[sdiy] OT: TL07x Friend or Foe?

David G. Dixon dixon at interchange.ubc.ca
Tue Sep 28 07:04:46 CEST 2010

Did you just try one specimen of TL074?  Are they all the same, or did you
get a bum one?  I've used dozens of the little beasties, and my system is
virtually noise-free and I don't notice any loss of high frequencies, but
then again I'm processing VCO-derived waveforms and not audio (and my
hearing peters out at about 13kHz owing to aural self-abuse).  I'd really
like to know WHY your compressor sounds this way.

Here is the list of opamps tried, reordered from best sound to worst sound,
with slew rates (in V/us), gain-bandwidth products (in MHz) and noise at 1
kHz (in nV/Hz^0.5) tabulated (all numbers rounded to nearest integer):

LT1359 - Hiss very low. Sounds very good.	  SR = 600   GBW = 25   N =
TLE2144 - Hiss very low. Sounds good.	  SR =  45   GBW =  6   N = 11
TLE2074 - Hiss very low. Sounds good.	  SR =  35   GBW =  9   N = 12
OPA404 - Hiss very low. Sounds good.	  SR =  35   GBW =  6   N = 15
OPA4131 - Hiss low. Sounds OK.		  SR =  10   GBW =  4   N = 15
TL074 - Hiss loud. Sounds poor.		  SR =  13   GBW =  3   N = 18

The LT1359 is obviously a really high-quality op-amp (and it costs about 10
times what TL074 costs as well).  There are piles of other specs one could
look at, but I would guess that these are the important ones for audio.
Still, even with a GBW of only 3 MHz, there should still be tons of
frequency headroom for audio at unity gain.   The noise numbers don't look
terribly different -- certainly not enough to give audible hiss in the
TL074.  The low slew rates could perhaps cause a bit of third-harmonic
distortion at high frequencies in signals with large amplitudes.  However, I
can't imagine that the effect of any of these numbers would even be
noticeable for audio signals.

I'd really like to hear what the gurus have to say about this issue.

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