Understanding Mcintosh Meters?

Thanks for all the responses.

Ok, Now I am little more confused. I thought there was greater danger having an underpowered vs a overpowered amp because the underpowered amp could clip, causing distortion, and that is what damages speakers.

Musicnoise: You said that as long as I am supplying less wattage than my speakers can handle I am fine. So does that mean if my amp is say 50-100 wpc (depending on speaker impedance) and my speakers are rated at 350w power handling, I dont have to worry about frying them with my amp? But then why does everyone say an underpowered amp is more dangerous?

Onemug: Yes, my amp does have powerguard. This means that my speakers should be protected against any spike in the music that may cause the amp to clip? I have already put my volume much louder than I could ever use it in my current living sitation and the powerguard lights did not illuminate, so I am just asking more for peace of mind and understanding.

Thanks for all the help guys.

Musicnoise is correct in the sense that McIntosh's Power Guard circuit will essentially keep the amp from clipping to any significant degree . . . and amplifier clipping causes the overwhelming majority of (power-related) speaker failures. And I agree with others that a simple wattmeter of any sort is of very limited use for preventing speaker damage . . . your ears and basic common sense are better things to go by.

But for your specific meter accuracy question, it varies from model to model. For the amps without autoformers (including IIRC the 6450), it's essentially a voltmeter, calibrated to a specific impedance load. So when your meter reads "100 watts", what it really means is "20 volts" . . . which is the same thing IF the amp is driving a 4 ohm resistive load . . . which your loudspeaker is obviously not. But in your case, "100 watts" on the meter will still be pretty close to the maximum power output when driving your speakers, so concept of "meter full scale" is the same, but the wattage calibration will be inaccurate.

For the later McIntosh amps that have autoformers, the meters are much more accurate, because it's the drive to the autoformer that's measured, not the voltage at the speaker terminals. They also have a current sensor that gives a certain amount of correction factor for the typical variations in a loudspeaker load. So if the load on, say, the 4-ohm autoformer tap is maybe within +/- 20% of 4 ohms, the meter reading will be very accurate. But if you were to put a 2-ohm load on the 8-ohm tap, this would be outside of the correction range of the current sensor, and the meter wouldn't be accurate at all.

But in the real world, the 6450 isn't a hugely powerful amp . . . and with clipping removed from the picture by the Power Guard, you really shouldn't need to worry a whole lot with an average domestic loudspeaker. The meters are handy for checking i.e. channel balance on a mono record, or verifying that your cables are hooked up correctly, or as an educational tool to understand the rough relationships between power and SPL, but they're not sufficient to really tell when you're about to damage your speakers.

Farjamed, first off in answer to your question about the 4 or 8 ohm concern, If you have autoformers then the meteres self correct for the impedance driven. I have own a number of Mc amps and find that the meters do indicate about how much power your are delivering at the outputs. How ever I emphasize "about" and really the appeal is the look. I think they are accurate as you can get in real world for what they are designed to do. The reason for the 200 on a 100 watt amp is to let you know when you are totally pegged and that is so close to the 100 watt mark that you won't really be able to decipher the difference between 100 and 200 while playing unless you leave it cranked all the way for anylength of time. At which point the Power Sentry will kick in and shut you down. Mc's power sentry /power guard circuits are designed to protect you from clipping and that is where you need to lose any concern about that occuring. As when you drive the amp to clipping the system will shut down the outputs and the lack of sound will get your attention far before the meters will ever indicate. I think the blue meteres are great to look at but I usually take my glasses off so I really can't see the needle. Have a glass of wine and relax and enjoy the music. They are much more romantic than a fireplace and never require cleaning ashes (unless you continue to crank it and the PG or PS circuits fail).

One last related question..

What about amps with a digital readout of the "volume" that shows a db reading. Are these accurate, in the sense that raising it from -35db to -32db is actually doubling the power?

Theo: I am with you, I can't see the needles from where i sit either, but i like the way they look.

Are these accurate, in the sense that raising it from -35db to -32db is actually doubling the power?

Yep, pretty much.

To be perfectly precise . . . it's actually the voltage gain that you're changing. So assuming that the input signal voltage and load impedance remain identical, than this is what happens.