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The Loudspeaker & Amplifier Interface

By Rod Elliot,Steve Feinstein

[Elliott]

Golden EarMany claim that the ear is one of the most finely tuned and sensitive measuring instruments known. I am not going to dispute this - not so that I will not offend anyone, but because in some respects it is true. Having said that, I must also point out that although extremely sensitive, the ear (or to be more correct, the brain) is also easily fooled. We can imagine that we can hear things that absolutely do not exist, and can just as easily imagine that one amplifier sounds better than another, only to discover that the reverse is true under different circumstances. Listeners have even declared one amp to be clearly superior to another when the amp hasn't been changed at all.

Could it be the influence of speaker cables, or even loudspeakers themselves? This is quite possible, since when amps are reviewed it is generally with the reviewer's favorite speaker and lead combination. This might suit one amplifier perfectly, while the capacitance and inductance of the cable might cause minute instabilities in other otherwise perfectly good amplifiers. Although it’s a fine theory to suggest that a speaker lead should not affect the performance of a well-designed amplifier, there are likely to be some combinations of cable characteristics that simply freak out some amps. Likewise, some amps just might not like the impedance presented by some loudspeakers - this is an area that has been the subject of many studies, and entire amplifiers have been designed specifically to combat these very problems [1].

Many published designs never get the chance of a review, at least not in the same sense as a manufactured amplifier, so it can be difficult (if not impossible) to make worthwhile comparisons. In addition, we sometimes have different reviewers making contradictory remarks about the same amp. Some might think it is wonderful, while others are less enthusiastic. Is this because of different speakers, cables, or some other influence? The answer (of course) is that we have no idea.

We come back to a common problem, which is that the standard amplifier tests are not necessarily appropriate. A frequency response graph showing that an amp is ruler flat from DC to daylight is of absolutely no use if everyone says that the highs are "veiled", or that imaging is poor. Compare this with another amp that is also ruler flat, and (nearly) everyone agrees that the highs are detailed, transparent, and that imaging is superb.

We need to employ different testing methodologies to see if there is a way to determine from bench (i.e. objective) testing, what a listening (i.e. subjective) test might reveal. This is a daunting task, but is one that must be sought vigorously if we are to learn the secrets of amplifier sound. It is there - we just don't know where to look, or what to look for ... yet. Until we have correlation between the two testing methods, we are at the mercy of the purveyors of amplifier snake oil and other magic potions.

 

[Audioholics]

HirschVeteran reviewer Julian Hirsch of Stereo Review tackled the subject of amplifier sound in an article in the 1980s. Hirsch was a meticulous reviewer, a formally-trained engineer who put his faith in rigorous, repeatable, scientifically-valid tests and procedures. He completely eschewed imprecise fads like green Magic Markers, Shun Mook M’pingo discs, and $1000/ft interconnects that had no discernable or measurable differences compared to standard cables.

After exhaustive testing of several amplifiers with several different speakers (some known to be “easy” loads and some known to be “difficult” to drive), Hirsch made some interesting observations that simultaneously explained why there were potential sound differences between amplifiers while at the same time remaining utterly faithful to the quantifiable, repeatable scientific process. It remains, in my view, the best explanation for this entire topic that I have yet seen.

Starting with the huge and all-important assumption that the amplifier under test is not being driven into distortion or being operated in a way that could elicit bad behavior, overheating, or engage any protection circuitry, Hirsch found the following:

  • Some pre-amp/power amplifier/speaker systems combined in such a way as to produce a very slightly rising or drooping frequency response across the entire 20-20 kHz audible spectrum. It could be a combination of the way the pre-amp and power amp combined on an input/output impedance basis, it could be because of the way a particular amp behaved with the specific load presented by that specific make/model of speaker, but there were system combinations where the frequency response showed a variation across the band.

These were not big variations. Perhaps on the order of + 0.5 dB in the bass to - 0.5 dB in the treble. A 1 dB total tilt, +/-. But over a wide range of frequencies—many octaves—that’s definitely audible. As a matter of fact, this phenomenon is formally known in audio engineering circles as “spectral tilt.” If system “A” is tilted up a dB across the band and system “B” is titled down a dB across the band, experienced listeners will definitely hear that, without question. Godehard Guenther, head of ADS Speakers in the 1970s, talked at length about this effect when voicing his then-top-of-the-line ADS 910 speaker. Extremely slight changes to the speaker’s “tilt” made a dramatic difference in its perceived bass-treble balance and “weight.” Therefore, depending on the program material being used, the interaction of the different components, etc, such a spectral tilt variation could unquestionably lead a listener or reviewer to characterize the amplifier’s sound as “harsh” or “sweet” or “musical,” etc.

  • Hirsch also found that small—minute, even—differences in amps’ comparative signal-to-noise ratios often had a tangible impact on their perceived sound.

The SNR difference was not audible as hiss per se. But he did find that a few extra dB of noise could subliminally mix with the program material and lend a very slightly “brighter” or “harder” character to the sound. Granted if amplifier “A” had a SNR ratio of 80 dB and amplifier “B” was 75 dB, both would be considered more than “quiet” enough for high fidelity purposes. But the difference between their SNRs could show up as a perceived difference in tonal character.

This was a great study, perhaps the best ever done on amplifier sound, for several reasons:

  • First, like the consummate scientific professional that he was, Hirsch did not enter his study with any preconceived notion as to how the study should come out. He did not try to fit the data to suit his preferred conclusion, as so many lesser “experts” do.
  • Second, his results were based on quantifiable, measurable tests. They were not “feelings” or “impressions” or anything else of an inexact nature.
  • Lastly, even though he didn’t subscribe to the “subjectivist” approach to reviewing, he had no hesitation in saying that there were indeed differences, and then he proceeded to explain why, in precise, concrete terms. Too many reviewers have very strong feelings about their approach or personal listening skills and these feelings too often prevent them from making truly objective observations, especially if the observation is contrary to their previously-stated position.

Hirsch suffered from no such ego-based limitations, and therefore his findings were more defensible from a strictly scientific standpoint. In my experience, this study presents the clearest tangible, measurable, repeatable evidence yet that there are definitive audible differences between amplifiers.

So, sometimes (not all the time) there are differences in the sound of amplifiers, but according to Hirsch, probably not for the reasons we’d like to think.  Are there other reasons for audible differences between amplifiers, such as the “gut feeling” one gets when viewing beautifully laid-out amplifier circuitry, massive heatsinks, storage capacitors with impressive uf ratings, and a large heavy torridal transformer, all accompanied by convincing technical literature text and an impressive array of reviewers’ quotes lauding the design’s sonic excellence?

It’s tough for any of us to say we’re unswayed by that.

 Pass Labs

Pass Labs X350.5 Two-Channel Amplifier

Measurable Performance Characteristics

[Elliott]

We need a way to correlate subjective versus objective testing. Both are important, the problem is that one is purely concerned with the way an amplifier behaves on the test bench, and a whole series of more or less identical results can be expected. The other is veiled in "reviewer speak", and although it might be useful if the reviewer is known and trusted, is not measurable or repeatable.

The whole object is to try to determine what physical factors cause amplifiers to sound different, despite that fact that conventional testing indicates that they should sound the same.

A detailed description of the more important (from a sound perspective) of the various amplifier parameters is given below.

  • Input Sensitivity: The signal level required to obtain full power at the amplifier's output. This is determined by the gain and power rating of the amp. A 10W amplifier requires far less gain than a 200W amplifier to obtain full power for the same input voltage. It would be useful if all amplifiers had the same gain regardless of power, but this is not the case. Sensitivities vary widely, ranging from about 500mV up to 1.5V or more.
  • Total Harmonic Distortion (THD): This is a measure of the amount of distortion (modification) of the input signal, which adds additional signal frequencies to the output that are not present in the input signal. THD is commonly measured as a percentage, and can range from 0.001% to 0.5% for typical hi-fi amplifiers. A theoretically perfect amplifier contributes no distortion.
  • Intermodulation Distortion: A form of audio distortion where the distortion products occur at frequencies that are sums and differences of the input signal. For example, if the input is 500 Hz and 2200 Hz, then the IM distortion products will occur at 1700 Hz and 2700 Hz. IM distortion is particularly objectionable, since the distortion is not related harmonically in any way to the original signal, unlike THD.
  • Frequency Response: The measure comparing the input signal to the output in terms of frequency versus amplitude. A perfect amplifier will amplify all signals equally, regardless of frequency. Realistically, an amplifier needs a response of about 5Hz to 50kHz to ensure that all audible signals are reproduced with minimal modification.
  • Phase Response: This indicates the amount of time that the input signal is delayed before reaching the output, based on the signal frequency. Variations in absolute phase are not audible in an amplifier system, but are generally considered undesirable by the hi-fi press. Since it is not difficult to ensure phase linearity, this is not generally a design issue except with valve amplifiers.
  • Output Power: This is most commonly measured into a non-inductive resistive load. This is not done to improve the figures or disguise any possible shortcomings, but to ensure that measurements are accurate and repeatable. Power should only be quoted as "watts RMS" over a specified frequency range at a specified level of distortion. Although “RMS” is not strictly technically correct, is accepted in the industry, and may be measured into 8 ohms, or other impedances that the amplifier is capable of driving.
  • Output Current: Not often measured, but sometimes quoted by manufacturers, this represents the maximum current the amplifier can supply into any load. It is rare that any amplifier will be called upon to deliver any current greater than about 3 to 5 times the maximum that the nominal speaker impedance would allow for the amplifier's supply voltage. Greater variations may be possible with some speaker designs, but (IMO) this represents a flaw in the design of the loudspeaker.
  • Power Bandwidth: This is usually taken as the maximum frequency at which the amplifier can produce 1/2 of its rated output power (this is the -3dB frequency). A 100W amplifier that can produce 50W at 10 Hz and 50kHz will be deemed as having a 10-50kHz power bandwidth.
  • Output Impedance : This is the actual output impedance of the amplifier, and has no bearing on the amount of current that can be supplied by the output stage. Valve amplifiers usually have a relatively high output impedance (typically 1 to 6 Ohms), while solid state amps will normally have an output impedance of a fraction of an Ohm. By use of feedback, it is possible to increase output impedance (> 200 Ohms is quite easy), or it can be made negative. Negative impedance has been tried by many designers (including the author), but has never gained popularity - possibly because most speakers react very poorly to negative impedances and tend to sound awful.

The difficulty is determining just how much of any of the problem items is tolerable, and under what conditions. For example, there are many single ended triode valve designs that have very high distortion figures (comparatively speaking), high output impedance and low output current capability. There are many audio enthusiasts who claim that these sound superior to all other amplifiers, so does this mean that the parameters where they perform badly (or at least not as well as other amps) can be considered unimportant? Not at all!

If a conventional (i.e. not Class-A) solid-state amplifier gave similar figures, it would be considered terrible, and would undoubtedly sound dreadful.

 

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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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