Amplifier Sound

7/24/2019 Amplifier Sound

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Amplifier Sound - What Are The Influences?

Introduction

The sound of an amplifier is one of those ethereal things that seems to defy

description. I will attempt to cover the influences I know about and describethe effects as best I can. This is largely hypothesis on my part since there

are so many influences that although present and audible are almost

impossible to !uantify. "specially in combination some of the effects will make

one amp sound better and another worse - I doubt that I will be able to even

think of all the possibilities but this article might help some of you a little -

at least to decipher some of the possibilities.

I don#t claim to have all the answers and it is !uite conceivable that I don#thave any $although I do hope this is not the case%. This entire topic is sub&ect

to considerable interpretation and I will try very hard to be completely

ob&ective.

'eader input is encouraged as I doubt that I will manage to get everything

right first time and there are some areas where I do not really know what the

answers are. The only &oy I can get from this is that I doubt that anyone else

can do much better. If you can let me know.

The Components of Sound

When people talk about the sound of an amplifier there are many different

terms used. (or a typical $high !uality% amplifier the sound may be described

as )smeared) or having )air) or )authoritative) bass. These terms - although

describing a listener#s e*perience - have no direct meaning in electrical terms.

"lectrically we can discuss distortion phase shift current capability slewrate and a myriad of other known phenomena. I don#t have any real idea as to

how we can directly link these to the common terms used by reviewers and

listeners.


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Some writers have claimed that all amplifiers actually sound the same and to

some e*tent $comparing apples with apples% this is )proven) in double-blind

listening tests. I am a great believer in this techni!ue but there are some

differences that cannot be readily e*plained. An amp that is deemed

)identical) to another in a test situation may sound completely different in a

normal listening environment. It is these differences that are the hardest to

deal with since we do not always measure some of the things that can have a

big influence on the sound.

(or e*ample+ It is rare that testing is done on an amplifier#s clipping

performance - how the amp recovers from a brief transient overload. I have

stated elsewhere that a hi-fi amplifier should never clip in normal usage - nice

try but it IS going to happen and often is more common than we might think.,se a good clipping indicator on the amp and this can be eliminated but at

what cost? It might be necessary to reduce the volume $and S% to a level

that is much lower than you are used to to eliminate a problem that you were

unaware e*isted.

/ifferent amplifiers react in different ways to these momentary overloads

where their overall performance is otherwise almost identical. I have tested

I0 power amps and was dismayed by the overload recovery waveform. 1yfaithful old 23W design measures about the same as the I0 in some areas a

little better in some a little worse in others $as one would e*pect%.

Were these two amps compared in a double blind test $avoiding clipping% it is

probable that no-one would be able to tell the difference. Advance the level so

that transients started clipping and a fence post would be able to hear the

difference between them. What terms would describe the sound? I have no

idea. The sound might be )smeared) due to the loss of detail during the

recovery time of the I0 amp. Imaging might suffer as well since much of the

signal that provides directional cues would be lost for periods of time.

Measurable Performance Characteristics


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A detailed description of the more important $from a sound perspective% of

the various amplifier parameters is given later in this article but a brief

description is warranted first. Items marked with a 4 are problem areas and

the effect should be minimised wherever possible. The parameters that

should normally be measured $although for those marked 5 this is rare

indeed% are as follows6

4 Important parameter

5 'arely measured

Input Sensitivity : The signal level re!uired to obtain full power at the

amplifier#s output. This is determined by the gain and power rating of

the amp. A 73W amplifier re!uires far less gain than a 833W amplifierto 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 933m: up to 7.9: or

more.

Total Harmonic Distortion (THD) :This is a measure of the amount

of distortion $modification% of the input signal which adds additional

signal fre!uencies to the output that are not present in the input signal.

T;/ is commonly measured as a percentage and can range from 3.337ero

crossover distortion. This is generally measured as a part of the T;/ of

an amplifier and becomes worse as power is reduced from the

ma*imum.

"re#uency $esponse :The amount of fre!uency versus amplitude

distortion in an amplifier. A perfect amplifier will amplify all signals

e!ually regardless of fre!uency. 'ealistically an amplifier needs a

response of about 9;> to 93k;> to ensure that all audible signals are

catered for with minimal modification.

Phase $esponse :This indicates the amount of time that the input

signal is delayed before reaching the output based on the signal

fre!uency. :ariations 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 e*cept with valve amplifiers.

%utput Po&er :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. ower should only ever be !uoted as )'1S) which

although is not strictly correct is accepted in the industry and may be

measured into @hms or other impedances that the amplifier is

capable of driving.

%utput Current ! :ot often measured but sometimes !uoted by

manufacturers this represents the ma*imum 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 B to 9 times the ma*imum that

the nominal speaker impedance would allow for the amplifier#s supply


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voltage. Creater variations may be possible with some speaker designs

but $I1@% this represents a flaw in the design of the loudspeaker.

Po&er 'and&idth :This is usually taken as the ma*imum fre!uency at

which the amplifier can produce 7D8 of its rated output power $this isthe -Bd= fre!uency%. A 733W amplifier that can produce 93W at 93k;>

will be deemed as having a 93k;> power bandwidth.

Sle& $ate ! :0losely related to power bandwidth the slew rate is the

ma*imum rate of change $measured in :olts per microsecond% of the

amplifier output. The higher the amplifier power the higher the slew

rate must be to obtain the same power bandwidth.

%pen oop 'and&idth ! :The bandwidth of the amplifier with no A0

feedback applied. :ery few amplifiers will have an open loop bandwidth

greater than a few kilo-;ert> but valve amps and some solid state

designs have a comparatively high open loop bandwidth.

%pen oop ain ! :'arely !uoted e*cept for /IE amps $and few of

them as well% this is the gain of the amplifier without any A0 signal

feedback. It is not really a helpful parameter for most people but can

be used to determine the ...

%pen oop Distortion ! :The T;/ of the amplifier with no feedback

applied. This should be as low as possible but realistically will usually be

!uite high by normal standards. The open loop distortion is reduced by

an amount appro*imately e!ual to the feedback ratio.

%pen oop %utput Impedance ! :The output impedance of the

amplifier with no A0 feedback applied. This may range from a few @hmsto 73 or more @hms depending on the design of the amplifier. :alve

amplifiers will normally have an open loop output impedance of 3.F of the

designed speaker impedance.


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"eedbac* $atio ! :;ow much of the open loop gain is fed back to the

amplifier#s input to obtain the sensitivity figure !uoted for the amp.

(or e*ample if an amplifier has an open loop gain of 733d= and a gain of

83d= then the feedback ratio is 3d=. The application of feedback will

o Increase bandwidth

o 'educe phase shift

o 'educe distortion

o 'educe output impedance

%utput Impedance :This is the actual output impedance of theamplifier and has no bearing on the amount of current that can be

supplied by the output stage. :alve amplifiers usually have a relatively

high output impedance $typically 7 to 2 @hms% while solid state amps

will normally have an output impedance of a fraction of an @hm. =y use

of feedback it is possible to increase output impedance $G 833 @hms is

!uite easy% or it can be made negative. egative impedance has been

tried by many designers $including the author% but has never gained

popularity - possibly because most speakers react very poorly tonegative impedances and tend to sound awful.

"very amplifier design on the planet has the same set of constraints and will

e*hibit all of the above problems to some degree. The only e*ception is a

0lass-A amplifier which does not have crossover distortion but is still limited

by all other parameters.

The difficulty is determining &ust how much of any of the problem items is

tolerable and under what conditions. (or e*ample there are many single

ended triode valve designs which 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


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they perform badly $or at least not as well as other amps% can be considered

unimportant? ot at allH

If a conventional $i.e. not 0lass-A% solid state amplifier gave similar figures it

would be considered terrible and would undoubtedly sound dreadful.

Although all the issues described above are separate in their own right many

can be lumped together under a single general category....

Distortion

Technically distortion is any change that takes place to a signal as it travels

from source to destination. If some of the signal )goes missing) this is

distortion &ust as much as when additional harmonics are generated.

We tend to classify distortion in different ways - the imperfect fre!uency

response of an amplifier is not generally referred to as distortion but it is.

Instead we talk about fre!uency response phase shift and various other

parameters but in reality they are all a form of distortion.

The bottom line is that amplifiers all suffer from some degree of distortion

but if two amplifiers were to be compared that had no distortion at all they

must $by definition% be identical in both measured and perceived sound.

aturally there is no such thing as a perfect amplifier but there are !uite a

few that come perilously close at least within the audible fre!uency range.

What I shall attempt to do is look at the differences that do e*ist and try to

determine what effect these differences have on the perceived )sonic

!uality) of different amplifiers. I will not be the first to try to unravel this

mystery and I doubt that I will be the last. I also doubt that I will succeed in

the sense that success in this particular area would only be achieved ifeveryone agreed that I was right - and of that there is not a chanceH

$;owever one lives in hope.%


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In this article I use the somewhat outdated term )solid state) to

differentiate between valve amps and those built using bipolar transistors

1@S("Ts or other non-vacuum tube devices.

I have also introduced a new $?% test method which I have called a SI1$Sound Impairment 1onitor% the general concept of which is described in the

appendi* to this article.

Clippin+ Distortion

;ow can one amplifier#s clipping distortion sound different from that of

another? 1ost of the hi-fi fraternity will tend to think that clipping is

undesirable in any form at any time. While this is undeniably true many of the

amps used in a typical high end setup will be found to be clipping during normal

programme sessions. I#m not referring to gross overload - this is !uite

unmistakable and invariably sounds awful - regardless of the amplifier.

There are subtle differences between the way amplifiers clip that can make a

very great impact on the sound. :alve amps are the most respectable of all

having a )soft) clipping characteristic which is comparatively unobtrusive. ow

feedback 0lass-A amplifiers are ne*t with slightly more )edge) but otherwise

are usually free from any really nasty additions to the overall sound.

Then there are the myriad of 0lass-A= discrete amps. 1ost of these $but by

no means all% are reasonably well behaved and while the clipping is )hard) it

does not have significant overhang - this is to say that once the output signal

is lower than the supply voltage again it &ust carries on as normal. This is the

ideal case - when any amp clips it should add no more nastiness to the sound

than is absolutely necessary. 0lipping refers to the fact that when the

instantaneous value of output signal attempts to e*ceed the amplifier#s powersupply voltage it simply stops because it cannot be greater than the supply.

We know it must stop but what is of interest is how it stops and what the

amplifier does in the brief period during and immediately after the clipping

has occurred.


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"i+ure , - Comparison of 'asic Clippin+ .aveforms

In (igure 7 you can see the different clipping waveforms I am referring to

with )A) being representative of typical push-pull valve amps )=) is the

waveform from a conventional discrete 0lass-A= solid state amp and )0)

shows the overhang that is typical of some I0 power amps as well as !uite afew discrete designs. This is a most insidious behaviour for an amp because

while the supply is )stuck) to the power rail any signal that might have been

present in the programme material is lost and a 733;> $or 783;>% component

is added if the clipping )stuck to rail) period lasts long enough. This comes

from the power supply and is only avoidable by using a regulated supply or

batteries. either of these is cheap to implement and they are rarely found

in amplifier designs.

Although (igure 7 shows the signal as a sinewave for ease of identification in

a real music signal it will be a sharp transient that will clip and if the amp

behaves itself this will be $or should be% more or less inaudible. Should it stick

to the supply rail the resulting description of the effect is unlikely to

accurately describe the actual problem but describe what it has done to the

sound - from that listener#s perspective. A simple clipped transient should not

be audible in isolation but will have an overall effect on the sound !uality.

Again the description of this is unlikely to indicate that the amp was clippingand regrettably few amps have clipping indicators so most of the time we

simply don#t know it is happening.

To be able to visualise the real effect of clipping we need to see a section of

)real) signal waveform with the lowest and highest signal fre!uencies present


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at the same time. If the amp is clipped because of a bass transient $this is the

most common% the period of the waveform is long. even if the signal is clipped

for only 9 milliseconds this means that 9 complete cycles of any signal at

7333;> is removed completely or 93 complete cycles at 73k;>. This

represents a significant loss of intended information which is replaced by a

series of harmonics of the clipped fre!uency $if clipping lasts for long enough%

or more typically a series of harmonics that is not especially related to

anything $musically speaking - all harmonics are related to something but this

is not necessarily musicalH%

I think that no review of any amplifier should ever be performed without some

method of indicating that the amp is clipping $or is sub&ect to some other form

of signal impairment% and this can be added to the reviewer#s notes - alongthe lines of6

)This amplifier was flawless when kept below clipping $or as long as the SI1

failed to show any noticeable impairment% but even the smallest amount of

overload caused the amp to sound very hard. Transparency was completely

lost imaging was impaired badly and it created listener fatigue very !uickly.)

ow wouldn#t that be cool? Instead of us being unaware $as was the reviewer

in many cases% that the amp in review was being overdriven - however slightly -

we now $all of us% have that missing piece of information that is not included at

the moment. I have never seen a review of an amp where the output was

monitored with an accurate clipping indicator to ensure that the reviewer was

not listening to a signal that was undistorted. I#m not saying that no-one does

this &ust none that I have read.

The ne*t type of overload behaviour is dramatically worse and I have seen

this in various amps over the years. 1ost commonly associated with overload

protection circuits the sound is gross. I do not know the e*act mechanism

that allows this to happen but it can be surmised that the protection system

has )hysteresis) a term that is more commonly associated with thermal

controllers steel transformer laminations and Schmitt trigger devices.

=asically a circuit with hysteresis will operate once a certain trigger point is


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reached but will not reset until the input signal has fallen below a threshold

that is lower than the trigger point. The typical waveform of an amplifier with

this problem is shown in (igure 8 and I believe it IS a problem and should be

checked for as a normal part of the test process. This type of overload

characteristic is not desirable in any way shape or form.

"i+ure / - Hysteresis %verload .aveform

In this case the additional harmonic components added to the original sound

will be more prominent than with )normal) clipping. As before I cannot even

begin to imagine how the sound might be described - all the more reason to

ensure that testing includes informing the reader if the amp was clipping or

not during the listening tests. The loss of signal with this type of distortion

will generally be much greater than simple clipping and the added harmonic

content will be much more pronounced especially in the upper fre!uencies.

Clippin+ Synopsis

Tests conducted as a part of any review would be far more revealing if the

clipping waveform were shown as a matter of course. After some learning on

our behalf we would get to know what various of the hi-fi press meant when

they described the sound while the amp was clipping versus not clipping or

what the amp sounded like when its overload protection circuits came into

action.

To this end I have designed a new distortion indicator circuit which not only

indicates clipping but will show when the amp is producing distortion of any

kind beyond an acceptable level. @ne version has been published as a pro&ect


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and I have chosen the acronym SI1 $Signal Impairment 1onitor% for this

circuit.

The SI1 will react to any form of signal modification and this includes phase

distortion and fre!uency response distortion. I do not believe that thisapproach has been used before in this way. It is not an uncommon method for

distortion measurement but has not been seen anywhere as a visual indicator

for identifying problem areas that an amp may show in use. This circuit will

also show when an amplifier#s protection circuit has come into effect.

Although the detector has no idea what type of problem is indicated it does

indicate when the input and output signals no longer match each other - for

whatever reason. @scilloscope analysis would be very useful using this circuitas with a little practice we would be able to identify many of the currently

unknown effects of various amplifier aberrations. ote that it is unable to

reveal crossover distortion unless it becomes !uite high which is e*tremely

unlikely in any modern amplifier design.

Crossover Distortion

0lass-A amplifiers have no crossover distortion at all because they maintain

conduction in the output device$s% for the entire waveform cycle and never

turn off. 0lass-A is specifically e*cluded from this section for that reason.

(or the rest a similar !uestion as the one before - how can one amplifier#s

crossover distortion sound different from another? Surely if there is

crossover distortion it will sound much the same? ot so at all. Again valve

amplifiers are much better in this area than solid state amps $at least in open

loop conditions%. When valves cross over from one output device to the ne*t

$standard push-pull circuit is assumed% the harmonic structure is comprisedof mainly low order odd harmonics. There will be some Brd harmonics a

smaller amount of 9th and so on.

Solid state amps tend to create high order odd harmonics so there will be the

Brd harmonic only a tiny bit less of the 9th harmonic and the harmonics will


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e*tend across the full audio bandwidth. Transistor and 1@S("T amps have

very high open loop gains and use feedback to reduce distortion. In all cases

the crossover distortion is caused because the power output devices are non-

linear. At the low currents at which the changeover occurs these non-

linearities are worse as well the devices usually have a lower gain at these

currents.

This has two effects. The open loop gain of the amplifier is reduced because

of the lower output device gain so there is less negative feedback where it is

most needed. Secondly the feedback tries to compensate for the lower gain

$and tries to eliminate the crossover distortion% but is limited by the overall

speed of the internal circuitry of the amplifier. This results in sharp

transitions in the crossover region and any sharp transition means high orderharmonics are produced $however small they might be%.

@ne method to minimise this is to increase the !uiescent $no signal% current in

the output transistors. With a linear output stage in a well designed circuit

crossover distortion should be all but non-e*istent with any current above

about 93 to 733mA $but note that if the !uiescent current is increased too

far overall distortion may actually get worse%. (igure B shows the crossover

distortion $at the centre of the red trace% and the residue as seen on anoscilloscope $green trace amplified by 73 for clarity% - this is the typical

output from a distortion meter with an amplifier that has noticeable

crossover distortion. If measured properly the distortion is highly visible

even though it may be barely audible. ote that the waveform below would not

!ualify for the last statement - this amount of crossover distortion would be

very audible indeed.


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"i+ure 0 - Crossover Distortion .aveform

If T;/ is !uoted without reference to its harmonic content then it is !uite

possible that two amplifiers may indicate identical distortion figures but one

will sound much worse than the other. /istortion at a level of 7W should

always be !uoted and the waveform shown. @nce the waveform can be seen it

is easy to determine whether it will sound acceptable or dreadful - before we

even listen to the amp. istening tests will confirm the measured results withgreat accuracy although the descriptive terms used will vary and may not

indicate the real problem.

Crossover Distortion Synopsis

Although this is one area where modern amplifiers rarely perform badly it is

still important and should be measured and described with more care than is

usually the case. While few amplifiers will show up badly in this test now

crossover distortion was one of the main culprits that gave solid state a badname when transistors were first used in amplifiers.

I do not believe that we can simply ignore crossover distortion on the basis

the )everyone knows how to fi* it and it is not a problem any more). I would

suggest that it is still a real problem only the magnitude has been reduced -


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the problem is still alive and well. Will you be able to hear it with most good

!uality amp? Almost certainly not.

"re#uency 1nd Phase Distortion

/istortion of the fre!uency 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 fre!uencies are amplified

e!ually and the first to go are the e*tremes at both ends of the spectrum. It

is uncommon for solid state amps to have a fre!uency response at low powers

that e*tends to anything less than the full bandwidth from 83;> to 83k;>.

This is not the case with some of the simple designs and single ended triode

$S"T% 0lass-A - as well as inductance loaded solid state 0lass-A amps - willoften have a less than ideal response.

I would e*pect any amplifier today should be no more than 3.9d= down at

83;> and 83k;> referred to the mid-band fre!uency $usually taken as 7k;>

but is actually about J39;>%. $1y preferred test fre!uency is KK3;> $concert

pitch A below middle 0% but none of this is of great conse!uence.% 3.9d= 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 73;> to over

93k;>.

(or reference the octaves included for )normal) sound are6

83 K3 3 723 B83 2K3 783 8923 9783 738K3 83K3 $all in

;ert>%

To determine the halfway point between two fre!uencies one octave apart we

multiply the lower fre!uency by the s!uare root of 8 $7.K7K%. The halfway

point is between 293 and 783;> or J3K.J2;>. Eou must be so pleased tohave been provided with this piece of completely useless informationH Lust

think yourselves lucky that I didn#t tell you how to calculate the distance

between the frets on a guitar.


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1ost amplifiers will manage well beyond the range necessary for accurate

reproduction at all power levels re!uired to cater for the re!uirements of

music. So why are some amps described as having poor rendition of the high

fre!uencies? 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 sub&ective and

there is no known measurement that reveals this !uality. We must try to

determine what measurable effect might cause such a phenomenon. There arefew real clues since amplifiers that should not be classified as e*ceptional in

this area are often described as such. @ther 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 e!uation since amps

with comparatively high distortion can be comparable to others with negligible

distortion. hase shift is also out of the !uestion since amps with a lot ofphase shift can be favourably compared to others with virtually none. @ne

ma&or difference is that typical S"T amplifiers have !uite high levels of low

order even harmonics. Although these will give the sound a uni!ue character I

doubt that this is the sole reason for the perceived high fre!uency

performance - I could also be wrong.

hase distortion occurs in many amplifiers and is worst in designs using an

output transformer or inductor $sometimes called a choke%. The effect is that

some fre!uencies are effectively delayed by a small amount. This delay is

usually less than that caused by moving one#s head closer to the loudspeakers

by a few millimetres. It is generally thought to be inaudible and tests that I

$and many others% have conducted seem to bear this out.


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"re#uency 1nd Phase Distortion - Synopsis

There must be some mechanism that causes multiple reviewers to describe an

amplifier as having a poor high fre!uency performance such as $for e*ample% a

lack of transparency. There are few real clues that allow us to determine

e*actly 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 )e*perimenter e*pectancy). 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.

@ne of the few variables is a phenomenon called slew rate. This is discussed

fully in the ne*t section.

Sle& $ate Distortion

This has always been somewhat controversial but no-one has ever been able to

confirm satisfactorily that slew rate $within certain sensible limits% has any

real effect on the sound. (igure K is a nomograph that shows the re!uired slew

rate for any given power output to allow full power at any fre!uency. To use it

determine the power and calculate the peak voltage and place the edge of a

ruler at that voltage level. Tilt the ruler until the edge also aligns with the

ma*imum full power fre!uency on the top scale. The slew rate is indicated on

the bottom scale.

(or e*ample if the peak voltage is 93: $a 793WD ohm amp% and you e*pect

full power to 83k;> the re!uired slew rate is 2:DMs. =ear in mind that no

amplifier is evere*pected to provide full power at 83k;> and if it did the

tweeters would fail very !uickly.


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"i+ure 2 - Sle& $ate 3omo+raph

Slew rate distortion is caused when a signal fre!uency and amplitude is suchthat the amplifier is unable to reproduce the signal as a sine wave. Instead

the input sine wave is )converted) into a triangle wave by the amplifier. This is

shown in (igure 9 and is indicative of this behaviour in any amplifier with a

limited slew rate. The basic problem is caused by the )dominant pole) filter

included in most amplifiers to maintain stability and prevent high fre!uency

oscillation. While very few amplifiers even come close to slew rate induced

distortion $ANA Transient Intermodulation /istortion% with a normal signal

this is one of the very few possibilities left to e*plain why some amps seem tohave a less than enthusiastic response from the reviewers# perspective.

If you don#t like the nomograph you can calculate the ma*imum slewrate if a

sinewave easily. The formula is ...

S' O 8 4 P 4 f 4 :p

Where S' is slewrate in :Ds and :p is the peakvoltage of the sinewave

$:'1S 4 7.K7K%

(or e*ample 83k;> at 8: '1S $733WD ohms% re!uires a slewrate of ...

S' O 8 4 P 4 83333 4 K3

S' O 93829K :Ds O 9.3B:DMs

We already know absolutelythat no music source will ever provide a full power

signal at 83k;> but to allow it the amp needs a slewrate of 9:DMs $close


19/29

enough%. Should someone claim that you need 733:DMs or better that their

amp can do &ust that and you#ll miss out on much of your music then you know

that the claims are fallacious. ;aving a higher slewrate than strictly necessary

does no harm provided that the design#s stability hasn#t been compromised to

achieve the claimed figure. All design is the art of compromise and some

compromises can be a giant leap backwards if the designer concentrates on

one issue and ignores others. I happen to think that stability

is e*tremelyimportant - no amp should oscillate when operated normally into

any likely speaker load ... everH

"i+ure 4 - Sle& $ate imitin+ In 1n 1mplifier

The red trace shows the amp operating normally and the green trace shows

what happens if the slew rate is deliberately reduced. Is this the answer

then? I wish it were since we could all sleep soundly knowing e*actly what

caused one amp to sound the way it did compared to another which should

have sounded almost identical.

A further test is to apply a low fre!uency s!uare wave at about half to BDK

power mi*ed with a low-level high fre!uency sinewave to the amplifier. At the

transitions of the s!uarewave the sinewave should simply move up and down -

)riding) the s!uarewave. If there is any mis-behaviour in the amp the


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sinewave may be seen to be compressed so its shape will change or a few

cycles may even go missing entirely. "ither is unacceptable and should not

occur.

This is an e*tremely savage test but most amplifiers should be able to copewith it !uite well. Those that don#t will modify the music signal in an

unacceptable way in e*treme cases $which this test simulates%. Again this is

an uncommon test to perform but may be !uite revealing of differences

between amps.

"re#uency 1nd Sle& $ate Distortion - Synopsis

We need to delve deeper and although there seems to be little $if any% useful

evidence we can use to e*plain this particular problem there is an answer andit therefore possible to measure the mechanism that causes the problem to

e*ist.

%pen oop $esponses

The performance of a feedback amplifier is determined by two primary

factors. These are

@pen loop performance

(eedback ratio

If the amp has a poor open loop gain and high distortion then sensible

amounts of feedback will not be able to correct the deficiencies because

there is not sufficient gain reserve. =y the time the performance is

acceptable it may mean that the amplifier has unity gain and is now

impossible to drive with any normal preamp.

1any amplifiers have a very high open loop gain but may have a restricted

fre!uency response. et#s assume an amp that has a gain of 733d= at 83;>

and K3d= gain at 83k;>. If we want B3d= of overall gain $which is about

standard% then there is F3d= of feedback at 83;> but only 73d= at 83k;>.

As a very rough calculation distortion and output impedance are reduced by


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the feedback ratio so if open loop distortion were B< $not an unreasonable

figure% then at 83;> this is reduced to 3.3379.

=ecause these figures are so rarely !uoted $and I must admit I have notreally measured all the characteristics of the 23W amp in ro&ect 3B - open

loop measurements are difficult to make accurately% we have no idea if

amplifiers with poor open loop responses are responsible for so many of the

failings we hear about. It is logical to assume that there must be some

correlation but we don#t really know for sure.

Ideally an amplifier should have wide bandwidth and low distortion before

global feedback is applied which will &ust make a good amp better. @r will it? Ihave read reviews where a very simple amp was deemed one of the best around

$this was !uite a few years ago% and I was astonished when I finally saw the

circuit - it was almost identical to the )"l 0heapo) amplifier $see the pro&ects

pages for more info on this amp%.

The only ma&or difference between this amp and most of the others at the

time was the comparatively low open loop gain and a somewhat wider

bandwidth than was typical at the time because it does not need a miller

capacitor for stability. So the amp was better in one respect worse in

another.

In the end it doesn#t really matter what the open loop response is like as long

as closed loop $i.e. with feedback applied% performance does not degrade the

sound. Again we have the same !uandary as before - unless we can monitor the

difference between input and output at all levels and with normal signal

applied we really don#t know what is going on. The usual tests are useful but

cannot predict how an amp will sound. I have heard countless stories about

amps that measure up e*tremely well but sound )hard and dry) and have no

)music) in them.


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,nless these measurements are made $or at least some modified form% we will

still be no further in understanding why so many people prefer one brand of

amp over another $other than peer pressure or advertising hype%.

@ne possibility is to measure the amp with a gain of K3d=. This is an easyenough modification to make for testing and the performance is far easier to

measure than if we attempt open loop testing. The difference between

measured performance at B3d= gain $about B8% versus K3d= $733% would be an

e*cellent indicator of the amp#s performance and it is not too hard to predict

the appro*imate open loop response from the different measurements. To be

able to do this re!uires that all measurements be very accurate.

Would these results have any correlation with the review results? We willnever know if someone doesn#t try it - work the techni!ues discussed here

thoroughly with a number of different amps. It would be useful to ensure

that the reviewer was unaware of the test results before listening to guard

against e*perimenter e*pectancy or sub-conscious pre&udice.

It is very hard to do a synopsis of this topic since I have too little data to

work with. @nly by adopting new ideas and test methods will we be able to

determine if the )golden-ear) brigade really does have golden ears or that

they actually hear much the same )stuff) as the rest of us but have a better

vocabulary. That is not intended as a slur &ust a comment that we have to find

out if there is anything happening that we $the )engineering) types% don#t

know about or not. ,nless we can get a match between measured and

described performance we get nowhere $which is to say that we stay where

we are on opposite sides of the fence%.

Spea*er - 1mplifier Interface

1any is the claim that the ear is one of the most finely tuned and sensitive

measuring instrument known. I am not going to dispute this - not so that I will

not offend anyone $I seem to have done this many times already% but because

in some respects it is true. ;aving said that I must also point out that

although e*tremely sensitive the ear $or to be more correct the brain% is also


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easily fooled. We can imagine that we can hear things that absolutely do not

e*ist and can &ust as easily imagine that one amplifier sounds better than

another only to discover that the reverse is true under different

circumstances. isteners have even declared one amp to be clearly superior to

another when the amp hasn#t been changed at all.

0ould it be the influence of speaker cables or even loudspeakers themselves?

This is !uite possible since when amps are reviewed it is generally with the

reviewer#s favourite 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 a fine theory to suggest that a speaker lead should not affect the

performance of a well designed amplifier there are likely to be somecombinations of cable characteristics that simply freak out some amps.

ikewise some amps &ust might not like the impedance presented by some

loudspeakers - this is an area that has been the sub&ect of many studies and

entire amplifiers have been designed specifically to combat these very

problems Q7R.

1any 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 notimpossible% 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 the same problem I described earlier which is that the

standard tests are not necessarily appropriate. A fre!uency response graph

showing that an amp is ruler flat from /0 to daylight is of absolutely no use if

everyone says that the highs are )veiled) or that imaging is poor. 0ompare

this with another amp that is also ruler flat and $nearly% everyone agrees that

the highs are detailed transparent and that imaging is superb.
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We need to employ different testing methodologies to see if there is a way to

determine from bench $i.e. ob&ective% testing what a listening $i.e. sub&ective%

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

&ust don#t know where to look or what to look for ... yet. ,ntil we have

correlation between the two testing methods we are at the mercy of the

purveyors of amplifier snake oil and other magic potions.

The SI1 distortion indicator is one possible method that might help us but it

may also react to the wrong stimulus. erhaps we need to add the ability to

detect small amounts of high fre!uencies with greater sensitivity but now a

simple idea becomes !uite comple* possibly to no avail. It is also important

that such a device has >ero effect on the incoming signal itself so some careis needed to ensure that there is negligible loading on the source preamplifier.

This is not the only avenue open to us to correlate sub&ective versus ob&ective

testing. =oth 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 e*pected. 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 ob&ect is to try to determine whatphysical factors cause amplifiers to sound different despite that fact that

conventional testing indicates that they should sound the same.

Impedance

The output impedance of any amplifier is finite. There is no such thing as an

amplifier with >ero 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. Lust 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 fre!uency.


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A genuine >ero 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 >ero regardless of

fre!uency 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.

Another approach might be to make the output impedance infinite and againthe load will have >ero effect on the amplifier itself. Alas this too is

impossible. Civen 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%.

A test method already e*ists for this and uses one channel of an amp to drive

a signal back into the output of another. The passive amplifier is the one under

test. It is also possible to use a different source amplifier altogether since

there is no need for it to be identical to the test amp. ,se of a )standard)

amplifier whose characteristics are well known is useful since the source will

be a constant in all tests. /ifferences may then be seen clearly from one test

to the ne*t.

The method is shown in (igure 2 and is a useful test of the behaviour of an

amp when a signal is driven into its output. This is e*actly what speakers do -

the reactive part of the loudspeaker impedance causes some of the power to


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be )reflected) back into the amplifier. Since one amplifier in this test is the

source the device under test can be considered a )sink)

"i+ure 5 - 1mplifier Po&er Sin* Test

I have used this test and although it does show some interesting results the

test is essentially not useful unless used as a comparative test method. The

amplifier under test is also sub&ected to very high dissipation $well above that

e*pected with any loudspeaker load% because the transistors are e*pected to

#dump# a possibly large current while they have the full rail voltage across

them. There is a real risk of damaging the amplifier and I suggest that you

don#t try this unless you are very sure of the driven amplifier#s abilities.

We may now ask )Why is this not a standard test for amplifiers then?). The

answer is that no-one has really thought about it enough to decide that this

will $or should% be part of the standard set of tests for ob&ective testing of

an amplifier. The results might be !uite revealing showing a signal that may be

non-linear $i.e. distorted% or perhaps showing a wide variation in measured

signal versus fre!uency. The result of this test with amps having e*tensive

protection circuits will be a lottery - most will react $often very% badly at only

moderate current.

If there is high distortion or a large fre!uency dependence then we have

some more information about the amplifier that was previously unknown. It

might be possible to correlate this with sub&ective assessments of the amp

and gain further understanding of why some amps supposedly sound better


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than others. We might discover that amps with certain characteristics using

this test are sub&ectively &udged as sounding better than others ... or not.

If this test became standard and was routinely allied with the SI1 tester

described above we may become aware of many of the problems thatcurrently are $apparently andDor allegedly% audible but for which there is no

known measurement techni!ue.

Conclusion

This article has described some tests that although not new are possibly the

answer to so many !uestions we have about amplifiers. The tests themselves

have been known for some time but their application is potentially of benefit.

We may be able to finally perform an ob&ective test and be able to predict

with a degree of confidence how the amp will sound. It may also happen that

these tests are not sufficient to reveal all the subtleties of amplifier sound

but will certainly be more useful than a simple fre!uency response and

distortion test.

Any change to the testing methods used is not going to happen overnight and

nor are we going to be able to see immediately which problems cause a

difference and which ones have little apparent effect. Time patience and

careful correlation of the data are essential if this is to succeed. There are

laws of physics and there are ears. Somewhere the two must meet in common

ground. We already know that this happens since there are amplifiers that

sound e*cellent - according to a large number of owners reviewers etc. - now

we need to know why.

There is a test method $or a series of methods% that will allow us to obtain a

suite of tests that makes sense to designers and listeners alike so we can getcloser to the ideal amplifier namely the mythical )straight wire with gain) but

from the listener perspective rather than the senseless repetition of tests

that seem to have no bearing on the perceived !uality of the amp. This is not

to say that the standard tests are redundant $far from it% but they do not

seem to reveal enough information.


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(or this to succeed the sub&ectivists must be convinced as must the

)ob&ectivists). 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 other testing

methods that may reveal the subtle differences that undeniably e*ist

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 opamp a

bipolar &unction transistor and a 1@S("T or =rand )A) versus =rand )=) and

those who claim that there is no difference at all.

The fact that there are differences is obvious. The degree of difference andwhy 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

e*planation 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 e*ample%

it is better to use one brand of capacitor in the signal path or another.

These testing methods can also be applied to the measurement of individual

components speaker cables interconnects and preamps particularly the SI1

tester. ,sing the amplifier power sink test with different cables and speakers

might give us some clues as to why so many people are adamant that one

speaker cable sounds better than another even though there is no measurable

difference using conventional means.

The greatest benefit of these tests is that they will reveal things we have not

been looking at $or for% in the past and may show differences that come as a

very great surprise to designers and listeners alike.


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(or information on the use of the SI1 and an initial article describing how it

works and my results so far please see )Sound Impairment 1onitor - The

Answer?)
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