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Frequency & Phase Distortion in Amplifiers

By Rod Elliot,Steve Feinstein

Tube AmpDistortion of the frequency response should not be an issue with modern amplifiers, but with some (such as single ended triode valve designs), it does pose some problems. The effect is that not all frequencies are amplified equally, and the first to go are the extremes at both ends of the spectrum. It is uncommon for solid-state amps to have a frequency response at low powers that extends to anything less than the full bandwidth from 20Hz to 20kHz. This is not the case with some of the simple designs, and single ended triode (SET) Class-A - as well as inductance loaded solid state Class-A amps - will often have a less than ideal response.

I would expect any amplifier today should be no more than 0.5dB down at 20Hz and 20kHz, referred to the mid-band frequency (usually taken as 1kHz, but is actually about 905Hz). (My preferred test frequency is 440Hz (concert pitch A, below middle C), but none of this is of great consequence.) 0.5dB loss is acceptable in that it is basically inaudible, but most amps will do much better than this, with virtually no droop in the response from 10Hz to over 50kHz.

Most amplifiers will manage well beyond the range necessary for accurate reproduction, at all power levels required to cater for the requirements of music. So why are some amps described as having poor rendition of the high frequencies? They may be described a "veiled" or something similar, but there is no measurement that can be applied to reveal this when an amplifier is tested. Interestingly, some of the simpler amplifiers (again, such as the single ended triode amps) have poorer response than most of the solid-state designs, yet will regularly be described as having highs that "sparkle", and are "transparent".

These terms are not immediately translatable, since they are subjective, and there is no known measurement that reveals this quality. We must try to determine what measurable effect might cause such a phenomenon. There are few real clues, since amplifiers that should not be classified as exceptional in this area are often described as such. Other amps may be similarly described, and these will not have the distortion of a single ended triode and will have a far better response.

We can (almost) rule out distortion as a factor in this equation, since amps with comparatively high distortion can be comparable to others with negligible distortion. One major difference is that typical single ended triode amplifiers have quite high levels of low order even harmonics. Although these will give the sound a unique character, I doubt that this is the sole reason for the perceived high frequency performance - I could also be wrong.

There must be some mechanism that causes multiple reviewers to describe an amplifier as having a poor high frequency performance, such as (for example) a lack of transparency. There are few real clues that allow us to determine exactly what is happening to cause these reviewers to describe the sound of the amp in such terms, and one may be tempted to put it all down to imagination or "experimenter expectancy". This is likely to be a mistake, and regardless what we might think about reviewers as a species, they do get to listen to many more amplifiers than most of us.

Axiom A1400

Frequency Response of Axiom Audio Class D Amplifier would change radically depending on the load impedance it was driving.


As a general observation, it’s amazing how often regular ol' frequency response turns out to be the causal factor in definitive, repeatable sound differences (assuming no gross distortion). The FR variations may have been caused by many different factors, such as interaction between components or connectors or other issues. The underlying equipment itself may not have any inherent FR errors or problems, but when used in combination with specific associated equipment, FR variations suddenly crop up.

This is one of the potential problems of assembling high fidelity component systems for which the average consumer has no solution or remedy. Pre-amp “A” from this company + power amp “B” from that company + speakers “C,” source unit “D,” interconnects “E,” and speaker wire “F” all seem on paper to add up to one terrific system.

But due to circumstances completely unforeseen and unavoidable, they combine for an ever-so-slight FR variance that leads the user to conclude that this or that component is to “blame,” especially when the variance occurs after the latest component change.

This is either a strength of so-called “closed” all-in-one systems, where every link in the system’s chain is controlled by the system designer and one can reasonably expect every part of the system to work correctly with every other part. Or…the all-in-one customer (shelf system, docking station, soundbar, etc.) is so uncritical that even if there are slight FR errors along the way, that customer never notices or cares.




The output impedance of any amplifier is finite. There is no such thing as an amplifier with zero output impedance, so all amps are influenced to some degree by the load. An ideal load is perfectly resistive, and has no reactive elements (inductance or capacitance) at all. Just as there is no such thing as a perfect amplifier, there is also no such thing as a perfect load. Speakers are especially gruesome in this respect, having significant reactance, which varies with frequency.  A genuine zero impedance source is completely unaffected by the load, and it does not matter if it is reactive or not. If such a source were to be connected to a loudspeaker load, the influence of the load will be zero, regardless of frequency, load impedance variations, or anything else.  Since this is not the case in the real world, the goal (or at least one of them) is generally to make the amplifier have the lowest output impedance possible, in the somewhat futile hope that the amp will not be adversely affected by the variable load impedance. In essence, this is futile, since there will always be some output impedance, and therefore the load will always have some influence on the behaviour of the amp.

 Denon Amp

Denon 10CH Amplifier Output Impedance is commendably low

Another approach might be to make the output impedance infinite, and again, the load will have zero effect on the amplifier itself. Alas, this too is impossible. Given that the conventional approaches obviously cannot work, we are faced with the problem that all amplifiers are affected by the load, and therefore all amplifiers must show some degree of sensitivity to the speaker lead and speaker.

The biggest problem is that no-one really knows what an amplifier will do when a reactive load reflects some of the power back into the amp's output. We can hope (without success) that the effects will be negligible, or we can try to make speakers appear as pure resistance (again, without success).


Skeptical Audioholics Reviewers Becoming Believers?


rBHWe had a few skeptical reviewers in house at the Audioholics Showcase Theater Room one day when we were testing the RBH T30-LSE speaker system.  Clint DeBoer was among the skeptics that believed all amplifiers sound the same if not overdriven.  With that I connected and level matched our Denon POA-A1HDCI (MSRP: $7500) 10CH amplifier and a Panasonic SA-XR50 Class D  receiver (MSRP: $299).  I purchased the Panasonic receiver after measuring it because I’ve never measured such lousy performance in an amplifier before. Also this is the very same receiver that some audiophiles over at Audioasylum and AVSforum were raving about.  They made comments such as “this is the closest sound to tubes you can get from solid state” and “incredible sound for the money”.  I wanted to use it as a reference going forward to really demonstrate how amps can sound different sonically.  I didn’t tell the listening panel how bad the Panasonic was.  I connected both amps to Zone 2 and Zone 3, respectively, of my Denon AVP-A1HDCI A/V processor and level matched them.  All I had to do was switch the speaker leads each time so the listeners could instantly compare the difference.  At no time were they aware of which amp they were listening to. 

We ran this test at low listening levels (70dB at the listening area) and at higher listening levels (80dB).  Each time I switched between the amps, the listeners all recognized quite a dramatic difference in sound.  The Panasonic actually conveyed more bass than the Denon but it was very boomy and lacked articulation.  At the higher listening level, things really got ugly for the Panasonic.  It was clearly running out of gas and it just sounded nasty.  The reason the Panasonic exhibited boomy bass was likely because of its high output impedance (measured over 1 ohm!) compared to the Denon that measured in milliohms.  The RBH speakers dip down to about 3 ohms so the system damping factor was really compromised with the Panasonic receiver. 

I asked them to now pay attention to high frequency detail and clarity.  I heard comments like “That amp sounds very harsh” or “The sound is very bright but lacks life and ‘air.’” Can you guess which amp these comments were attributed to?  It was again they Panasonic Class Dl.  Even at low power levels where both amps were operating unclipped, there was a clear preference by our listening panel for the better-designed amplifier.  Clint was no longer a skeptic and closed minds were opened.  As an engineer myself, I still recognize that not everything we can measure matters and not everything that matters is necessarily always properly measured. 

This example goes a long way towards proving that amps that measure definitively different (including the very tangible output impedance difference between the two) will sound different—not exactly a big surprise.

However, the audible differences between amplifiers that “golden ears” speak of occur between equipment acknowledged to be quite good by any standards, with excellent measured performance in all the so-called “important” areas. This Bryston may have 0.05% THD and IM and a SNR of 90 dB, and that Adcom may have THD and IM of 0.05% and a SNR of 90 dB, but some people will swear that one or the other is “sweeter” or “harsher” or “veiled,” etc.  With the Denon vs. the Panasonic, we can see the measured differences and accept intellectually why the sonic differences exist.

That’s not what this article is exploring. We’re trying to figure out why Amp A and Amp B sound different despite the fact that all their measured parameters are not only very close, but supposedly past the point of audible relevance. No one can hear the difference on a complex musical waveform between 0.073% and 0.055% THD or between 94 and 91 dB SNR. But you’ll hear the difference blindfolded at 50 paces between 3.2% upper-order THD of the Panasonic when it’s clipping and 0.076% lower-order THD of the Denon. No wonder the Panasonic sounded lousy when it was “running out of gas", but its poor audio performance at unclipped listening levels was also quite interesting.




OpinionWill we ever be able to finally perform an objective test, and be able to predict with a degree of confidence how a given amp will sound?

Any tests that might be devised to do so must satisfy both the subjectivists’ and the objectivists’ camps. We are all looking for the same thing - the flawless reproduction of sound - but the two camps have drifted further and further apart over the years.

These are my musings, and I am open to suggestions for testing methods that may reveal the subtle differences that undeniably exist between amplifiers. At the moment we have a chasm between those who can (or think they can) hear the difference between a valve and an op-amp, a bipolar junction transistor and a MOSFET, or Brand "A" versus Brand "B", and those who claim that there is no difference at all.

The fact that there are differences is obvious. The degree of difference and why there are differences is not. It would be nice for all lovers of music (and the accurate reproduction of same) if we can arrive at a mutually agreeable explanation for these differences that is accurate, repeatable, and measurable.

If these criteria are not met, then the assessment is not useful to either camp, and the chasm will simply widen. This is bad news for all - it is high time we all get together and stop arguing amongst ourselves whether (for example) it is better to use one brand of capacitor in the signal path or another.

New testing methods can also be applied to the measurement of individual components, speaker cables, interconnects and preamps.



The sound variation between amplifiers is as controversial and inexact a subject as any in audio. It is one of the only topics where a large, well-qualified, well-educated segment of the audio community doesn’t even think the subject exists!  Does it exist? Do amplifiers—when used within their undistorted performance envelope—exhibit real sonic differences from each other?  If so, are these differences due to design approach/circuitry differences and component quality? Or are the sound differences caused by their interaction with other equipment?

We’d love to hear your experiences and your impressions as to why.



Many thanks to Rod Elliot of ESP for his contributions to this article.


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Recent Forum Posts:

davidscott posts on April 18, 2021 17:10
PENG, post: 1477146, member: 6097
Funny but unfortunately true that if someone says something enough times, someone (lots) will believe it is true whether it is or not, eventually..
And facts don't matter anymore at that point unfortunately
PENG posts on April 18, 2021 16:43
davidscott, post: 1477139, member: 86172
And covid is a hoax? And Trump won the election? And the earth was created in 7 days? Ill just keep watching reruns of the XFiles thank you.

Funny but unfortunately true that if someone says something enough times, someone (lots) will believe it is true whether it is or not, eventually..
davidscott posts on April 18, 2021 16:32
mtrycrafts, post: 1477143, member: 5380
Well, but way back then a day's length was not measure properly, no atomic clocks around.
No they weren't created till day 7
mtrycrafts posts on April 18, 2021 16:25
davidscott, post: 1477139, member: 86172
And covid is a hoax? And Trump won the election? And the earth was created in 7 days? Ill just keep watching reruns of the XFiles thank you.
Well, but way back then a day's length was not measure properly, no atomic clocks around.
davidscott posts on April 18, 2021 16:22
And covid is a hoax? And Trump won the election? And the earth was created in 7 days? Ill just keep watching reruns of the XFiles thank you.
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