Can load dependency be wholly corrected with EQ?

[shuppatsu]

I've asked this question a couple times in review threads but either it gets buried in the mix or no one knows:

The ASR graphs suggest that correcting for load dependency is a simple peak filter once you've made some measurements of your speaker in the room. But is this actually the case? I assume those graphs are made with a pure tone sweep. How does impedance change with actual music, and does this impact how the frequency response is? For example, if there were a 200Hz note and 15KHz note at the same time, would the 15KHz note be boosted in the same way that it might if the 15KHz note were played by itself?

FWIW, ChatGPT gives a rather plausible answer that EQ cannot wholly correct load dependency issues (expand spoiler if interested). But ChatGPT doesn't give any sources and draws from the audiophile community, which is full of BS. "Plausible" doesn't mean correct.

Spoiler: Chat-GPT answer

From an audio objectivist perspective—like those seen at Audio Science Review (ASR)—we’d approach the question analytically, focusing on measurable performance, amplifier design principles, and the effectiveness of equalization in addressing load-dependent behavior.

Load Dependency in Class D Amps​

In Class D amplifiers, the output stage uses high-frequency switching to modulate the audio signal, which can lead to load-dependent frequency response variations. This occurs because:

1. The output filter (low-pass filter) is designed to remove the high-frequency switching noise, and its performance depends on the connected load's impedance.
2. Impedance variation at different frequencies changes how the filter interacts with the signal, leading to deviations in the frequency response.

This means that, especially with speakers of varying impedance (e.g., dipping to 4 ohms or lower), the amplifier might not produce a perfectly flat response, particularly at high frequencies.

Can Equalization Fully Correct Load Dependency?​

Theoretical Perspective:​

In theory, equalization could address some aspects of load-dependent frequency response, but it’s not a complete solution for several reasons:

1. Global vs. Local Equalization: Equalization (EQ) is typically applied in a global manner—adjusting the input signal before amplification. However, load dependency is a localized effect at the amplifier’s output stage, meaning EQ adjusts the signal in advance, but can’t directly influence how the amp’s output filter interacts with the speaker load in real-time.
2. Dynamic Nature of Load Dependency: The impedance of a speaker is not constant; it varies across frequencies and, in some designs, with volume levels and crossover behavior. EQ would have to be incredibly precise and adaptive to handle these dynamic changes. Standard EQ typically applies static adjustments, which can't follow the complex, non-linear impedance shifts in real-time.
3. Phase and Time Effects: Load dependency can also affect the phase response of the amplifier, which EQ does not usually correct. EQ can flatten the frequency response in amplitude, but phase shifts or time-domain artifacts introduced by the interaction between the amplifier and the load might remain uncorrected.

Practical Perspective:​

From a practical, measurable standpoint, which is the foundation of an objectivist approach, the correction possible through EQ is often limited:

• Low-frequency deviations caused by impedance changes may be somewhat correctable since EQ can be effective in handling amplitude differences at lower frequencies.
• High-frequency deviations, where Class D amps often show the most significant load dependency, are harder to fully correct with EQ. The interaction between the output filter and the speaker impedance introduces effects that EQ alone may not mitigate effectively.

Real-World Testing (ASR Approach):​

Looking at objective measurements from sources like Audio Science Review, load-dependent Class D amps often show frequency response variations depending on the load, particularly in the high frequencies (e.g., roll-off with lower impedance loads). Measurements show that:

• The frequency response changes by several dB at specific frequencies with different speaker loads.
• Attempting to correct these using EQ would require precise compensations that may only be valid for one load condition.

Moreover, real-world listening tests sometimes corroborate that even after EQ is applied, residual artifacts from load dependency (such as distortion or phase shifts) can still affect subjective audio quality. While equalization can flatten amplitude response, it doesn’t fully address the underlying causes of load dependency, especially in Class D designs.

Conclusion:​

From an objective standpoint, EQ can mitigate some aspects of load dependency in Class D amplifiers, but it cannot completely correct it, especially for more complex impedance interactions. An ideal solution would involve a better amplifier design—one that minimizes load dependency through a robust output stage or a load-invariant design, rather than relying solely on post-hoc EQ adjustments.

 

[SIY]

ChatGPT has done a fine job of distilling nonsense into its essence.

 

[DVDdoug]

At low-enough levels, yes. Or if you are cutting rather than boosting, yes. When you boost, there is the possibility of clipping your amplifier, or digital clipping (with digital EQ) if you aren't "careful".

If you have a +6dB bump you can fix it with a -6dB cut, no matter the cause.

The ASR graphs suggest that correcting for load dependency is a simple peak filter once you've made some measurements of your speaker in the room.

That shouldn't happen with a reasonably good amplifier and "normal" speakers. It can happen with tube amplifiers (higher damping factor) but it doesn't have to. I'm pretty-sure it's not a problem with McIntosh tube amps.

It's a more common with headphones since headphone impedance (and headphone amplifier design) varies all over the place.

or example, if there were a 200Hz note and 15KHz note at the same time, would the 15KHz note be boosted in the same way that it might if the 15KHz note were played by itself?

The different simultaneous frequencies don't interfere with each other (again assuming no clipping or other distortions). ...Well, they CAN, but EQ of one frequency band doesn't affect the other bands.

 

[AnalogSteph]

The ASR graphs suggest that correcting for load dependency is a simple peak filter once you've made some measurements of your speaker in the room. But is this actually the case? I assume those graphs are made with a pure tone sweep. How does impedance change with actual music, and does this impact how the frequency response is? For example, if there were a 200Hz note and 15KHz note at the same time, would the 15KHz note be boosted in the same way that it might if the 15KHz note were played by itself?

Assuming an adequate degree of linearity, the superposition principle is your friend. That assumption should be true for any speaker with at least 2 ways at any kind of sensible levels.

 

[Sokel]

Assuming you now exactly what to correct as any interaction with different speakers can vary (a lot)
You can find great info here,along with examples and ABX test proving audibility:

Complex impedance load amp FR influence - WiiM Amp review by Erin

Was pleasantly surprised today when I read the review of the WiiM Amp on Erin's website. Erin will be using two complex impedance profiles in all of his amplifier reviews in the future to measure their impact on the frequency response of the tested amplifiers. My only criticism is that the...

www.audiosciencereview.com

 

[solderdude]

I've asked this question a couple times in review threads but either it gets buried in the mix or no one knows:

The ASR graphs suggest that correcting for load dependency is a simple peak filter once you've made some measurements of your speaker in the room. But is this actually the case? I assume those graphs are made with a pure tone sweep. How does impedance change with actual music, and does this impact how the frequency response is? For example, if there were a 200Hz note and 15KHz note at the same time, would the 15KHz note be boosted in the same way that it might if the 15KHz note were played by itself?

Load dependent EQ can only correct properly when the whole actual system is characterized. You cannot do that from measurement results taken in just 1 or 2 different resistive loads.

Not load dependent EQ can be used to correct a response based on measurements.

When transducers come into play a myriad of issues comes to life (dynamic compression, distortion that changes at different levels).
At least one can correct some issues which are most likely the most important ones (FR)

FWIW, ChatGPT gives a rather plausible answer that EQ cannot wholly correct load dependency issues (expand spoiler if interested). But ChatGPT doesn't give any sources and draws from the audiophile community, which is full of BS. "Plausible" doesn't mean correct.

Conclusion:
From an objective standpoint, EQ can mitigate some aspects of load dependency in Class D amplifiers, but it cannot completely correct it, especially for more complex impedance interactions

Yep, correct conclusion.
As soon as things are load dependent you cannot correct it unless the whole (electrical) system is characterized.
Then you can correct electrical errors but will still have acoustical errors.
These can be (partly) corrected at a specific position (where a calibrated measurement microphone was).

In any case .... correction, even if not entirely correct, should give improvement.

 

[shuppatsu]

ChatGPT has done a fine job of distilling nonsense into its essence.

Do you speak in generalities or do you think its response here is nonsense?

That shouldn't happen with a reasonably good amplifier and "normal" speakers. It can happen with tube amplifiers (higher damping factor) but it doesn't have to. I'm pretty-sure it's not a problem with McIntosh tube amps.

I'm referring to amplifiers with load dependency, such as cheap Class D designs without PFFB.

Assuming an adequate degree of linearity, the superposition principle is your friend. That assumption should be true for any speaker with at least 2 ways at any kind of sensible levels.

I'm slow, could you say whether the superposition principle implies that you can or cannot correct for load dependency with EQ?

Load dependent EQ can only correct properly when the whole actual system is characterized.

Are you referring to a kind of EQ that applies different EQ based on what the load is at any given time? Or are you saying that given the right kinds of (hard to obtain) measurements, one could come up with a PEQ profile that completely handles the FR issues caused by load dependency?

At any rate, thanks y'all for the responses. It looks to me that PFFB is worthwhile even with room correction.

 

[solderdude]

Are you referring to a kind of EQ that applies different EQ based on what the load is at any given time? Or are you saying that given the right kinds of (hard to obtain) measurements, one could come up with a PEQ profile that completely handles the FR issues caused by load dependency?

The latter... one can, when the amp + load has been measured (characterized) come up with an EQ that completely corrects the frequency response.

At any rate, thanks y'all for the responses. It looks to me that PFFB is worthwhile even with room correction.

PFFB will lower the distortion a bit but still have peaking, just not load dependent and above the audible range.
There can still be differences with certain capacitive loads though.
One could use EQ prior to the amp to lower the (still present) HF peaking in those amps.

 

[shuppatsu]

One could use EQ prior to the amp to lower the (still present) HF peaking in those amps.

What are the implications of HF peaking? Seems to me that +2.5dB at 30KHz would do nothing. Is the concern increased chance of IMD? Is this only an issue for hires content, since the reconstruction filter would attenuate the high frequency anyway at 44.1.

 

[kemmler3D]

What are the implications of HF peaking? Seems to me that +2.5dB at 30KHz would do nothing. Is the concern increased chance of IMD? Is this only an issue for hires content, since the reconstruction filter would attenuate the high frequency anyway at 44.1.

If you have peaking outside the audible range, it is still wrong, but you don't need to worry about it because you won't hear it.

The one thing you might want to think about is if you have very high IMD coming in right above 20khz, AND you are playing high-res content, then you might get something bleeding back into the audible range. I think this would be a rare situation at best.

 

[SIY]

Do you speak in generalities or do you think its response here is nonsense?

Both. Though to be fair, I used it to generate my mandatory DEI statement at work and it did well enough for me to get past that requirement. But as for this specific thing... solderdude nailed it for you.

The latter... one can, when the amp + load has been measured (characterized) come up with an EQ that completely corrects the frequency response.

 

[solderdude]

What are the implications of HF peaking? Seems to me that +2.5dB at 30KHz would do nothing. Is the concern increased chance of IMD? Is this only an issue for hires content, since the reconstruction filter would attenuate the high frequency anyway at 44.1.

It's not an issue IRL but you wanted to know about corrections that are system wide so just mentioned it and that it can be corrected for as well.
Note that any HF in recordings that might cause IM are already low in level anyway so even when there are IM products they will be low and HD for treble above 8kHz also will be inaudible.

 

[voodooless]

Just apply room correction as a whole, and ignore the load dependencies. No need to characterize the system, measure impedance or any other complex stuff

 

[solderdude]

Just apply room correction as a whole, and ignore the load dependencies. No need to characterize the system, measure impedance or any other complex stuff

Room correction ONLY can correct the entire system at the exact mic position(s) to that mic and the gear it is connected to and may not be valid when not in that exact position or circumstances.
The measurements also has quite a lot of tolerances and measurement errors in it. On top of that one has to choose a target and not all targets are equal.

Even when the (electric) part is fully characterized and compensated electrically you still have to do EQ to correct for the transducers and condition errors and choose a target.
So in that sense there is little gain in characterizing the electric part other than knowing that part is fine and has lower distortion and proper response. This latter fact may 'ease' the mind that this part is optimal and may well enhance enjoyment by itself.

Isn't audio fun... he could also just enjoy what he has and realize that the transducers and usage of them is magnitudes worse that whatever in the system was 'optimized' for.

The original question 'Can load dependency be wholly corrected with EQ?' can only be answered with 'that depends on what kind of correction is used and how that was done'

 

[voodooless]

Room correction ONLY can correct the entire system at the exact mic position(s) to that mic and the gear it is connected to and may not be valid when not in that exact position or circumstances.

Sure, but that amp doesn't change that fact.

The measurements also has quite a lot of tolerances and measurement errors in it. On top of that one has to choose a target and not all targets are equal.

You should do all this anyway, and these caveats apply regardless of what the amp does, as well.

 

[solderdude]

I don't know if one really should. Plenty of people just enjoy speakers in their rooms just the way it is.
Doing the room correction may well change it. No way to tell if that is going to be more enjoyable. Could be, could also not be the case. Highly case by case and individual dependent.
For 'accuracy' to a specific target on the listening position one should though (which is your point I reckon)

The original question 'Can load dependency be wholly corrected with EQ?' can only be answered with; that depends on what kind of correction is used and how that was done. With characterization and the correct compensation one can increase signal fidelity. That may be audible or not.

With non PFFB cheap and popular load dependent amps the treble response can be audible different depending on the speaker used compared to a non-load dependent design (regardless of the class) assuming that amp is not 'colored' in some way. That may or may not turn out to be 'more pleasant'.
Using EQ based on an impedance plot and knowing how to inter/extrapolate response based on 4 and 8 ohm resistive measurements can give some improvement but may not be 'accurate'. Characterizing it can give even more improvement in these particular cases. Impossible to predict how audible that all will be because of the actual load.

 

[voodooless]

Characterizing it can give even more improvement in these particular cases. Impossible to predict how audible that all will be because of the actual load.

I can make the same argument as you did:

I don't know if one really should. Plenty of people just enjoy speakers in their rooms just the way it is.
Doing the room correction may well change it. No way to tell if that is going to be more enjoyable. Could be, could also not be the case. Highly case by case and individual dependent.

The point is that it doesn't matter what correction you make or how: you may not like the end result, regardless of whether it is more accurate or not.

 

[Willem]

But how about simply avoiding problematic amplifiers and speakers? Good load independent amplifiers are now pretty cheap, and there are enough speakers with a benign impedance response as well.

 

[voodooless]

But how about simply avoiding problematic amplifiers and speakers? Good load independent amplifiers are now pretty cheap, and there are enough speakers with a benign impedance respoinse as well.

Sure, why not Give it another generation or two of cheap Chinese amps, and they will all have PFFB anyway

 

[MaxwellsEq]

The ASR graphs suggest that correcting for load dependency is a simple peak filter once you've made some measurements of your speaker in the room. But is this actually the case? I assume those graphs are made with a pure tone sweep. How does impedance change with actual music, and does this impact how the frequency response is? For example, if there were a 200Hz note and 15KHz note at the same time, would the 15KHz note be boosted in the same way that it might if the 15KHz note were played by itself

Can load dependency be wholly corrected with EQ?

Not with a simple EQ.

Load dependency in the graphs is generally based on two fixed non-reactance resistors. A true loudspeaker normally has capacitance and inductance which interact and create a complex impedance curve throughout the frequency response. A non-load-tolerant amplifier doesn't care about any of this (assuming it doesn't run out of current and voltage). A load-tolerant amplifier will have peaks and troughs throughout the frequency range. You could try and model and then EQ for that amplifier feeding this speaker... Personally I'd just buy a proper amplifier that's not load tolerant.