High damping factor or no damping factor?

Wharfedale EVO 4.1 4.2 Speakers

gobble

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I was researching the power amp upgrade path for my full rangers, and was trying to figure out the logic behind current driven power amps. In the process I found some conflicting information that I would like sorted out by the electronics experts on this forum


For example this article (http://www.passdiy.com/pdf/cs-amps-speakers.pdf) seems to indicate that a power amp with high output impedance and low damping factor would work great with sensitive full rangers, when in fact it is commonly held that output impedance of source needs to be lower than input impedance of receiver by a factor of 10.

To summarize: "Full rangers are easily over-damped, resulting in excessive loss of bottom end. Over-damping has good transient bass control but also suffers a significant loss of bottom end response. This partially explains the preference for tube amps with such loudspeakers. Our amplifier had a high output impedance and thus no damping factor. The Son of Zen operates without feedback and has an output impedance of about 16 ohms. This nets a damping factor of 0.5, miniscule compared to the 100 to 1000 you can achieve with regular solid-state amplifiers... With the low damping factor, the Fostex became a totally different speaker. It suddenly had bottom end response and a better top end".

Contrast this with (Live Sound: Understanding The Relationship Between Amplifier Damping Factor, Impedance & Cable - Pro Sound Web)

A system where the damping factor of the entire loudspeaker/wire/amplifier circuit is very low will exhibit poor definition in the low frequency range. Low frequency transients such as kick drum hits will sound muddy instead of that crisp punch we would ideally want from the system

or this one (Amplifier Output Impedance Why It)

"The only way to achieve constant voltage drive with a changing load impedance is by having very low amplifier impedance... The lower the amplifier output impedance, the lower the speaker produced voltage is. The lower the speaker produced voltage, the less effect it has on cone motion. A good rule to follow it that amplifier output impedance must be at least ten times lower than speaker impedance for good control of cone motion."



So what design philosophy or type of product should I choose?

TIA
Regards
 
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gobble

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Finally, it is very easy to mod a regular SS amp for current feedback if you know what you're doing. A simple way to find out if a low DF is for you is to insert a small resistor in series with your speaker positive line. This artificially increases the output impedance of the amplifier, and creates a low DF situation. Around 0.5 ohms, will give you a DF of 16, 1 ohm will give you DF of 8, and so on, a resistor of 8 ohm will give you DF of 1.

The resistor should be 10-40 watts, and a 'non-inductive wirewound' or MOX type - higher resistor values will need to have higher rating as they will be dissipating some of the load and it's safe to oversize the resistor, as smaller wattages may cause distortion in transients.

Note that most tube amps tend to have an output impedance of around 1-2 ohms due to the impedance of the transformer, but I've seen bigger amps (90watts +) with DF as high as 20 and 30. Those work very well, but the transformer$ are expen$ive.

Nice!! Will do and test on my setup thanks! :)



Cheers
 

partho

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Sorry for my ignorance. I have heard higher the damping of the amplifier better the control on the speaker. Is it true?
Does damping improve musicality of a amplifier?
What type of damping (high/medium/low) is suitable for different kind of music?
Regards,
Partho.
 

gobble

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Finally, it is very easy to mod a regular SS amp for current feedback if you know what you're doing. A simple way to find out if a low DF is for you is to insert a small resistor in series with your speaker positive line. This artificially increases the output impedance of the amplifier, and creates a low DF situation. Around 0.5 ohms, will give you a DF of 16, 1 ohm will give you DF of 8, and so on, a resistor of 8 ohm will give you DF of 1.

The resistor should be 10-40 watts, and a 'non-inductive wirewound' or MOX type - higher resistor values will need to have higher rating as they will be dissipating some of the load and it's safe to oversize the resistor, as smaller wattages may cause distortion in transients.

Note that most tube amps tend to have an output impedance of around 1-2 ohms due to the impedance of the transformer, but I've seen bigger amps (90watts +) with DF as high as 20 and 30. Those work very well, but the transformer$ are expen$ive.

Is a current feedback amp typically one with low DF?
Are these 10-40 watts, and a 'non-inductive wirewound' or MOX type available locally? Is there a circuit diagram out there (both for converting my NAD 325 into a current feedback one and another one for altering DF) I can use to model one?

TIA
Regards
 

gobble

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If that was Greek, don't attempt this. You need the service manual, and some understanding of what you are about to achieve. Also the risk of destroying your amplifier is very real - a lot of commercial amps do not use standard topologies and you may not fully understand the implications of what you are doing if you can't read a service manual and know instinctively what to do.


That was greek for me, I was expecting a local DIYer to cook something assuming this could be achieved in line with speaker cables, like for the DF trick. I wont try it for sure now :)

Thanks very much for the clarification.

PS: So I dont need anything audiophile grade to accomplish this DF thingie? Just buy two resistors, once for each L/R terminal?

Regards
 

gobble

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Cranky

What brand or vendor do you recommend for the resistors? I heard non-inductive types are hard to find thats why...

Regards
 
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