Amplification - The importance of damping factor

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The importance of damping factor

Printed From: Graham Slee Hifi System Components
Category: And the rest
Forum Name: Amplification
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URL: https://www.hifisystemcomponents.com/forum/forum_posts.asp?TID=538
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Topic: The importance of damping factor

Posted By: Analog Kid
Subject: The importance of damping factor
Date Posted: 25 Jun 2009 at 10:06pm

How important is the damping factor when choosing an amplifier? For example, will an amplifier with a damping factor of 400 sound better than one rated at 100 or 200?

I got the impression that a high damping factor is desirable, but what i am curious to learn is what audible differences one will be able to detect between two amplifiers that have low and high damping factors, respectively.

I was reading the entry on damping factor on Wikipedia and became slightly confused about the matter.

http://en.wikipedia.org/wiki/Damping_factor - http://en.wikipedia.org/wiki/Damping_factor

It says:

"A high damping factor (which requires low output impedance at the amplifier output) very rapidly damps unwanted cone movements induced by the mechanical resonance of the speaker, acting as the equivalent of a 'brake' on the voice coil motion (just as a short circuit across the terminals of a rotary electrical generator will make it very hard to turn). It is generally, though not universally, thought that tighter control of voice coil motion is desirable as it is believed to contribute to better quality sound. A high damping factor indicates that an amplifier will have greater control over the movement of the speaker cone, particularly in the bass region near the resonant frequency of the driver's mechanical resonance."

But then further down it says:

"...high "damping factor" values do not, by themselves, say very much about the quality of a system; most modern amplifiers have them, but vary in quality even so. Given the controversy that has long surrounded the use of feedback, some extend their distaste for negative feedback amplifier designs (and so a high damping factor) as a mark of poor quality. For them, such high values imply a high level of NFB in the amplifier. Tube amplifiers typically have much lower feedback ratios, and in any case almost always have output transformers which limit how low the output impedance can be. Their lower damping factors are one of the reasons many audiophiles prefer tube amplifiers."

This is confusing. First it is implied that a high damping factor is desirable and then it says some people prefer valve amplifiers because they have lower damping factors??? So is a high damping factor good or bad?

Graham, I hope you can shed some light on this matter.

Replies:

Posted By: Graham Slee
Date Posted: 26 Jun 2009 at 5:57am

Damping Factor is imaginary, it is not a measurable thing.

See my replies on AC vs DC coupling.

Damping Factor calculations completely ignore Kirchoff's Law.

It would be better to choose an amplifier with the largest power supply capacitors - if they would only put that in their specifications.

As regards negative feedback, here is another myth. Basically, designers don't use negative feedback when they've made a mess of a design: if the design is unstable, taking the negative feedback out stops it oscillating...

Bell Labs, way back in history, discovered that approx. 40dB negative feedback (100 times) reduced distortion to unnoticeable levels. However, that can only work in a stable amplifier. Ours are stable, and they use similar levels of negative feedback - if it was so bad do you think we'd have such a good following? The truth is that negative feedback used properly results in an incredibly good musical performance - and the sad truth is there are a lot of "well respected" dabblers making Hi-Fi.

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Posted By: Graham Slee
Date Posted: 26 Jun 2009 at 3:17pm

More info...

Take a look at these 16 year old reviews. Understanding the circuit topologies and graphical data like I do, I can see how these correlate with the listening panel's conclusions.

Also you should see that the amount of damping factor doesn't really tell you that an amp is any better or any worse.

The RF and noise plots say it all for me...

These are four of the original ten Hi-Fi Choice reviews in that issue.

Surprisingly one of the least expensive gets the best marks - the "high-end" fail miserably - nothing changes apart from the increase in BS required to sell the "high-end"...

We can talk about RF and noise if anybody likes?

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That none should be able to park up and enjoy the view without a smartphone and the knowledge in how to use apps