Hi, head-fiers
I have some comments about the AD8065 op-amp used as IC1 in WNA mkII headphone amplifier (the IC in the rail splitter is a LM6171). I’ve modify my amp with new wiring to make it very easy to change from the standard cap-coupled output to a direct output (see pictures)and I’ve extensively compared the “capless” AD8065 with the LM6171 (plus the C4 mandatory output cap).
When first heard through the AD8065 all recordings seem more detailed, dynamic, direct and alive, but extended hearing reveals some shortcomings. True, the AD8065 is more detailed (perhaps simply due to the absence of a big electrolytic cap in the signal path) but also a little “dry”, with the music perhaps too “in-my-face”. Normally it is not artificial or etched, but in some recordings this closer perspective is uncomfortable. The soundstage is a little flat, without the depth I think suits better to the classical music I listen to (I know that headphone listening “soundstage” is a controversial issue, but my Grado HP-2 are very spacious-sounding). Perhaps the AD8065 really needs crossfeed (I’m going to try the Linkwitz crossfeed circuit available in the net as soon as possible). The bass is extremely precise and taut, not loose at all, but this is again a disadvantage more often than not for classical, which need IMHO a more expansive and deeper, however less controlled, bass (the bass that a symphony orchestra produces in a big concert hall is deep and a little loose, very different I think to the rock, pop or jazz bass which has more punch).
The LM6171 is less detailed (but not much if the C4 output cap is bypassed with a good film type), and more forgiving. It is laid-back, rounder, a little more soft-sounding (a rose-tinted view, as Mike said), and more emotionally involving for me (I’m trying to be as objective as I can, but this is the word that comes to my mind for describing the LM6171 sound). Perhaps the use with other musical styles or headphones, or playing with other elements (adding crossfeed or using different cables), could lessen the AD8065 shortcomings, but at the moment I prefer the LM6171.
And now I have a couple of doubts I hope you can help to clarify. A very tempting experiment is to avoid the output cap C4 when LM6171 is used. I don’t understand how the “100k trimpot + 10M resistor” trick works, but searching in the old WNA MkI posts I’ve found this opinion of Dr White himself:
“As a couple of people have pointed out you can increase the value of the feedback resistors ( say X10 ) to fix the dc offset problem and guarantee unconditional stability, as others have done with the LM6171. The problem with doing this is that it slugs the bandwidth of the LM6171 and it doesn't sound so good. I've tried using a couple of resistors and a potentiometer to zero out the dc offset but its not very good because the offset drifts with temperature. The only satisfactory way to zero out the offset is by using another opamp as a servo. Unfortunately the effect of dc servos is even worse, sonically, than using coupling capacitors. So, inelegant though it is, it looks like the coupling capacitor on the output is the best all round compromise."
Do you think the “trimpot trick” does slug the LM6171 bandwith (perhaps the feature responsible of their very special sound)? When using the trimpot, is it mandatory to use the input cap C1? If so, the problem is that with LM6171 there is always a cap in signal path, in input side (with the trimpot) or in the output side (as standard), or even in both sides (worst-sounding but bullet-proof).
And, lastly, I have a doubt (or more precisely, a concern) about the AD8065. When I powered it for the first time, I heard a thump at turn-on (not too worrying, I thought). I left the amp on continuously for two weeks or so, burning-in, enjoying it and taking the listening notes I’ve posted above, but when I turned it off I heard a very, very loud thump that really scared me. Just for test it, I turned on and off the AD8065-WNA amp with output caps with the same result: a thump in both channels at turn-on and a much more loud thump at turn-off.
Is this normal? With LM6171 these thumps are nearly inaudible. I was so afraid that, in fact, another time I wanted to hear to the AD8065-WNA amp I switched it on with the headphones disconnected (there are dummy resistors in its output socket) and I plugged them in 10 seconds later with no music playing. Is this dangerous for the amplifier?
Thank you for your help and best regards
Jose
