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Replace resistor by low-inductance resistor - Is it audible?

Francis Vaughan

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For those who have a clue what the loopgain stability is, there is a comparison of 0R22 resistor non-inductive and with 1uH inductance, as a Re resistor in output stage of the class AB amplifier with global negative feedback. I am sorry I would be tired to argue with anyone that resistor inductance does not matter.
That is actually pretty cool. Really nice example of where things can come unglued. The difference in stability margin would give one pause for thought.
 

solderdude

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I am sorry I would be tired to argue with anyone that resistor inductance does not matter.

NO one here argues that inductive resistors IN an amplifier (certainly with overall feedback) does NOT matter. It does, it is logical, measurable and obvious. No one disagrees with you about this. No one is against you on this.

This thread is NOT about resistors IN an amplifier IN the audiopath IN a feedback loop... this matters ... no dispute nor denial.
The argument and measurements are of resistors IN the loudspeaker filter.
There only power rating and tolerance is of any significant effect.
No amplifier in the world will EVER become unstable if a resistor IN A SPEAKER is wirewound and has an inductance as cables and speakers have much higher inductance by themselves.
 
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March Audio

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For those who have a clue what the loopgain stability is, there is a comparison of 0R22 resistor non-inductive and with 1uH inductance, as a Re resistor in output stage of the class AB amplifier with global negative feedback. I am sorry I would be tired to argue with anyone that resistor inductance does not matter.View attachment 55606
Give it a rest mate, no one is arguing or disputing this.

It is just totally off topic and irrelevant.

Please start another thread if you want to discuss this.
 
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SIY

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Give it a rest mate, this is totally off topic and irrelevant.

Please start another thread if you want to discuss this.

Pavel likes to design old-fashioned power amps. So do I (even MORE old-fashioned).

Unfortunately, that means that, irrespective of the topic (in this case, resistors used in loudspeaker passive crossover networks), he will only talk about it with respect to designing old-fashioned power amps. If you see me do the same thing ("But look what happens in a tube amp!"), feel free to metaphorically club me. :cool:
 

DualTriode

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Sorry, again no. There is no active damping. ....

Hello,

There is absolutely active damping.

At resonance the output device, the driver, is a motor that generates AC voltage at the amplifier output. From the amplifier output the Back EMF goes back through the feedback loop to an inverting input. The amplifier now outputs AC voltage that is 180 degrees out of phase to the voltage generated by the driver motor. That is active damping

Now the current crop of amplifiers includes active devices in the feedback loop. That is what we call error correction.

Thanks DT
 

SIY

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Hello,

There is absolutely active damping.

It would have been nice if you'd read and understood the rest of this. However, it's TOTALLY irrelevant to the ludicrous claim that an extra couple of microhenries in a speaker crossover is of any significance. I'm sorry, you're just absolutely wrong about that, absolutely wrong that the speaker is in the amp's feedback loop, and absolutely wrong that a couple of microhenries in a wirewound resistor of crossover size is unusual. Just... wrong.

Active devices in the feedback loop are NOT "error correction," though they can be used that way. Plain old resistors in a feedback loop are also "error correction." Or is this more of your private terminology?
 
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Francis Vaughan

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Active damping has a quite clear technical definition. It does not cover what is described here. It sort of sounds a bit like the feedback loop is "actively" doing something, but that activity is restricted to acting like a voltage source, which means it tries to look as close to a zero impedance as possible. The back-emf is a force that is attempting to push it away from that, so it counters it. But, and be clear, that countering never exceeds the potential needed to stop the output node of the amplifier looking like it has zero impedance. So the damping of the speaker never improves over that seen by shorting the terminals of the loudspeaker. Compared to steam driven amplifiers of old, ones that had quite significant output impedances (and including modern abominations like zero feedback SET) this is a big step up.

Active damping, as the term is generally accepted to mean, requires that there is a control network that is predicting movement of the cone, and is generating countering forces, or otherwise varying system parameters, that will control the resonance faster/better than the intrinsic internal resistive losses will. The obvious example is automotive active suspension, or active noise cancellation. That typically is done in loudspeakers by adding analogues of the mechanical properties of the speaker to another control system. This is usually in a feedforward network, although there are other ways. The REL subwoofer reviewed here has some clever tricks that have a similar effect. There are other ways as well, for instance using acceleration feedback (via an accelerometer) can be used to move the effective mass of the driver. Lots of interesting possibilities are available. But they require additional circuitry and sensors. In the extreme there are suggestion for using Kalman filters to predict cone motion, and Klippel himself has a blue sky paper talking about possibilities for self calibrating feedforward correction.
 

Francis Vaughan

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Get out your frequency generator and oscilloscope and test it.
You do too.
Test it with an amplifier and then with a resistor across the speaker terminals with a value the same as the output impedance of the amplifier. They will be the same.
 

DualTriode

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You do too.
Test it with an amplifier and then with a resistor across the speaker terminals with a value the same as the output impedance of the amplifier. They will be the same.

You are thinking about it.

On one hand you have a single end triode amplifier with little or no damping.

On the other hand you are speaking about advanced motion controls with precision accelerometer controlled servos. Bolt on after market stuff, however an integral part of the feedback system.

What I am speaking about is somewhere in between the two. Driver Back EMF injected into the amplifier feedback loop.
 

DualTriode

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It would have been nice if you'd read and understood the rest of this. However, it's TOTALLY irrelevant to the ludicrous claim that an extra couple of microhenries in a speaker crossover is of any significance. I'm sorry, you're just absolutely wrong about that, absolutely wrong that the speaker is in the amp's feedback loop, and absolutely wrong that a couple of microhenries in a wirewound resistor of crossover size is unusual. Just... wrong.

Active devices in the feedback loop are NOT "error correction," though they can be used that way. Plain old resistors in a feedback loop are also "error correction." Or is this more of your private terminology?

Okay we do agree; Plain old resistors in a feedback loop are also "error correction."

We can likely also agree that the Benchmark AHB2 is an example of the new crop of amplifiers that include active devices for error correction. No new terminology needs to be defined.

Personally my preference is to actively crossover at line level and leave out the capacitors, inductors and resistors between the driver and amplifier output. All those passive devices placed in between actually do interfere with damping.

Now if you will go back and read my comments about microhenries and inductance, I have made no claims about audibility. I have only asked how much is audible.

It is much the same as pursuing low distortion. At what level is distortion audible? Yet we chase and pay for low distortion beyond the limits of hearing.

Now take your strawman and go out in the garden and play with the fairies.
 

JohnYang1997

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Okay we do agree; Plain old resistors in a feedback loop are also "error correction."

We can likely also agree that the Benchmark AHB2 is an example of the new crop of amplifiers that include active devices for error correction. No new terminology needs to be defined.

Personally my preference is to actively crossover at line level and leave out the capacitors, inductors and resistors between the driver and amplifier output. All those passive devices placed in between actually do interfere with damping.

Now if you will go back and read my comments about microhenries and inductance, I have made no claims about audibility. I have only asked how much is audible.

It is much the same as pursuing low distortion. At what level is distortion audible? Yet we chase and pay for low distortion beyond the limits of hearing.

Now take your strawman and go out in the garden and play with the fairies.
Last sentence is unnecessary.
 

Francis Vaughan

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You are thinking about it.

On one hand you have a single end triode amplifier with little or no damping.

On the other hand you are speaking about advanced motion controls with precision accelerometer controlled servos. Bolt on after market stuff, however an integral part of the feedback system.

What I am speaking about is somewhere in between the two. Driver Back EMF injected into the amplifier feedback loop.

So am I. The others are examples of different systems that we are not talking about the central point, really to emphasise why a voltage source is the key to conventional speaker analysis. Try reading what I wrote again.

Go and do the experiment. I have been quite explicit for at least the last day odd. A conventional amplifier with negative feedback configured as a voltage source (ie 99.9% of all audio power amplifiers) is indistinguishable from a resistor with the same impedance across the speaker terminals when considering damping of the cone. This is to say that the amplifier provides no active damping, as is conventionally understood by the term.

Now go and do the experiment. It isn't exactly hard.
 

Francis Vaughan

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We can likely also agree that the Benchmark AHB2 is an example of the new crop of amplifiers that include active devices for error correction. No new terminology needs to be defined.

The ABH2 is a feedforward design. This is quite well understood technology, and in domestic HiFi goes all the way back to 1978 and the Quad 405. Feedforward as an idea actually predates feedback by about 4 years. Both were invented by Harold Black and are close to 100 years old.
http://www.aes.org/e-lib/online/browse.cfm?elib=4007

Note, a feedforward design is not a feedback design. They are intrinsically different. You can of course build a hybrid.
 

solderdude

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What I am speaking about is somewhere in between the two. Driver Back EMF injected into the amplifier feedback loop.

Experiment time:

Take a stereo amp (or 2 different amps). Connect L and R output via an 8 Ohm resistor. Put a 1V signal on one amp, try low and high freq.
Short the input on the other channel (amp) and measure the actual voltage change on the voltage amp part (so before the output voltage followers) of the amp with the shorted input. There you can actually see (measure) what the feedback path does. It shows the 'active damping'.
Post pictures of the results and setup to make your point.

Personally my preference is to actively crossover at line level and leave out the capacitors, inductors and resistors between the driver and amplifier output. All those passive devices placed in between actually do interfere with damping.

I don't think many will disagree with this. Especially large inductors in series with the woofer will seriously f' up the damping factor. Using a DF 10,000 or DF 50 amp. won't make much difference here.

If the DF were that important every speaker manufacturer would be using sense cables connected at the speaker terminals like Kenwood tried long ago with their Sigma drive.

Of course this all has nothing to do with series resistor types in the speaker filter.
 

KSTR

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My impression is that many audio designers don't fully understand the amplifier--speaker interface. Especially they don't fully understand "damping" and they are not aware that while there is feedback, it is local degenerative feedback in the speaker driver itself.

Damping is defined by the total terminating impedance the voice coil sees. Most of that is its own static impedance, described by its DC resistance Re plus some static semi-inductance. The amp's output impedance comes on top of that and is pretty much irrelevant, even in an active speaker, because Re is orders of magnitude greater. With series elements in between in case of passive crossovers, it's even more irrelevant. "Damping factor" is a red herring and no serious designer would ever use that term, it was invented for marketing only.
The above describes the standard case where the amp is designed as a voltage source with low and stable output impedance. The topology of the amp does not matter for this.

Amplifiers can be designed with arbitrary output impedance, though. For a practical system, the real part of the impedance can even be negative, but not more than -Re for obvious stability reasons. At the other end there are no restrictions, the output impedance may reach positive infinity, representing a true current source.

Now what does that imply for the damping and the feedback it establishes?
The current drive case is easy to explain. The driver is operated in pure force-controlled mode, the current creates a force F=BL*i that is impressed. The microphonic voltage (aka "back-EMF", a term that shouldn't be used as it only creates confusion) can develop freely and simply goes into the voids. Think of it like a signal generator in series, the current source doesn't care, it still injects the current demanded by the amp's input signal. The only damping present is the mechanical damping of the driver in the given build. At resonance, only little current is needed to exite the movement which means the current profile must have a proper shaping (EQ) to get a flat frequency response. This technique is fraught with problems because of the instablilities, at least for standard drivers in standard enclosures that are to be operated at their resonance frequency and have little mechanical damping. Above resonance, or if resonance is well controlled mechanically for example with resistive air load in horn designs, it is feasible though and actually has some significant benefits.
Current drive means no local feedback in the driver, as the microphonic voltage has zero effect on the output, the driver is free to move, only steered by the force generated by the impressed current.

Let's look at the opposite corner case, terminating the voice coil with very close to zero effective impedance (output impedance of amp = -Re + some small safety marging, lets assume to use 99% of -Re). The voltage of a moving voice coil always is E = i*Re + k*dx/dt, in words, the sum of the voltage drop along the static impedance plus the microphonic voltage which is proportional to voice coil velocity (1st derivate of the voice coil displacement x). Because the effective Re now is only 1% of the Rdc of the driver, the i*Re term is quite insignificant. The amplifier now controls the velocity of the driver directly. When the microphonic voltage produced by the motion does not match the required voltage as per amp's input signal, even a small voltage difference produces a huge correction current because the effective Re is so small. This a true feedback mechanism, the driver is operating in velocity-controlled mode by the feedback. Because sound pressure is proportional to cone acceleration (below the point the cone starts beaming), the input signal must be EQ'd to a falling 20dB/decade slope vs frequency to get the required velocity profile.
Again, full feedback of this kind isn't practical for many reasons, the most important one being the static impedance not being stable, the DC resistance has thermal drift and the inductive part is instable, current and position dependent, etc. On top of that, the sensor itself is not linear, the microphonic voltage starts to drop when the VC starts to leave the gap, leading to a nasty overshoot reaction (the feedback "thinks" it must correct for an apparantly too low velocity and impresses counteracting current until the senses voltage matches the input, but then the actual velocity is too high).

Which leaves us with usable ranges for the output impedance somewhere in between these extremes. It can be found empirically that for a given driver in a given enclosure design there is a terminating impedance profile vs frequency that gives the best results overall with the lowest distortion and a stable large-signal behavior, notably a clean and quick overdrive recovery, in most cases this impedance does *not* result in zero output impedance of the amplifier. Therefore, it often is a good idea to have some series elements in the path to the driver even in active systems with standard voltage amplifiers, and it is good for noise as well. This is also a reason why a passive design may actually measure and sound better than its active replica with all drivers terminated with zero impedance.

Special acoustic loadings need special treatments wrt to terminating impedance, for example a ported box needs a properly adjusted mechanical damping of the Helmholtz resonator to make it effective. Notably too high damping of the driver (brickwall behavior) is a bad thing, make the port Q too high and its output contribution too narrow-band. Too low damping isn't good eiher, making the port ineffective. Closed boxes or open dipoles typically work best when the excursion shows close to critical damping in its step response which means the driver settles fast from its own error signal, notably after an overdrive event. Too small damping results in ringing at the resonance whenever the cone is exited by an external signal (including its own errors), EQing the input signal doesn't help here because the error develops after that. This is one of the reasons why current drive doesn't work nice at resonance.
 
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kach22i

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Hello,

There is absolutely active damping.

At resonance the output device, the driver, is a motor that generates AC voltage at the amplifier output. From the amplifier output the Back EMF goes back through the feedback loop to an inverting input. The amplifier now outputs AC voltage that is 180 degrees out of phase to the voltage generated by the driver motor. That is active damping

Now the current crop of amplifiers includes active devices in the feedback loop. That is what we call error correction.

Thanks DT
I was told that all tube amps listen to the room through the speakers, Bob Carver is marketing his current works as such.

An amplifier that actually listens to the room.
https://www.bobcarvercorp.com/copy-of-crimson-raven

Same thing as what you are describing?

Loudspeakers are electric motors. Electric motors all kick voltage back to the power source whether it’s a loudspeaker, a refrigerator or an electric drill.
 
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