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.