Other Gear > Headphones
Audeze LCD 4 - Which amp?
Deke609:
Thanks Doc.
Ready!
Microphonics, if it becomes an issue, are something I can probably fix. My rack rests on spring-loaded self-leveling feet. I've read the threads you started about sorbothane feet and lead tube rings. And Jamie R's recent posts about his vibration control box using an inflatable cushion and heavy plate. So there are lots of potential, stackable solutions.
cheers and thanks,
Derek
Deke609:
Order for Beepre and BeeQuiet placed :) I had originally earmarked this money for a SS amp, also American made, but canceled my order of same after hearing just how good the SII is. The SS amp was for greater bass extension - but now I'd rather focus my efforts on getting more of the BH sound from my system. (That said, if BH ever makes the Kaiju iron upgrade for the SII, I will be the first to order - I bet there are enough SII owners on this forum that would jump at the upgrade to make it do-able).
Deke609:
Guys - Now that the amp has 100+ hours on it, and I've experimented with all 4 output impedance configurations, I'd like to return to the power/distortion and damping issues. And just to be clear, I'm not trying to "correct" anything. I am perfectly happy with the amp - it sounds stellar at 4, 8 and 16 ohms configurations (in comparison with which, 2 Ohms sounds a bit dry and lifeless - but I was perfectly happy with 2 Ohms before hearing 4, 8 and 16 :D). So this is just for fun and education - if I somehow manage to squeeze out a bit more SQ improvement from all this, that's just a bonus.
You previously suggested building a stereo L-pad that would present a fixed 8 ohm load to the amp. I have the required parts, but I don't know whether I should proceed because I'm unsure whether my headphones need the amp output to be "padded down". I suspect not, given that I can comfortably listen to them with the amp configured for 16 Ohms with the volume attenuators at -18 dB, at -9 dB when configured for 8 ohms, and pretty much wide open at 4 Ohms.
But I may well be missing something - so maybe it's helpful to return to something PB wrote a month ago in reference to using an L-pad:
--- Quote from: Paul Birkeland on July 17, 2018, 08:39:34 AM ---If the resistor values get too large, you'll lose damping of the headphones and the bass will get lumpy on you. If the resistor values are too small, you'll load the amp down and it will distort.
--- End quote ---
Based on some reading I've done, my understanding is that the damping/lumpy-bass issue is a function of a headphone's impedance curve across the audible frequency bandwith. My guess is that if the headphone impedance dips appreciably at certain bass frequencies, a high output impedance could result in loss of damping at those frequencies and the bass will be loose and boomy. Do I have it right?
If my understanding is correct, then I don't think I need to worry about damping. The LCD 4s have an input impedance of 200 Ohms and the impedance versus frequency plot is dead flat - literally a straight line parallel to the x-axis for all frequencies. This has been tested and verified by various reviewers (Inner Fidelity, Stereophile, etc.). My understanding is that this is a common characteristic of planar magnetic headphones.
So my current thinking is that (a) no padding down is necessary, and (b) headphone damping is not a problem. BUT I have come across numerous references in online forums about "keeping an amp happy" by putting a resistor in parallel with each headphone channel -- e.g., 2 8 ohm resistors for an amp configured for 8 ohm speakers.
1st issue: Does the SII need to "see" the resistance for which it is configured to be "happy"? If so, is this what PJ was referring to when he mentioned "parafeed circuit resonance affected by amp load impedance"? And what PB meant when he mentioned too little resistance "loading the amp down" and causing it to distort?
One other thing I don't understand is how power hungry headphones can be be driven fully if resistors having a value much smaller than the headphone's impedance are placed in parallel with the headphones. While the headphones will still get the same voltage, wouldn't most of the current flow across the parallel resistor, bypassing the headphones? My thinking is that if voltage is fixed by the OTs and the amp cannot exceed 3.5 to 4 Wpc, then current is necessarily limited such that, with the parallel resistors in place, the headphones may not be able to draw as much current as they need. But maybe I misunderstand this too.
2nd issue - Distortion. I've tried to read up on this, but there are so many different types of distortion and I don't know which one or ones I should be educating myself about - what kind(s) of distortion are we talking about? And is it produced at the amp or the headphones?
Many thanks,
Derek
Paul Birkeland:
--- Quote from: Deke609 on October 25, 2018, 03:07:09 PM ---I suspect not, given that I can comfortably listen to them with the amp configured for 16 Ohms with the volume attenuators at -18 dB, at -9 dB when configured for 8 ohms, and pretty much wide open at 4 Ohms.
--- End quote ---
Yeah, no reason to pad things down. If you were all the way down on coarse and fine and things were loud, then you'd want to pad down the output a bit. If the general noise floor of the amp was too high, the padding would also remedy that.
--- Quote from: Deke609 on October 25, 2018, 03:07:09 PM ---Based on some reading I've done, my understanding is that the damping/lumpy-bass issue is a function of a headphone's impedance curve across the audible frequency bandwith.
--- End quote ---
When load impedance gets lower than output impedance, fluctuations in the load impedance become very audible. For example, an 8 ohm rated speaker that dips to 4 ohms and up to 50 ohms will have the potential for lumpy bass issues unless run from the 4 ohm setting (or lower).
--- Quote from: Deke609 on October 25, 2018, 03:07:09 PM ---If my understanding is correct, then I don't think I need to worry about damping. The LCD 4s have an input impedance of 200 Ohms
--- End quote ---
Yeah, plenty of damping on any impedance setting.
--- Quote from: Deke609 on October 25, 2018, 03:07:09 PM ---I have come across numerous references in online forums about "keeping an amp happy" by putting a resistor in parallel with each headphone channel -- e.g., 2 8 ohm resistors for an amp configured for 8 ohm speakers.
--- End quote ---
Our amps remain quite happy with just about anything you want to load them with. They are unconditionally stable.
--- Quote from: Deke609 on October 25, 2018, 03:07:09 PM ---1st issue: Does the SII need to "see" the resistance for which it is configured to be "happy"? If so, is this what PJ was referring to when he mentioned "parafeed circuit resonance affected by amp load impedance"? And what PB meant when he mentioned too little resistance "loading the amp down" and causing it to distort?
--- End quote ---
What PJ is describing and what I brought up are two different issues. There's no harm in putting loading resistors in parallel with the headphones, I wouldn't pretend to be able to predict whether you'd like the outcome or not. A load that is too low in impedance relative to what the amp is expecting will generate extra distortion and a little more power. You don't need either.
--- Quote from: Deke609 on October 25, 2018, 03:07:09 PM ---One other thing I don't understand is how power hungry headphones can be be driven fully if resistors having a value much smaller than the headphone's impedance are placed in parallel with the headphones. While the headphones will still get the same voltage, wouldn't most of the current flow across the parallel resistor, bypassing the headphones? My thinking is that if voltage is fixed by the OTs and the amp cannot exceed 3.5 to 4 Wpc, then current is necessarily limited such that, with the parallel resistors in place, the headphones may not be able to draw as much current as they need. But maybe I misunderstand this too.
--- End quote ---
The resistor L-pad reduces the power available to the headphones. Putting an 8 ohm resistor in parallel with each headphone driver will slightly reduce the output of the amp, so you will see slightly reduced power to your headphones as well.
--- Quote from: Deke609 on October 25, 2018, 03:07:09 PM ---2nd issue - Distortion. I've tried to read up on this, but there are so many different types of distortion and I don't know which one or ones I should be educating myself about - what kind(s) of distortion are we talking about? And is it produced at the amp or the headphones?
--- End quote ---
We don't really go into a whole lot of discussion here on distortion generated by transducers. The distortion we are mentioning is harmonic distortion.
Deke609:
Many thanks again, PB.
It's good to know that my SII is happy no matter the load.
If I were to play around with parallel resistors, what value(s) would you suggest? I think you previously stated that twice the intended load was a good starting point.
And thanks for clarifying that it is harmonic distortion that we are talking about. That might be something fun to experiment with. In a 1972 paper for the AES (see link in "Why do I like tube amps" thread under "General Discussion), Russell Hamm suggested that it may actually be harmonic distortion that is responsible for why many people find tube sound to be fuller bodied and punchier than SS. As Hamm explained:
"Transistor amplifiers exhibit a strong component of third harmonic distortion when driven into overload. This harmonic produces a "covered" sound, giving a recording a restricted quality. Alternatively a tube amplifier when overloaded generates a whole spectrum of harmonics. Particularly strong are the second, third, fourth, and fifth overtones, which give a full-bodies "brassy" quality to the sound. The further any amplifier is driven into saturation, the greater the amplitude of the higher harmonics like the seventh, eighth, ninth, etc. These add edge to the sound which the ear translates to loudness information. Overloading an operational amplifier produces such steeply rising edge harmonics that they become objectionable within a 5-dB range. Transistors extend this overload range to about 10 dB and tubes widen it to 20 dB or more. Using this basic analysis, the psychoacoustic characteristics stated in the beginning of this paper can be related to the electrical harmonic properties of each type of amplifier."
In the 1972 article, Hamm noted that significant/audible distortion only occurs when the amp is overloaded - which might lead one to think that at normal listening levels, harmonic distortion isn't an issue and that tube and SS should sound pretty much the same. However, Hamm points out that transients -- the attack of a note, drum hit, etc. -- often overload the amp, if only momentarily - which suggests (to me) that tube amps should sound more dynamic even at normal (non-overloading) listening levels -- which is something that I find.
I can't claim that this is a definitive explanation of the difference between tube and SS, but it is very interesting.
I wonder whether through tweaking the amp, it is possible to reduce 2nd harmonics that produce a "covered" sound, while leaving the other harmonics undisturbed? And would this be a good thing? Or is the 2nd harmonic only detrimental to SQ when out of balance with the other harmonics, such that removing it or lessening it from the full range of tube harmonics would remove some of the body of the sound. Dunno. But it would a very interesting to experiment with, were it possible to do so.
cheers,
Derek
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