Quad Op Amp for Audio

[Martin Czech]

:::The noise figure for op-amps is an important issue for audio.
:::The 741 type op-amp has a huge 35 nv/Hz figure.
:::Fairchild RC4136 is in the 10 nv/Hz range.
:::NE553x is in the 5 nv/Hz range.
:::Anything below 20 nv/Hz is usable for +/-15 volt supplies and
:::+10 dB (10 v peak to peak) signal levels.
:::

It is current and voltage noise.  Bipolar input stages have
low voltage noise, this is good for low impedance sources. FET inouts
have low current noise, which is good for high source impedance.
So, for a ota state variable a FET type might be a good choice.

:::Do not use them for building guided missiles as they do
:::not like to operate at 125 degrees C (input leakage
:::currents go up significantly over military temperature ranges).
:::

Even at moderate temperature FET inputs pull more bias current then
bipolar things (break even about 50C Ta). See LM11 app notes/data
sheet. The current goes up exponentially. MOSFET inputs are thought to be
better, but they usually have a zener/diode junction as ESD protection,
which also draws exessive currents if the chip temp rises.

Offset voltage and offset voltage drift is another problem of
FET devices (this may not matter in audio appl.).

m.c.