Does Op-amp Rolling Work?

[perceptionist]

Purifi just released their deluxe EVAL8, EVAL9, & EVAL10 with FET-input discrete OPAMPs. Will they roll?

 

[BeeKay]

You will not persuade the believers.

Which is why this planet is doomed on so many levels.

 

[solderdude]

Purify's motivation for their discrete op-amp design.

“Discrete is an old audiophile hobby horse,” said Lars. “It proves nothing. Consider writing a few words.”


Yes, precisely. Doing things discretely has no inherent value. The normal procedure is that you have a set of requirements and you go looking for a solution. These days, that solution is usually available as a chip, particularly for something as bread and butter as an op amp. Duplicating that with separately packaged transistors will add exactly zero value. You only do that if you want something that chip makers aren’t offering. It’s the circuit that matters, not whether it’s on a die or on a circuit board.


The specific challenges lie in the application. We’re looking for an op amp to be used in a low gain buffer stage.

• Input impedance should be high, or it wouldn’t be a buffer. The op amp must be used in a noninverting configuration.
• It should also be linear. If the source impedance and the impedance of the feedback network are not matched, non-linear capacitances of on-chip ESD diodes can cause more distortion than the entire op amp. That problem goes away if you can match the impedances on both inputs. If the circuit is inverting, the inputs don’t swing so the problem goes away too. But as I said, we’re looking at a noninverting buffer and we can’t second-guess the source impedance.
• The output must be able to drive a heavy load without causing distortion to increase. The noise of a low-gain buffer is dominated by thermal noise in the feedback network, not the op amp input noise. So, to get low noise the feedback network has to have a pretty low impedance.
• We also don’t want to resort to class A. The idea of an input stage that has greater idling losses than the attached power stage does not appeal. This means dealing with distorted currents flowing through the power pins and that distortion couples magnetically into the inputs. What muppet came up with the idea of putting an op amp’s power pins as far away from each other as possible? I despair when I see people market discrete op amps that simply copy that pinout. If any IC people are reading: please put the power pins next to each other in the future. The sum of both currents is undistorted so all you need to do is run the +/- power traces directly on top of each other.
• Finally, the traditional op amp has no explicit output reference. Crudely put, the output voltage is GBW times the integral of the input voltage, but that’s relative to what potential precisely? It’s whichever rail the transimpedance stage sits on. Noise on that rail is coupled directly to the output, and it’s left to the feedback loop to remove it. And so, op amp chips have two PSRR curves. The worst of the two is essentially equal to loop gain. I like to see an explicit reference pin that I can tie to ground and that way get much better PSRR. That reference pin should be directly adjacent to the output pin (a shout-out here to the OPA1622 which does have an explicit ground pin, and power supplies sensibly close together – it has other shortcomings unfortunately).

That’s quite a shopping list, and since there doesn’t seem to be a chip around that ticks all boxes, discrete it is.

Will people try to roll these input buffers in their amplifiers ?
I am sure some will try.
This requires creating a 'DIP-8 converter' to start with.

It has to do with signal quality (performance)... op-amp rollers are all about 'the magic' and 'the sound quality increase' they can clearly hear but not explain.

 

[caught gesture]

The brain is excellent at processing information and extrapolating from available data to fill in the blanks from sensory inputs. For example, the optic nerve attachment to the retina creates a large "blind spot" in our field of vision, yet we are generally unaware of the missing information (until we collide with a bicyclist that we didn't see). The ear is a relatively crude transducer - think of a $5 microphone plugged into a $5,000,000 Cray computer. I diagnose this as a case of opamp OCD.

The information throughput of a human being is about 10 bits/s. In comparison, our sensory systems gather data at ~ 10^9 bits/s. The stark contrast between these numbers remains unexplained and touches on fundamental aspects of brain function: what neural substrate sets this speed limit on the pace of our existence? Why does the brain need billions of neurons to process 10 bits/s? Why can we only think about one thing at a time? The brain seems to operate in two distinct modes: the ‘‘outer’’ brain handles fast high-dimensional sensory and motor signals, whereas the ‘‘inner’’ brain processes the reduced few bits needed to control behavior.

https://meisterlab.caltech.edu/documents/30100/Zheng_2024_The_unbearable_slowness_of_being-_Why_do_we_live_at_10_bitss.pdf

 

[KSTR]

Purify's motivation for their discrete op-amp design.

Their points are valid but could have also been fully addressed with a suitable composite opamp. Total effort might be comparable to a fully discrete circuit, though, depending on the details.
But I'm skeptical that it's 100% technical reasons only why they went the discrete route.

 

[Sokel]

Rolling aside, there's a real danger about quality as well.
If one is not absolutely certain about origin and if the OPA's are not bought through official channels (Mouser, TME, DigiKey, etc) can easily end up like this:

Free distortion generator anyone???


Edit: a frightened friend here asked me if it's outputting DC as well, I checked and it's not, totally negligible.

 

[antcollinet]

Rolling aside, there's a real danger about quality as well.
If one is not absolutely certain about origin and if the OPA's are not bought through official channels (Mouser, TME, DigiKey, etc) can easily end up like this:

View attachment 490109


Free distortion generator anyone???

where was that op amp from?

 

[Sokel]

where was that op amp from?

That's a supposed OPA2134 from the known East Asia market, of the ones that sell 1 euro each, or less
At least, one can tell it's not real, even physically, this one even misses the dot that marks the No1 pin at the original BB ones.
Tell me if you want photo.

 

[Sokel]

And the comparison with a real one at the same silly circuit I have built (an unbal to bal converter, it's at the DIY subforum):

Same results at an el. x-over as well, even worst at lows.

So....

 

[amirm]

Their points are valid but could have also been fully addressed with a suitable composite opamp. Total effort might be comparable to a fully discrete circuit, though, depending on the details.
But I'm skeptical that it's 100% technical reasons only why they went the discrete route.

I suspect they are sitting there, wondering how they can match what Topping has done with composite op-amps.

 

[Sokel]

I suspect they are sitting there, wondering how they can match what Topping has done with composite op-amps.

They are forced to use higher gain and input impedance though (at finished products), it's hard to combine all at the same time.

 

[SnakeLord]

Purify's motivation for their discrete op-amp design.

There is an pretty recent Bruno P. interview on YT which i cannt find atm. Its divided in 4 parts.
Bruno is speaking about the "real" reasons on that interview which in your case are just hidden behind a bunch of technical snake oil blablabla quotes. Even if the general idea of it is right, yet it can be applied to basically every single aspect of the module/board design - so its not an valid argument for a discreet op-amps solution. You can make same working/effective design as an fully integrated design like ICEpower does for example. It might be complicated design which requiers some know-how but its solved!

So what Bruno is basically saying: Amp-Companies asked him to give them at least something they can customize on an amp designs besides the casings. So Purifi/Bruno came up with the custom Buffer and discreet op-amps idea since those amp-Brands needed their unique selling points and they try to differ from each other.

Thats basically it, no magic here to find again...

 

[Sokel]

So what Bruno is basically saying: Amp-Companies asked him to give them at least something they can customize on an amp designs besides the casings. So Purifi/Bruno came up with the custom Buffer and discreet op-amps idea since those amp-Brands needed their unique selling points and they try to differ from each other.

Thats basically it, no magic here to find again...

Linear PSUs for some as well.
Until Purifi outs its own PSU of course, which is highly expected, mostly for built quality, western certifications, etc.

 

[solderdude]

So what Bruno is basically saying: Amp-Companies asked him to give them at least something they can customize on an amp designs besides the casings. So Purifi/Bruno came up with the custom Buffer and discreet op-amps idea since those amp-Brands needed their unique selling points and they try to differ from each other.

Thats basically it, no magic here to find again...

The buffer circuit which is used is not a simple pin compatible DIP socket so still is not 'swap-able' nor can it easily be applied to any other design either without making DIP adapters.
So basically just integral part of the design and not a matter of simply swapping the buffer for some generic/audio op-amp.
At least not in the current designs.

 

[pma]

Purify's motivation for their discrete op-amp design.

All true, IF the points are really important .

 

[cgallery]

I was comparing the 5532 to the LME4562 and just when I thought I had a "tell," at least one of the 4562 units started popcorning.

So frustrating.

 

[JSmith]

I was comparing the 5532 to the LME4562 and just when I thought I had a "tell," at least one of the 4562 units started popcorning.

So frustrating.

This is exactly why people that aren't circuit designers (or unable to measure) shouldn't be swapping opamps... it's oscillating and unstable.


JSmith

 

[cgallery]

This is exactly why people that aren't circuit designers (or unable to measure) shouldn't be swapping opamps... it's oscillating and unstable.


JSmith

No, the 4562 is often used in this particular amp and the manufacturer has even sold the amp with the 4562. And I've had the 4562 flake in devices supplied with that op amp.

 

[JSmith]

No, the 4562 is often used in this particular amp and the manufacturer has even sold the amp with the 4562. And I've had the 4562 flake in devices supplied with that op amp.

You haven't even specified what amp and what devices... but are you just randomly swapping opamps for no reason, or?


JSmith

 

[Sokel]

I was comparing the 5532 to the LME4562 and just when I thought I had a "tell," at least one of the 4562 units started popcorning.

So frustrating.

OPA's origin?
Put in an adapter with nice long pins I guess.

Schematic?