Pass Labs Distortion/Feedback Tech Article -Transparency?

Before narrowing the amp short list ( or maybe as a way to shorten the short list ) I’ve been hitting the books a bit to learn some amplifier 101. Since distortion and feedback seems to be one of those things that is talked about I went looking and stumbled on this excellent Pass Labs Tech article.
[Pass Labs Tech Article Audio Distortion] Pass LabsAudio distortion and feedback - Pass Labs
Aside from all the technical stuff I found this passage to be particularly interesting: “ Anecdotally, it appears that preferences break out roughly into a third of customers liking 2nd harmonic types, a third liking 3rd harmonic, and the remainder liking neither or both. Customers have also been known to change their mind over a period of time.”
I think about this statement and wonder what all this talk of transparency and neutrality is all about.


Very interesting article - Nelson Pass is a really smart guy. Distortion profiles, amount of feedback, and improving transparency are all issues I’ve been reading about in the world of Class D amps. Here are the thoughts of Ralph Karsten from Atma-Sphere regarding his new Class D amp (which has 35db of feedback):

Atma-Sphere Class D in production, preliminary

Class D monoblocks, 2 x 100 Watts

The Class D project is something we started in 2017; it was about then that we started hearing class D amps that were demonstrating good potential so we started our own research. Within about 6 months we had a prototype that was showing us that this was worthy of pursuit. Our goal was a solid state amp that had all the positive attributes of a good tube amplfier, and since our amps have been getting great reviews and awards in the high end press for the last 30 years or so we felt we were in a good position to know if the goal was achived. The result is a solid state amp that has all the smoothness in the mids and highs that you expect from an excellent tube amp but with considerably lower distortion, output impedance and obviously higher efficiency.

The Class D is based on GaNFET output devices and accepts both single-ended and balanced inputs. The circuit is entirely of our own design and manufacture- it does not use anyone else’s modules. Tube amps generate a fair amount of either 2nd or 3rd harmonic which the ear regards as innocuous. These harmonics mask the higher ordered harmonics and intermodulations that cause an amplifier to sound harsh and bright, resulting in smooth sound. The Class D amp does the same thing, but with far less distortion so its more transparent with a relaxed, organic presentation. In direct comparisons to our OTLs its easier to make out details in the rear of the soundstage; its more focused. This amp has a very nice first watt!

1 Like

Hmm. A GaN implementation that’s making some noise. Interesting that their website doesn’t include any images and none of the dealer websites have any mention of it that I can find.

Great article. Thanks for the link. My favorite quote so far:
“Accusations are occasionally made that objectivists can’t hear, and conversely that subjectivists hear things that aren’t there. This being the entertainment industry, I hope everyone is having a good time.”


This graph is very telling when trying to wrap your head around THD (misleading figure of merit) vs. higher-order harmonic distortion vs. amount of feedback. Leave it to Nelson Pass to take something very complex and successfully explain to us “ordinary folk”.

And, finally starting to understand IM distortion and its importance. Really great article indeed.

Loads more great stuff from Nelson here: Pass LabsTechnical Articles - Pass Labs


If you Google “Atma-Sphere Class D” you can find some pictures of their new amp. There’s been some discussion of it recently on Audiophile Style and a few other online forums. Apparently, it was demo’d at a recent show in Florida and will be at AXPONA next month.

In addition to wanting to check out the sound quality, one of my concerns about this amp is that the power output is not especially high, at least compared to a lot of other Class D amps like the M700. Output is 100w per channel into 8 ohms and 200w per channel into 4 ohms.

I could not agree more. I’ve read a couple of the articles and have appreciated how accessible they are.
One advantage I hadn’t anticipated when I took my first step away from the commercial though respected brands is how accessible the people behind the companies are. How easy it is to understand their values and what they aim to deliver, the cultures they strive to cultivate.

I built a Pass DIY Pearl 2 Phono Preamp from a PC board kit and talk about bang for the buck. That thing cost around $600 total and gave some far more expensive phono preamps a run for their money.

1 Like