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Old 6th January 2012, 09:16 AM   #601
SY is offline SY  United States
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Guys, do you think we can switch back to the topic of "Sound Quality versus Measurements"? We have other threads here more suitable for the OT stuff.
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Old 6th January 2012, 10:31 AM   #602
DF96 is offline DF96  England
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Quote:
Originally Posted by ThorstenL
Class B/AB crossover distortion is not monotonic. This was the angle I commented on. And this kind of distortion (which D. Self comming from Class B calls GM Doubling distortion and I, coming from a Class A angle would call GM halving distortion) is endemic in almost all modern amplifiers.
Crossover distortion, unless extremely bad (both output devices cut off), is monotonic. Gm-doubling distortion is definitely monotonic.

My understanding of monotonic is that the output direction does not change sign, so for a non-inverting amp the output never goes more -ve when the input goes more +ve. Strictly monotonic is stronger, and also rules out stationary output.

What do you understand by 'monotonic'?
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Old 6th January 2012, 10:42 AM   #603
wahab is offline wahab  Algeria
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Generaly , it s monotonic decrease of the harmonics amplitudes
in respect of frequency.
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File Type: gif AWB55 LP THD1.gif (31.0 KB, 197 views)
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Old 6th January 2012, 10:46 AM   #604
DF96 is offline DF96  England
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OK, now I understand where the confusion arose. I was talking about a monotonic transfer function.
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Old 6th January 2012, 11:04 AM   #605
wahab is offline wahab  Algeria
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Rather , a caracteristics function , the transfer function being its laplace
transform.

You will notice that a transfer function can be non monotonic
with GNFB if ever the amp/device loop gain is above 1 for phase output
being at -180.

As for a caracteristics function , as you point it ,it can hardly be non monotonic.
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Old 6th January 2012, 11:05 AM   #606
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Hi,

Quote:
Originally Posted by DF96 View Post
What do you understand by 'monotonic'?
Monotonic is used in two ways known to me for amplifiers, one refers to monotonic ordering of distortion products, the other to the nature of the distortion, the second one implies that distortion changes in step with the signal, that is more signal more distortion, less signal less distortion without any sudden changes.

Example - the distortion of a class A amplifier is monotonic even if it is odd order harmonics dominant, yet the distortion of a class AB ampliifer is never (cannot be) monotonic if dominated by the output stage, EVEN if the decay of the distortion products is always even order dominant.

The implication of monotonic in any context is a smoothly increasing or decreasing sequence.

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Old 6th January 2012, 11:23 AM   #607
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by DF96 View Post
My understanding of monotonic is that the output direction does not change sign...
Maybe that gets to the heart of the disagreement. I thought it meant that distortion always decreases as signal level reduces.
[edit: ...or Wahab's definition. I think that may be more important]

I had a look at the schematic for the Rotel RA-840BX4 (couldn't find RA840). One interesting thing is it has a double EF output stage, and the biasing instructions say to set the voltage drop across the emitter resistors to 2.5mV.

This is ten times lower than suggested e.g. by Doug Self, so I would expect fairly high crossover distortion. OTOH, the topology is pretty much as Thorsten described, so (unlike e.g. the "Blameless") it probably doesn't have distortion that increases badly with increasing frequency, or artificial emphasis on high-order distortion products.

Perhaps this gives a clue as to what's important. I'm not saying crossover distortion is OK, just that it may not be top of the "bad stuff" list.

Wide open-loop bandwidth, modest GNFB, no miller compensation, multiple output pairs, and it sounds OK despite having a bit of crossover distortion. Well, assuming this is representative of the Rotels that sound OK.

Last edited by godfrey; 6th January 2012 at 11:26 AM. Reason: cross-post
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Old 6th January 2012, 11:34 AM   #608
DF96 is offline DF96  England
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Quote:
Originally Posted by wahab
the transfer function being its laplace
transform.
Yes, my mistake.

In the Geddes/Lee paper they use 'transfer function' with the same meaning that I did, so at least I have company! Sorry for causing confusion.
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Old 6th January 2012, 12:33 PM   #609
wahab is offline wahab  Algeria
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Quote:
Originally Posted by DF96 View Post
Yes, my mistake.

In the Geddes/Lee paper they use 'transfer function' with the same meaning that I did, so at least I have company! Sorry for causing confusion.
Indeed , i agree that your meaning is more logical , and i use it
myself generaly.....


As for crossover distorsion , it s easy to get rid of it but often
there s designs that do not explicitly adress this issue.

One thing important is , as you also pointed it, in PP output stage for exemple ,
to keep a meaningful voltage difference between the two halfs
command bases, as it s mandatory that at least one device is conducting
at any given moment, as if both device stop simultaneously conducting ,
the amp s open loop gain can collapse to the point that it will
be lower in the up of the audio band than the amp s supposed
closed loop gain , thus GNFB will be absent and crossover distorsion
will be awfully high.
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Old 6th January 2012, 03:41 PM   #610
DF96 is offline DF96  England
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OK, here is my suggestion for a THD-type metric. The idea is that higher orders sound worse, and amps spend much of their time well below max output (same as GedLee).
1. Find/pick the max output level for the amp - perhaps dominant harmonic reaches 5%?
2. Drop down 50% in voltage (-6dB) and measure harmonics, then combine using the Shorter weighting (harmonic n gets multiplied by n<sup>2</sup>/4).
3. Drop to 10% (-20dB from max) and repeat.
4. Drop to 1% (-40dB from max) and repeat.
5. Combine these three numbers in quadrature, so the highest one dominates the result.
With modern test equipment this should be possible. For a 40W amp this means testing it at 10W, 100mW and 1mW. For smooth amps such as genuine Class A should be dominated by the first figure, whereas P-P Class B/AB may get worse at lower levels. Could also do this as a function of frequency.

I know THD is not everything, so something similar for IMD is needed too (and other things).

IMD test proposal:
Generate three tones as follows: 5000Hz, 5001Hz both at 25% of max voltage to provoke non-linearity at 1Hz, and 270Hz at 10%. Look for 1Hz sidebands on 270Hz tone. Repeat at lower signal levels, and at other difference frequencies. The idea is to look for both normal 3rd-order IM, and the effect of recycled 2nd-order IM. Could also look for sidebands at mains frequency and harmonics.
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