Op amp buffer

[TimWaldvogel]

I am building a couple op amp buffers. I just ordered a few OPA134's and a few LF351N's, any reason one would be better for a buffer? GUS recommended a Jfet op amp for a buffer if I was gonna do an op amp buffer

Opinions/experiences?

[earthtonesaudio]

The datasheets say the OPA134 is quieter.

[TimWaldvogel]

That's what I thought, but I wasn't sure if a 2m2 input impedance would change that factor

[merlinb]

That's what I thought, but I wasn't sure if a 2m2 input impedance would change that factor

Earthtones is right, the OPA is the quieter of the two in this application as it has the lowest current and voltage noise. Both are very good though- you won't hear the difference or anything.

[Ben N]

If you're not interfacing with a hi-Z source like a guitar, you may be able to lower the value of the input Rs to improve noise performance. Then again, if Z-in is not an important consideration in your application, you may be able to use something like a 5532, which is very low noise with a suitable source and can drive almost any load.

[amptramp]

That's what I thought, but I wasn't sure if a 2m2 input impedance would change that factor

For noise calculation purposes, the input impedance of the buffer is in parallel with the output impedance of the guitar.  I would run the guitar at maximum gain so that the impedance becomes the 2M2 of the buffer in parallel with the ~500K of the pot on the guitar in parallel with the resistance of the pickup.  That is on the order of 7000 ohms, so the total noise resistance would be maybe 6800 ohms.  Any coupling capacitance disconnects the parallel resistances so the noise may go up at low frequencies, but low frequencies are usually cut in a guitar amp or pedal chain.  If you want to control volume, do this at the output of the buffer (meaning it may be a good idea to put the buffer in the guitar) - but a lot of people get by without going to these extremes, so don't sweat it.

[TimWaldvogel]

would stacking op amps in a buffer make much of a difference ?