Right, so I saw the NAD and thought it was the real deal with bells on.
For the naive simpleton (asking for a friend) seeking to differentiate between those that do and those do not colour, what are the key words to look out for? I'm worried about simply avoiding "anything commercial".
Unrelated Q: if the "right sort" sound so good, why would commercial players like NAD make them sound worse? I have no issue with people adding a hefty price - they have marketing costs, costs of a lovely case, offer a warranty, support, etc. Them be market forces.
I appreciate it's hard to see why I steered in that direction. "Anything commercial" is not the right wording really. I simply mean a Purifi amp shifted from its reference performance point by input buffers claiming to improve on the default performance.
Look at ASR measurments of a Purifi Eval amp and it matches what Purifi claim. Look at the Purifi-based NAD measurements and you realize it's worse. Yes measurements like SINAD aren't super informative as to how something sounds, other than to say less noise and distortion is always preferable. But two similar SINAD scores for amps might mean they sound different if the distortions are notably different where it may matter more. For instance, lower IMD from a multi-tone test is generally going to benefit perceived SQ. It's interesting that people generally prefer Purifi over NCore when they are broadly similar, but Purifi achieves lower IMD generally. The same can be said for DACs... there are some at or above 120db SINAD but struggling to stay under -130db IMD, while some poorer SINAD score DACs, like a Chord Qutest are poorer overall but have IMD down at -140db. Where distortion resides is important.
Back to your question...
Input buffers using an off the shelf LM4562 or OPA1612, or the EVAL input buffers maintain the performance level of the Eigentakt.
Sparkos, Sonic Imagery opamps and others tend to 'flavour' the sound with some added harmonics, albeit not much. They will sound different, but not by much. The question, which my view answers, is, are these worth paying for?
To me, it makes sense to start with the best objectively measured amp as per the Eigentakt. If that sort of transparency cannot be tuned by yourself to how you like it, then look further afield. At least you didnt spend large.
FWIW, I have a Pass amp (Nigel, you know what these sound like) and I can make my Purifi monoblocks sound like it, but even cleaner and more resolving. I can also make my monoblocks sound more like a brighter stable of amps.
When I first got them, I thought they were bright relative to the Pass. I then measured the room and found that I had a broad dip in in-room frequency response from 80 to 200Hz. It was not a null, but I applied a -3db cut across all frequencies and then added a single parametric EQ boost of 6db to fill the hole in the bass. The result was a naturally warm and articulate sound. But even so, if I basically produce more bass hump, or change the tilt of the treble response versus the Harman Curve, I can get these things to tonally sound however I wish. The one thing I cannot do is make them less resolving, less detailed, less articulate, or less capable of demonstrating the distortion in the Pass amp when I go back to it.
Nigel, I noted in your Pass listening with Nick, you marvelled at the Pass sound. Well I am not selling my Pass just yet as I am still tuning my newer system and it acts as a checkpoint back to where I was before. Also with cost of living crisis, I probably cannot shift it for as much as I otherwise could before. But it will be going in due course.
Now I have not answered why an OEM would make a purifi sound worse. The right question is why use Purifi modules at all? The answer to that bit is two fold.
1) The Purifi is very cost effective for the performance it achieves, such that it enables higher SQ than traditionally offered at such a price point
2) You can try to make a traditional Class A or AB amp with similar performance, but the relative price (cost of production and what it needs to sell at) is a disaster.
Due to (2), manufacturers are queuing up with Purifi to use their modules as the main gain stage in their units, but then how is their brand differentiated from others doing the same?
The answer is the tuning of sound is intended to be by the input buffer. Whether using different opamps for the input gain, or discrete, or using a Linear Power Supply and huge reservoir of caps over the SMPS PSU. As Keith of Purite would put it, it's an effects box.
Purifi or specifically Bruno Putzey, intended the Purifi module to be a statement piece and set a new reference point as'is. He didn't skimp on achieving its best performance when using OPA1612 in the input buffer. It was chosen specifically because it's basically optimal, otherwise he would have chosen a different opamp. He conveys this in an interview on YouTube as well as explaining why he didn't design a discrete opamp instead.
Now, back to NAD et al... if the tweak results in something with better SINAD and more importantly a better IMD measurement, then it's an improvement.
But based on ASR measurements of Purifi Eval version and Purifi-based NAD version, the NAD SINAD appears too far adrift for the NAD to be better sounding. In the end, the stock Purifi always seems to be more signal, less noise and distortion. If somebody likes something else, and many will, then they like a type of distortion.