PPA distortion

[JMS]

I've always found my PPA to sound shrill and metallic, but couldn't figure out why. I thought maybe I screwed something up, but it seemed to measure well, as on tangent's site. Recently I did another comparison using RMAA5.4, and I noticed one measurement, THD, in which the PPA measures worse than the other options. Could this be the culprit?

I measured using RMAA5.4, default settings except with "analyze noise and distortion only on 20-20khz" checked, loopback method on an Audigy 2 USB, 96khz/24-bit. The amps I used for comparison were:

PPA (4-4-4 Intersil, OPA2132PA -- tried AD8610's too, same sound)
Audigy's headphone out (different from line out)
MF X-can v3

The load was KSC-50's, with impedance of 80 ohms.

The THD numbers were:

line-out loopback (no load): 0.0030%
headphone out (loaded): 0.0039%
line-out to PPA (loaded): 0.0064%
line-out to X-can (loaded): 0.0055%

This result for the PPA seems consistent with that posted on tangent's site, where he has 0.0042% for a 300-ohm load and 0.0080% for a 32-ohm load.

Yes, everything else about the PPA measures as well or better, especially in terms of stereo separation, but I'm beginning to suspect that the cause of the shrill/metallic sound is reflected in the THD numbers.

(I'd post the results, but it's too big to attach to this message.)

 

[drewd]

The distortion values seem vanishingly low to me. Have you measured the frequency response? Are the opamps biased with the FETs? If they are, have you tried changing the value of R9?

Have you listened to the PPA with a different set of headphones? Also, I'm sure that you meant that you were using the OPA132...if you thought that it was shrill, I'm sure that the 8610 was worse! Have you tried a "smoother" opamp, like the 227? It may be that you can solve the problem with some opamp rolling.

Another thing to check is to make sure that the output ground of your headphone jack is isolated from the input ground of the amp - that will certainly make the sound unpredictable.

Maybe one of those suggestions will help...it's kind of a shot in the dark. Good luck!

-Drew

 

[tangent]

It's an op-amp and system matching thing. The AD8610 is way way too revealing for many systems in a PPA, especially with HA3-5002 buffers. I'm not sure why you'd say that the OPA132 sounds the same to you, though...it's a quite different beast from the 8610.

The OPA627 is quite laid back in many systems, but a lot of people think it's nicely balanced in a PPA. Another to try, if the 627 is too expensive or too laid back for you, is the AD8065.

You might also consider getting one of the two available diamond buffers. I've only heard the Glassman buffer, and it does help. The Larocco buffer should do even better.

 

[Voodoochile]

I feel the same way about the AD8610 in a PPA. Especially with the diamond buffer and a discrete attenuator, the sound is just too harsh for me. I would not call it distorted, but harsh for sure, to my ears. I think it's just not a sympathetic region for your ears.

On a budget, the 8610 with DIP buffers and an Alps would balance very nicely I think (with many heavier cans), but a diamond buffer and/or attenuator and/or silver wiring allows the darker 627s to really shine nicely, and I prefer th overall sound of that better- the midrange is sweeter with the 637s, I think, and the detailing is cleaner and more balanced (again in my opinion).

Of course with low-z cans, the 8610 is pretty unbearable to me.

 

[Wodgy]

You should consider taking a look at your source. The Audigy has a naturally shrill and metallic sound. The tubes in the X-Can (your basis for comparison to the PPA) will inject a bit of euphonics to cover this up.

That's not to say that the PPA itself doesn't have a solid state sound -- it does, regardless of the choice of op-amp -- and a certain hardness, but in comparison to a lot of other amps it's a very high quality solid state sound. My gut feeling is that your amp just outclasses your source, and you're hearing it.