Impedance and current-source amplification

[454Casull]

I realize that a current-source amp doesn't care about the impedance in that the magnitude of the current delivered is indepedent of the impedance; however, what about the phase? The current across an inductor lags behind the voltage, so from a conventional voltage-source amp the motion of the speaker would lag behind the voltage waveform going across its voice coil (I think).

Is it the same way with a current-source amp?

[Svante]

Originally posted by 454Casull
I realize that a current-source amp doesn't care about the impedance in that the magnitude of the current delivered is indepedent of the impedance; however, what about the phase? The current across an inductor lags behind the voltage, so from a conventional voltage-source amp the motion of the speaker would lag behind the voltage waveform going across its voice coil (I think).

Is it the same way with a current-source amp?

If you drive a loudspeaker with an ideal current source, the voice coil inductance and resistance does not affect the acoustic output at all. I think that answers your question?

With the normal near-ideal voltage source the output is greatly affected by both. The voice coil inductance acts as a lowpass filter, typically of order 0.7 or so*, and the resistance keeps the response from peaking at fs (the resonant frequency of the driver).

The resistance effect is highly desirable, but the inductance might not be.

*Yes, filters are not only of integer orders. Since the voice coil inductance has an impedance that typically is proportional to f^(0.7) one could say that the filter order becomes 0.7.

[454Casull]

Originally posted by Svante


If you drive a loudspeaker with an ideal current source, the voice coil inductance and resistance does not affect the acoustic output at all.

Seriously? The voice coil then looks like it has no inductance at all?

[Svante]

Originally posted by 454Casull

Seriously? The voice coil then looks like it has no inductance at all?

Yes. No. The inductance is still the same, but if the inductance varies, the output voltage of the current source will vary to compensate for that. Imagine that an ideal current source has an infinite output impedance. You can connect any impedance in series with that, and the output impedance will still be infinite, ie no change.

This has another positive effect; if the inductance is non-linear and causes distortion in the constant-voltage drive condition, it causes NO distortion in the constant-current drive condition. Funnily, the current source will produce a distorted voltage and this distortion compensates for the distortion caused by the non-linear inductance.

[kelticwizard]

Okay, that might be true for an ideal current source amp.

Not that I am any expert on this, (far from it), but I am given to understand that in real life current source amps the output will vary quite a bit with the impedance variation. This is in direct contrast with most solid state amps with near ideal voltage source behavior.

Is this true?

[janneman]

Originally posted by kelticwizard
Okay, that might be true for an ideal current source amp.

Not that I am any expert on this, (far from it), but I am given to understand that in real life current source amps the output will vary quite a bit with the impedance variation. This is in direct contrast with most solid state amps with near ideal voltage source behavior.

Is this true?

Since Vout = Iout * Rload, with current amps, Iout is fixed for a specific level, Rload changes therefore Vout varies all over the place. But the idea is that Vout doesn't matter with current drive, it's the current that matters. And that is a fixed ratio to input signal.
So, yes, it's true, but who cares?

Jan Didden

[Svante]

Originally posted by kelticwizard
Okay, that might be true for an ideal current source amp.

Not that I am any expert on this, (far from it), but I am given to understand that in real life current source amps the output will vary quite a bit with the impedance variation. This is in direct contrast with most solid state amps with near ideal voltage source behavior.

Is this true?

No.

You can make a very-near ideal current source. It will of course have the same limitations regarding maximum output voltage and current as the typical near-ideal voltage source (=most normal amplifiers).

Here is a proof-of-concept design:

[kelticwizard]

Originally posted by 454Casull
I realize that a current-source amp doesn't care about the impedance in that the magnitude of the current delivered is indepedent of the impedance; however, what about the phase? The current across an inductor lags behind the voltage, so from a conventional voltage-source amp the motion of the speaker would lag behind the voltage waveform going across its voice coil (I think).

I think you are right, but I am not sure. I wouldn't mind some verification on that myself.

I do remember reading in Audio magazine an interview with a prominent designer, (I forget his name). He said that he thought in a few years, the cone loudspeaker would disappear from the high end of the audio market because in one octave, the signal comes at you as if it is ten feet in front of you, but in another octave the signal comes at you as if it was much farther back. I think that might be what he was referring to, but once again I am not entirely certain.

[kelticwizard]

Jan and Svante:

I think I botched up the wording of the post you both replied to.

When I said:

Not that I am any expert on this, (far from it), but I am given to understand that in real life current source amps the output will vary quite a bit with the impedance variation. This is in direct contrast with most solid state amps with near ideal voltage source behavior.

I was thinking of the SPL from a traditional moving coil speaker or speaker system , not the output the amp delivers to the speaker. If a speaker has an irregular impedance curve but a smooth SPL output when hooked up to a solid state voltage driven amp, I am given to understand that when the same speaker is hooked up to a current driven amp the SPL will vary with the impedance plot.

Am I correct on this?

[Svante]

Originally posted by kelticwizard
Jan and Svante:

I think I botched up the wording of the post you both replied to.

When I said:


I was thinking of the SPL from a traditional moving coil speaker or speaker system , not the output the amp delivers to the speaker. If a speaker has an irregular impedance curve but a smooth SPL output when hooked up to a solid state voltage driven amp, I am given to understand that when the same speaker is hooked up to a current driven amp the SPL will vary with the impedance plot.

Am I correct on this?

Ah, I see! Yes, that is the big problem with current drive; it will peak at the system resonance(s). Effectively, what happens is that Qts of the driver becomes Qms and Qes is made completely unimportant.

The output towards higher frequencies will increase too, due to the voice coil inductance.