Rp-280f impedance curve modification?

Discussion in 'The Klipsch Korner' started by LinderK, Jan 29, 2019.

  1. LinderK

    LinderK New Member

    Messages:
    10
    Hello,
    I recently purchased a pair of these, having been impressed by the dynamics Klipsch offer. As I use a Scott 299b as my daily driver, I also was excited about having more efficient speakers. In listening at high levels, particularly bass heavy material, I was noting clipping. This lead me to look at the minimum impedance, which apparently is low enough (3.1ohms) that my amp doesn't like it too much. My question is: Is there an electrical (crossover) or physical (port modification or internal volume) adjustment that may be effective at addressing this?

    Thanks,
    John
     

     

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  2. Gazdatronik

    Gazdatronik Super Member

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    No, not really. 3.1 minimum is pretty reasonable for what they are. Are you using the 4 ohm taps?
     
    Last edited: Jan 29, 2019
  3. LinderK

    LinderK New Member

    Messages:
    10
    I'm using the 8ohm taps, as that was the listed average impedance. I suppose I could give it a shot to see if the clipping improves.
     
  4. Gazdatronik

    Gazdatronik Super Member

    Messages:
    1,502
    Location:
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    I would start there. If the problem persists, there could be something up with the amp internally, an aged capacitor or a circuit out of spec that might be achieving a resonance/feedback thing.

    I had a newer Scott receiver, coincidentally, that would work fine, but one day , I pulled off a speaker grille, and the woofer was pulsating at a very low frequency while nothing was playing.
     
  5. LinderK

    LinderK New Member

    Messages:
    10
    So I tried switching the impedance tap, still having the issue. Just to be sure I tried 2 other SS maps I have, same issue. Keep in mind, this is occurring at the highest peaks, at the highest volumes. It's somewhat ironic (for me), as it was my speakers that would give out previously...now it's my amp! As a background also, the Scott has been completely refurbished, along with Dave G.'s excellent modifications, and is actually quite robust. Maybe I should've held on to my Coda 11; that thing could bring just about anything to its knees!
    Thanks for chiming in!
    John
     
  6. Gazdatronik

    Gazdatronik Super Member

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    1,502
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    Those speakers sound fine with 40w, I can attest. Have your verified your source is ok?
     

     

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  7. LinderK

    LinderK New Member

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    Yes, the source (s) are ok. I checked the Scott, and 1 of the SS amps on my other speakers(Totem Staffs), and no issues. Interestingly, I've since read another report of the same issue I'm experiencing. I agree they sound great with lower powered amps, but again, this at the highest volumes, on the most demanding material.
    I still wonder if there may be away to soften the dip in impedance. It seems there could be something that could be done?
     
  8. Gazdatronik

    Gazdatronik Super Member

    Messages:
    1,502
    Location:
    Chattanooga TN via Chicago
    Might need a different amp or different speakers at this point, the Klipsches are designed to feel at home with a modern high-current amplifier. I would have thought for their efficiency rating that 21w would be enough for them, but it appears you have found the limit.

    The impedance dips are pretty low, maybe not dramatic today, but probably ridiculous in 1960.
    [​IMG]
    Whether the crossover is underdesigned or the cabinet design is to blame will be up to the eggheads to work out, but by that point, you would be making a different speaker. There is a chance that the drivers themselves have a low impedance too. Too bad you are not in a position to Bi-amp them.
     
  9. twiiii

    twiiii Addicted Member

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    The Heritage line is much nicer to older tube stuff. I'd want a set of Cornwall I with the EV tweeter myself. Smoother high end than the 280's, The older Cornwalls have a smoother impedance curve which would help your Scotts and are a little more efficient as they are a true 8 ohm speaker not 4 ohms like the 280. I owned Scott and Fisher and if I were you I'd be looking for a restored 275 if you want to keep the same speakers. top notch 240 would give you 55 watts per channel, Depending on which version of 275 anywhere from 90 to 100 watts per channel if up to snuff. Sometimes you can find a nice MK4 for a price. A 275 would give another 6 db.
     
  10. LinderK

    LinderK New Member

    Messages:
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    It's a lesson learned, efficiency doesn't necessarily equate to easy to drive. All in all, I'm not displeased with the speakers, but I think on Monday, I'll be inquiring about returning them. I do have some recently acquired Bogen MO-100A monoblocks that would surely handle them, but they are in need of a full restoration, which will take some of course, putting me outside of a return window.
    Thanks so much for chiming in to offer your help!

    John
     
  11. LinderK

    LinderK New Member

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    So in contemplating my dilemma, I had a further thought. It appears, as twiii mentioned, in order to get the output I'm trying to achieve, I would need to get a more powerful amp. My thought is, what if I use the rumble filter to roll off the low frequencies sooner and sharper, then connect an outboard sub and adjust the gain to it accordingly? That would certainly alleviate the Scott from dealing with the frequencies that have the lowest impedance and require the most current. It seems in a sense, similar to bi-amping as Gadzotronik suggested.
    Any thoughts as to why this couldn't work?
     

     

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  12. Gazdatronik

    Gazdatronik Super Member

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    Maybe, except using a rumble filter, the sub may not get anything, since a 299b has no pre-outs. Well, it has one, the center channel output, but you could do it only if the rumble filter does not affect this output.

    in more simple terms, the sub out has to be upstream of the rumble filter, maybe possible with a tap before that circuit which would mean a little soldering.

    also it may not solve the issue entirely since the impedance dips low in the mid frequencies too, but its worth a try.
     
  13. LinderK

    LinderK New Member

    Messages:
    10
    My thought was to run the sub off of the speaker outs via a 1k resistor. The center channel could work but would need rewiring to send a summed output.This might be something to do later if this works ok. I'm hoping that since the needed current in midrange on up to achieve a high spl is less, it will be easier on the amp, despite the lower impedance . Also, I mistyped initially- it's a "C" model 299, not "B". Sorry for that, I didn't catch that until now. Additionally, it looks like (hopefully ), the rumble filter would still give enough of a frequency profile for the sub to work with if the gain has enough range.

    I have a cheapie Yamaha sub that I can try this with, so I should at least be able to see how well it works (or doesn't) without having to buy something new before I call the shop on Monday about returning them.
    Will give an update tomorrow...fingers crossed!
     
  14. twiiii

    twiiii Addicted Member

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    You think you have issues now adding sub just really nesses up the entire situation. If you really want an all tube sound and can't afford Mac, Fisher made a model 400 pre-amp that was nice and a pair of Dyna MK III's should keep you running for a long time. I liked the MK 111. They will tone down Klispch's modern drivers which can be a little ragged on the top octaves or so. I like speakers that respond smoothly to 25 hz and can move a lot of air. Unfortunately Klipsch comes up a little shy as does Altec, Frazier and EV. JBL made some 4300 and 4400 series speakers that can do the job, but they will take up some floor space. You can always stand the 4435 on end and rotate the horn 90 degrees to save floor space and raise the tweeter to ear height. Two 15's move some air.
     

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