learn how to do the measurements now and post it here, although that lacks the comparison.

    I agree they were great value, I was happy to have them in my dorm room way back then. That said, its big brother RS3000 has the Zobel on the woofer built into the crossover and the next gen RS2001 had a cap in parallel in the crossover to the woofer so I think there's something to the mod.

    http://www.infinity-classics.de/technik/manuals/RS_2000_technical_sheet.pdf
    http://www.infinity-classics.de/technik/manuals/RS_3000_technical_sheet.pdf
    http://www.infinity-classics.de/technik/manuals/RS_2001_technical_sheet.pdf
     
  5. purr1n

    purr1n Desire for betterer is endless.

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    Wow. Infinity was a good company and I liked how companies back then provided schematics.

    Surprised RS2000 didn't have zobel because sometimes two more cheap parts could reduce the need for an expensive higher value part such as an inductor.

    The Zobel may help the woofer sound better, play more within the frequencies it's designed. Depends on how well behaved the woofer is and how it rolls off. One thing to check to make sure there isn't a hole in the midrange response because the slope of the woofer is steeper, with the response falling well short of the original acoustic XO point with the tweeter. You may have to tweak the tweeter or woofer XO.
     
  6. zobel

    zobel New

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    That's cool that you have the XO diagrams for those three models. Your model, which didn't have a Zobel, might have been made to have more apparent bass by simply limiting the woofer's output somewhat into the mid-range. That Zobel presents a flat impedance to the coil. Eliminating the woofer's rise in impedance allows the coil to roll off electrically at six db / octave..which changed the frequency response to what you prefer....(less mids with the same lows)......
    and therefore, presenting a more apparent low end. The Zobel could have also smoothed out the woofer's response through the top of it's range. You would have seen this in a measured SPL / frequency chart, along with the overall impedance chart, had you been able to make them at the time.. Sounds good?...GREAT....no need to measure now. XOs are fun, aren't they? :)
     
    Last edited: Feb 28, 2022

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