Understanding & Interpreting Solid State Amplifier Measurements - Frequency & Power Response

FREQUENCY RESPONSE MEASUREMENTS

Frequecy Response Measurements indicate how the output varies with frequency with ALMOST NO LOAD.

If you look closely, the top of the graph says the measurements were taken at 2.83 Volts into 4 or 8 ohms.

This implies that the amplifier was putting out ( a token )

2.83 x 2.83 divide by 8 = 1 Watt .

( not much compared to its 190 Watts per channel max output with 1% distortion )

The frequency response therefore typically portraits the Best ( idealistic ) performance that the Amplifier is likely to provide on a theoritical basis, and not its ACTUAL Performance, feeding loudspeakers & delivering substantial power to the speakers.


To fully understand the frequency response measurements, we need to know the following :

1. THE AUDIO SPECTRUM ( 20 Hz to 20,000 Hz )

2. THe Decibel

3. -3 dB points

4. Logarathmic Ratios

5. The "Flat Frequency Response"

6. Any other pointer I have probably missed.


Can I request other forum member to write / contibute on the above points ( 1 to 6 ) ?

Enquiring minds are also invited to ask on anything I have writtten or forgotten to write about above...

Like I said earlier,this is going to be one of the most interesting, informative & useful of threads in a long, long time-
Go Amp Go!

THANKS for your kind words of encouragement, Kamal.


1. THE AUDIO SPECTRUM ( 20 Hz to 20,000 Hz )

The Human ear is Supposed to hear sounds that range in Freq from:

20 Hz ( VERY Low, deep bass, that is often felt physically more than it is heard )

to

20,000 Hz or 20 Kilo Hertz i.e. 20 KHz

(remember Kilo is an abreviation for 1000 eg Kilogram = 1000 grams)

Sadly, due to thickning of the eardrum ( am I correct, doc ? ) ears more than 40 years old, rarely can hear 15 KHz or higher.


OCTAVES
The audio frequecy spectrum spans 10 Octaves.

Each octave is a doubling of frequency. ie

20 Hz to 40 Hz

40 Hz to 80 Hz

80 Hz to 160 Hz

160 Hz to 320 Hz

320 Hz to 640 Hz

640 Hz to 1280 Hz

1280 Hz to 2560 Hz

2560 Hz to 5120 Hz

5120 Hz to 10,240 Hz ( ie 5.12 KHz to 10.24 KHz )

10,240 Hz to 20,480 Hz ( ie 10.24 KHz to 20.48 KHz )

You will observe the the size of each octave grows at higher frequencies.

THE DECADE
A decade interval is also sometimes used.

A decade is a x10 span of frequencies.

eg , there are 3 decades between 20 Hz & 20 Khz, viz :

20 Hz to 200 Hz

200 Hz to 2 KHz

2KHz to 20 KHz

Our hearing is logarathimic, and our ear responds accordingly ( more when someone addresses Log hearing / ratios )


All Hi Fi Equipment therefore strives to provide impecable performance ATLEAST for 20 Hz to 20 KHz.

2. The Decibel

The unit of sound was called a "Bell" as a tribute to Alexander Gram Bell.

One tenth of a bell was where it all began.... it was the smallest change volume in level perceptible to a ( non audiophile ? ) person.This unit was called a "decibel."


Strangely ( ? or have we evolved over the past 100 years or so ?? ) today, measurements get down to 0.1 decibel (dB ) and some claim to be able to hear down to that minute level....

Hence going by the listning capabilities of our forefathers... any differences of less than 1 dB would / should be in-audible.

3. -3 dB points

Due to the log ratio of the decibel (dB) a 3 dB change represents Half / double the power !

Hence if an amplifier is delivering 100 Watts at say 1 KHz, and is 3 db down ( i.e. Minus 3 dB i.e. -3dB ) at 20 KHz....

...it implies that it can deliver only 50 Watts at 20 KHz.


On the other hand, if your use tone controls to provide a 3 dB bass boost at 20 Hz ( i.e. +3 dB at 20 Hz ), the amp would be called on to pump out 200 Watts at 20 Hz, while delivering 100 Watts at 1 Khz.

Tone controls often provide a boost or cut of 12 dB or even 20 dB.

See the Tone Control graphs for the same NAD Amplifier:



The graph above shows that the Tone controls can provide a max boost or cut of 9 dB at 10 Hz to 30 Hz

Similarly, they can boost or cut 7 dB of treble at 20 KHz.



Also observe in the graphs how the vertical lines are VERY unevenly spaced every decade... Far apart between say 10 and 20 and then getting progressively closer towards 70, 80 & 90.

BACK TO THE FREQ RESPONSE GRAPH




Stereophile magazine said :

The left channel's output impedance at the speaker terminals was 0.1 ohm in the bass and midrange, rising slightly to 0.14 ohm at 20kHz. As a result of this low value, the modification of the amplifier's frequency response by the usual Ohm's Law interaction between its source impedance and that of the loudspeaker will be small (fig.1, top trace at 2kHz). This graph, taken with the tone controls defeated, also indicates that the amplifier has a wide small-signal bandwidth, the –3dB point into 8 ohms lying at 200kHz. This bandwidth did not significantly change with different volume-control settings, but it did decrease slightly into 2 ohms (fig.1, bottom dashed trace).

Feedback, questions, or comments on all the above welcome.

If you prefer some other line of explanations, let me know,

Thanks Amp Nut, A good beginning to our amplifier analysis . From what I understood from your post, it would be dangerous to use a large bass boost (as the amp would be overloaded beyond its normal power rating). Right?
Corollary: Tone controls should be sparingly used?

I didn't understand the part about the placement of the vertical lines. Do explain please.

The published freq response graph is ideal at 1W. At higher watts, there will be aberrations (clipping), especially in the high freq spectrum. Right?

Logarithmic ratios: Here's what I found on the net. Can you simplify it?
What is a logarithmic scale?
A ruler is a linear scale: it has marks on it corresponding to equal quantities of distance. One way of expressing this is to say that the ratio of successive intervals is equal to one. A logarithmic scale is different in that the ratio of successive intervals is not equal to one. Each interval on a logarithmic scale is some common factor larger than the previous interval. A typical ratio is 10, so that the marks on the scale read: 1, 10, 100, 1000, 10000, etc. Such a scale is useful if you are plotting a graph of values which have a very large range.

Since many aspects of perception are related to proportional change, logarithmic scales are very common in psychophysics. A graph of many perceptual scales against the logarithm of the stimulus size is a straight line over some range (this is known as Weber's law). A scale of perceptual pitch against log (Hz) is a good example.

What is the difference between loudness and intensity?
Loudness is a perceptual or subjective quality of a sound; intensity is a physical or objective property. Although changes in intensity can cause changes in loudness, they are clearly two different scales. In particular, sounds which are below the threshold of audibility have a non-zero intensity, but zero loudness.

Intensity is measured in Wm-2, but we usually prefer to use the Sound Pressure Level scale (dBSPL). Loudness can be measured in units called phons, where 10 phons is the perceived loudness associated with a pure tone of 1000Hz at 10dB above the threshold of audibility.

Why do we need to multiply by 20 in the decibel scale?
The number 20 has two causes: one that gives us a multiplier of 10, and one that gives us a multiplier of 2.

The factor of 10 is easy - we are working in decibels not bels. One bel (named after Alexander Graham Bell, by the way) is rather a large unit, roughly equal to a tripling in amplitude. So we multiply by 10 and work in tenths of a bel to give us more sensitivity.

The factor of 2 is there because we have ignored the fact that the correct definition of decibels is as a logarithmic ratio of powers not amplitudes.

Why doesn't 0dB mean that nothing is measured?
The decibel scale is a logarithmic ratio scale: we start with a ratio of pressures, then take the logarithm and finally multiply by 20. If the ratio is a number greater than one, then the logarithm must produce a value greater than 0 (e.g. log(10)=1). If the ratio is a number less than one, then the logarithm must produce a value less than 0 (e.g. the log(0.1)=-1). If the ratio is equal to one (i.e. that the two pressures are equal) then the logarithm returns 0 (since log(1)=0). Thus the zero point on the decibel scale is simply the point at which the measured amplitude is equal to the reference amplitude.

Err, of course,elementary, my dear Watson.......
Seriously, I'm feeling technically drowned.
Ampji, Neutralji, I beg you to please consider-
you are addressing engineers as well as BA/MA guys.
Alongside the Hitech stuff, plz also give simple explanations which us poor arty types can follow.
Your passion for the subject of course, comes thru loud & clear.