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Crossover Distortion, the truth
Crossover Distortion, the truth
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Old 4th December 2017, 12:46 PM   #1
OnAudio is offline OnAudio
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Default Crossover Distortion, the truth

There are three major classes of amplifiers, class A, A/B and D. Class A does not suffer from crossover distortion, nobody cares about it in class D, we actually put a dead time. Class A/B may suffer from it if not well biased. Intresting solutions to mimic class A include ibias by Krell. Attempts to shift the crossover point of a class A/B from zero such as class XD by Self, trying to mimic single ended class A. A good bias point does eliminate the problem in class A/B leading to better THD figures.
Electronic components do produce structural noises. Class A holds a steady current throughout the amplifier which is converted to heat, providing the much needed heating in winter. Many urban dwellers have nostalgic memories of winter nights spent huddled together around a class A master piece. This steady current also keeps the output transistors and rectifier capacitors structural noise in check. The birth of MOSFETs was what class A was waiting for.
Class A/B and class D share the relationship that the output devices are switched on/off , the difference is that class A/B is switched by the audio signal while in class D we switch the output devices on/off using a higher frequency than the audio frequency, keeping the devices cooler and shifting their structural noise, a side effect is bus pumping by the half bridge, creating an unintended boost converter. A class D amplifier will give you no warmth during winter, however a few cool LEDs might just fool you.
The choice between a switch mode power supply and linear power supply dictates the structural noise that the power supply produces. If your below the age of 25 years you may hear your computers switch mode power supply sing. Everything else around us hums at 50/60 Hz including the hum you see or hear emanating from your body when you touch an oscilloscopes probe or power amplifier input.
A linear power supply is bulky, heavy and expensive, but if well designed blends in with the environmental noise. A switch mode power supply is smaller, lighter and cheaper but sings at a different frequency from the environment probably necessitating EMI treatment.
The modern generation is more accustomed to class D sound from the modern electronic devices such as BT speakers, laptops, TVs, xphones, etc. It may be difficult to pickout a well designed class D amplifier from a class A amplifier. One might even argue that a young person is already conditioned to find class D sound more 'normal'. Class A/B amplifiers are the grail of simplicity and are often coupled with a high quality switch mode power supply with or without rail switching for cooler higher current operation. A popular setup for car audio and professional PA.



Feel free to share your expertise on this matter
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Old 4th December 2017, 02:12 PM   #2
DF96 is offline DF96  England
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Quote:
Class A holds a steady current throughout the amplifier which is converted to heat
Class A average current is steady. In some cases the instantaneous current varies.

Quote:
Electronic components do produce structural noises.
What is structural noise? Is it what everybody else calls excess noise or shot noise or partition noise or flicker noise?

Quote:
A linear power supply is bulky, heavy and expensive, but if well designed blends in with the environmental noise.
What is environmental noise? Is it what everyone else calls hum or RFI?

You claim to offer us the truth about crossover distortion, then hardly mention it in your post.
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Old 7th December 2017, 09:27 AM   #3
OnAudio is offline OnAudio
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Quote:
Originally Posted by DF96 View Post
What is structural noise? Is it what everybody else calls excess noise or shot noise or partition noise or flicker noise?
Not quite, caps sing, transformer laminations sing, everything sings
Quote:
Originally Posted by DF96 View Post
What is environmental noise? Is it what everyone else calls hum or RFI?
Both plus more
Quote:
Originally Posted by DF96 View Post
You claim to offer us the truth about crossover distortion, then hardly mention it in your post.
Its an open ended discussion for further interaction, theres mention of bias, most people with heat-sink real estate may prefer to bias their A/B amplifier where the 1st Watt is class A
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Old 7th December 2017, 11:03 AM   #4
DF96 is offline DF96  England
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Originally Posted by OnAudio
Not quite, caps sing, transformer laminations sing, everything sings
Ok, so when you say "noise" you don't actually mean 'noise'. That knowledge may help future communication.

Quote:
Its an open ended discussion for further interaction, theres mention of bias, most people with heat-sink real estate may prefer to bias their A/B amplifier where the 1st Watt is class A
You started the thread. Maybe you should tell us what you know about crossover distortion? Otherwise it could be misunderstood as being merely some sort of publicity stunt?
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Old 7th December 2017, 12:11 PM   #5
Guitar.mod is offline Guitar.mod  Europe
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I think that MOSFET output is a better choice for class AB Amp (D. Self calls it "Optimal Class B", that means this Bias current is relatively small, for example 15-30mA). Because BJT output needs precise "optimal bias current" regarding D. Self and B. Cordell. Some variations (+-15mA) introduce Higher crossover distortion. Even Higher current - if it is not A class - does not help here, because of the Gm doubling (when two devices conduct) and droop (when one device conducts) effect. Mosfets do not produce Gm doubling, so when it is more bias current, there is less crossover distortion. When bias current is relatively big (for example 300-500mA) a little Bias variations (+-15mA) because of the thermal compensation slowness does not produce more/noticeable distortion.
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Old 7th December 2017, 12:22 PM   #6
AKSA is offline AKSA  Australia
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DF,
Why do you attack people who are practical, creative and generous?
Examine Harrison's history in this forum and tell me I am wrong.
Is it to demonstrate you are well educated?

HD
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Old 7th December 2017, 12:47 PM   #7
billshurv is online now billshurv  United Kingdom
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Crossover Distortion, the truth
I think there are some posts that were adjusted on this thread that make things a bit disjointed. I certainly cannot understand the Original post.
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Old 7th December 2017, 12:51 PM   #8
scottjoplin is offline scottjoplin  Wales
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I'm getting a distinct feeling of deja vu over some of the wording
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Old 8th December 2017, 09:58 AM   #9
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Crossover Distortion, the truth
OT posts removed.
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Old 8th December 2017, 09:59 AM   #10
mchambin is offline mchambin  France
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Nothing new in the OP.
IMO a publicity scheme, as I see too many on DIYaudio.
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