I would instead look at the output impedance ratio to the nominal impedance of the speakers the amp will drive. You want a ratio of at least 10 between nominal speaker impedance/amp output impedance. Anything less than this will usually cause a drop off in some portion of the frequency response, depending on the impedance curve of the speaker. |
I plan to do that, but right now I'm just asking for a list. |
CAT JL-2 has a DF of 10. Why do you want a low DF? I thought I high DF was desireable... |
Higher damping factors are not a free lunch; something has to be traded off to get there, and that something is the ear-friendly distortion behavior of a low (or zero) global negative feedback amp.
Most SET and OTL amps have a low damping factor, but they work fine on speakers designed with such amps in mind.
I suggest reading this paper before you select an amplifier; it will be helpful regardless of what type of amp you go with:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php
Duke
dealer/manufacturer |
What are you trying to achieve? |
Since people will be listing all of the tube amps made, would anyone mind also seperating them by black or silver faceplate? I will only buy silver to match my rack and this will cut down on my research. |
I appreciate that people are willing to share their expertise... but I'm just hoping that someone will be generous enough to share the fruits of their labors so I don't have to do the work myself. A list for the rough parameters indicated is all I'm looking for. |
Plenty has been written on the subject of impedance matching and the relationship between damping, feedback, distortion, etc... no need to reinvent the wheel. Thanks. |
I'm trying to achieve a list. |
Let's just say the list is wanted for data-mining regression analysis. |
I would think it much easier to make of list of tube amps with a dampening factor higher than 20. |
(Limited) damping in tube amps is not due to the power output tubes, but to the magnetic hysteresis (lag) of the power output transformers. SS amps (*most of them) where the speaker (load) is connected directly to the power output transistors, can have damping factors of 1000 or more! (Which is why SS amps are prefered for woofers and subwoofers.
* One exception I know about (in SS amps) are those McIntosh SS amps which employ an 'autoformer' at their output. If you check the specs on those, you will see they show a rather modest damping factor, just like tube amps; but again, this is due to the transformers. |
I think the information in the posts by by Duke and Nsgarch is worth keeping in mind.
There are speakers that are designed and voiced with specific amps or types of amps. That doesn't mean other amps can't be used, but the designer had a goal in mind when developing the speaker.
Here is an example. I own Music Reference EM-7 12v SET amps. They have 6 ohm taps. If I hook up the wires leading from the transformer in parallel I get 3 ohm output, series 12 ohm.
My speakers are made by Duke (Audiokinesis) and their impedance curve is between 8 - 12 ohm and very smooth. If I followed Stevecham's theory I'd need a solid state amp or a tube amp with a fair amount of negative feedback (like my Transcendent Sound T-16 with 1 ohm output) to satisfy the desired 10 to 1 ratio. Duke used the Atma-Sphere S-30 to voice his amp. It provides nowhere near the desired ratio due to it's highish output impedance (close to 7 ohms IIRC) and very low feedback (mine is configured with zero feedback). However, it's an outstanding amp for the speaker.
Getting back to the EM-7's. I've tried both configurations and prefer the 12 ohm output. As far as I can hear nothing is getting rolled off.
If all you want is a list you can start with most OTL amps, SET amps, or other Class A tube amps with zero or very low negative feedback. It will be a fairly long list. |
Compared to ss amps most tube amps have lower damping factors. Wolcott tube amps appear to be capable of a rather high damping factor for a tube amplifier.
Some amps with a high damping factor have little or no global feedback. |
In tube amps the loop negative feedback is a big factor in the resulting damping factor. For example an OTL can be built with a damping factor of 50:1 if you have enough feedback. IMO such a large amount of feedback does nothing to improve the resulting sound, and also it should be considered that no speaker made needs more than 20:1, and many of them like a lot less.
An example of the latter are open baffle designs- in order to get them to play bass the amp might have a damping factor of 0.5:1 or less and yes, that's not a typo. IOW, a *lot* depends on the speaker and the designer's intent! |
Are there any tube amps with DF higher than 20? You don't see that much, I don't think. |
The Wolcott's are capable of a df higher than 20, much higher. |
ralph noted...a *lot* depends on the speaker and the designer's intent!
AMEN. Look backwards from any driver after a crossover and let me know exactly how high (or low) DF can be a meaningful figure of merit.
dave |
Highly overrated spec? I hope so - my CAT JL2 and current amp have tiny little DFs, yet I had/have well controlled, tuneful bass. High DF might very well make my speaker sound constipated. |
And a low DF might very well make another's speaker sound flatulent?:-) |
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