Understanding Mcintosh Meters?

This is probably a stupid question, or atleast one I should be able to answer myself, being that I did graduate with a degree in computer engineering, but I recently got a mcintosh ma6450, and was wondering how I make sense of the meters. I understand what they measure, and what it means, but how do i know when I am close to clipping.

Let me explain further. the amp is rated 100wpc, in to 4 ohms and the meters read from .01 to 100 watts with the final mark being 200 watts. But what if my speakers are 8ohms? I know that means my amp produces about 50 watts in to 8ohms, but does that mean that I can only take the meters about 1/2 way before I will clip, or do I go by the decibels, meaning 0db is really the max before clipping, only instead of producing 100wpc (0db corresponds to 100w watts on the meters), the amp is only producing ~50wpc since the speakers are 8ohms.

Hope I explained my question well enough. Thanks for your help.

First, the meters are logarithmic, not linear; so 1/2 way between two marked points is not a value arrived at by taking the linear mid value. In other words, 1/2 way between '40' and '400' is not 220, rather it works out to about 125. There are a couple sites that can explain how to make the calculation or look it up in a math text. Second, clipping refers to distortion of the waveform from hitting the rails. I believe that the amplifiers determine clipping by comparing the 'input' waveform to the 'output' waveform and turning on a light when clipping occurs, simultaneously reducing gain - in other words, you cannot really clip, for other than a few milliseconds, with a mac amp. However, you can fry speakers with a mac amp by simply providing too much current to the speaker. In short, if you are worried about damaging your speakers, find out from the manufacturer the maximum wattage your speaker can handle, then keep your gain set so you don't exceed that wattage, as properly read from the meter. Trying to base your analysis on the impedance of your speakers vs the taps on the amp is not fruitful because the impedance spec'd for your speakers is nominal - no speaker has the same impedance over the frequency range of interest. So the method I have suggested is general. There are a host of other issues that I am sure other responders will mention - including, but not limited to, the accuracy of the meter reading at any one point, and the duration of the signal at a given wattage.
As to the last post - I am not familiar with the model amp you mentioned, my explanation was based on current amps sold by the company - you may want to look at the manual, or call the company, to find out if the clipping protection I described is in your model.
Hi farjamed, I had posted a question on power level DB meters a while back, do a search for "Amplifer DB meters".

Turns out that they are essentially useless and have little real world meaning. They are a nice marketing tool. I just taped over the meters to prevent them distracting me. Unless you think they look cool just ignore them.

"Blue Meter" with glass front, nice looking, but totally worthless!! Don't worry about the meters they will never tell a story on your speaker or systems capability, if it sounds good go with it, you will know just like everybody else having amps with NO meters when there is an issue of being overdriven. I owned several mcintosh pieces over the years, basically you can buy white ipod or you can buy a fancy "Skined" Ipod to make it look better, thats all the meters do for you, they give you a unique look.
Totally worthless? Don't know about that. Does a car need a tachometer? No, but I like knowing what's going on engine wise. Same with audio. They will let you know when you're about to "redline" the amp (not the speakers though).

To the OP: Don't know the 6450 that well, but I think it's new enough that it should have a "power guard" or "power sentry" monioring circuit that would take the guess work out of meter calculations.

Above all else, your ears are the best meters. Turn up the volume slowly until you begin to hear stress out of the speakers, note where the meters are and keep it under that level. It doesn't take long to burn out a tweeter or part of a crossover it you go into clipping too long.
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.
“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”

The theory as to why an underpowered amplifier is potentially more damaging to your speakers than an adequatly powered amp is that the amp will tend to clip when "overdriven", and this will occur at lower volumes than for an adequatly powered amp. Clipped waveforms are waveforms that sit at the rail voltage of the amp (not an intended aspect of the signal). For example, consider a sine wave with a peak of 10 volts as opposed to a clipped sine wave with a peak of 10 volts. The greater clipping, the longer per cycle the waveform sits at 10 volts. In the extreme, the waveform becomes a square wave. Impressed across the same load, more power is dissipated by the clipped waveform than the unclipped waveform. More power means more heat dissipated , and at some point, damage may occur. However, that does not mean that an adequately powered amplifier cannot cause damage just because it does not clip. If the speaker draws more current than it can handle, damage can result.
They are put there to sell product. When I was a dealer I was often told that products with meters invariably outsold those without. My own experience bore this out, my best selling amp was a Hitachi mosfet power amp with large meters. I was quite a good amp, probably better than the Haffler I was also selling in many ways. But people bought it for the meters, not the sound. If you really want to track output LEDs are much better , but not as much fun.
>> 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?

Yes, but I would not push it. Amps and speakers can be repaired, your ears can't.
Ok, so i thought I understood the meters. But apparently they arent as useful as I thought. They arent really measuring anything. They pretty much are just there for looks. Let me explain.

Someone said, the meters could help tell whether your system was hooked up properly, but as long as there is a source being fed into them, they will react to the music regardless of whether or not speakers are even connected.

My left meter has been reading sligtly less than my right meter. I thought maybe when i moved the amp around recently the left speaker wire came a bit loose from the connection. Upon inspection I realized they have nothing to do with speakers being connected or not.

I am confused now.
If you want to see what the meters are measuring and at what point, the schematics for all of the currently sold amplifiers are available from Mcintosh for a very nominal feel. Suggest buying the schematics and studying them - which will dispel the idea that the meters measure nothing.
I had the MA 6450 and the meters come in handy to the extant that you know when you are coming close to overdriving the amp. As far as reading them, the 6450 has an 8 ohm and 4 ohm overlay. You read the top or bottom, depending on the impedance of your speakers. It's close enough to tell what is going on. Plus the powerguard is nice. You should see the red lights flash if it's protecting it from clipping.