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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    Hello, everyone. Many years ago, I acquired a MC2120 power amp for a very good price. At the time, I was running tubes in my system, and knew little about high-end solid-state gear (or McIntosh in general). My quest for knowledge led me to join AK where I found the McIntosh section, made my very first post in the gear thread, and was somewhat intimidated by some of the audio systems owned by folks around here. A few months later, I acquired a Pioneer SPEC system, and after the MC2120 developed an issue with the right channel, I ended up selling it off in one of my not-so-smart moments. :oops:

    Fast-forward nearly 10(!) years later. I'd moved on from the SPEC gear (which I still have) to Dynaco tubes by way of a big scroe. Two weeks ago, I stopped into a local electronic component store which occasionally gets some vintage equipment in, and what should I spot sitting near the back of the store but a MC2505 and C26! :eek: I asked the guy who runs the store about them, and was told they were part of an estate cleanout. After some discussion and research, he gave me a figure of $800 for the pair. I told him I'd think about it.

    Two days later, I returned to the store (just ahead of a snowstorm!) to do a more proper listening test. I disconnected the warbly cassette deck which the owner had been using as a test source, and connected my 1G iPod. Cued up Donald Fagen - The Nightfly, and started listening. I liked what I heard, handed him the money, wrapped the gear in trash bags, hauled them to my car, and started the slow drive home amidst the snowflakes which had coated the roads since the test began. Fortunately, I made it without incident.

    After the snow died down, I unloaded the gear from my car, set them up in my main stereo, hooked them to my Paradigm Studio Monitors, and again was impressed. :music: The wooden cabinets have some cosmetic issues, and the C26's front panel has the usual bubbling around the headphone jack (plus a missing bezel around the buttons), but they're in pretty good shape overall. Apart from some minor issues, they seem to work perfectly. Anyway, here are some pictures of the gear, plus one of my revamped main system:
    [​IMG]
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    Last edited: Jan 20, 2017
  2. butch4695

    butch4695 AK Subscriber Subscriber

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    Very nice. Good deal
     
  3. Mike Gibson

    Mike Gibson Modulator Super Mod Subscriber

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    Welcome back Adam.
     
  4. nolasally

    nolasally AK Subscriber Subscriber

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    Nice grab. You paid about .50 on the dollar for that gear:thumbsup:
     
  5. motorstereo

    motorstereo the wonder of it all Subscriber

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    Nice pickup Adam:thumbsup:. Another plus is the slant leg cabinets. I'd love to find something of that caliber around here especially for what you paid. Now we need to know.......how does it compare to your Spec gear??
     
  6. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Thanks for the replies! As I often say, you never know what you're going to find where, and for how much! :D Thus far, the MC2505 and C26 sound quite good, though there are some minor issues (more on that below). I'm toying with doing some bi-amping; I'd likely use the MC2505 for the lows, and my Dynaco Mark III tube monoblocks for the mids and highs. I'm hoping that I won't have to mess around with crossovers since the amps should be similar power-wise, but I'm not sure if there might be any x-factors. What say ye?

    I sometimes get a thump in both channels when I turn the C26 off, with the MC2505 plugged into one of its switched outlets. Any ideas what might cause this? I'm pretty sure turning the MC2505 off using its power switch doesn't cause much of a thump, though I haven't done any thorough testing as of yet. Roger Russell's page mentions something about reducing turn-off thump as part of the C26's design, so I'm guessing it's not typical.

    As for how it compares to the SPEC gear, that's a good question. I have yet to do any sort of comparison, but I wouldn't be surprised if the Mc gear holds its own even though the MC2505 puts out only a third of the SPEC-4's output power. I have yet to give the MC2505 much of an excursion volume-wise, though it seems to have plenty of 'pep'. I'll have to get a speaker switcher, and see what results. :idea:
    -Adam
     
  7. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Some more updates:

    The power-off thump definitely seems to be linked to the C26. Turning off the MC2505 while the C26 is still powered up does occasionally result in a mild thump, but not nearly as loud as when I turn everything off using the switch on the C26's volume control. The position of the C26's input selector knob doesn't seem to have a definite effect on the thump, nor does turning off the tuner before shutting down everything else. Any ideas?

    Anyway, earlier this evening, I stopped by the electronics store I'd bought the MC2505 and C26 from. When I walked in the door, the store owner handed me a small plastic bag. Inside was the little metal bezel which goes around the C26's buttons! :) Apparently, the guy who'd helped the owner out with the estate cleanout had found it in his car. I'm damn glad about that too, since it's probably not an easy part to find (McIntosh didn't even bother to list a part number for it in the C26 service manual). Does anyone have a good idea about what sort of adhesive I should use to reattach it to the faceplate? It appears that they originally used some sort of double-stick tape, or perhaps more of a peel-and-stick arrangement.

    Anyway, I had an odd experience during one of my many tests of this equipment. I noticed a volume imbalance between the left and right channels, as indicated by the peak LEDs on my ever-present Realistic APM-200 power meters (is there such thing as too many meters? :biggrin:) lighting more for the left channel than the right, even with the C26's mode switch set to mono. I started fiddling with the controls on the MC2505 and C26, to rule them out as a cause before assuming worse.

    Once I got to the tone control knobs, I counted the number of click-stops between each extreme of each control and the center line on the faceplate. Sure enough, the bass control for the left channel went 6/4 instead of 5/5, meaning that the 'center' position for the knob's pointer line was actually +1! :eek: I pulled the knob off, and saw that the metal 'D' ring in the center was off-position (guessing said ring was originally glued in place, or something; any ideas?). Tweaked it a bit using a pair of needlenose pliers, reinstalled the knob, turned it to the (real) center position, and now the two channels are much closer level-wise. Gotta love the simple fixes. :cool:
    -Adam
     
    Last edited: Jan 25, 2017
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  8. mnmmt

    mnmmt Member

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    I am so paranoid about my unobtainable on/off/volume pot going bad on my mx preamp that i leave the volume control on all the time and turn it on and off with a surge protector. As for the thump, I have been told that it is best to turn on the source first, then the preamp, then the power amp, and turn them off in reverse. Having the power amp on first (or simultaneously) and off last amplifies whatever transients the preamp puts out when it turns on, sending bad stuff to your speakers. All my gear makes some kind of noise at startup--even my modern all digital pre/pro sends a click and occasional chirp through the amp.
     
  9. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    That's a very good point. I've seen a couple of threads regarding power switch issues in some of the power amps, but wasn't sure if the sort of volume control switch used in the C26 was also susceptible to failure. I have an Optimus "Professional Series" Home Theater Power Center (catalog number 61-2236) which has three sets of outlets on the back: two unswitched, two "filtered for audio", and two "filtered for video", the latter two sets controlled by a button switch on the front. Not sure if this thing is particularly great, but it should at least work to take some stress off of the C26's power switch until I pick up something better (I know a store which has some Adcom ACE-series power centers, and possibly some Monster power conditioners).

    Getting back to my other topics, does anyone have any comments about my previously-posted idea for bi-amping? Here is a crudely-drawn diagram of my proposed setup:

    [​IMG]

    Since the MC2505 and Dynaco Mark IIIs would appear to have roughly the same power output levels (the Mark III was de-rated to 50W when the FTC rating system was introduced), I didn't think there would be a problem with using them straight-up with the tri-amping terminals on the back of my Paradigm Studio Monitors. However, I've been told that a tube amp used in this manner (i.e. being fed with a full-range signal, but the output connected to a crossover which filters out part of the spectrum of said signal) may be bad for the output transformer(s) in the amp(s).

    A few years ago, I tried a similar experiment where I used my pair of Mark IIIs to feed the woofers, and a Dynaco ST-35 to feed the mids and highs, but the only issue noted was that the ST-35 ran out of steam as I cranked up the volume, as the mids/highs lagged behind the lows in intensity (as one might expect when pitting a 17WPC amp against a 60WPC one). No known damage was done to the ST-35 or its output transformers, to the best of my knowledge. Do I have reason to be concerned, and if so, what can be done to protect the output transformers in the Mark IIIs? Again, thanks in advance!
    -Adam
     
  10. seacliffe301

    seacliffe301 Active Member

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    Nice find! Don't be discouraged by the rough look of those cabinets. Sanded properly, stained, and refinished, and you'll have $400. worth of cabinets there.

    As for the audible thump, have you tried turning off your mains on the front of your C-26 before powering down? I would also utilize the remote path to see if it is also effected.
     
  11. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    The MC2505 is connected to one of the switched outlets on the C26, so I haven't been able to test that as of yet. When I install the 'new' power center, I should be able to test that. Not sure what you mean by the 'remote path', unless you're referring to said power center. I'm pretty sure that most 'remotes' still used ultrasonic control when the C26 came out...
    -Adam
     
  12. seacliffe301

    seacliffe301 Active Member

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    I was referring to the "main" & "remote" speaker on & off controls on the front panel pushbuttons on the C-26. I once had a C-28 (with a 2505) that utilized an SCR-2 speaker control relay to switch the speakers on & off from the front panel of the preamp. I know that the C-26 does not utilize this relay, but I was wondering if any "on-off" speaker switching from the front of your preamp (before powering off) would eliminate that thump you're experiencing.
     
  13. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Understood. There are indeed main/remote speaker on/off switches on the front, though they work the opposite from what I originally expected (had them depressed at first, and was wondering why I didn't hear any sound... :oops:). I do indeed have the output of the MC2505 connected to the Amplifier Output terminals on the back of the C26, and the speakers connected to the Main Speakers terminals. I'm guessing I'll get the same results from having the Main speaker switch turned off on the C26 as I would from switching the MC2505's power switch off before powering off the C26. However, we shall see.
    -Adam
     
  14. c_dk

    c_dk Super Member

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    The power supply for both the amp and preamp are 15-20 years overdue for a rebuild. The single rail type supply in the C26 and others of that vintage are know for some turn on- off transients.

    The rule back then applies even more today.....pre on first then amp, amp off first then pre.

    It is also helpful to turn down the amp's gain to 12:00 until you get the power supply rebuilt. Typically the preamp S/N will also be improved with the volume control tracking between say, 9:00 and 1:00.
     
  15. TSmith8605

    TSmith8605 Senile Member Subscriber

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    The way I understand it is if you hook up a tube power amp to nothing, or to nothing below your speaker’s crossover frequency in this case, the output current for the bass energy (in this case) is blocked from the speaker and so a bunch of current (all the bass-frequency energy) circulates inside your output transformer coils (since it can’t go to the speakers). Since there is so much energy in the bass frequencies, your amp’s power supply has to work very hard to supply this wasted power. So you are heating up your output tubes and transformers and possibly overloading your amp’s power supply capacity just to drive this waste energy around inside the output transformer of the tweeter amp.

    I’m not sure, but I suspect that this will happen also with the autoformers in the Mc2505. (The same thing probably also happens to the high frequencies in the output transformers of the woofer amp but in that case there is not nearly as much energy in the high frequencies so it does not matter.)

    It may be this is why your ST-35 amps “ran out of steam” as it’s power supply was overloaded pushing the wasted base energy around inside its output transformers.

    I don’t think this will cause any damage (famous last words) but it will likely degrade the sound and possibly shorten the life of you output tubes.

    I am shooting from the hip here, so if anyone else knows more about this, please chime in and make appropriate corrections or additions.

    I think this can be easily fixed with a simple passive high pass filter consisting of a single, good sounding capacitor placed in line between our pre amp and tweeter amp. You should choose the value of the capacitor that will give you a cutoff frequency somewhat below that of your speakers crossover frequency - maybe an octave or half-octave below so as not to interfere with the band going to the mid / tweets. This web site will show you how to make the filter, Note on the website’s filter diagram, Vin is the output from your preamp, C is the capacitor you are adding, R is the input impedance for the Mk III and Vout is what goes to the MK IIIs.

    This way, you filter out most of the bass energy from the tweeter amp without affecting what’s otherwise going on with the speakers crossover network.

    With this passive cross-over (capacitor) installed between your preamp and tweeter amp, you otherwise hook up the system as your picture shows still relying mostly on the speakers cross over.

    BTW, I have owned some MK IIIs and some ST-70s and I would bet your system will sound better with the Mc2505 running full range by itself than with the bi-amp set up. I think the Mc2505 (if working to spec) has a nicer top end than any tube amp I’ve ever used including an Audio Research tube amp I once had. Just IMHO.

    Good luck and have fun.
     
    AdamAnt316 likes this.
  16. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Thanks for the very informative reply. I'd been thinking about something like this, but wasn't sure exactly how it'd work in the given application. According to this page, the input impedance of the Mark III is 500KΩ. According to the brochure for my Paradigm Studio Monitors, the mids are crossed over at 2.5kHz. According to the formula 1/(2πFR), I'd need a 127pF capacitor (1.27323954473x1^-10), if I did my math correctly. Does this sound about right? And what sort of capacitor would suffice?

    Thanks for the recommendation. I know that the MC2505 is a very nice sounding amp, but I'd at least like to experiment with bi-amping to see what it might sound like. I was hoping it'd be as simple as connecting the amps as shown in the diagram I drew up, since they should have roughly the same power output, but as usual, X-factors tend to get in the way... Anyway, again, thanks!
    -Adam
     
  17. TSmith8605

    TSmith8605 Senile Member Subscriber

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    I would set the frequency of this filter at a lower point than the 2.5 kHz - probably around 1 kHz. Doing so will get a lot of the bass energy out of the tweeter amp as intended but any effects of this in-line filter will be well below the high-pass passband (above 2.5 kHz) of the speaker's filters. If you put your filter at 2.5 kHz, you will add another 6 dB per active roll off to the speaker's existing roll off which was not intended by the speaker's designer. And I won't check your math, I'd probably screw it up.

    Experimenting is a better and much more fun way to learn how things sound than taking advice from some guy on the Internet. So have fun and post results.
     
  18. TSmith8605

    TSmith8605 Senile Member Subscriber

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    Forgot to say that since this cap will be in the signal path, I'd use a good polypropylene cap rated at 20 to 30 Volts. Also the 1 kHz is not exact or critical so that if you cannot find the exact cap value that yields 1 kHz, find something close and run it through the equation to verify where the freq cut-off lands.

    Also, to bi-amp like this, the two power amps do not need to have the same power rating, they need to have the same gain. You should be able to find the gain specs for the amps and as I said, the Mc has gain controls so you can adjust to match the MK IIIs gain.
     
  19. damacman

    damacman Blown and Injected Subscriber

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    A far simpler approach would be a quality two way electronic crossover network.
     
  20. AdamAnt316

    AdamAnt316 Collector of heavy things Subscriber

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    Again, thanks for the reply! I redid the math, and that brings me closer to 318pF. Guessing I should be able to find a cap for either 300 or 330pF, which would still put me in-line for 1kHz. Not sure if polyprop caps are made in these sorts of ranges, but who knows. My main concern is how close the input impedance spec on the above-linked post is to the reality of these Mark IIIs, so some experimenting may indeed be in order if things seem off.
    The gain thing is a good point. According to the specifications, the sensitivity of the MC2505 is 0.5V, while the Mark III is closer to 1.5V according to the link I posted above. Guessing I'll have to dial the MC2505 back a bit to keep them in-line.
    Thanks for the suggestion. I'll definitely keep that in mind, though I'd like to try the passive approach at least at first. Not sure whether or not this will be a long-term experiment; more of a simple attempt at pairing tubes with solid-state, so I'd rather keep the whole thing inexpensive for the time being.
    -Adam
     

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