PPA Instability

A well worn topic here that never seems to go away.
It is the hissing that results from using low impedance high sensitivity phones with the PPAv2.
It is not a gain issue. I tried a gain of 2 which actually increased the hiss over a gain of 11 using grado gs-1000s. To me this indicates that the PPA requires large global feedback to minimize instability.
Ok so what I tried.
All opa627 - hiss definitely present and annoying
All opa827 - hiss is loud enough to drown out quiet passages in music (ie really bad).
Using 100pF silver mica in c6. Tried a ceramic 100pF which was slightly worse.
Using recommended MJE243/253.
3.3 ohm buffer resistors.
Buffer biased to 30mA.
Interesting note is the hiss goes away completely if I connect a 10 foot extension cable between the PPA and grados.
Any help would really be appreciated

What I haven't tried is replacing the pot. I'm using this stepped attenuator. As a last resort I could try to replace that.



EDIT Just want to add that this is a great amp and sounds amazing with k701s and hd650s. Somehow it just doesn't like low impedance phones (at least mine doesn't).

Ok, so I've tried swapping opa134 in right and ground (I only have two) and that had no noticeable effect.
I tried putting 100pF (what I had on hand) in the feedback loop (of the right channel op-amp) and that increased the hiss significantly.
So in summary lowing the gain increases hiss. Adding a feedback cap increases hiss. Swapping op amps does nothing noticable. Adding a headphone extension cable practically eliminates all hiss.
The cable itself when plugged in measures 0.4 ohm from the buffer output (OG for example) to the female end of the cable. I'm thinking it's the complex impedance that results in stability.
Thanks for the comments so far. Any further thoughts?

Potential Solution Found.

Adding a 100pF cap in parallel with r11 kills the hiss completely. How does this sound as a solution?
It would be nice to hear from Tangent, PPL, or Morsel on this one as they were the design team afaik.

EDIT 1
Reducing r11 to 100 ohm has the same effect of killing the hiss. This makes me feel like I've found the culprit. Maybe having r11 at 1k is just causing too much reflection between the op-amp and buffer? 200 ohm does the job too... though their might still be a tiny hiss, but definitely barely perceptible.

I've replaced r11 with 220 ohm resistors and the hiss is completely gone... at least with the grado gs-1000i - which is very sensitive. Haven't tried iems yet.
I'm interested to hear some thoughts on this.

4.32k from the pot/attenuator to the op-amp non-inverting input and 1M from the non-inverting input to ground.
There schematic is here.

I appreciate the comments guys, but a properly implemented amp shouldn't need those kind of hacks. They come at a cost.

The right interstage impedance is always the solution to coupling a low output impedance stage to a high input impedance stage.
I can't say why 1k is too high, but a complete analysis of the small signal model would probably reveal the culprit somewhere in the complex impedance of the buffer stage.

I agree with the cable comments. My guess is that it's the capacitance to ground that kills the high frequency noise.

Thanks to Tangent for weighing in on this. It's always nice to hear from the actual designer(s) as their hands on experience in testing is better than our (un)educated guesses. Also, I'm very cautious about even slight modifications to other people's designs.

EDIT I wanted to add that I don't think the actual design is in any way flawed. The issue is might be a function of different current gains for different transistors. It is my understanding that in a configuration like the diamond buffer the input resistance is determined almost entirely by the current gain of the transistors. As we know current gain has a huge range in any given transistor. If the stability is related to the current gain then it may be an issue that different r11s are needed for different builds.
I may be way off track here too