How To Measure "Current" In An Amplifier?

Erik, can you give examples?:-)
It's hard for me to understand why a speaker designer would want to discourage potential sales just to make a point. 

Sadly, these impedance issues don't often discouarage sales.  Instead they are sold at a premium for being more "discriminating."  with the idea that somehow they will reveal more in the music.

I did a thorough electro-acoustical analysis of a Focal Profile 918. And by this I mean I measured each driver electrically and acoustically as well as reverse engineered and simulated the crossover.

To simplify it, perhaps too much, the bass section had an array of 8 power resistors (8 x 10 = 80 watts) to deliberately leave the midbass with a very low impedance (3 Ohms).

Replacing these resistors and associated parts with a coil left the complex frequency response identical but raised the impedance to similar amounts as the Thiel you mentioned, around 4.5 ohms.

Since then I've noticed a wide variety of Focal speakers, regardless of woofer or woofer count seem to share the same impedance profile, though not as blatant an approach.

Best,

Erik

@unsound

Take a look at my blog post about it, and scroll down to the "Bogus Woofer Cap" section. The original minimum impedance for this speaker was 2.6 Ohms.

http://pqltd.blogspot.com/2016/01/focal-profile-918-ultimate-upgrade-guide.html

There is 1 cap and 4 resistors that are simply completely unnecessary. They are the same type of conjugate network Thiel used, but instead of being for the buyer, are for the seller.  It's designed to waste up to 40 watts of power in an area where it's easy to hear.



Best,

Erik

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
 

@unsound

You probably already know this, but take a look at my LM-1 speaker measurements. It is NOT tube friendly like the Thiel. Notice the 24 Ohm peak in the bass and 16 Ohm impedance peak where the woofer and tweeter response meet.

http://speakermakersjourney.blogspot.com/2016/05/lm-1-bookshelf-measurements.html

These peaks are what the Thiel crossover was trying to avoid.

On the other hand, a solid state design would play beautifully with the LM-1

Best,

Erik