Axiom Home Page Forums General Discussion Stereo Power into M80s from my cheap Denon

My bad, I just figured this being an Axiom forum and this little tid bit "The question of course is to what degree? If you are putting 300W into a speaker, is 150W going into heating?" The M80s are the only speaker here rated for such power.

The 400W spec is a conservative figure, I believe Alan and Ian have mentioned this in the past. They have been driven with 700w continuous for 24hr/day for 2wks straight with peaks exceeding 1100+ watts in recent tests.

The post started out about my receiver and then I drifted off babbling about speakers. My bad, not yours .

But for all the powerheads out there, upgrading to M100s may not be a bad idea .

To respond to Alan, my buddy came over earlier today with his scope primarily to determine at what power level the Denon starts to clip. So here's what we found out.

Recall that we are injecting a 50Hz tone in order to extract the maximum possible current out of the amp. The M80s have an impedance minimum of 4Ohms at this frequency. By the way, we did confirm the symmetry, period and "cleanliness" of the Realtraps 50Hz tone.

The Denon started square-waving at +9dB which represented a complex power of 225W/channel. However, well prior to this at -2.5dB, which represented 112W/channel, we noticed the onset of slight frequency or slew rate distortion on the positive half of the sine wave. The negative-going portion of this top half lost symmetry by decaying slightly from 90 to 135 degrees and then assumed proper symmetry from 135 to 180 degrees. The effects of this artifact were not audible (at least not to us). We did not see any other distortion artifacts throughout our tests such as cross-over distortion, hysteresis or frequency shifting. Recall that my amp is rated a "clean" 90W into 8Ohms.

We made some other observations:

• The left and right channel DC levels were -10mV and -20mV respectively. These levels are practically insignificant in my opinion as I see no obvious "sucking in" of the cone.
  
  
• At maximum gain with no source, we saw 6mV peak-to-peak, 60Hz noise with some high frequency artifacts on top. Again, this has practically no consequence.
  
  
• Speaking in a normal voice 2 feet away from the M80 caused a 2mV p-p signal to be picked up at the speaker terminals. When we noticed this, the concern was cross-talk from one speaker to the other. But we didn't dig any more into this and now in retrospect we should have tried to identify how this may vary with sound pressure. The yard and beer beckoned however .
  
  
• The Radio Shack meter that I used for the initial readings is accurate for sine waves.
  
  
• I listen to stereo at levels of less than half-a-watt per side.

The conclusions are:

• I have headroom of more than 21dB.
  
  
• The amp exceeds spec at least at 50Hz.
  
  
• Under realistic music conditions, I would expect the amp to start square-waving at a higher power level
  
  
• I am happy with my amp.

On another note, does anyone know why the charts are missing in the original post? I still have them on photobucket.

Mo, very interesting results again, which confirm your prior conclusions(the original chart and graphs still appear on my screen). Incidentally, the slight distortion of the waveform noted around 112 watts wouldn't seem to be related to slew rate, since at 50Hz the slew rate required would be extremely low even for much higher power levels.

Quote:

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I am happy with my amp.

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The most important thing imo

Hi John,

Thanks. The graphs are back on my screen now. Maybe I'm having these problems with the site because I use Opera. The problems have appeared in the last month.

The big difference with my prior conclusions is the power level after +9dB. I had made the assumption (Wid doesn't like it when I assume ) that the waveform remains sinusoidal right up to maximum volume. That assumption has now been proven wrong. Therefore, the voltage and power levels in my chart after +9 should be discounted since the waveform is being clipped.

As for the slew rate, I have to admit that I don't know the technical term for what we witnessed but it looks like the picture below. I'm wondering if the issue is related to a leaky power supply cap on the positive rail. If this was appearing at lower volume levels, I'd have my Denon apart right now but I just don't see this issue as being a practical concern.

Mojo,

Where the hell do you get the time to do this stuff??

Hey Mike,

Audio has been a long repressed passion that has now been re-awakened. Now I have to subdue my passion before it subdues me .

Mo, when I said that your previous conclusions were confirmed, I meant your belief that you had more than enough clean power for your use. Clipping as the level went over 200 watts doesn't change that. By the way, a quick slew rate calc(6.28 x frequency x peak voltage)/1,000,000, indicates that at 50Hz and that power level about .01 V/microsecond is adequate. Modern equipment typically has rates of 10V/microsecond or more and has a slew rate far more than adequate for the full audio range.