Can a McI be safely run at a 2:1 ohmage mismatch?

Can a McI be safely run at a 2:1 ohmage mismatch?

Postby Jon S. » Tue Dec 10, 2013 2:37 pm

I'm sure I'm asking this poorly but here's what I mean. If I run my Mc50 from the 8 ohm tap into an 8 ohm load, I'm golden. But what if I ran instead:

1. from the 8 ohm tap into a 4 ohm load? -or- 2. from the 4 ohm tap into an 8 ohm load?

If it were a vintage Fender or Traynor amp, I wouldn't give it second thought. But a Mc'I's so different, I really have no idea if this is safe with one.

And my question, I'm embarrassed to admit, isn't purely hypothetical. My MC50 has been modded to have a pair of 1/4" outs, one is supposed to be 4 ohms, the other 8. Unfortunately, they're not labeled, and though I was told upon receiving the amp back which one is supposed to be 4 ohms out and which is supposed to be 8 ohms out, it took a couple of emails back and forth to get a response and I've always had a nagging wonder as to whether I got it straight. Anyway ...

Until this past weekend, I ran out from what I'd thought is the 8 ohm out into an 8 ohm load (1X12 w/a E120). Sounded great.

This weekend, I added a second 8 ohm cab (K120) in parallel (4 ohm total load). This sounds even better! BUT the sound from what I'd think is the 4 ohm out into the 4 ohm load surprised me by being less loud than I'd anticipated. And when I switched to what I think is the 8 ohm out with the same 4 ohm load, it's louder and I actually like the tone better this way.

So I guess my precise questions are, first, would 8 ohms out into a 4 ohm load be expected to be louder than 4 ohms out into the same 4 ohm load (same power amp and all other things being equal)? AND can I safely continue to run out of what I think is the MC50's 8 ohm out into my 4 ohm load, since I like the way it sounds better this way, without damaging the MC50?

Whew, I'm wiped just thinking about and typing this! Thanks for whatever insight y'all can provide.
"For me, I think the only danger is being too much in love with guitar playing. The music is the most important thing, and the guitar is only the instrument." Jerry Garcia
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Re: Can a McI be safely run at a 2:1 ohmage mismatch?

Postby Jon S. » Tue Dec 10, 2013 2:50 pm

UPDATE: I found the MC50 Owner's Manual on line: http://vintagevacuumaudio.com/schematic ... rranty.pdf It says, at page 2,

"The only adverse effect on the operation of a McIntosh amplifier when it is improperly matched is reduction in the amount of distortion-free power available to the loudspeaker. Close impedance matching is desirable for maximum distortion-free power."


And at page 4,

"The amplifier is completely stable when connected to any loudspeaker system or even to any reactive loads. The MC50 has special circuits to prevent damage by any short circuit or open circuit of the output load, or by any amount of output impedance mismatch."


I believe (correct?) that this puts my issue to bed - at least in terms of allowing me to safely use either output jack with my 4 ohm load.
"For me, I think the only danger is being too much in love with guitar playing. The music is the most important thing, and the guitar is only the instrument." Jerry Garcia
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Re: Can a McI be safely run at a 2:1 ohmage mismatch?

Postby NSP » Tue Dec 10, 2013 3:36 pm

Not long ago at a gig by mistake I ran the 8ohm tap on my Mc250 to a 4ohm load and didn't realize until I was tearing down at the end of the night. I was a bit concerned (Terry DeWick brought it back to spec last year) so I asked Brad S what his take was...

"You're ok with what you did, but don't do it again if you can avoid it. Generally, if you aren't pushing the amp into full power clipping/distortion, you're reasonably safe running that mismatch. But ideally we want them matched.

The damage could have come by amp overheating and hard driving, but the Mac has a protection circuit in there if you had gotten close to hurting it. They are very robust and can handle abuse.

I know some people do that mismatch on purpose to get a certain type of distortion, but I wouldn't do that to them. Too precious. You are always safe doing a mismatch the other way, like an 8 ohm cab on the amp's 4-ohm tap. Upward mismatches like that are totally safe. It's the (8:4) 2:1 downward mismatch that can be risky because the too-low-impedance of the cab can ask the amp to move double the current, double the heat."

So, based on that I'll be running mine matched. Hope this helps.
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Re: Can a McI be safely run at a 2:1 ohmage mismatch?

Postby Jon S. » Tue Dec 10, 2013 3:51 pm

Was your experience that running the 8 ohm output into the 4 ohm load was noticeably louder, at the same volume knob setting, than running the 4 ohm output into the 4 ohm load?

P.S. You find interesting stuff on the 'net. Like this (I "think" I understand it :o ) : http://www.pstracks.com/pauls-posts/mci ... nsformers/
"For me, I think the only danger is being too much in love with guitar playing. The music is the most important thing, and the guitar is only the instrument." Jerry Garcia
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Re: Can a McI be safely run at a 2:1 ohmage mismatch?

Postby waldo041 » Tue Dec 10, 2013 4:18 pm

I have noted that Jerry ran a 2.66 ohm speaker load into a 4 ohm tap. The Mcintosh Autoformer allows for this operation, and even though some warn against it it is actually OK with the Mac. Yes, the amp will be louder and you will be able to clip it faster. Removing the Sentry Monitor allows the output transistors to actually clip versus clipping the Sentry which is the Mcintosh protection circuit that protects the amp against direct shorts at the output taps. It "senses" the current and voltage at the output of the output transistors, before the Autoformers primary, and bleeds to ground anything over the threshold to ground at the the drivers. With no protection circuit a direct short at the output will kill the output transistors and possibly fry the windings of the Autoformer because it tells the amp to draw ALL voltage and current. To be clear, a direct short is equal to a no ohm load on any of the taps. Like a direct short at the output, this type of mismatching is reducing the ohms the amps sees on the mismatched tap and tells the amp to draw more voltage and current. So when you ask the amp to draw more voltage and current at the output by mismatching in this fashion you are certainly working this VI limiting circuit(sentry) if it is in place.

I have had my sentry removed on my Mc2100 for quite some time and have been mismatching like this with no problems or overheating. I have even used my 4 ohm load on my 16ohm tap which is the loudest and where the amp clips the easiest, but i do find the 2:1 ratio on the 8ohm tap to be better to my ears. This all said, with the Sentry removed DO NOT SHORT THE OUTPUTS!

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Re: Can a McI be safely run at a 2:1 ohmage mismatch?

Postby Jon S. » Tue Dec 10, 2013 4:26 pm

Well, I think waldo's last post resolves this issue. Thanks again, Mike, for your latest, appreciated information & reassurance. I think I'll stick with 8 ohms out into the 4 ohm load - it sounds so very good!
"For me, I think the only danger is being too much in love with guitar playing. The music is the most important thing, and the guitar is only the instrument." Jerry Garcia
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Re: Can a McI be safely run at a 2:1 ohmage mismatch?

Postby zambiland » Tue Dec 10, 2013 4:49 pm

I'll just add that Mac amps can take a huge amount of abuse. When I was in high school I had a rig with a 2105 and a double 15 cabinet. I decided to run each cabinet off its own channel by making each of the 1/4" jacks on the back of the cabinet a separate run to each driver. It would run great for about a half hour, although super hot and then shut down. I couldn't figure it out until I realized that the mac amps don't have common -'s on the speaker outputs and the two channels were shorting through the metal jackplates. Still, no harm was done to the amp.

In general, amps with output transformer designs like the Macs don't follow the behavior of OT-less solid state amps where the power increases when the impedance of the speaker drops. The most efficient transfer of power is when the impedance matches. It's one of the cool things about the big Mac amps, where they have 1/2 ohm output taps so you can run a bunch of speakers in series.
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