Amp damping factor?
Magfan, Yes, that all sounds right. As you imply by referring to instantaneous ratio, the damping effect is frequency dependent (since speaker impedance varies with frequency). By convention, though, the numerical value specified for the damping factor of an amplifier is its output impedance divided into 8 ohms. Putting the RDON of the output devices within a feedback loop will lower the effective output impedance, with consequent side-effects as Duke and I noted. By producing output devices with lower RDON, the amount of feedback which may otherwise be needed is reduced. Kijanki: Inductor in-series with the woofer has resistance approx. 0.08 ohm limiting DF to 100. Good point! Which brings to mind that the dc resistance of the woofer voice-coil itself will also (probably even more significantly) limit the damping which can be achieved, and make extremely high amplifier damping factors meaningless. Since the path of the current which flows as a result of back emf has the amplifier output impedance, the speaker cable, the crossover inductor, and the woofer voice-coil in series, resistance anywhere in that path will affect that current similarly. Regards, -- Al |
Bifwynne, most tube amps have much lower damping factors than most solid state amps. Some very highly regarded tube amps have damping factors in the vicinity of 2, although most tube designs are somewhat higher than that. What is important is that damping factor and output impedance (which unless otherwise specified is equal to damping factor divided into 8 ohms) should be chosen to be suitable matches for the particular speaker. Mismatches can occur in two ways: 1)Tonal imbalances may result if the speaker design, particularly its variations of impedance as a function of frequency, reflects the expectation that it will be used with a solid state amp and it is used with a tube amp, or vice versa. See this Atma-Sphere white paper. What he refers to as a "power paradigm" amplifier would be one with high output impedance, and what he refers to as a "voltage paradigm" amplifier would be one with low output impedance. 2)Speakers rely to varying degrees on the amplifier for damping of "back emf" produced by the woofer. Inadequate damping will result in "loose" bass, because the woofer cone will tend to continue moving after the signal has stopped or changed. However, as indicated earlier in this thread, once a damping factor is provided that is adequate for the particular speaker, which is rarely if ever higher than a two-digit number, further increases in damping factor will not be helpful, and in some cases may be reflective of excessive feedback that would be sonically harmful. The key thing is the matchup of speaker and amp. Some speakers are good matches for both tube and solid state amps, but some are not. A simple way to help narrow that down is to research what amps others have used successfully with the particular speaker. Regards, -- Al |
Can one say that if a speaker has a nominal impedance of 8 ohms, there should be a good speaker/tube-amp match? Is speaker efficiency a relevant factor? Any other good rules of thumb that might provide helpful guidance?Bifwynne, it's probably not possible to give a reasonably simple all-purpose answer to your question, because there are many variables involved. It is probably fair to say that a LOOSE correlation exists between higher nominal impedance + higher efficiency and suitability for use with a tube amp. However, a more telling factor is often the impedance vs. frequency curve of the speaker. Keep in mind that an amp having low output impedance, such as most solid state amps, will supply current into a given load impedance that to a close approximation (and assuming the amp is operated within its limits) increases in direct proportion to a reduction in load impedance, while the voltage it outputs will be essentially unaffected by load impedance. Therefore if speaker impedance goes down at some frequencies, power delivery at those frequencies will increase (Power = Voltage x Current, oversimplifying slightly). An amp having high output impedance, such as most tube amps, will not behave that way, and it's power delivery will vary to a lesser degree as load impedance varies. In general, the flatter the curve (i.e., the less variation of impedance with frequency), the more likely it is to be a good match for a tube amp. There are some exceptions to that, such as electrostatic speakers, which commonly have impedances that decline to very low values in the upper treble, and which are often much better matches to tube amps than to solid state amps. The low output impedance of a solid state amp would cause it to deliver more power into that lower impedance than a tube amp with higher output impedance would deliver, causing the upper treble to be over-emphasized with that kind of speaker. At the other extreme, many dynamic speakers have low impedances in the bass region, and higher impedances in the mid-range and treble. That can sometimes be an indication that the speaker is intended for use with a solid state amp, where the designer is counting on increased power delivery into the low impedance to reinforce the bass. I couldn't find an impedance curve for the Paradigm Signature S8 v2 which you indicated in another thread that you are using, but I'll assume it is similar to the curve for the earlier version of the speaker, shown near the bottom of this page. As you'll see, the impedance curve is anything but flat. It ranges from about 3 ohms in a lot of the bass region, to a 21 ohm peak in the mid-range, and is in the general vicinity of 8 ohms in most of the treble region. That would indicate that in comparison to a solid state amp, a tube amp would de-emphasize the bass, emphasize the mid-range significantly, and provide some de-emphasis of the treble, but to a lesser degree than the bass. Another thing to watch out for, in the plot of impedance phase angle, is highly capacitive phase angles (i.e., angles that are significantly less than 0, approaching say 40 degrees or so), that occur at frequencies where the impedance is low. The S8 has that combination in the area of 60Hz. That results in the amplifier having to supply relatively large amounts of current at that frequency. I suspect that would not be a problem for your VS115, but it might be for some less powerful tube amps. Your VS115, btw, has a damping factor of 8, corresponding to an output impedance of 1 ohm on its 8 ohm tap. So the effects I have described would be smaller in degree than in the case of many other tube amps that have higher output impedances. A way of reducing those effects further, if you find it to be sonically preferable, would be to use the 4 ohm taps. That would provide an output impedance of 0.5 ohms, although maximum power capability would be reduced significantly. Hope that helps, -- Al |
Bifwynne, I did some quick calculations to provide a quantitative perspective on the effects I described above. In comparison to an amp having negligibly small output impedance, the 1 ohm output impedance of the 8 ohm taps on your amp will result in an emphasis of the mid-range frequencies for which the speaker has an impedance approaching 21 ohms, relative to bass frequencies where speaker impedance is around 3 ohms, of about 2db. Using the 4 ohm taps would reduce the 2db figure to about 1.1db, although it would significantly reduce maximum power capability, as I indicated. Regards, -- Al |
In summary, I love the sound of my ARC VS 115 amp and Paradigm S8 v2 speakers, using the 8 Ohm taps. Maybe I shouldn't, but ignorance can be bliss.BIF, thanks for the update. I don't think there is any reason that you "shouldn't" prefer the configuration which sounds best to you. The impedance curve of your speakers seemed to suggest that trying the 4 ohm taps would be a worthwhile experiment, but clearly there are an enormous number of other variables in the system, the room, and your own listening preferences that can overshadow what MIGHT be best in terms of that single variable. Best regards, -- Al |
