MC 754

welcomdmat

Super Member
I have been lucky enough to buy a pair of MC754s that were very recently serviced by Audio Classics. I plan to use them in their mono setting, but I have never done this before.

Do I need to concern myself with the length of speaker wire, the length of the RCAs, or anything else that I don’t know about?

They were both serviced at the same time/ the same trip.
 
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My RCAs will be under 3 ft and my speakers will be about 4 and 8 ft if I put the amplifiers on either side of my preamp.

Part of this is just being new to me. I was able to get them only open from Audio Classics for photographs that I requested, and I am very excited. I think the excitement has me worried about proper setup.
 
If those were the ones recently listed on the auction site, they appeared to be in outstanding condition.

Best of luck with them.
 
Do I need to concern myself with the length of speaker wire, the length of the RCAs, or anything else that I don’t know about?

Don't worry about slight differences in speaker cable lengths...a foot here and there is not going to matter. Unless you need a really....really...long run of RCA interconnects, there are no worries there. I have a 15' run of Canare cables between my C15 and MC752....zero issues or problems and it sounds great. I believe in shorter speaker cables and longer interconnections, it also also me to have my rack on the side of the room and my amps on the floor between the speakers. Just use appropriate gauge cables with good terminations and all will be fine.

One of the few "sleeper" Mc amps, still available for less than $10/watt! I didn't know that they could be strapped for mono...

Yes. The 754 has a switch on the rear to allow the user to select stereo or mono. The 752 did not have that feature.
 
Old post, but I recently acquired a second 754 and plan on using both to push 4ohm speakers. The manual does not say that can't be done, but rather just lists the power output for a mono bridged setup at "200 watts across 8 or 16ohm loads". There is no rating for a 4ohm scenario but no language in the manual restricting a bridged 4ohm setup.

It also requires that the common speaker terminals not be used for mono operation, and states the pos/neg speaker wires be inserted into both left and right terminals on each amp. I'm not sure if that is applicable for a bridged setup, but whether I do that or leave them normal, I am getting clipping at high volumes when there are sudden bass blasts (The power guard lights turned on on each amp and clipping occurred when listening to the 1812 Overture at high volume).

Any thoughts?
 
The MC752 and MC754 amps were hefty McIntosh utility amps that we would install to supplement a Mac owners main rig to supply power to multi rooms throughout the home. Ussually buried in the bottom of a cabinet or utility closet, i can not think of ever using them as the systems main amp.

Blinking PG lamps tell you two things........the amp is putting out 1/2 % distortion in this series at this volume level and/or you need to open your wallet and buy a bigger amp.

Your tweeters are probably thanking you because the PG circuit protected them from burning out and only a bench test by a tech will let you know if the amp is in need of service or if you just need a bigger amp to play your music at that volume.

The amp has got to be 25 years old and just outside of the expected lifespan of some of the capacitors used. I recently posted a picture in this forum of a MA6200 (basically the same amp section) only able to put out 10 watts under a bench test, because of power supply parts failure.
 
Old post, but I recently acquired a second 754 and plan on using both to push 4ohm speakers. The manual does not say that can't be done, but rather just lists the power output for a mono bridged setup at "200 watts across 8 or 16ohm loads". There is no rating for a 4ohm scenario but no language in the manual restricting a bridged 4ohm setup.

It also requires that the common speaker terminals not be used for mono operation, and states the pos/neg speaker wires be inserted into both left and right terminals on each amp. I'm not sure if that is applicable for a bridged setup, but whether I do that or leave them normal, I am getting clipping at high volumes when there are sudden bass blasts (The power guard lights turned on on each amp and clipping occurred when listening to the 1812 Overture at high volume).

Any thoughts?

Don't run a 754 bridged into a 4 ohm speaker. The amp will see this as a 2 ohm load. That's why the referenced to 8 ohm loads is referenced in the manual.
 
Thanks Techguy
After some research that's what I learned. I will say, though, that I ran them at normal volume for 3 hours last night and they sounded excellent - huge improvement in soundstage/seperation.

I may just have to bring some 8ohm speakers into the setup.
 
Old post, but I recently acquired a second 754 and plan on using both to push 4ohm speakers. The manual does not say that can't be done, but rather just lists the power output for a mono bridged setup at "200 watts across 8 or 16ohm loads". There is no rating for a 4ohm scenario but no language in the manual restricting a bridged 4ohm setup.

It also requires that the common speaker terminals not be used for mono operation, and states the pos/neg speaker wires be inserted into both left and right terminals on each amp. I'm not sure if that is applicable for a bridged setup, but whether I do that or leave them normal, I am getting clipping at high volumes when there are sudden bass blasts (The power guard lights turned on on each amp and clipping occurred when listening to the 1812 Overture at high volume).

Any thoughts?
You’re most certainly not getting clipping as Power Guard prevents that. However, the signal is being compressed because the amplifiers lack the power to reproduce the full dynamics given your average listening level. Wanna play the 1812 Overture at realistic levels? Buy MC1.2kws. Or, just realize that your expectations are unrealistic with this media and reduce the average listening level.

To educate yourself a bit more about why this is happening, research the topic of crest factor as well as the dynamic range Telarc captures in that recording. In the meantime, you’ll likely be able to enjoy the combo on nearly anything else you listen to. Me, I’m not so worried about running the amps at 4 Ohms bridged ... but heat management becomes a problem which can cause the amps to shut down prematurely.
 
I know its not recommended to drive 4 ohm loads with the bridged mode. But if the amp stays cool and you don't flash the power guard lights, why not try. If the Power guard lights flash or the amp gets warm, then obviously driving a 4 ohm speakers is not a good idea. As 4 ohm speakers may dip below 4 ohm in some cases or in other cases only rarely reach 4 ohms you are going to have to give it a try, but being careful. If the speakers stay way above 4 ohms above 1000 HZ you should be OK. I will admit that recent amps and current amps are designed to handle more complex impedance curves that the 754 in the bridged mode was not. But that said we used them driving complex loads in commercial systems where 25 volt transformers were used and never had an issue. I will say we were careful to roll off the bass below 50 HZ to prevent the reactive component of the speaker transformers from over taxing the amps.
 
Here are a few things you'll want to read:

https://www.stereophile.com/content/recording-october-1979-telarc-1812-overture

http://www.audioheritage.org/vbulletin/showthread.php?22243-Telarc-1812-and-the-L300-s!

Now, let's point out some engineering that allowed your gear to live to play another day:

1 - You ignored the OM and ran the amps at 4 Ohms bridged anyway.
2 - Your average listening level on the media was too high given the dynamic range present. No worries with Power Guard. Cannon shots that would have normally destroyed the drivers in your speakers when played back at the same volume with an similarly powerful amplifier lacking Power Guard were simply compressed by Power Guard. This effectively eliminated the possibility of dramatically overpowering your speakers by prohibiting the waveform at the speaker terminals from becoming square in shape and incredibly powerful.

Think about this for a second. Trying to reproduce cannon shots at realistic levels with an audio system is somewhat ludicrous. While most of us have a copy of this recording (I do on CD), it's really nothing more than a parlor trick. I've never heard it played by any audio system where I thought - you know, that sounds just like it would had I been standing next to the cannon (and obviously now deaf). In my teens and 20s, I bought every single new Telarc disc as it came out - Time Warp, Chiller, Happy Trails, and The Great Fantasy Adventure CDs all have dynamic range that is just off the chart. We used them to demonstrate high powered car audio systems. When I loaned any out, stuff was destroyed.

Play some music on your system and enjoy it.
 
You’re most certainly not getting clipping as Power Guard prevents that. However, the signal is being compressed because the amplifiers lack the power to reproduce the full dynamics given your average listening level. Wanna play the 1812 Overture at realistic levels? Buy MC1.2kws. Or, just realize that your expectations are unrealistic with this media and reduce the average listening level.

To educate yourself a bit more about why this is happening, research the topic of crest factor as well as the dynamic range Telarc captures in that recording. In the meantime, you’ll likely be able to enjoy the combo on nearly anything else you listen to. Me, I’m not so worried about running the amps at 4 Ohms bridged ... but heat management becomes a problem which can cause the amps to shut down prematurely.

Every stereo piece I own I fell into or bought non-working and repaired, so unless I find a pair of those 1.2kws on the curb...

I listened to the setup at normal volume, then loud for some time as I use this room for movies too. I graduated to the Telarc recording when I figured they could handle it. That recording is always a final test for me.

When I put the CD in to test, the digital cannons caused issues with the woofers (my VMPS have 15" and 12" woofers in each cab) and the power guard lights turned on on each amp.

I had a Pioneer 1250 in here at one point that handled those cannons nicely without ever tripping it's relay, but, I can live without that recording and listen to everyday music given how much I like this sound at the moment...

After the 3 hours of listening last night the amps were almost room temp - much, much cooler than that 1250 would have been had I left it going for that amount of time, so I'm not worried about overheating moving forward.
 
Every stereo piece I own I fell into or bought non-working and repaired, so unless I find a pair of those 1.2kws on the curb...

I listened to the setup at normal volume, then loud for some time as I use this room for movies too. I graduated to the Telarc recording when I figured they could handle it. That recording is always a final test for me.

When I put the CD in to test, the digital cannons caused issues with the woofers (my VMPS have 15" and 12" woofers in each cab) and the power guard lights turned on on each amp.

I had a Pioneer 1250 in here at one point that handled those cannons nicely without ever tripping it's relay, but, I can live without that recording and listen to everyday music given how much I like this sound at the moment...

After the 3 hours of listening last night the amps were almost room temp - much, much cooler than that 1250 would have been had I left it going for that amount of time, so I'm not worried about overheating moving forward.

Chances are the same recording at the same loudness may well have triggered the PG lamps on a pair MC2205 or even larger. Main point, PG did its job protecting the speakers under vigorous test conditions unlikely to be replicated under normal listening conditions.

There is also every possibility the SX1250 was clipping without your knowledge as is often the case - but the duration wasn't long enough to result in damage. In any case, there's little doubt a pair of MC754 can deal with loads far higher than the SX1250 is capable on its best day. Cool operation after extended play also a good sign. There's little cause for concern here.
 
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Chances are a pair or MC2205 may well have triggered the PG lamps of an MC2205 or even larger. Main point, PG did its job protecting the speakers under vigorous test conditions unlikely to be replicated under normal listening conditions.

There is also every possibility the SX1250 was clipping without your knowledge as is often the case - but the duration wasn't long enough to result in damage. In any case, there's little doubt a pair of MC754 can deal with loads far higher than the SX1250 is capable on its best day. Cool operation after extended play also a good sign. There's little cause for concern here.

Thanks guys
I have to think now that I got away with murder showboating that recording for years with the 1250. My dad used to play the 1812 Overture to show-off his 80s A/V amp and Kenwood speakers, so I always liked demoing my system with that recording as a way to 1-up the old man...

Although the cannon booms were always clear while shaking the house with that 1250, the constant ringing bells at the end of the recording impressed me more than anything else on that Telarc LP/CD. I can still listen to it at low levels and appreciate the range of frequencies.

Today I'm going to move forward and hook up the 754s with a Threshold SL-10 preamp and modded Systemdek IIX. Now that I understand the practical boundaries of the bridged setup I can really listen without any nagging insecurities...
 
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