How To Measure "Current" In An Amplifier?

Gdhal 9-26-16
Atmasphere:  "Regarding your amplifier, the specs don’t add up as presented so there is more to the story. The 45 amp figure seems to refer to output. Converted to RMS as Al did (so it makes more sense) the 15.9amps does not gel with 220 watts. Into 8 ohms at 220 watts the current is 5.24 amps. The 15.9 amp figure makes more sense if the load impedance is slightly less than 3 ohms. At this point the amp would be making a bit over 660 watts. Does that sound right to you? "

I agree at face value the numbers don’t add up. I have sent a question to Musical Fidelity, but wouldn’t hold my breath for a response.

Not sure if it answers Ralph's question, but FWIW it appears that the 220 watt capability of the amp into 8 ohms is voltage limited, not current limited (or thermally limited, for that matter).  The manual indicates that the max output voltage at the onset of clipping, across the 20 Hz to 20 kHz frequency range, is 42 volts RMS, which corresponds almost exactly to 220 watts into 8 ohms.

Best regards,
-- Al
   

Hello Al,
Your Daedalus speakers are designed to work well with either tube or SS amplifiers. Your VAC REN 70/70 amplifier provides a choice of several  NFB settings  and  zero feedback.  I'm assuming that you have tried the various feedback settings,  which one sounds best with the Daedalus Ulysses?
Thanks, 
Charles,  

Hi Hal,

For a resistive load:

Power = (Current Squared) x Resistance = (Voltage Squared) / Resistance

15.9 amps into 4 ohms = 1011 watts
15.9 amps into 2 ohms = 506 watts

42 volts RMS (the amp’s maximum output voltage rating) into 4 ohms = 441 watts
42 volts RMS into 2 ohms = 882 watts

FWIW my guess is that the 4 ohm and 2 ohm power numbers stated in the review and the dealer writeup you linked to, which roughly correspond to the 42 volt max output rating, were informally provided to those parties by MF (since those numbers do not appear to have been officially published by MF), and I would guess that they do not represent what the amp can do **continuously.**

I note also that the dealer writeup includes the word "peak" just after the mention of 800 watts into 2 ohms, which casts some doubt on the meaningfulness of that number.

Also, the SoundStage review mistakenly says "MF claims that the M6i can pass 45 amps peak current." What MF claims is 45 amps peak-to-peak, which for a sine wave or other waveform that is symmetrical above and below zero corresponds to 22.5 amps peak.

None of which means, BTW and IMO, that your amp isn’t a fine match for your speakers :-)

Best regards,
-- Al

Hello Al,
Your Daedalus speakers are designed to work well with either tube or SS amplifiers. Your VAC REN 70/70 amplifier provides a choice of several NFB settings and zero feedback. I’m assuming that you have tried the various feedback settings, which one sounds best with the Daedalus Ulysses?
Thanks,
Charles,

Hi Charles,

Yes, that’s true on all counts. Not surprisingly I found that the zero feedback setting sounds best, although the difference between that setting and the lowest one or two possible feedback values (of five that are provided, in addition to the zero feedback setting) was fairly minor. "Not surprisingly" in part because the impedance of the Daedalus Ulysses has exceptionally little variation over the frequency range. Lou Hinkley hasn’t ever officially published an impedance curve, as far as I am aware, and I haven’t ever seen published measurements of its impedance characteristics, but I recall him commenting in a post elsewhere that the nominal 6 ohm impedance varies within a tolerance of +/- 1 ohm at all frequencies above 100 Hz, and gradually rises to about 10 ohms in the bottom octave.

Best regards,
-- Al

These reviews indicate the Musical Fidelity can do 440 watts into 4 ohms and 800 watts into 2 ohms. I’m not stating that is or is not true, but IF true, would that support the 45 peak-to-peak amperage claim (or the RMS conversion to 15.9)? And, to the point of this thread, that it can "easily" drive a low impedance speaker?

'Yes' to the latter, no worries. 

To the former, 800 watts into 2 ohms is 20 amps. Thats 56.6 amps peak to peak. If that is so, why would they state that 45 amps peak to peak is the spec?