Current sense measurements on amp/loudspeaker

[Gerrit Boers]

Correct me if I'm wrong, but isn't it the current through the voice-coil that makes the sound? So, why are we always measuring voltages on the output of amps? To makes things worse, most of the time a purely resistive dummy load is used for these measurements.

To see what happens current wise I used a 0.1 ohm 'current sense' resistor to measure the current. Three channels were captured and used for analysis: input voltage, output voltage and output current. A dual FFT analyzer was used for the analysis. Sampling rate was 192KHz, FFT size 768000, pseudo-random noise excitation, rectangular window.
Two amps were used for the measurements, my single ended tube amp and a NAD C320 solid state amp. The load was a Fostex208S in a Replikon BLH/BVR enclosure.

The graph shows the impulse response (voltage and current) for both amps. I'm definitely operating at the limit of what is possible with this setup, we're looking at individual samples 5 microseconds apart, but I think the result is still interesting.

My interpretation (open for discussion):
The back EMF from the driver is more pronounced in case of the tube amp and also reflected more in the current response. The graph for the NAD shows feedback in action, the amp is working like mad to get the voltage down.

[Gerrit Boers]

The next one is perhaps even more interesting: input output cross-correlation. I must admit that I'm still not clear on how to interpret the results, but one thing is certain: Voltage and current each show a different picture. Looking at the voltage cross-correlation I would say that the NAD looks much better. For the current the picture is less clear, I think the tube amp has the advantage here (look at how the tail decays).

[sreten]

Originally posted by Gerrit Boers
Correct me if I'm wrong, but isn't it the current through the voice-coil that makes the sound? So, why are we always measuring voltages on the output of amps? To makes things worse, most of the time a purely resistive dummy load is used for these measurements.

Hi,

A perfect voltage amplifier has zero output impedance and the
voltage level drives the speaker by producing whatever current
is necessary. Not so for a less than perfect amplifier (valve).
Of course its the current that moves the drivers but any look at
an impedance curve shows that the current is all over the place
for voltage drive of typical two way speakers.



The behaviour of the amplifiers current delivery into various loads
is best explored by various lower resistive loads, the complex loads
of speakers, e.g. Z= 6 ohm / phase angle = 45 is ~ equivalent to 4R
loading.

Measuring the current for a driver can tell you things, see :
http://klippel.de/pubs/Klippel%20pap...ymptoms_06.pdf

/sreten.

[KSTR]

This (quite controversial) paper by Bill Perkins (www.pearl-hifi.com) might be interesting:
http://www.pearl-hifi.com/06_Lit_Arc...Distortion.pdf

Over in the "Distortion Preception" thread this topic was briefly hit also (that current is the only thing that matters, and voltage drive may be flawed by design. Nonetheless the voltage interface has become the standard and is what current drivers are designed for).

- Klaus (big fan of higher impedance drive, espcially for full-range drivers)

[Gerrit Boers]

I'm familiar with the work by Klippel, it's one of the things that prompted me to start this investigation.
The difference in output impedance between the two amps can be clearly seen in the amplitude response for the output voltage. The current response shows the impedance of the loudspeaker, with the inclusion of some nasty peaks.
It is my intention to take a closer look at distortion products in the current at several frequencies to see how the amps cope with the peaks in the impedance plot of the loudspeaker.

[planet10]

Originally posted by KSTR
Nonetheless the voltage interface has become the standard and is what current drivers are designed for.

Largely a consequence of a big turn -- some say a wrong turn -- that was made with the fairly concurrent introduction of small inefficient acoustic suspension systems and the transisitor with its cheap watts.

Fortunately systems that tend to prefer current drive are on the way back and in many cases redefining bang-for-the-buck and state-of-the-art.

dave

[planet10]

Originally posted by sreten
A perfect voltage amplifier has zero output impedance and the
voltage level drives the speaker by producing whatever current
is necessary. Not so for a less than perfect amplifier (valve).

Valve amplifiers are certinly capable of being very good voltage amps... but why bother. You'd have to include nelson Pass' F1 & F2 into your less than perfect amplifier (only less than perfect because you are making the assumption that a perfect amp has to be a voltage amplifier). A more reasonable approach is to look at the amplifier/cable/speaker as a system (which it is). The ad hoc amplifier driving the ad hoc speaker is were perfection breaks down.

dave

[Gerrit Boers]

Originally posted by KSTR
This (quite controversial) paper by Bill Perkins (www.pearl-hifi.com) might be interesting:
http://www.pearl-hifi.com/06_Lit_Arc...Distortion.pdf

- Klaus (big fan of higher impedance drive, espcially for full-range drivers)

Interesting, I will have to study this.

Originally posted by planet10

A more reasonable approach is to look at the amplifier/cable/speaker as a system (which it is). The ad hoc amplifier driving the ad hoc speaker is were perfection breaks down.

dave

I like to look at the amp-speaker-room as one system.


Attached is the phase response for current and voltage. Notice that there are just four points where voltage and current are in phase.

[planet10]

Originally posted by KSTR
paper by Bill Perkins

actually by Kurt Steffensen (in the foreground)



(Bill edited a post(s) Kurt made on the JoeList)

dave

[planet10]

Originally posted by Gerrit Boers
I like to look at the amp-speaker-room as one system.

Yes

dave