Another view of damping factor?

[head_unit]

You can troll around and find more or less two views of damping factor:

A) A high DF doesn't matter because the [typical] amplifier's output impedance is swamped by the speaker wire and voice coil resistance.

B) Amps with high DF have "more control" over cone motion and sound better.

Sometimes there is a middle view
BA) High DF is good but doesn't matter once great than [insert magic number here].

I was talking with a friend who opined that high DF was not important for its own sake. But his thesis was that designing an amp* for good sound, particularly with subs, results in a high damping factor. That by shooting for wideband frequency response and low noise and especially indifference to back EMF, a high DF often results as a byproduct.

What do you think?


*The context of the discussion was mainstream solid-state designs, including more lately Class D designs which often don't have good damping due to the physical output filters.

[stratus46]

Originally Posted by head_unit

You can troll around and find more or less two views of damping factor:

A) A high DF doesn't matter because the [typical] amplifier's output impedance is swamped by the speaker wire and voice coil resistance.

B) Amps with high DF have "more control" over cone motion and sound better.

Sometimes there is a middle view
BA) High DF is good but doesn't matter once great than [insert magic number here].

I was talking with a friend who opined that high DF was not important for its own sake. But his thesis was that designing an amp* for good sound, particularly with subs, results in a high damping factor. That by shooting for wideband frequency response and low noise and especially indifference to back EMF, a high DF often results as a byproduct.

What do you think?


*The context of the discussion was mainstream solid-state designs, including more lately Class D designs which often don't have good damping due to the physical output filters.

IMO it's hard to really 'use' the damping factor beyond 100. Your speaker wire and connectors would have to be less than 0.08 ohms for an 8 ohm system to exceed 100. I have heard high damping amplifiers that sounded 'constricted' (I hate 'descriptions' of sound). That same amp sounded dramatically better when the feedback was reduced. No, the gain wasn't changed but the open loop gain _was_ reduced so the feedback was less. The bass was tight, solid and well defined. I apologize for _more_ of these descriptive terms. The owner was as pleased as I with the result.

I wish I had measured the damping after the change but I would guess it was lower.

G²

[planet10]

Damping factor -- or better to consider output impedance -- needs to be considered in the light of the speaker (and wires), it is a system with.

High damping factor can result in an overdamped system with some speakers. We are staring to see more & more speakers that are happy with amplifiers with high output impedance (low damping factor).

dave

[TerryO]

Damping Factor, as well as THD are numbers that are just about useless as far as the quality of sound is concerned. SET amps have very little, if any, negative feedback and very low DF as a consequence. Yet as Nelson Pass, and others have demonstrated, these designs can actually have better bass response (within their power limitations) than SS amps with far higher DF.

I grew up with "the Iron Grip that a high DF provides", and like discovering the secret about Santa Claus, it was a great disappointment when I learned the truth.


Best Regards,
TerryO

[jimangie1973]

In my experience, the damping factor of the amp is very important, but for a different reason. As the output impedance of the amp changes, so does the frequency response of the system, because most speakers have such a highly frequency dependent impedance. We are very sensitive to relative amplitudes at different frequencies within the audio band. I'm pretty sure I can detect 0.1-0.2 dB variances. I've had to modify crossover values in my speakers to account for damping factor differences after changing amps. I think these output impedance differences are the main reason for different amps sounding different (assuming well designed amps).

[DF96]

View BA is nearest to the truth, with the caveat that it depends on the loudspeaker. Obviously a very high DF is swamped by wiring. A very low DF will result in frequency response variations, as few speakers have a level impedance.

Given a high enough DF that frequency response variations are small, the issue then becomes bass damping. This can be a mix of mechanical and electrical, complicated by the usual audio issue of 'accuracy' versus 'preference'. Maybe the speaker designer should specify what DF he intended.

High DF is usually achieved by some mixture of local and global NFB. My preference would be for as much as reasonably possible to come from local NFB (e.g. follower output, with a low impedance driver) with global NFB then adding a bit more.

[head_unit]

Interesting comments, but straying from my question which I must have put poorly.

So, let me re-phrase, since I'm really curious about the opinions of folks who design and build amps.

Never mind if damping factor is important or not, and never mind about the speaker wires and voice coil impedance.

What do you think about the thesis that designing for good sound will generally result in a high damping factor/low output impedance?
(not for tube or some other exotic designs, of course, he meant typical solid state)

[forr]

Whatever the damping factor of the amplifier, even infinite, the limiting factor in the damping of a driver resonance is the voice coil resistance which, for an 8 Ohm unit, may be something like 5 Ohm.

However there is a way to circumvent this limitation : using an associated circuit such that the amplifier presents a negative output resistance, the resistance of the circuit in then decreased (for example, the initial 5 Ohm value can be safely reduced to 2 Ohm).

This is a neglected but powerful technique.

[sreten]

Hi,


With solid state power amplifers its almost impossible not to end up with
a decent / high damping factor. The only exception I can think of is for
output inductorless designs that use a resistor instead, typically 0.22R,
this limits damping factor to around 40 (for 8 ohms) by default. Its also
possible to manipulate the output impedance for driving subwoofers.

rgds, sreten.

[sregor]

My 2 cents. High damping factor is nearly always a consequence of a large amount of NFB, whether the amplifier design is good or bad. A good design with a lot of NFB will have a high damping factor, a design with low (or no) feedback will generally have lower damping factor. Even class D amps can have high damping factor, if the feedback goes around the inductors. To me, the issue goes back to the speaker designer - if a speaker is designed to be flat with an ideal voltage source, then high DF is desirable. I am curently toying with larger boxes and lower amplifier output impedances, for increasing bass response with high Q woofers. There is some good reading in the Rod Eliot audio site for higher output impedance amplifiers, as well as here, but this is digressing from the orignal question. GOOGLE "Dick Pierce Damping" for a good read on damping factor.