Raimund:
Thanks for the info!
As I remember, the optimal noise performance of an op amp occurs when the s=
ource
impedance is equal to the op amp's voltage input voltage noise divided by t=
he
input current noise. For the INA163 this is 1500 - 2500 ohms, depending on =
what
value you pick for Ein (it is frequency dependent). This is certainly not 1=
50
ohms, so I wonder what the actual real-world noise performance of this op a=
mp
will be. The data sheet states "near theoretical", but stops at giving real=
figures. So maybe it is good, but maybe not as good as they would like one =
to
believe. In any case, I doubt that it will be showing up in devices such as=
an
LS-xx recorder because of the supply rail requirements.
Transformer coupling at the input of an op amp can be beneficial for a whol=
e
host of reasons. By picking a good, low-noise op amp and then picking the
transformer to match the=C2=A0source (mic)=C2=A0impedance properly to the o=
p amp's Ein/Iin
figure, one can obtain an excellent mic preamp. I have done this in the pas=
t
with very good results. The subtle nonlinearities of transformers are also=
considered to have beneficial effects to the sound. So I don't believe that=
transformer-coupled mic preamps will die out in he near future.
Utlimately. it will be the end user that will decide what is best, assuming=
that
the products available will give them that choice.=C2=A0I would like to see=
a really
good op amp-only circuit topology that covers all of the bases for low nois=
e and
excellent performance under all input conditions (especially with very low=
levels), but I haven't seen one yet.
One other issue that I see with the INA163 is the slew rate of 15 V/uS. Tha=
t is
not terribly good for a preamp that may be called on to amplify frequencies=
above 20 KHz. THAT Corp. has=C2=A0a new op amp, the 1583, that looks promis=
ing:
http://www.thatcorp.com/1583_Low-Noise_Differential_Audio_Preamplifier_ICs.=
shtml
Time to roll up the sleeves and start building and testing this stuff!
Best regards,
Greg O'Drobinak
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