Single or dual differential?

[dimitri]

And my wish is let us follow the topic (that words in black bold in the upper part of the screen, just below diyaudio.com)
I understand that you all have that much to type here, but please don’t allow Nonsense Expands To Fill The Bandwidth Allotted To It

[Steve Eddy]

Originally posted by hitsware
I=V/Z is a linear relationship BUT
the Z of a driver (or any reactance)
is (by definition) not. Something to
do with 'rate of change'

So what if the Z isn't linear versus frequency? That's not what the author said. What the author said was that current and voltage will only have a linear relationship if the circuit is purely resistive.

Here, read it again:

By Ohm's law, the current in a speaker voice coil is the amplifier output voltage divided by the speaker impedance. Therefore, the current can only be linearly related to the voltage if the speaker presents a purely resistive load to the amplifier.

I'm sorry, but Ohm's Law is a linear equation whether you're using resistance or reactance or impedance and therefore there will be a linear relationship between current and voltage even in a reactive circuit.

I = V/R, V = I x R, R = E/I

I = V/X, V = I x X, X = V/I

I = V/Z, V = I x Z, Z = V/I

se

[Steve Eddy]

Originally posted by fdegrove
Seems to me some people have difficulty in differentiating DC from AC behaviour...

Back to the kindergarten and Thevenin for some of us I reckon??

Eh? Since when does Ohm's Law become non-linear under AC conditions? The relationship between current and voltage is linear whether DC or AC.

Here's your kindergarten lesson for today:

Ohm's Law for resistance:

I = E/R, E = I x R, R = E/I

Ohm's Law for reactance:

I = E/X, E = I x X, X = E/I

Ohm's Law for impedance:

I = E/Z, E = I x Z, Z = E/I

Since all of these equations are linear, how does voltage somehow come to have a non-linear relationship to current under AC conditions?

se

[hitsware]

Lets start another thread about it and I'll respond best I can.
My fault this one digressed enough to make the tone of some of the responses get sort of aggressive.
(or I may be flattering myself )
This forum (new to me ) is a little differant than the FR forum in that more digression is wellcomed over there.
I'd like to see this thread explore active differential
(NOT .....
Counting xfmrs .....
Simply to make it comfortable and stimulating for guru types.
(which i think you and i can agree on ..... we ain't no JC, or NP
or even fred))
front ends to the max.
Off to Raleys ........... mike

[Steve Eddy]

Originally posted by hitsware
Lets start another thread about it and I'll respond best I can.

Here you go:

Ohm's Law 101

se

[dimitri]

>Lets start another thread

... thanks God (and Fred ) ....

[lumanauw]

Maybe this is not about the main topic. But since there are many expert here, I would like to ask another question.
Once I read an article by Japanese about current power amp. He said that what drives a speaker really is current, not voltage.
But all this time we define audio power amp as a device to amplify voltage. That is like there is sinusoidal 1Vpp input, the power amp is amplifying the voltage to certain value, like 30Vpp.
If the speaker is purely resistive, we can get clean sinusoidal current from sinusoidal voltage, since the load is pure resistive. But since the speaker is Impedance (zr+zl+zc), not pure resistive, the current shape certainly be different from the voltage shape, cause of Zspeaker depends on frequency.
The idea is this. What happens if we make an audio power amp, that sense voltage shape input (like sinusoidal), but gives output of current, with that particular input shape.
Is this what we know as "current feedback" (usually using opamp like the ssm- Analog Device current feedback power amp), or is it something else?

[Steve Eddy]

Originally posted by lumanauw
Maybe this is not about the main topic. But since there are many expert here, I would like to ask another question.
Once I read an article by Japanese about current power amp. He said that what drives a speaker really is current, not voltage.

Yes, ultimately it's the current flowing through the loudspeaker's voice coil which produces the time-varying magnetic field which interacts with the fixed magnetic field in the gap and causes the cone to move.

But all this time we define audio power amp as a device to amplify voltage. That is like there is sinusoidal 1Vpp input, the power amp is amplifying the voltage to certain value, like 30Vpp.

Yes. That's because pretty much all loudspeakers are designed to be driven by a voltage source rather than a current source.

If the speaker is purely resistive, we can get clean sinusoidal current from sinusoidal voltage, since the load is pure resistive. But since the speaker is Impedance (zr+zl+zc), not pure resistive, the current shape certainly be different from the voltage shape, cause of Zspeaker depends on frequency.

No, if you feed a sinusoid into a reactive circuit you'll get a sinusoid output.

The idea is this. What happens if we make an audio power amp, that sense voltage shape input (like sinusoidal), but gives output of current, with that particular input shape.

You'll get a sinusoid either way.

The problem is that you'll get a frequency response that resembles the speaker's impedance curve.

For example, the image I've attached is of the frequency response and impedance plot of a Fostext FE208E Sigma full range driver.

The plot along the top is the driver's frequency response as driven from a voltage source amplifier and the plot below is its impedance versus frequency.

If you drove the driver from a current source, its response would resemble its impedance plot, and instead of the gradual rolloff of its low frequency response, you'd get a big peak in its low frequency response.

se

[lumanauw]

What will it sound?
My idea of current amp is like this. I'll take an example from schematic in post no.81 (by hitsware). Let's assume that this is a working amp (not experimental).
The output is taken from the drain junction of SK and SJ. Push-pull amp will have this kind of output, usually taken in the junction of 0.22ohm/5W resistor (after emitors, if it is EF)
To get current sensor, before going to loud speaker, between the junction of those drains before output to sepaker, we put R, like 1ohm, to detect what is the current delivered to the speaker.
Then, this current figure is compared to the voltage signal in the differential input.
This way we don't need the 10k and 510ohm voltage feedback divider, since we are detecting the output current in the 1ohm resistor drop. That is the data for the differential to measure the difference in signal input and current output. Maybe this is "voltage input - current output" audio power amp. But is this possible? What will it sound?
This idea is coming from the statement of japanese I mentioned before that speakers are driven by current, not voltage.

[dimitri]

Dear lumanauw,
this topic name: single or dual differential

here is something about current drive
Hawksford wrong on amp topology for current driving loudspeakers?
Damping Factor >1000