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Carver Crimson 275 Review (Tube Amp)

Rate this amplifier

  • 1. Poor (headless panther)

    Votes: 379 95.0%
  • 2. Not terrible (postman panther)

    Votes: 5 1.3%
  • 3. Fine (happy panther)

    Votes: 6 1.5%
  • 4. Great (golfing panther)

    Votes: 9 2.3%

  • Total voters
    399

pseudoid

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...zero NFB...
You mean like NO 'local' or 'global' feedback at all?
Are there many such beasts that do not rely on NFB?
They are probably all tube amps...
I always thought that (such stability w/o FB) was an audio fantasy.
 

fpitas

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atmasphere

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You mean like NO 'local' or 'global' feedback at all?
Are there many such beasts that do not rely on NFB?
They are probably all tube amps...
I always thought that (such stability w/o FB) was an audio fantasy.
FWIW negative feedback is a destabilizing design feature. If you want to see a stable circuit condition, apply positive feedback.
Amps without feedback can be quite stable; very resistant to oscillation, since negative feedback and phase margin issues don't play well together.
Class D amps can be built with zero feedback as well.
 

solderdude

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It is actually worse than 1.7ohm as it is load dependent due to the current feedback.... ahem... 'the back EMF and room sensing feedback circuit'.
 
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antcollinet

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FWIW negative feedback is a destabilizing design feature. If you want to see a stable circuit condition, apply positive feedback.
Amps without feedback can be quite stable; very resistant to oscillation, since negative feedback and phase margin issues don't play well together.
Class D amps can be built with zero feedback as well.
:facepalm:

You know what a nuclear detonation is? Positive feedback. As is any explosion - explosives not often described as being stable.

Anything you apply positive feedback to goes to limits.


My room is getting hot - feed back that positively to the heating systme so that it puts out more heat. That is postive feedback.

Ever heard of thermal runaway - that is positive feedback also.
 
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SIY

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FWIW negative feedback is a destabilizing design feature. If you want to see a stable circuit condition, apply positive feedback.
Amps without feedback can be quite stable; very resistant to oscillation, since negative feedback and phase margin issues don't play well together.
Class D amps can be built with zero feedback as well.
For any competent designer, stability is absolutely no issue- stability criteria have been well-understood for nearly a century. To quote Klaatu, "I find it works well enough to get me from one planet to another."
 

Gringoaudio1

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FWIW negative feedback is a destabilizing design feature. If you want to see a stable circuit condition, apply positive feedback.
Amps without feedback can be quite stable; very resistant to oscillation, since negative feedback and phase margin issues don't play well together.
Class D amps can be built with zero feedback as well.
You’ve got it backwards mate.
Turn in your EE degree.
 

atmasphere

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:facepalm:

You know what a nuclear detonation is? Positive feedback. As is any explosion - explosives not often described as being stable.

Anything you apply positive feedback to goes to limits.


My room is getting hot - feed back that positively to the heating systme so that it puts out more heat. That is postive feedback.

Ever heard of thermal runaway - that is positive feedback also.
The statement was meant as humor; I was using the Chaos Theory definition of the term ;)

So, or 'but', depending on how you look at it:

A stable state is one that does not change. An amplifier in oscillation is in a stable state insofar as its output is concerned. FWIW positive feedback is the basis of self oscillating class D amps (like the Purifi) although at audio frequencies that feedback is negative. This BTW is why self-oscillating class D amps cannot be induced to oscillate when subjected to a strange load like one that is heavily capacitive. Its already oscillating... and therefor stable...

Of course there's more to it; apparently you missed this comment, perhaps when you did your facepalm:

Amps without feedback can be quite stable; very resistant to oscillation, since negative feedback and phase margin issues don't play well together.
(Emphasis added)

The reason negative feedback is destabilizing is due to phase shift. Phase shift is responsible for a design aspect all amplifier designers of amps with feedback must face, which is called 'phase margin'. If you exceed the phase margin of said amplifier, oscillation occurs. So it would appear that the amp is stable if that isn't done... except when the load on the amplifier's outputs messes with the feedback loop, which is a critical design in any amplifier with feedback (a simple resistor is rarely good enough...). Far too many amplifiers have been produced that can oscillate and fail when subjected to a slightly odd load, for example a Quad ESL57.
 

antcollinet

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The statement was meant as humor; I was using the Chaos Theory definition of the term ;)

So, or 'but', depending on how you look at it:

A stable state is one that does not change. An amplifier in oscillation is in a stable state insofar as its output is concerned. FWIW positive feedback is the basis of self oscillating class D amps (like the Purifi) although at audio frequencies that feedback is negative. This BTW is why self-oscillating class D amps cannot be induced to oscillate when subjected to a strange load like one that is heavily capacitive. Its already oscillating... and therefor stable...

Of course there's more to it; apparently you missed this comment, perhaps when you did your facepalm:


(Emphasis added)

The reason negative feedback is destabilizing is due to phase shift. Phase shift is responsible for a design aspect all amplifier designers of amps with feedback must face, which is called 'phase margin'. If you exceed the phase margin of said amplifier, oscillation occurs. So it would appear that the amp is stable if that isn't done... except when the load on the amplifier's outputs messes with the feedback loop, which is a critical design in any amplifier with feedback (a simple resistor is rarely good enough...). Far too many amplifiers have been produced that can oscillate and fail when subjected to a slightly odd load, for example a Quad ESL57.
You know what phase shift does to negative feedback?


Turns it into positive feedback.
 

mhardy6647

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You mean like NO 'local' or 'global' feedback at all?
Are there many such beasts that do not rely on NFB?
They are probably all tube amps...
I always thought that (such stability w/o FB) was an audio fantasy.
That is correct.
A certain, small amount of localized NFB is baked into a vacuum tube (as I understand it, which is barely), but no added local nor global NFB.
Thus the high-ish output impedance (and obviously some limitation of "flat" bandwidth, too) of this (and similar) amplifier.

JELSimple45-2A3schematic.jpg
 

Doodski

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That is correct.
A certain, small amount of localized NFB is baked into a vacuum tube (as I understand it, which is barely), but no added local nor global NFB.
Thus the high-ish output impedance (and obviously some limitation of "flat" bandwidth, too).

JELSimple45-2A3schematic.jpg
Did you make that DIY schematic? The hand written text is pretty good. Takes lots of practice to get the text justtt right.
 

mhardy6647

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Did you make that DIY schematic? The hand written text is pretty good. Takes lots of practice to get the text justtt right.
Negative. (no pun intended)
That is Joseph Esmilla's own schematic.

Joseph is a musician, photographer and hifi DIYer of... well... no mean repute in... umm... certain circles. :)

JE Herb and Joe.jpg


You'll, at least, recognize the fellow in the center, I reckon. :cool:
 

sergeauckland

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That is correct.
A certain, small amount of localized NFB is baked into a vacuum tube (as I understand it, which is barely), but no added local nor global NFB.
Thus the high-ish output impedance (and obviously some limitation of "flat" bandwidth, too) of this (and similar) amplifier.

JELSimple45-2A3schematic.jpg
That'll never work! There's no ring round the tube electrodes, so all the vacuum will leak out.

S
 

cybernaut667

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Listen to it? Why do that? Audio Science Review is about sine waves and powering resistors, not music powering loudspeakers.
If its cheap and built to test good, its golden, no need to be musical. Audio reviews without listening. If you don't actually listen to an audio product in a review, your name might be more accurately
Now that's the words of a dealer !

"The need to be musical" is the best part
 

pseudoid

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There has always been some controversy about/with introduction of feedback, especially in audio circuit designs [...more so in marriages?].
I think of 'negative feedback' as a necessary evil and each FB element is, after all, just another filter in circuit design that has the goal of keeping the circuit at unity.
Some designs use feedback to cover the shortcomings of a design. Some designers use excessive feedback that ends up introducing other in/out-of-band problems.
Cut @atmasphere some slack! :)
That'll never work! There's no ring round the tube electrodes, so all the vacuum will leak out.
@sergeauckland,
Smilies are free of charge (and free for use) @ASR.;)
 
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