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Why Let an Amplifier Sound Good when You can Force it to?
Why Let an Amplifier Sound Good when You can Force it to?
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Old 27th January 2018, 02:06 PM   #21
MrMagic is offline MrMagic  Greece
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There is no such a thing as a "zero feedback amplifier", -at least one that uses transistors, mosfets or tubes for amplification.

Simply because transistors, mosfets and tubes are highly non-linear devices (=imperfect, useless devices for faithful amplification, which become useful only with the application of feedback). So feedback, local, global or multiple-nested, is required in order to linearize those imperfect devices.
Here is where all the anti-feedback arguments collapse!


But there are many other ignorance-based myths (or even promoted by makers of mediocre amps because they only do a minimum R&D as many buyers usually are mostly convinced by rumors rather than scientific facts), like "too much feedback raises odd and higher order harmonics and the amp sounds bad", "local feedback is good and global feedback is bad", etc.

Nothing of this is true for various reasons, but mainly because it all depends on the specific design which might be very different, NOT on the amount and type of a single parameter in a complex system. It's like judging a sportscar only by the amount of gasoline that runs through the gas pump, which of course would be incredibly stupid for obvious reasons.

The amplifiers' performance is what matters, which is the product of tens or hundreds of parameters, put together not randomly, but methodically after research. By focusing on a single parameter, or on the component types etc, you are missing the important thing, which is the actual measured output performance.




Last edited by MrMagic; 27th January 2018 at 02:10 PM.
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Old 27th January 2018, 02:41 PM   #22
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by MrMagic View Post
There is no such a thing as a "zero feedback amplifier", -at least one that uses transistors, mosfets or tubes for amplification.

Simply because transistors, mosfets and tubes are highly non-linear devices (=imperfect, useless devices for faithful amplification, which become useful only with the application of feedback). So feedback, local, global or multiple-nested, is required in order to linearize those imperfect devices.
Here is where all the anti-feedback arguments collapse!


But there are many other ignorance-based myths (or even promoted by makers of mediocre amps because they only do a minimum R&D as many buyers usually are mostly convinced by rumors rather than scientific facts), like "too much feedback raises odd and higher order harmonics and the amp sounds bad", "local feedback is good and global feedback is bad", etc.

Nothing of this is true for various reasons, but mainly because it all depends on the specific design which might be very different, NOT on the amount and type of a single parameter in a complex system. It's like judging a sportscar only by the amount of gasoline that runs through the gas pump, which of course would be incredibly stupid for obvious reasons.

The amplifiers' performance is what matters, which is the product of tens or hundreds of parameters, put together not randomly, but methodically after research. By focusing on a single parameter, or on the component types etc, you are missing the important thing, which is the actual measured output performance.



All true, and it gets repeated constantly. But people would rather listen to slick marketing jargon than engineers that actually design the equipment, because I guess salesmen know more than engineers.

At least once a week I try to point a newbie in the right direction right here on this website. Some of them just disappear; some of them have flamed me. A few listen. I say good luck if you want to design audio equipment based on sales brochures.

I have only visited the local upper crust hi fi store once. The salesmen aren't as bad as the big box salesmen (MORONS) but they still lay it on thick. I probably couldn't sell that stuff because I'd make the customer's eyes glaze over.
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Old 27th January 2018, 02:43 PM   #23
Mark Johnson is offline Mark Johnson  United States
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Why Let an Amplifier Sound Good when You can Force it to?
Here's an amplifier whose mid-frequency gain is roughly 160X (44dB).

Is this a feedback amplifier?

Why, or why not?

_
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Old 27th January 2018, 02:52 PM   #24
MarcelvdG is offline MarcelvdG  Netherlands
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Probably someone will argue now that the base width modulation (Early effect) caused by the collector-base depletion region is a kind of feedback, and when you draw the same circuit with a triode they will argue that the effect of the anode on the current is a kind of feedback, and of course you use feedback to stabilize the bias point, but in my opinion it is not a feedback amplifier in the sense of Black's patent. At signal frequencies there is no feedback path around the active device.
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Old 27th January 2018, 02:53 PM   #25
Wavewhipper is offline Wavewhipper  United States
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Mainly only bias stabilization at the emitter, the emitter cap negates most of any signal feedback. I didn't explain in my first post that I was referring to output fed back to the input closed loop type of feedback. The Hfe curve could prevent runawy or excess gain by it's cliff like curve.
So far I like it best. Troubleshooting from a schematic and no voltage references can be difficult. Finding the origin of an error in a closed loop has been tricky for me personally. And I've seen others struggling with a packed circuit board in such a situation.

Last edited by Wavewhipper; 27th January 2018 at 02:57 PM.
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Old 27th January 2018, 02:57 PM   #26
Reodor Felgen is offline Reodor Felgen
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Quote:
Originally Posted by Eva View Post
- Feedback is an ideal universal mechanism, only imperfections in the elements comprising the feedback loops contribute to imperfections in the output of the system.
- Feedback is everywhere, even inside electronic gain devices such as transistors, at electron and molecular level.
- Feedback requires some element outputting a magnitude which is the sum or difference (and/or other math operations) between other 2 or more magnitudes: The error amplifier. Feedback requires the output of the system to be fed back to input of error amplifier.
- Feedback happens inherently in every amplification circuit, even in follower configurations (the simplest case). The conditions for a follower to be unstable do exist, and can be created in practice.
- In linear electronic circuits: there are lower and upper practical limits for feedback. Lower NFB loops have to work with higher error magnitudes, requiring wider linear range of operation of error amplifier (and earlier amplification stages). Higher NFB loops encounter the opposite problem, error magnitude becomes low enough to be disturbed by other magnitudes, as by: parasitic inductive and capacitive coupling between circuit elements, noise floor of components, drifts in error amplifier characteristics due to temperature.
- In switched-mode amplifier circuits: feedback is employed too. "It does not need to be stable in order to be linear". The interest for switched-mode amplification arises when the practical limits of feedback are explored (the raw conditions where linearity improvement takes place).

The following simulation pictures (based in a real circuit) show a class-D amplifier (modulator type: self-oscillating post-filter single-integrator) doing <0.007% 3rd harmonic at 5khz @ 50W @ 8ohm. The circuit is sized for up to 800W/2ohm output, using two N-ch TO-220 power MOSFET. This matches the performance of finest class AB projects shown in best solid state threads, while not being a stable NFB loop, but it is an unstable NFB loop constructed to be highly linear (this proves the 1st statement in this post).
Hi Eva

This is exactly right and a very good post.

BTW: In my experience with switching amplifiers (AKA Class D) you can also get a lot less Noise than in classic A or AB Amps.
BTW2: It seems like you are using about 400kHz (maybe a bit less) Switching Frequency.
I have designed my own Class D Amp, it's an all discrete design with "my own discrete comparator"
I'm using a bit higher switching frequency (and ofcourse my own discrete comparator) and I'm achieving a lot less THD at the same output level and in the 1ppm level at low outputlevel.

Cheers
Reodor
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Old 27th January 2018, 07:34 PM   #27
cbdb is offline cbdb  Canada
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Quote:
Originally Posted by Mark Johnson View Post
Here's an amplifier whose mid-frequency gain is roughly 160X (44dB).

Is this a feedback amplifier?

Why, or why not?

_
Yes it's feedback. But not much. Emitter resistors cause feedback, even if it's only the transistors internal emitter resistance. Show us the THD? Than figure out the gain and THD with a different emitter impedance, these will change the same as any other feedback scheme.
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Old 27th January 2018, 11:29 PM   #28
Wavewhipper is offline Wavewhipper  United States
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Don't take the comparator (differential amp) statement I made as official, I made that assumption and became confident enough to write that as if from official sources. Ot really should have been a question.

Quote:
Originally Posted by Wavewhipper View Post
I was just going to say using negative feedback for linearization requires a precision comparator stage for the magic to work. Differential amp pairs on one die are a good idea for that. Sort of funny how the errors wiggle their way through the system until they stabilize at the comparator. There has to be some sort of high speed hunting and settling time, well above audio range.

I could not find but a few transistors even somewhat able to produce linear gain over the functional range without feedback. And they probably deteriorate with time and heat.
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Old 28th January 2018, 01:45 AM   #29
mchambin is offline mchambin  France
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What do you mean by precision comparator ?
Is this the long tail pair most amps and op amp have at the inputs ?
The only aging I know with a bjt ltp comes from input overdrive that can simply be avoided with two diodes shunting the inputs.
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Old 28th January 2018, 04:36 AM   #30
xx3stksm is offline xx3stksm  Japan
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Quote:
Originally Posted by Mark Johnson View Post
Here's an amplifier whose mid-frequency gain is roughly 160X (44dB).

Is this a feedback amplifier?

Why, or why not?

_
The emitter voltage of Q1 is almost 1.9V(12*(27/127)-Vbe). The emitter current of Q1 is 1.9/2.2(R1)=0.86mA, which means the equivalent emitter resistor of Q1 is 25/0.86=29ohm. The gain of Q1 is 4700(R2)/29=162.
In this situation, I'm sure Q1 has positive DC feedback because the negative coefficient of Vbe makes emitter current increasing. The positive feedback stops at the point where radiation of heat is equal to generation.

But I'm not sure AC feedback exists or not. It's true AC gain becomes large according to temperature rise because the equivalent emitter register becomes low. Temperature and the gain are a positive feedback to some extent. But AC feedback in an electrical circuit usually means between the input signal and the gain. I can regard temperature and the gain are positive feedback in a wide sense. The input signal doesn't have any feedback on the gain in this situation, unlike NFB of an OP amp. This has feedback or not is up to you, IMHO.
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