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|28th July 2012, 05:36 PM||#1|
Join Date: Jul 2012
Choosing op amps for Essence STX I/V stage
I recently bought a ASUS Essence STX, although it's far better than the integrated soundchip on my mainboard, I'm thinking about changing better op amps on the card.
I find some good op amps but choosing one from them seem to be a difficult trade-off on the electrical characteristics, the followings are what I found:
OPA627 (or similar ADA4627): Maybe the most well known audio op amp ever,
I found the PCM1704 from TI uses OPA627 in the I/V stage in the datasheet example diagram.
But its capacitive-load driving capability is very poor, I don't know whether it will cause ringing problem when driving the LPF stage.
AD8620: Has similar characteristics from the OPA627/ADA4627, but has a little bit higher input bias current and a little bit lower dynamic performance.
The capacitive-load driving capibility is far better (there is a comparison on the capacitive-load driving between OPA627 & AD8620 in datasheet p.14.)
AD827: A very fast op amp, originally designed for video purposes, but many people also use it in audio systems.
It really seems good for DAC output I/V conversion.PCM1794 datasheet says that
"Dynamic performance such as the gain bandwidth, settling time, and slew rate of the operational amplifier affects the audio dynamic performance of the I/V section."
And it can drive heavy capacitive-loads (With lots of internal compensations.)
But its noise is high in some extent for modern op amps.
LT1364: Similar to but faster than the AD827, and has the same questions.
LME49990/LME49720/AD797: These low noise and distortion series may be ideal for the preamp stage, many people also put them in the I/V stage and got satisfactory results.
But the dynamic performance is not so good as those ones above.
(I don't know how fast is really just enough for this kind of application.)
ADA4898: Transimpedence amplifiers do the job of "I/V conversion" so is it good to use those op amps with these kind of characteristic (such as low input resistance?)
(If it is true, then FET-input parts seem to be bad choices?)
High dynamic performance combined with low input resistance:
55V/µs, 65MHz (-3dB freq.), Ri=5kΩ.
And its input noise is very low, too.
Or some people could recommend other good ones for me?
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