How To Measure "Current" In An Amplifier?

What is probably the best indication of an amp's ability to drive low impedances and/or impedances that have highly capacitive phase angles at some frequencies (which would also increase the amount of current that has to be supplied) is if the amp has a continuous power rating into 4 ohms that is twice its rating into 8 ohms, and if it has a rating into 2 ohms (if that rating is provided or is indicated in the measurements section of published reviews) that equals or approaches twice its rating into 4 ohms.

Current specs that may be provided in manufacturer literature should be ignored in most cases. They usually represent how much current can be supplied into a dead short (zero ohms) for some unspecified number of milliseconds (thousandths of a second). And according to a post I recall that was made a while back by Atmasphere in another thread on this topic, that spec is commonly not even based on the short being placed at the output of the amp, but at the output of the power supply instead.

Regards,
-- Al

I would say that since they are excluding 4 ohm measurements and damping factor they are not particularly proud of it’s output into low impedances.

Erik, just as a point of info the M6si manual indicates a damping factor of 180.

I’ll mention also that once damping factor gets above say a few tens (in other words, away from tube amp territory) I would not expect much if any correlation between damping factor and performance into low impedances.

Best regards,
-- Al

Gdnrbob 9-26-16
I wasn’t quoting you saying High DF was bad, only referring to Almarg’s response that a high DF would wreak havoc on tube amps.

Hi Bob,

No, I didn’t say that. Or at least I didn’t mean that :-)

I suspect you are referring to this statement:

I’ll mention also that once damping factor gets above say a few tens (in other words, away from tube amp territory) I would not expect much if any correlation between damping factor and performance into low impedances.

Tube amps often have damping factors in the single digits (i.e., less than 10), and in some cases even less than 1. The highest damping factor I can recall ever seeing for a tube amp was in the vicinity of 25 or so. While of course solid state amps commonly have damping factors in the hundreds, and sometimes even above 1000 (especially in the case of many class D amps).

I read a Wikipedia page explaining it, and it sounded like a high damping factor allowed an amp to control a speaker cone better than a low one.
So, what makes having a high damping factor bad?

Speakers vary widely in the amount of damping that is optimal, not only in terms of "control" but with respect to the tonal effects resulting from the interaction of their effective output impedance (which is inversely proportional to damping factor, as Erik indicated) with the speaker’s variations of impedance over the frequency range.

The majority of speakers are of course designed these days with the expectation that they will be used with solid state amplifiers, and those speakers will perform well with amps having relatively high damping factors. However once the damping factor gets above a few tens (arguably as little as 20, but 50 or 75 so would seem to be a safe ballpark) further increases in damping factor will **in themselves** make little or no difference in terms of both control and tonality. I say that despite beliefs to the contrary that are prevalent among many audiophiles, and claims to the contrary that can be found in a lot of marketing literature.

Many other speakers, though, are equally happy or even more happy with tube amps having low damping factors. It depends on the intention of the designer, as Ralph has said in a number of past threads. And some speakers are equally happy with tube and solid state amps, and with high or low damping factors. In those cases the intrinsic sonic character of the particular amp can be determinative of which is most optimal.

So how can a high damping factor be bad?

1)It may be a poor match for the particular speaker, the result being too much damping and "control," and/or tonal balances that were not intended by the designer.

As Erik indicated, a low damping factor, corresponding to a high output impedance, will result in output voltages from an amp that may vary significantly as a function of a speaker’s impedance variations over the frequency range. However some speakers will sound best when that variation **is** significant, i.e., when the variation of amp output as a function of load impedance variations results in power being held closer to constant than voltage. See Ralph’s paper on that subject.

2)High damping factors (and also very low total harmonic distortion) are accomplished in some designs by means of heavy-handed application of feedback, which can result in objectionable amounts of transient intermodulation distortion (that is not normally specified, and for which measurement standards do not exist), and/or increases in certain harmonic distortion components that are particularly offensive (even though **total** harmonic distortion may be reduced).

Hope that clarifies more than it confuses :-)

Best regards,
-- Al

Erik_Squires 9-26-16
... they were making speakers deliberately for the crowd that feels a speaker that can tell the difference between a low and high current amplifier was more "revealing" and therefore better for music listening, which is nonsense.

Excellent point. As I've said in a number of past threads, the ability of a component or system to resolve musical detail and its ability to resolve hardware differences are two different things, and may even be inversely correlated in some cases.

Gdhal 9-26-16
So would one conclude that given that current rating of 45, it [the Musical Fidelity M6si] can drive a low impedance load, or can't drive a low impedance load?

Hi Hal,

Per my earlier comment, as well as Ralph's (Atmasphere's) further explanation, that rating is essentially meaningless, and should be ignored. FWIW, though, I'll mention that for a sinusoidal waveform a current rating of 45 amps peak-to-peak corresponds in the RMS terms that are more commonly used to about (45/2) x 0.707 = 15.9 amps.

I have no particular knowledge of how well the Musical Fidelity M6si would do with a speaker having difficult impedance characteristics. But despite the manufacturer's statement that its "combination of high power, high current and extraordinary stability means that it can drive any loudspeaker with ease," several factors suggest to me that it would not do particularly well with such a speaker, in terms of sonics. Those factors include: Lack of a specified 4 ohm power rating; its relatively light 36 pound weight ("relatively light" given its 220 watt/8 ohm power rating and that it is apparently a class AB amplifier, and also given that it includes a preamp, a phono stage, and a USB DAC), which suggests that it is not designed in an especially robust manner; and its ~$3K price, which seems modest given all of the functionality it provides.

Best regards,
-- Al
 

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

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 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

Good find, Hal, as the review makes clear that the design of the M6si draws significantly on the design of the M5si, and the relevant specs that are provided for both are not greatly different.

The total gain of its line-level and power amplifier sections can be calculated from the specified 270 mv sensitivity for a 150 watt output (presumably into 8 ohms) as being about 42 db. That is somewhat highish, but not unreasonably so, and given also the 91.3 db/2.83 volts/1m sensitivity of your speakers as measured in the Stereophile review (which corresponds to about 88 db/1 watt/1m in the mid-bass and lower mid-range regions where speaker impedance is around 4 ohms) seems consistent with the lowish volume control settings you are using. Especially if you are using digital sources, that typically have higher output levels than an analog phono stage would.

Possible indications of too much gain are overloading of circuitry that precedes the volume control in the signal path, which would manifest itself as obvious distortion, and depending on the design of the volume control mechanism the possibilities of excessively coarse resolution of volume adjustments that may occur at low settings, introduction of channel imbalances that would not be present at higher settings, and some degree of sonic degradation. But you would almost certainly have already perceived these kinds of things if they were occurring to any significant degree.

As far as power and low impedance drive capability are concerned, I note the following in the M5si review:

Rated at 150W/8ohm, the latest in Musical Fidelity’s ’super integrated’ amplifiers achieves 2x165W/8ohm and 2x265W/4ohm with 200W, 355W and 545W possible under dynamic conditions into 8, 4 and 2ohm loads, respectively.... While its output into the lowest 1 ohm loads is protected at 275W or 16.6A ... the M5si still looks capable of grappling with any likely partnering loudspeaker.

And I note that for a 2 ohm load the 16.6 amp figure corresponds very closely to the 545 watt maximum dynamic (non-continuous) power capability that is indicated.

So while the design falls significantly short of doubling its continuous power capability into 4 ohms relative to 8 ohms, and the considerably higher capability into 2 ohms appears to be limited to intermittent dynamic demands, it seems to me that the compromises that were introduced into the design in the interests of keeping the cost at the desired level were chosen judiciously and intelligently. Especially in the context of an application such as yours where the speakers have a combination of reasonably good sensitivity and the ability to handle copious amounts of power.

Best regards,
-- Al