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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 26th January 2018, 01:19 PM   #1
Wavewhipper is offline Wavewhipper  United States
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Default Why Let an Amplifier Sound Good when You can Force it to?

Some people say feedback amplifiers sound best, some say they have some sort of characteristic sound. I am all confused and figure I'll try to see it in an enlightened manner. LOL

Looks to me like an amplifier that is forced to perform by feedback may at worst fail in a cascade of burned components if the feedback loop goes crazy and measures weren't taken to assure the amplifier can't overdrive stages outside the design safety zones.
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Old 26th January 2018, 01:32 PM   #2
Mooly is offline Mooly  United Kingdom
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Why Let an Amplifier Sound Good when You can Force it to?
Well this will end up a bun fight before the day is out

Assuming we are talking about power amplifiers then I think there is some truth in saying that various topologies and component choices do influence the subjective outcome.

As to feedback loops 'failing' in some way, well I would counter that by asking why would they (fail) ? They are passive parts, usually lightly stressed, they would be the last suspect's under a fault condition in my experience.

If you mean failing to maintain stable operation when faced with adverse drive conditions, well that's a different ball game. A competent design should be just fine when overdriven to any normal extent. If it flips and latches to a rail for example, in the face of modest overdrive then the designer has failed in their job.

You can spin all this a 1001 ways and still not be happy, or still try and argue for one implementation over another.
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Old 26th January 2018, 01:36 PM   #3
Fast Eddie D is offline Fast Eddie D  United States
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Quote:
Originally Posted by Wavewhipper View Post

Looks to me like an amplifier that is forced to perform by feedback may at worst fail in a cascade of burned components if the feedback loop goes crazy and measures weren't taken to assure the amplifier can't overdrive stages outside the design safety zones.
That's why engineers have slide rules. They are charged with making sure this stuff doesn't happen.

A properly damped amplifier with carefully parsed gain structure will exhibit minimum latch up.
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Old 26th January 2018, 02:06 PM   #4
Mark Johnson is offline Mark Johnson  United States
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Why Let an Amplifier Sound Good when You can Force it to?
There's a famous audio circuit wizard called Nelson Pass, who designs and builds and sells amplifiers under the brands "Pass Labs" and "First Watt". He's also very active in the DIY audio movement, participates regularly on this website, and is the driving force behind the Burning Amp festivals.

I grabbed the image below from the Products page of his First Watt website. Notice that many of his amplifiers use feedback, and many use no feedback at all. Think about that for a few minutes.

You could search the internet for listening reviews of a few First Watt feedback amps, and a few of their non-feedback amps. What do reviewers say? You could ask your audio buddies who own one (or more!) of the First Watt amps, will they please let you listen to the amp, using music that YOU are familiar with?

Bob Cordell's power amp book gives example schematics for amplifiers with feedback, and other amplifiers with no feedback at all. You could experiment with those, either on the lab bench, or in circuit simulation software, or both.

_
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File Type: png firstwatt.png (31.1 KB, 1312 views)
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Old 26th January 2018, 02:25 PM   #5
DF96 is offline DF96  England
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There are two approaches to feedback:
1. talk about it
2. learn about it
Sadly, many people choose option 1 and carefully avoid option 2. Doing it the other way round (2, then 1) means you have something useful to say.
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Old 26th January 2018, 02:35 PM   #6
scottjoplin is offline scottjoplin  Wales
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Quote:
Originally Posted by Wavewhipper View Post
Some people say feedback amplifiers sound best, some say they have some sort of characteristic sound. I am all confused and figure I'll try to see it in an enlightened manner. LOL
Often people say random stuff to confuse, it's a way to control you
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Old 26th January 2018, 03:01 PM   #7
mchambin is offline mchambin  France
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Listening tests at the HiFi levels of nowdays mostly deal with placebo effect, if not totally biased not really done double blind.
Serious unbaised tests prove they are all the same. Simply because audio sources, loudspeakers and audotoriums are of a much lower quality.
Save your money and saliva and go for simple well proven designs, stay away from sensational assertions especially with stratospheric LTSpice results as well as hundred of pages books.
Here is all you need:
ESP Ron Elliot Elliott Sound Products - The Audio Pages (Main Index)
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Old 26th January 2018, 04:08 PM   #8
Fast Eddie D is offline Fast Eddie D  United States
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I haven't seen any amplifiers that have no feedback.

Even an emitter resistor, or cathode resistor, provides local feedback. That resistor also can make the device operate in a much more linear way.

Antique tube circuits never had global feedback. They had local degeneration. Early transistor circuits, which were not that different from tube circuits, used local degeneration just like their tube predecessors.

Even the super simple MOSFET follower has local degeneration because of its source resistor.

the Nelson Pass "Stasis" amplifier is a two stage amplifier. The output stage is a push pull Sziklai pair. It is not included in the global feedback. It definitely used local feedback.

An amplifier with no degeneration at all would not provide predictable performance. It would also be very nonlinear.
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Old 26th January 2018, 07:20 PM   #9
LBHajdu is offline LBHajdu  United States
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I don't think this can be answered. There are different kinds and amounts of feedback that can be applied to various gain stages which in turn give rise to different harmonic compositions at different loads and power levels. This then in turn favors or discriminates against different types of music or sounds. Example people pay big money for a 2A3 amp with 1% distortion because it makes simple vocal music sound really smooth and intimate but try the same amp on a big orchestral peace and you will say it sounds really congested.

Here are three companies, some of them use feedback and others don't, they are all commercially successful:

Ayre Acoustics - never uses any feedback loops, only local degeneration
Mr. Pass (Pass Labs, First Watt, ...) - sometimes yes sometimes no depending on the circuit
Mark Levinson - uses lots of global feedback

Then that takes you down the road of asking, are you in the precision instrumentation business or the entertainment business. Mr. Pass says he is in the entertainment business and I think what makes him successful is that he -> builds -> listens -> tweaks and improves -> listens again -> once he feels he has done the best he can reasonably do after some number of iterations he moves on to something new.
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Old 27th January 2018, 12:04 AM   #10
Wavewhipper is offline Wavewhipper  United States
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As far as runaway amps that can burn up a transistor I was thinking of a closed loop voltage feedback amp, used to flatten cheaper not so flat transistors. Sometimes cold solder joints, a bad resistor lead connection etc could break the loop open and allow the amp to drive to saturation. Mainly worried about using inexpensive driver transistors that could be driven to excessive wattage by a design no longer restricting the driver to a narrow low current, flat gain part of the hfe graph.

I figured out how to stretch the cheaper curvy transistors with voltage feedback and using some of them in the <100ma (Ic) range as a part of a Darlington. If the loop were to break open, some configurations could push the delicate transistors to unintended high drive levels and even start a cascade of cooked transistors.

I backed out of that and decided to use more robust and expensive components since I am not making a million of them. I seem to be encountering many analog power amps that could be called a high power opamp.
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