[regal]

There seems to be a growing trend for low damping factor amps in the speaker world, especially w/ full range drivers. Now, dynamic headphones are full range drivers. I see a lot of people powering their Grado's with DV337's, which on paper makes no sense to me. But maybe there is something to going with a low damping factor. Are headphones a reactive load? Are headphone impedance curves flat? What are peoples thoughts on this, especially regarding tube amps ?

 

[gyrodec]

regal, one thing to remember with the fullrange speakers is that they are very often horn loaded, at least as a rear loading for bass extension. This adds a great deal of resistive air damping to the driver (at least in the more critical bass frequencies) which will have a similar effect to a higer damping factor from the amp - it will damp down any overshoot or ringing of the driver. Unfortunately we can't do this air-loading in normal cans for obvious physical reasons.

Another thing is that multi-way speakers put the passive crossover in between the amp and driver, and so the damping factor seen by the driver is not that of the amp, especially not in the cut-off regions of the filter. It will actually be much lower. As a full-range driver has no crossover to alter the amps damping factor this part is avioded.

Finally, and low damping factor also means highish output impedance and so a frequency dependant voltage divided effect will be created with the non-flat impedance of the driver and so you will get frequecy response variations. (See the fr plots of most tube amps in Stereophile for an idea of how much this can be).

At the end of the day, people like many non-flat and non-ideal amp/speaker combinations. I loved the details from my Linn Kans, but I know they weren't flat - I measured them at work (KEF).

So, a high damping factor is the correct way. That can't be debated by anybody with any engineering knowledge in that area. BUT, it doesn't mean a low damping factor setup won't sound wonderful.

 

[regal]

Thanks, great post.

However you have to admit that it is possible to over dampen a speaker. And that an amp with too low an output impedance could conceiveably sound dry and lifeless?

I guess my question is how high of a damping factor is needed for the average headphone driver? Surely not as much as speakers? There isn't as much mass/inertia to control. Is 100 best, 10 OK, 2 OK? I mean I am always amazed at people who love driving their Denon D5000's with a tube amp. Either they're nuts, or there is something we are missing.

 

[gyrodec]

regal, I think the fact that the property is called "Damping Factor" can confuse people. It makes them think that too much will dampen the music making it, as you say, dry and lifeless or some other bad thing. Its not the case. All the damping factor is doing is damping down unwanted cone movement - overshoot and ringing. Hence the speaker will more accurately follow the amps signal and it won't dampen the dynamics and micro-detail with unwanted movement. So higher damping factor will actually be more lively and dynamic than low damping factor. This, along with current drive ability, is the reason soldid state amps are generally better at bass definition than tube amps. The same holds all the way up the frequency range, but the effects are more subtle. Hence, more is always better than less, but the law of diminishing returns means anything over 100 is about the same infinite. As headphone drivers are low-mass, they don't respond as much to higher damping factror and so the effect is even more subtle. I don't know what a good minimum might be, and whatever it is it will probably be outweighed by the frequecy response issues cuased by the OP impendance anyway. So, I think for cans its not the most important thing, but I would always want more and not less.

 

[gyrodec]

regal, forgot one point. The dry and lifeless than SS has against tube is usually because tubes have higher amounts of even order harmonics and these add life, glow, shimmer and all that non-dry stuff. Not usually due to the damping factor issue.

 

[Mazuki]

For that matter, it wouldn't make that much sense driving Grados with a Bijou either

 

[regal]

The Bijou has negative feedback and can get a decent damping factor for Grados, at least compared to other tube amps.

 

[xabu]

Well it's an somewhat old thread, but I just found this in connection with my research about damping factor relevance and the combination of Hifiman HM-801 GAME Amp (32 ohm out) and beyerdynamic T 50 P (alomost flat impedance curve close around 32 ohm).
 
Maybe it is of some general interest:
 
"Is an amplifier's damping factor important to headphone performance?

      With loudspeakers, the lower the amplifier's output impedance, the higher the damping factor into the rated load. Damping factor is given as the ratio of loudspeaker impedance to the amplifier's output impedance. As the theory goes, the higher the damping factor, the better the amplifier's ability to control a loudspeaker's low frequency response (when the motional reactance of the system is at maximum), because the low output impedance of the amplifier allows any back-emf generated by the loudspeaker to be absorbed by the amplifier. That theory has been discharged by members of the audio community as unsubstantiated.

      However, even if the theory were correct for loudspeakers, its applicability to headphones is suspect. John Woodgate, a contributor to The Loudspeaker and Headphone Handbook (1988), had the following to say about the effect of damping factor on headphone performance:

            Headphone transducers are resistance-controlled, not mass-controlled like loudspeaker drivers above the main resonance. In any case 'damping factor' is largely nonsense - most of the resistance in the circuit is the voice-coil resistance and reducing the amplifier source impedance to infinitesimal proportions has an exactly corresponding effect on damping - infinitesimal.

            However, the source impedance affects the *frequency response* of a loudspeaker because the motional impedance varies with frequency, and thus so does the voltage drop across the source impedance. This means that the source impedance (including the cable) should be less than about one-twentieth (not one two-hundredth or less!) of the rated impedance of the loudspeaker, to give a *worst-possible change* in frequency response from true voltage-drive of 0.5 dB.

            The motional impedance of headphone transducers varies very little (or should vary very little - someone can always do it wrong!) with frequency, so the source impedance can be high with no ill effect.

      The IEC 61938 international standard specifies that headphones should be driven by a 120 ohm source - regardless of the impedance of the headphones themselves. If the headphones were designed to this standard, then an amplifier's high output impedance should have little effect on the sound of the headphones. In general, headphones with a flat impedance curve over the audio range will not be affected by high output impedance. For example, in May 1995, Stereo Review published a review of the Grado SR125 headphones. The impedance curve of the SR125s, which have a nominal impedance of 32 ohms, varied from 31 to 36 ohms over the entire 20Hz to 20kHz spectrum. Not all headphones may be as well behaved as the Grados, but neither do they usually have the roller-coaster impedance runs of a loudspeaker. Tube amplifiers (with their high output impedances), it should be noted, have very low damping factors."
 
 
This can be found here:
 
http://gilmore2.chem.northwestern.edu/faqs.htm#amp

 

[regal]

Quote:

Well . In general, headphones with a flat impedance curve over the audio range will not be affected by high output impedance. For example, in May 1995, Stereo Review published a review of the Grado SR125 headphones. The impedance curve of the SR125s, which have a nominal impedance of 32 ohms, varied from 31 to 36 ohms over the entire 20Hz to 20kHz spectrum. Not all headphones may be as well behaved as the Grados, but neither do they usually have the roller-coaster impedance runs of a loudspeaker. Tube amplifiers (with their high output impedances), it should be noted, have very low damping factors."
 
 
This can be found here:
 
http://gilmore2.chem.northwestern.edu/faqs.htm#amp

Then explain why Grado's sound terrible with high output impedance OTL amps? (playing devils advocate)

 

[jcx]

an OTL amp with small receiver tubes could have only a few 10 mA of total current swing - giving single digit mW rms drive into 32 Ohms - with dynamic music clipping due to current starvation could be common at anywhere near live event SPL levels for many types of music
 
also some commercial OTL tube amps have too small a value output coupling capacitor for 32 Ohm loads - you can lose the bottom few octaves to the RC high pass filter formed by the output C and load R
 
 
I believe I've seen a fairly "lumpy" impedance curve posted somewhere around here for one of the new "high end" multi-driver iem - this would cause more FR irregularity with higher output Z amps

 

[xabu]

Quote:

Then explain why Grado's sound terrible with high output impedance OTL amps? (playing devils advocate)

Apart from what's said by jcx, it's perhaps because of the "... In general, ..."

 

[regal]

Check out dsavitsk's  pentode amp,   it has like zero damping factor with Grados,  I'm guessing roughly 1k ohms output impedance.   It is a current source more than an amp.  Well worth reading and just adds to the mystery of the mighty Grados:
 
http://www.ecp.cc/CagedFrog.html

 

[nikongod]

 
Quote:

Then explain why Grado's sound terrible with high output impedance OTL amps? (playing devils advocate)

Measure an OTL amp with no global feedback and a 32 ohm load. The fact that the amp cant drive 32 ohms clean dosnt mean people wont try. Non global feedback OTLs fail with 32 ohm headphones because of how significantly they distort driving even mildly difficult loads.
 
The golden ears of head-fi said OTLs were better than a transformer coupled tube amp ("I can hear an obvious 1 octave rolloff on both ends of the range with the transformer coupled amp that I cant hear with the OTL") and it was so. 
 
Edited:
Im gathering parts for the SS Caged Frog

 

[regal]

Quote:

 

Measure an OTL amp with no global feedback and a 32 ohm load. The fact that the amp cant drive 32 ohms clean dosnt mean people wont try. Non global feedback OTLs fail with 32 ohm headphones because of how significantly they distort driving even mildly difficult loads.
 
The golden ears of head-fi said OTLs were better than a transformer coupled tube amp ("I can hear an obvious 1 octave rolloff on both ends of the range with the transformer coupled amp that I cant hear with the OTL") and it was so. 
 
Edited:
Im gathering parts for the SS Caged Frog

Sounds like they were listening to cheap opt transformers? 
 
Anyway I guess I learned damping factor as a fundamental decades ago,  as you know I have argued both sides of its importance with headphones.  I think the Grado drivers are a bit of a mystery as to designing a tube amp for them.  With the  Magnum drivers my hybrid and its huge damping factor excel,  while the stock Grados tend toward sterile/dry bass.    What I'm trying next is low output impedance B+, transformer coupled to give a 2ish damping factor.