Amplifier Output Impedance (Damping Factor) and Speakers

[Ingenieur]

But not when the speaker is in motion.
In the voice coil structure, the Re is also dependent on frequency, time, level and temperature.

Is it?
Does the resistivity of copper change?
The length or diameter of the wire?
Or does the apparent resistance change when measured due to load and back EMF?

We do not talk about the R of motor coils when running, only when stopped.

 

[pogo]

Let me say it another way. What portion of Re is involved when the following applies to a motion and that with a condition that has not been steady-state:
Each turn of the voice coil couples individually and proportionately, so the resistance is distributed.

 

[Ingenieur]

When looking at a speaker Z plot think of phase as V/I difference.
So if V ^ due to emf, Z ^
If I decreases due to higher R, Z increases

Also the crossover is reflected in the plot and the frequency determines what portion of R is exposed at the terminals (I is bypassed, filtered).

 

[pogo]

Also the crossover is reflected in the plot and the frequency determines what portion of R is exposed at the terminals (I is bypassed, filtered).

The plot was determined with a nearly steady state test signal. But what does the whole thing look like in the time domain with transients?

 

[Suffolkhifinut]

Let me say it another way. What portion of Re is involved when the following applies to a motion and that with a condition that has not been steady-state:
Each turn of the voice coil couples individually and proportionately, so the resistance is distributed.

Until the winding gets hot, then the resistance of each turn will depend on where they are in the winding.

The plot was determined with a nearly steady state test signal. But what does the whole thing look like in the time domain with transients?

 

[Ingenieur]

The plot was determined with a nearly steady state test signal. But what does the whole thing look like in the time domain with transients?

A frequency sweep is not steady state.
It is quasi-time domain, 1/f, to get to the frequency domain you need a transform.

The Z should not change, it is V/I
Magnitude and phase
V test signal
I load dependent

Resistance is not referred to as distributed
Capacitance is

I guess you could say a 100 turn coil with R = 1 Ohm, each turn = 0.01 Ohm
But why?
You only care about terminal V(unless a tapped xfmr) and I is the same thru each,

 

[Suffolkhifinut]

A frequency sweep is not steady state.
It is quasi-time domain, 1/f, to get to the frequency domain you need a transform.

The Z should not change, it is V/I
Magnitude and phase
V test signal
I load dependent

Resistance is not referred to as distributed
Capacitance is

I guess you could say a 100 turn coil with R = 1 Ohm, each turn = 0.01 Ohm
But why?
You only care about terminal V(unless a tapped xfmr) and I is the same thru each,

1/f is called periodic time, the time taken for 1 cycle of an alternating waveform.
Regarding a steady state let’s look at a coil with an inductance and resistance given in an earlier post, L=1 mH and R at 4 Ohms.
At a frequency of 1 kHz Z = 7.45 Ohms
At 100 Hz Z = 4.03 Ohms.
So at lower frequencies coil inductance has little effect on current.

 

[Ingenieur]

1/f is called periodic time, the time taken for 1 cycle of an alternating waveform.
Regarding a steady state let’s look at a coil with an inductance and resistance given in an earlier post, L=1 mH and R at 4 Ohms.
At a frequency of 1 kHz Z = 7.45 Ohms
At 100 Hz Z = 4.03 Ohms.
So at lower frequencies coil inductance has little effect on current.

Gotcha, trying to express in simple terms.
The sweep is transient in freq/time, eg, the sweep may take 1 sec., never the same freq twice. The amplitude may be constant V, which I assume is how they do it? Since an amp is a voltage gain/source device, I being determined by load. The ratio being Z.

Period T = 1/f , 1 cycle, 360 deg

X = 2 Pi 100 0.001 = 0.63 Ohm
Sqrt(16 + 0.395) ~ 4.05
Phase arctan(X/R) ~ 5.6 deg, PF ~ 0.995
Almost purely resistive

Capacitance is a different can of worms.
But typically in //.

I just turn my hifi on and listen.

 

[Ingenieur]

I think one of the confusions is the plot shows a magnitude of 20 Ohm, phase 30 deg, net 'R', but really a reflection of active power, is 17 Ohm.

Very simplistic equivalence, please be kind lol
2 things:
-There are other R in the xover's
-Consider the difference between the 17 and 8 (less losses of other R) as the work the speaker is doing,
Similar to a motor:
115 V, 1 HP, 16 A
P = 746 W = 1 HP
S = 115 x 16 = 1840 VA
pf + eff ~ 746/1840 ~ 0.4
Net pf = 0.4 / 0.9 (10% eff) ~ 0.45, 66 deg
Z = 115 / 16/ 66 = 7.2 / 66 Ohm
R = 2.9 Ohm
X = 6.6 Ohm

We know the motor windings are not 2.9 Ohm. That is over 800 W wasted heat.
It's fractions of an Ohm.

P = R x I^2 ~ 2.9 x 16^2
~ 742 W ~ 746 W ~ 1 HP

 

[egellings]

I had a chance to hear a pair of Acoustat 'stats' with direct coupled H.V. tube amps using sweep tubes driving them. Those things were downright spooky in their realism.

 

[Kijanki]

No just the opposite! The voltage induced it the speaker must always oppose the applied voltage. Otherwise you would get out electrically more than you put in and in any machine, even a speaker efficiency can never reach 100% never mind exceed it.

Not the voltage but the current has to flow in the opposite direction. Lenz law says that current induced by the motion has to oppose changes in magnetic field, meaning that flux produced by this current has to be in opposite direction. This means that current has to be in the opposite direction to produce this opposite flux. If current induced by free membrane motion would be in the same direction as amplifier`s current that causes motion in the same direction, then there would be no braking action. Induced current would propel membrane even further.

 

[Ingenieur]

Not the voltage but the current has to flow in the opposite direction. Lenz law says that current induced by the motion has to oppose changes in magnetic field, meaning that flux produced by this current has to be in opposite direction. This means that current has to be in the opposite direction to produce this opposite flux. If current induced by free membrane motion would be in the same direction as amplifier`s current that causes motion in the same direction, then there would be no braking action. Induced current would propel membrane even further.

Depending on phase, etc. current or voltage can be either 'direction'. Power can flow either way also. So it can add to supply.

Simple example, no R.
You can see the supply is 1, but drop across Xl is 2, therefore the rise across Xc must be 1. The only thing that changes with R is phase and magnitude. If Xc > Xl it would contribute.

 

[Suffolkhifinut]

Not the voltage but the current has to flow in the opposite direction. Lenz law says that current induced by the motion has to oppose changes in magnetic field, meaning that flux produced by this current has to be in opposite direction. This means that current has to be in the opposite direction to produce this opposite flux. If current induced by free membrane motion would be in the same direction as amplifier`s current that causes motion in the same direction, then there would be no braking action. Induced current would propel membrane even further.

Agreed the induced voltage tries to feed current back into the amplifier’s output stage. Worth remembering when it comes to inductance current is always the reference, with capacitance it’s voltage. Suppose it’s just custom and practice to refer to them that way?

 

[Ingenieur]

Agreed the induced voltage tries to feed current back into the amplifier’s output stage. Worth remembering when it comes to inductance current is always the reference, with capacitance it’s voltage. Suppose it’s just custom and practice to refer to them that way?

L = magnetic field = current
C = electric field = voltage

 

[Suffolkhifinut]

L = magnetic field = current
C = electric field = voltage

Yet you can’t have one without the other! If you look at the exponential charging curves for capacitors. They show both current and voltage going in the opposite direction, both are valid and correct. Current leads voltage by 90 degrees.

 

[Ingenieur]

Yet you can’t have one without the other! If you look at the exponential charging curves for capacitors. They show both current and voltage going in the opposite direction, both are valid and correct. Current leads voltage by 90 degrees.

ELI the ICE man

 

[pogo]

Here is an interesting measurement report about the switchable damping factor of the A200 (page 4, bottom left), which can be found on the T+A website: Link

 

[Geert]

Here is an interesting measurement report about the switchable damping factor of the A200 (page 4, bottom left), which can be found on the T+A website: Link

Measurements show a 0,4dB amplitude difference between a 2 ohm and 8 ohm load when switching the damping factor to 60. With the damping factor on 700 there's no difference. What's so interesting about that, it's basic math?

 

[pogo]

What's so interesting about that, it's basic math?

Interesting is that these frequency response measurements and the simple math for it do not correlate at all with the sound representation in reality

'A high damping factor tends to produce a more clearly defined, very precise and analytical sound image, whereas a reduced damping factor produces a more warm and softer sound image.' <- extract from the T+A A200 manual

Actually, an amp with low DF should sound like less low frequency power regarding the frequency response measurement. But exactly the opposite happens!
Frequency response measurement in steady state with a test signal on a non real load is not sufficient here.
The decisive factor here is the acoustic energy emitted over a certain period of time, i.e. the swing out behavior of the chassis and maybe a stronger excitation of the room modes.

 

[SIY]

Interesting is that these frequency response measurements and the simple math for it do not correlate at all with the sound representation in reality

'A high damping factor tends to produce a more clearly defined, very precise and analytical sound image, whereas a reduced damping factor produces a more warm and softer sound image.' <- extract from the T+A A200 manual

Marketing copy is always a better source than actual engineering analysis.