Back in the McGame! MC2505 and C26

Discussion in 'McIntosh Audio' started by AdamAnt316, Jan 20, 2017.

  1. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Update: I built the dual 2.5kHz high-pass filter using a pair of 330pF ceramic disks. However, before I could use it, I came to the realization that I'd designed it for the wrong crossover frequency. As I've mentioned before, the Paradigm Studio Monitors are tri-ampable, and the crossover has two frequencies, 275Hz and 2.5kHz. Shortly after building the filter, I realized that I should've designed it for the 275Hz frequency instead! :oops:

    I re-ran my calculations, and the proper capacitor for a 275Hz filter would be 1,157pF. Returning to the local electronics store, the closest I could find was 1500pF. Running this through the formula, this capacitor value would give me a filter for 212Hz, which should be a decent safety floor. What do you think?

    My next question is, how would I test that this filter does what it's supposed to? The closest I have to a spectrum analyzer is the one in my BSR EQ-3000 equalizer. Would this be suitable? If so, where should I connect it? Finally, how would I use it to test the filter? I know that the EQ-3000 has a built-in white noise generator, but I don't know how it'd come into play if, say, I have the EQ wired between the filter and the Mark IIIs. Thanks in advance!
    -Adam
     
  2. damacman

    damacman Blown and Injected Subscriber

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    Given how you're going about this, I don't see an issue with the difference in crossover points before and after the amp - theoretically. Listening results will be difficult to predict ...

    You won't be able to analyze the performance of such a filter at the line level with your BSR EQ as its dependent on the input impedance of the device its plugged into. If your BSR EQ has a mic then you could use a CD with pink noise and put the mic in front of the speaker to get a rough idea of its effectiveness by viewing the output on the spectrum analyzer.
     
    Last edited: Feb 8, 2017
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  3. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Thanks for the reply. I'm mainly trying to make sure that the Mark III doesn't have to amplify the frequencies which won't be going into the speakers it's connected to, and at the same time prevent any holes in the frequency spectrum if possible. I was hoping to just be able to run full-range signals from the C26 straight into the amps, but as others have indicated, that probably wouldn't be the best idea for the Mark IIIs.

    That's a good point about the EQ. I was hoping to just put the BSR between the filter and amp to see what was getting through, but it'd probably throw off the impedance. I do have a mic to go with the EQ-3000, but have yet to get the spectrum display to work as expected when going from the instructions regarding the use of the microphone. I have another audio spectrum analyzer I could use, but it's a bit archaic, so I dunno if it'll work well for the purpose...
    -Adam
     
  4. TSmith8605

    TSmith8605 Senile Member Subscriber

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    No quite. You do not want to make sure that the Mark III doesn't have to amplify the frequencies which won't be going into the speakers it's connected to. What your are trying to do is reduce the amount of bass energy going to the MK IIIs below the (now 275 Hz) cut off and at the same time prevent any holes in the frequency spectrum if possible.

    The lower in frequency you go, the more energy is required and all you want to do is take a big chuck out of as much that as that as you can. You do not want to match the 275 Hz crossover because you will, in that case, change the cross over slope to be steeper than what was designed by the speaker's design engineer. But you don't need to.

    Don't over think it. We started out saying it would work with just direct connection but would not be optimal. The more energy you can knock out, the better, but you don't have to knock out everything below 275 Hz.

    I would set the cap for around 100 to 150 Hz. That should be low enough to not affect the speaker's existing crossover performance and still take a good chuck out of the bass energy going to the MK III.

    I assume the speaker allows you to bi-amp with the MK III running the mid and tweeter and the Mc running the woofers (as opposed to tri-amp only)
     
  5. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Again, thanks. I get at least some of the gist of what's going on. To me, the biggest variable is exactly what the input impedance of the Mark III is. Is there a way to measure it? Should it at least be close to the 500KΩ spec seen in the Dynaco manuals?

    I'm trying not to over-think things. I'll play around with various connection methods, and see what seems to work best. In all likelihood, I'll probably just stick with using the MC2505 by itself, but it'd be nice to see what results I get when I pair it with my tube amps, at least if I can bring the MC2505's gain in-line with that of the Mark IIIs.

    As I stated in my earlier post, the currently present 1500pF capacitors (both vintage Aerovox mica stamps, for what it's worth) should give me a cut-off of approx. 212Hz. If it seems too steep, I can parallel in the 330pF ceramic disks I bought earlier, and that would theoretically lower the drop-off point to 173Hz. If worse comes to worse, I might pick up some 'padder caps' if it seems like some fine-tuning might be necessary.

    To the best of my knowledge, these speakers should allow for bi-amping. There are three sets of terminals, which are currently bridged together with straps. I'm pretty sure it'll work if I leave the mids and tweeters strapped together, as it didn't seem to cause a problem the previous time I tried bi-amping using the aforementioned Mark III/ST-35 combo.
    -Adam
     
  6. TSmith8605

    TSmith8605 Senile Member Subscriber

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    The 173-Hz point would be better than 212 Hz. The schematic shows a 470-kOhm resistor across the input and specs say 500k so I'd just go with that.
     
  7. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Understood, thanks. Once I get around to trying this out, I'll have to experiment with jumpering the cap in to see if it makes an audible difference. If it does, I'll try paralleling some more caps across the existing 1500pF caps until I no longer notice it.

    In other news, I finally got around to re-attaching the bezel around the buttons on the front of the C26. Started out by cleaning the remnants of the old crud from around the button panel on the front panel using 91% rubbing alcohol and cotton swabs, then started the same process on the back of the bezel. Used some coins to gently scrape off as much of the old adhesive as possible, which took several rounds of swabbing and scraping, after which I let it dry.

    Once that was finished, I applied double-sided mounting tape to the back of the bezel, and carefully trimmed the excess tape off using an X-acto knife. Once I'd trimmed away as much of the tape as I could, I test-fit the bezel to the front panel to make sure the buttons weren't going to get jammed, peeled off the tape's backing, and very carefully put the bezel into place on the glass panel. After applying pressure to the bezel, it seems to be stuck onto the panel very nicely.
    -Adam
     
  8. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Minor update:

    I have yet to try the bi-amping experiment, since I've been fairly content listening to the MC2505 as-is for the time being. :music: The volume control on the C26 may have some imbalance issues, so I'll have to look into fixing that before going too crazy. Anyway, here is a picture of the C26 with the bezel re-applied, as mentioned in my earlier post:
    [​IMG]
    Anyway, I've since picked up an MR73 to go with the MC2505 and C26! :banana: The glass has some bubbling in the paint, and the endcaps and knobs were incorrect when I got it, but it now has (almost) correct knobs on it thanks to our esteemed Mike G. The AM dial is a bit off-alignment, but it seems to work fine otherwise. Here's a picture of it:
    [​IMG]
     
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