Several times while listening tests I have observe the same.
Over the years the most important question was for me, why the traditional measurements don't show this effects. In the meantime I know, that there is one diagram, that show this effect:
The distortion and noise waveform with fundamental notched out.
I have upload this diagram of two different power amplifiers from stereophile magazine in post #5 under
Linn SMPS for LM3886
I don't think a listening test is required to find out, which of both power amplifier sounds better and more ear-friendly - as long as those diagrams present. The same applies analogously to integrated operational amplifiers - I am convinced of that (in general - not the THD-value determines the sound character by all listening tests - rather the kind of distortion; this means 0,5% THD may be better than 0,001%).
The suited frequencies for creating this above mentioned diagrams by measuring op-amps should be 1Khz, 10KHz and 50KHz - so I think.
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Thanks Russell, I will consider the OPA1642 on my next Mouser shopping. What about OPA1656 as buffer?
Of course! And all in all 8 people (without me) in the tests, 2 of them professional musicians (classical). One was an audiophile shop worker.
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I use this config:
MAINS----------MF---IT--->PC
MAINS----------MF---IT--->DAC + ---MF--->PreAmp + Tuner + RIAA + Turntable
MAINS----------MF-------->PowerAmp
MF = Mains Filter
IT = Isolating Transformer
Absolutely right! I remember some British experts in the 80's claimed & proofed that THD is more or less meaningless. Much more important is how the harmonics were put together.
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Yes, I believe you are correct---the sonic 'signature' of the distortion determines its sound quality more than the actual total THD number. Hard to put a number on this, though. A good test and maybe a way to quantify this might be to use Rod Elliott's idea of subtracting the input from the output signals of a device, adjusting to even out levels, and then amplifying this difference and plotting it with a spectrum analyzer.
I can highly recommend the opa1656, use it in my newest DAC with great result.... Low noise, high speed and high performance CMOS part. Also good as output driver, typ Iout 100 mA.
Only problem is that it's SMD only, and seems to be hard to source, at least right now....
That's good. How close in decibel did you match levels, how did you make it double blind and how was the switching done?
Oh, hey that's come out recently. It has some specs even better than the 1642. I haven't played with any, nor have I talked to anyone who has. Looking at the specs, I would use if for gain, but not for buffering. I would be afraid of that chip in noninverting unity gain buffers. I has very little phase margin at unity gain.
I think I'll stand my my statements: OPA1612 for buffering and places where base current can be tolerated, and for gain if low value input resistors can be used. OPA1642 for filtering. I will adjust my statement and recommend the OPA1656 for gain when you need to use larger input resistors.
The slew rate is at 40 V/uSec!!! Fuckotastic!!! But that is not s guarantee for good sound.
Buy OPA1656s when you can. There has been a little shortage of these very fine OpAmps recently. So if you are ordering from Mouser or Digikey, do yourself a favour and grab at least 10 of them while you are at it 
Low price, fairly low noise across a wide range of source impedances, high output drive capability, high open loop gain for a unity gain stable OPA - what's not to like?
Low price, fairly low noise across a wide range of source impedances, high output drive capability, high open loop gain for a unity gain stable OPA - what's not to like?
Thank you Knoppers! Well, the good thing is that my huge Mouser cart is more or less ready just add some 1656 to it and voila! I'm curious about the sound. Maybe I also should put some of them in my Onkyo T-4970 tuner, the LME49720 in comparison to the original opamps made a huge difference already.
What a difference an opamp makes
Using only little powers
Brought the sun and the flowers
Where there used to be rain
What a difference an opamp makes
There's a rainbow before me
Skies above can't be stormy
Since that moment of bliss and never missing the hiss
What a difference an opamp makes
Using only little powers
Brought the sun and the flowers
Where there used to be rain
What a difference an opamp makes
There's a rainbow before me
Skies above can't be stormy
Since that moment of bliss and never missing the hiss
I think the problem I'm seeing with the OPA2156, and to a lesser extent the OPA1656 is that the 1/f knee for the noise is much higher than for the OPA1612 and OPA1642.
1/f corners:
OPA1612 ---- 10 Hz
OPA1642 ---- 20 Hz
OPA1656 ---- 1 kHz
OPA2162 ---- 1 kHz
It also looks like the OPA1642 and OPA2162 are not happy driving 600 ohms, but the OPA1612 and OPA1656 are. For DAC-output IV converters, I'd choose the OPA1656.
1/f corners:
OPA1612 ---- 10 Hz
OPA1642 ---- 20 Hz
OPA1656 ---- 1 kHz
OPA2162 ---- 1 kHz
It also looks like the OPA1642 and OPA2162 are not happy driving 600 ohms, but the OPA1612 and OPA1656 are. For DAC-output IV converters, I'd choose the OPA1656.