It is most likely used to increase the quiescent current in class A of the op-amp.
I just ordered and will test the OPA1641, the OPA16xx SoundPlus family looks exciting.
Yes, and I just noticed that they interestingly chose to use a P-channel device there, rather than the expected N-channel.
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Having experimented with NE5532's in the last while they have an odd sonic character, unless some form of pull-up is used on the output to the positive rail. This can be just a light resistor (~ 1mA) if signals are small, to >5mA using active elements if signals are larger and the output is loaded. There isn't one value for all occasions, as sonics can deteriorate in "seemingly" going too far into "class A" when signals are too small in voltage and current. Nevertheless it does seem they need a minimum of about 1mA to remove the oddity of its character.
In applying such "compensation", for exactly what I am not entirely sure, they compete with others listed here, in fact I prefer them depending on how they are used. It seems that NE5532's can dig into sonics more deeply than all the others tried... reaching into a darker background. Unfortunately they seem also to reveal the limitations of themselves.
By the way, a top contender was the LT1364, but I went back to the NE5532 only because of the pull-up current, with no desire yet to go back.
I've whittled down my favourites list to OPA1612 and AD8022. OPA1612 has the better treble, AD8022 is nice and balanced, but they're both really good. Good piano tone, good micro details and ambience around voices. Honourable mention to LT1364 and LT1361 - they are both dynamic and involving even if the tonalities are rather more rounded. The AD8066 is rather flat in comparison and OPA1642 more so. Both FET if that makes any difference. Both are neutral with good tone, but just less involving.
OPA1656 ( modern clean smooth sound, nice "deep" bass, which is "the" pro for me for this opamp )
OPA627 ( goes actually into the oldschool direction, it has similar attributes as the burson V6 vivid, but latter sounds more refined )
OPA1612 ( pretty clean, but for me it sounds abit boring at times, less smooth and less deep/slammy bass than the OPA1656 )
OPA1622 ( similar to the OPA1612 from what i remember)
OPA2134 ( sounds flat but it doesnt get more boring than this... )
OPA1692 ( definitely has its own sound, but imo other opamps sound "overall" better )
LM4562 ( ok-ish, but a bit bright sounding )
OPA627 ( goes actually into the oldschool direction, it has similar attributes as the burson V6 vivid, but latter sounds more refined )
OPA1612 ( pretty clean, but for me it sounds abit boring at times, less smooth and less deep/slammy bass than the OPA1656 )
OPA1622 ( similar to the OPA1612 from what i remember)
OPA2134 ( sounds flat but it doesnt get more boring than this... )
OPA1692 ( definitely has its own sound, but imo other opamps sound "overall" better )
LM4562 ( ok-ish, but a bit bright sounding )
OPA2810 (headphone amp)
AD8512 (line outs and mic inputs)
OPA1656 (mic inputs)
all three are in my Black Lion Audio Revolution 2x2 Interface
imo from sonics the AD8512 on the line outs is definitely worth a shot, its a very simple output stage just containing one dual opamp per channel,
also the OPA2810 as headphone driver doesnt seem bad
i just looked into the device again to document it here, i havent A/B compared these opamps (beside the OPA1656) as they are soldered SOIC8
AD8512 (line outs and mic inputs)
OPA1656 (mic inputs)
all three are in my Black Lion Audio Revolution 2x2 Interface
imo from sonics the AD8512 on the line outs is definitely worth a shot, its a very simple output stage just containing one dual opamp per channel,
also the OPA2810 as headphone driver doesnt seem bad
i just looked into the device again to document it here, i havent A/B compared these opamps (beside the OPA1656) as they are soldered SOIC8
Attachments
Don't let that discourage you by no means...
This is my Roland Super UA-S10, which I use exclusively as a DAC and headphone amp. It has a great sound! I replaced the original SOIC8 NE5532 (TPA6120A2 driver) with my favourite AD8066. Working with SOIC8 is a bit of a hassle, but it is not a major issue by any means.
The outboard power supply is my low-noise linear power supply; I also placed the 2 x {1LT3045 - 1A} LDOVR regulators (connected in parallel) inside the unit... that's supposedly good for 2A. A fantastic overall improvement.
Few OpAms to to add to the "good list":
Alex.
- regular: OPA2156, OPA2140, AD8397
- FDA: OPA1632, LME49724, ADA4595
Alex.
Funny with these kind of comparison threads, be them about op-amps, tweeters, power amps or even cables... there is never a distinct consensus - almost every single product will come up as someones favourite 🙂 It never goes like; yes, milk taste better than gasoline in coffee - and everybody agrees... except for maybe one petrol head out of 30 judges.... 🙂 which really don't think so but ....
This all probably tells a message that there aren't any big differences really - thats the only conclusion I think one can draw... it's a non critical choice. Take something modern and recent with proper specs and you are golden.
And it just takes a little tilted balance or anomaly in the rest of the used system for the evaluation, to skew the result a bit...
Sorry for the downer... 🙂 but I couldn't help the observation.
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This all probably tells a message that there aren't any big differences really - thats the only conclusion I think one can draw... it's a non critical choice. Take something modern and recent with proper specs and you are golden.
And it just takes a little tilted balance or anomaly in the rest of the used system for the evaluation, to skew the result a bit...
Sorry for the downer... 🙂 but I couldn't help the observation.
//
no downer really, i think you are specially right with this:
there are many good recent opamps it really comes down to preference imo, different styles of tastes, music, system synergy etc..
i was a bit surprised the that the OPA627 (a somewhat older design) is the closest sounding one to the discrete opamp i tested (burson V6 Vivid)
and while the opa627 probably would still count as "audible transparent" it really begs the question how much of somewhat miniscule measuring changes are audible, for example a distortion profile with peaks at -100db
this would for example explain why a h2/h3 dominant distortion profile is preferred over a more random one with overall lower noise figures...
Specs alone might not suffice, yes.
But proper measurement would show the differences, in the literal sense (subtractive measurement -- aka null testing -- with any audio signal). So far no one has succeeded in showing significant differences for well-designed standard opamp circuits with good parts (no 741's etc) like the one under discussion here, including myself...and I really tried hard, maybe even harder than anyone else for more than two decades, now being able to expose differences 140dB(!!!) below signal level (a substantial effort).
Also, the moment proper controls are used in comparative listening tests ("yeah, proper blind testing, baby!"), all the differences seem to go away. This is good news as it allows us to freely choose our favorites without risks.
BTW, I'm not saying that people who perceive differences are making things up. The perception, and the perception difference, is real and happening (neurologically in the brain) but the actual audio signal at the ear drums may not have been any different at all.
yes actually quite similar experience, they did sound very close to me, close enough that its really hard to tell which one is more correct, but i cannot deny that there are some audible difference anyway
My experiences exactly. Some of my short list were very close indeed, like AD8066 and OPA1642. Others had audible differences - the LT1364 and LT1364 were distinctly different from OPA1612 and AD8022, more dynamic but also more rounded and with less ambience around voices. I listen several times to recordings of the grand piano - that's always a very good test. These differences do matter - you obviously choose the best sounding op-amps to live with in your equipment. I listened to a variety of very well-known tracks over a few days to make my choice.
I'm quite satisfied that these small differences are not "just in my head". That's a well-worn old fall-back position for those who can't go beyond measurements and is completely un-verifiable. I'm a pro musician so I'm very sensitive to the sound of acoustic instruments which I've lived with all my life, and I'm also a psychologist and I'm well acquainted with the various forms of bias and the steps you routinely take to lessen or eliminate them.